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Tunable microring resonators using light-activated functional polymer coatings

Amado Velazquez-Benitez and Juan Hernandez-Cordero

DOI: 10.1364/OL.399411 Received 04 Jun 2020; Accepted 25 Sep 2020; Posted 25 Sep 2020  View: PDF

Abstract: We demonstrate tunable microring resonators (MRs) based on light-activated functional polymer coatings deposited on glass optical fibers. MRs were fabricated using two layers of polydimethylsiloxane-based compounds: one incorporating an azobenzene dye and one using a fluorescent ytterbium and erbium-doped sodium yttrium fluoride powder. The latter yields a photoluminescent composite producing green upconversion emission under infrared pumping. This visible emission then allows for triggering photoinduced birefringence effects in the azobenzene layer, thereby modifying the spectral features of the MR devices. The shift in the resonance peaks as a function of pump power is linear yielding a tuning range of 1.3 nm. Aside from the photoinduced effects observed in these tunable MRs, we also discuss the photothermal effects involved in the tuning mechanism.

Dual-output fs/ps burst-mode laser for MHz-rate rotational coherent anti-Stokes Raman scattering

Michael Smyser, Erik Braun, Venkat Athmanathan, Mikhail Slipchenko, Sukesh Roy, and Terrence Meyer

DOI: 10.1364/OL.404984 Received 13 Aug 2020; Accepted 25 Sep 2020; Posted 25 Sep 2020  View: PDF

Abstract: A burst-mode laser system is developed for hybrid fs/ps rotational coherent anti-Stokes Raman scattering (RCARS) at MHz rates. Using a common fs oscillator, the system simultaneously generates time synchronized 1061 nm, 274 fs and 1064 nm, 15.5 ps pulses with peak powers of 350 MW and 2.5 MW, respectively. The system is demonstrated for two-beam fs/ps RCARS in N2 at 1 MHz with a signal-to-noise ratio of 176 at room temperature. This repetition rate is an order of magnitude higher than previous CARS using burst-mode ps laser systems and two to three orders of magnitude faster than previous continuously pulsed fs or fs/ps laser systems.

Fiber-laser-pumped picosecond optical parametric generation and amplification in MgO:PPLN

Biplob Nandy, Chaitanya Kumar Suddapalli, and Majid Ebrahim-Zadeh

DOI: 10.1364/OL.403699 Received 24 Jul 2020; Accepted 24 Sep 2020; Posted 25 Sep 2020  View: PDF

Abstract: We report the generation of tunable high-repetition-rate picosecond pulses in the near-infrared at high average power with record conversion efficiency using singe-pass optical parametric generation (OPG) and amplification (OPA) in MgO:PPLN, for the first time to our knowledge. By deploying a mode-locked Yb-fiber laser at 1064 nm providing 21 ps pump pulses at 80 MHz, and a cascade of two 50-mm-long MgO:PPLN crystals, we generate up to 8.3 W of total average output power at a conversion efficiency of 59% over a tunable range of 513 nm, across 1902-2415 nm, with a record threshold as low as 600 mW (7.5 nJ). The two-stage OPG-OPA scheme provides control over fine wavelength tuning and output spectral bandwidths, enabled by independent control of phase-matching in each crystal. The OPG-OPA output exhibits high spatial beam quality and excellent passive power and central wavelength stability better than 0.9% rms and 0.1% rms, respectively, over 1 hour. The output pulses have a duration of ~11 ps, with 10dB bandwidth of ~350 nm at 2107 nm.

Twisted ribbon carousels in random, three-dimensional optical fields

Isaac Freund

DOI: 10.1364/OL.404574 Received 04 Aug 2020; Accepted 24 Sep 2020; Posted 25 Sep 2020  View: PDF

Abstract: The instantaneous electric vector E in a random three-dimensional optical field is shown to generate twisted ribbon carousels that spin about their axes. The ribbons can be right or left handed, and can unwind and rewind in time, changing their handedness during an optical cycle. Analytical formulas describing this behavior are presented.

Efficient low-brightness-pumped Raman amplification of a single high-order Bessel-mode in 335-m of 70-μm-diameter silica-core step-index fiber

Sheng Zhu, Yutong Feng, Pranabesh Barua, and Johan Nilsson

DOI: 10.1364/OL.404602 Received 07 Aug 2020; Accepted 24 Sep 2020; Posted 24 Sep 2020  View: PDF

Abstract: We experimentally demonstrate Raman amplification of signal pulses in a high-order Bessel mode (LP06) at a wavelength of 1121 nm in a 335-m step-index fiber with a 70-μm diameter, 0.227-NA pure-silica core. This was pumped by 5-ns multimode pulses at 1065 nm from an Yb-doped fiber MOPA. The mode purity of the amplified pulses is well preserved to dB of average-power gain, to 774 W of peak power in 2 ns pulses at 20 kHz repetition rate, when pumped with a peak power of 942 W. The pump depletion as averaged over the signal pulses reaches 59%. We believe that this is the first demonstration of stable mode propagation and Raman amplification of a single Bessel-like higher order mode in a fiber of hundreds of meters. This shows the potential for efficient power scaling of a single signal mode with low-brightness pumping, comparable with that from continuous-wave multimode diode lasers.

Sub-ppb detection of benzene using cantilever-enhanced photoacoustic spectroscopy with a long-wavelength infrared quantum cascade laser

Juho Karhu, Hadrien Philip, Alexei Baranov, teissier roland, and Tuomas Hieta

DOI: 10.1364/OL.405402 Received 13 Aug 2020; Accepted 23 Sep 2020; Posted 24 Sep 2020  View: PDF

Abstract: We report a novel photoacoustic spectrometer for trace gas sensing of benzene. A quantum cascade laser emitting at the wavelength 14.8 μm is used as the light source in the spectroscopic detection. This wavelength region contains the strongest vibrational band of benzene, which is free of spectral overlap from common trace gases, making it a strong candidate for sensitive benzene detection. Cantilever-enhanced photoacoustic spectroscopy is used for detection. This simple and robust measurement setup can reach a benzene detection limit below 1 ppb.

Frequency ratio of an ¹¹⁵In+ ion clock and a ⁸⁷Sr optical lattice clock

Nozomi Ohtsubo, Ying Li, Nils Nemitz, Hidekazu Hachisu, Kensuke Matsubara, Tetsuya Ido, and Kazuhiro Hayasaka

DOI: 10.1364/OL.404940 Received 10 Aug 2020; Accepted 23 Sep 2020; Posted 23 Sep 2020  View: PDF

Abstract: We report on the first frequency ratio measurement of an ¹¹⁵In+ single ion clock and a ⁸⁷Sr optical lattice clock.A hydrogen maser serves as a reference oscillator to measure the ratio by independent optical combs.Over more than 90 000 seconds of measurement time, the frequency ratio f_{In^+} / f_{Sr} is determined to be 2.952 748 749 874 863 4(21) with relative uncertainty of 7.0×10^{-16}.The measurement creates a new connection in the network of frequency ratios of optical clocks.

3D porous graphene–assisted capsulized cholesteric liquid crystals for terahertz power visualization

Lei Wang, Ruiwen Xiao, Shengxin Yang, Hongsong Qiu, Zhixiong Shen, Peng Lv, Caihong Zhang, Wei Hu, Makoto Nakajima, Biao-Bing Jin, and Yanqing Lu

DOI: 10.1364/OL.405695 Received 18 Aug 2020; Accepted 22 Sep 2020; Posted 24 Sep 2020  View: PDF

Abstract: We demonstrate a high-efficiency visualized terahertz (THz) power meter based on the THz-photothermochromism of capsulized cholesteric liquid crystals (CCLCs) embedded in three-dimensional porous graphene (3DPG). The graphene is a broadband perfect absorber for THz radiation and transfers heat efficiently, and its black background is beneficial for color measurement. Quantitative visualization of THz intensity up to 2.8×〖10〗^2 mW/〖cm〗^2 is presented. The minimal detectable THz power is 0.009 mW. With multi-microcapsule analysis, the relationship between THz power and the average hue value of CCLCs achieves linearity. The device can convert THz radiation to visible light and is lightweight, cheap, and easy to use.

Terahertz pulse-altered gene networks in human induced pluripotent stem cells

Takehiro Tachizaki, Reiko Sakaguchi, Shiho Terada, Ken-ichiro Kamei, and Hideki Hirori

DOI: 10.1364/OL.402815 Received 16 Jul 2020; Accepted 22 Sep 2020; Posted 24 Sep 2020  View: PDF

Abstract: Terahertz (THz) irradiation has been exploited for biomedical applications in the non-invasive manipulation of living cells. Here, we develop an apparatus for studying the effects of THz pulse irradiation on living human induced pluripotent stem cells (hiPSCs). The spot of focused THz pulse covers the entire cell-culture area, where the maximum electric field reaching 0.5 MV/cm. RNA sequencing of global gene-expression analyses reveals that many THz-regulated genes are driven by zinc-finger (ZNF) transcription factors. These results suggest that the local intracellular concentration of metal ions, such as Zn2+, was changed by the effective electrical force of our THz pulse owing to its quasi-unipolar temporal shape.

Subcycle spatiotemporal compression of infrared pulses in χ² semiconductors

Andrew Hofstrand and Jerome Moloney

DOI: 10.1364/OL.403502 Received 27 Jul 2020; Accepted 22 Sep 2020; Posted 23 Sep 2020  View: PDF

Abstract: Using a full-field propagator model, we report on the emergence of highly-localized, subcycle solitonic structures for few-cycle long-wave-infrared (LWIR) pulses propagating through optical semiconductor materials with efficient quadratic nonlinearities and broad anomalous transmission windows. We briefly discuss the theoretical basis for the observed spatiotemporal carrier-wave dynamics and compare with simulations of a weakly perturbed pulse's propagation through two currently grown, low-loss IR semiconductor crystals.

A chirp-compensated pulsed Titanium-Sapphire oscillator-amplifier laser for precision spectroscopy

Wim Ubachs, Joel Hussels, Cunfeng Cheng, and Edcel John Salumbides

DOI: 10.1364/OL.401703 Received 01 Jul 2020; Accepted 22 Sep 2020; Posted 22 Sep 2020  View: PDF

Abstract: Active frequency-chirp compensation for a narrowband pulsed Titanium-Sapphire laser system is demonstrated using an intra-cavity electro-optic modulator resulting in improved spectral resolution and stability.With referencing to an optical frequency comb and further residual frequency chirp detection from shot-to-shot measurements, the resulting laser pulses are frequency up-converted for high-precision spectroscopy measurements in the VUV regime, where the relative uncertainty contribution due to frequency chirp is pushed to the $5 \times 10^{-11}$ level.

UVC lasing at 263nm from Ba2LaF7:Yb3+,Tm3+ upconversion nanocrystals microcavities

Ting Wang, Bitao Liu, Yue Lin, Qihua Yang, wei gao, Mingjie Li, Jianbei Qiu, Xue Yu, Xuhui Xu, and Siu Yu

DOI: 10.1364/OL.401768 Received 03 Jul 2020; Accepted 22 Sep 2020; Posted 22 Sep 2020  View: PDF

Abstract: It is a daunting challenge to realize ultraviolet C (UVC) lasing (i.e. has a wavelength range from 200 to 275 nm) from upconversion nanocrystals due to their low upconversion efficiency. Here, we fabricate Ba2LaF7:Yb3+(90 mol%), Tm3+(5 mol%) upconversion nanocrystals from amorphous borosilicate glass to support emission at ~263 nm under 980 nm ns laser excitation. The excitation threshold can be further reduced from ~130 to ~26.5 mJ/cm2 by using a cylindrical microcavity. We also found that the growth of defect-free Ba2LaF7 nanocrystals with a high concentration of co-doping Yb3+ and Tm3+ ions inside high optical damage threshold borosilicate glass is the key to achieve room-temperature UVC upconversion lasing under high-intensity excitation.

Single pixel hyperspectral Cherenkov-excited fluorescence imaging with LINAC X-ray sheet scanning and spectral unmixing

Xu Cao, Shudong Jiang, Jason Gunn, Petr Brůža, and Brian Pogue

DOI: 10.1364/OL.401286 Received 30 Jun 2020; Accepted 22 Sep 2020; Posted 23 Sep 2020  View: PDF

Abstract: Cherenkov light induced from MV X-rays during external beam radiotherapy serves as an internal light source to excite phosphors or fluorophores within biological tissues for molecular imaging. The broad-spectrum of Cherenkov light leads to spectral overlap with these luminescence emissions. To overcome this overlap problem, single pixel hyperspectral Cherenkov-excited fluorescence imaging is demonstrated using hyperspectral detection and spectral unmixing, coupled with light sheet scanning and filtered back projection (FBP) reconstruction. Thin scanned sheets of MV X-ray produced Cherenkov light to illuminate the planes within the tissue-simulating media. A fluorescence probe was excited by Cherenkov light, and the overlapped spectrum was detected by a time-gated spectrometer. A complete hyperspectral sinogram of the data was obtained through translation and rotation, and hyperspectral 2D images finally were reconstructed by FBP. Through linear spectral unmixing it was possible to resolve hyperspectral images of both Cherenkov and the resulting fluorescence intensity.

Localized structures formed through domain wall locking in cavity-enhanced second-harmonic generation

Carlos Mas Arabi, Pedro Parra-Rivas, Tobias Hansson, Lendert Gelens, Stefan Wabnitz, and François Leo

DOI: 10.1364/OL.399658 Received 08 Jun 2020; Accepted 22 Sep 2020; Posted 22 Sep 2020  View: PDF

Abstract: We analyze the formation of localized structures in cavity-enhanced second-harmonic generation. We focus on the phase-matched limit, and consider that fundamental and generated waves have opposite sign of group velocity dispersion. We show that these states form due to the locking of domain walls connecting two stable homogeneous states of the system, and undergo collapsed snaking. We study the impact of temporal walk-off on the stability and dynamics of these localized states.

Enhancing third harmonic generation by mirror-induced electric quadrupole resonance in a metal-dielectric nanostructure

Jin Yao, yan yin, Longfang Ye, Guoxiong Cai, and Qing Huo Liu

DOI: 10.1364/OL.400593 Received 22 Jun 2020; Accepted 22 Sep 2020; Posted 22 Sep 2020  View: PDF

Abstract: Electric quadrupole resonance (EQR), a commonly available high-order Mie-type resonance in all-dielectric nanoparticles, suffers from weak field enhancement and thus the inferior third harmonic generation (THG). In this work, according to the intrinsic centrosymmetry of current distribution, the mirror-induced EQR in a silicon disk is effectively generated by introducing a bottom metal film with perfect electric conductor (PEC) mirror effect, manifesting the preeminent capabilities of far-field scattering and near-field enhancement. The beneficial THG by mirror-induced EQR is enhanced by more than two orders of magnitude as compared to that of the typical EQR without PEC mirror effect. Furthermore, the influence of silicon Kerr effect on THG is investigated under the increasing pump intensity, achieving a maximal efficiency of 2.2×10^-4 under pump intensity I_0 = 3 GW/cm^2. This work opens new possibilities for exploring new mirror-induced Mie-type resonances in hybrid nanostructures, and finds their important applications in frequency conversion, spectroscopy and sensing at the nanoscale.

Fabrication of antireflection microstructure on the surface of GaSe crystal by single pulse femtosecondlaser ablation

Andrey Bushunov, Andrei Teslenko, Mikhail Tarabrin, Vladimir Lazarev, Lyudmila Isaenko, Alexander Yelisseyev, and Sergei Lobanov

DOI: 10.1364/OL.404515 Received 03 Aug 2020; Accepted 22 Sep 2020; Posted 22 Sep 2020  View: PDF

Abstract: GaSe crystals are promising as nonlinear optical con-verters in the mid- and far-IR ranges. However, it is impossible to increase the GaSe surface transmittance of 77% with conventional antireflection coatings because of poor surface quality, leading to coating adhesion problems. Antireflection microstructures (ARMs) offer an alternative way of increasing surface transmittance. In this work, ARMs were fabricated on the surface of a GaSe plate by single-pulse femtosecond laser ablation. An average GaSe surface transmittance of 94% in the 6–13 μm range and a maximum transmittance of 97.8% at 8.5 μm were obtained. The proposed method can be used to increase the efficiency of GaSe-based non linearconverters.

Dynamic polarization control for free-space continuous-variable quantum key distribution

Shiyu Wang, Peng Huang, Tao Wang, and Guihua Zeng

DOI: 10.1364/OL.404589 Received 07 Aug 2020; Accepted 22 Sep 2020; Posted 22 Sep 2020  View: PDF

Abstract: Free-space quantum key distribution (QKD) is attractive for the establishment of future global-scale quantum networks. However, it can be quite difficult for dynamic polarization control required in continuous-variable QKD systems to work properly in the presence of channel fading. Here we propose a dynamic polarization control scheme and verify its validity via simulations and an experiment performed over a 150-m free-space channel. The results indicate the capability of the scheme to effectively control the states of polarization for free-space continuous-variable quantum communication.

Giant nonreciprocal transmission in low-biased gyrotropic metasurfaces

Na Liu, Jia Zhao, Liuge Du, chuanning niu, Chonglei Sun, Xiangpeng Kong, zuojia wang, and Xun Li

DOI: 10.1364/OL.404765 Received 06 Aug 2020; Accepted 21 Sep 2020; Posted 22 Sep 2020  View: PDF

Abstract: Strong magneto-optical effect with low external magnetic field is of great importance to achieve high-performance isolators in modern optics. Here, we experimentally demonstrate a significant enhancement of magneto-optical effect and nonreciprocal chiral transmission in low-biased gyrotropic media. A designer magneto-optical metasurface consists of a gyrotropy-near-zero slab doped with magnetic resonant inclusions. The immersed magnetic dopants enable efficient nonreciprocal light-matter interactions at the subwavelength scale, providing a giant macroscopic nonreciprocity and strong robustness against the bias disturbance. Microwave measurements reveal that the metasurface can act as a chiral isolator for circular polarization, with extremely weak intrinsic gyromagnetic activity. We also demonstrate its capability of signal isolation for circularly polarized antennas. Our findings provide an experimental verification of nonreciprocal photonic doping with low static magnetic fields.

Visible light OCT improves imaging through highly scattering retinal pigment epithelial wall

Tingwei Zhang, Aaron Kho, Robert Zawadzki, Ravi Jonnal, Glenn Yiu, and Vivek Srinivasan

DOI: 10.1364/OL.405398 Received 13 Aug 2020; Accepted 21 Sep 2020; Posted 22 Sep 2020  View: PDF

Abstract: Here, we provide a counter-example to the conventional wisdom in biomedical optics that longer wavelengths aid deeper imaging in tissue. Specifically, we investigate visible light Optical Coherence Tomography (OCT) of Bruch’s Membrane (BM) in non-pathologic eyes of humans and two mouse strains. Surprisingly, we find that shorter visible wavelengths improve the visualization of BM in pigmented eyes, where it is located behind a highly scattering layer of melanosomes in the retinal pigment epithelium (RPE). Monte Carlo simulations of radiative transport suggest that, while absorption and scattering are higher at shorter wavelengths, detected multiply scattered light from the RPE is preferentially attenuated relative to detected backscattered light from BM.

High azimuthal angle tolerant dual-channel wavelength filter from visible to NIR using conically mounted guided mode resonance structures

Pankaj Sahoo, Jaiyam Sharma, Ryoji Yukino, Adarsh Sandhu, and Joby Joseph

DOI: 10.1364/OL.405171 Received 11 Aug 2020; Accepted 21 Sep 2020; Posted 21 Sep 2020  View: PDF

Abstract: We present a concept to design narrow linewidth dual-channel wavelength filters using the principle of wavelength tuning under conical mounting of guided mode resonance structure. The general procedure for the design of such filters from visible to NIR wavelength range is presented and validated experimentally. We show that, already fabricated guided mode resonance structures which do not show dual wavelength filtering at these wavelengths in classical mounting can exhibit dual wavelength filtering in conical mounting. Using this principle, we design high azimuthal angle tolerant guided mode resonance dual wavelength filters at C-band communication wavelengths (1310nm and 1550nm) that are insensitive to azimuthal angle over a range of up to 20 degrees

High Quality factor micro-ring resonator for strong atom-light interactions using miniature atomic beams

Ali Eshaghian Dorche, Bochao Wei, Chandra Raman, and Ali Adibi

DOI: 10.1364/OL.404331 Received 05 Aug 2020; Accepted 21 Sep 2020; Posted 22 Sep 2020  View: PDF

Abstract: An integrated photonic platform is proposed for strong interactions between atomic beams and annealing-free high quality factor (Q) microresonators. We fabricated a thin-film, air-clad SiN microresonator with a loaded Q of 1.55×10^6 around the optical transition of 87Rb at ~780 nm. This Q is achieved without annealing the devices at high temperatures, enabling future fully integrated platforms containing electro-optic circuitry. The estimated single-photon Rabi frequency (2g) is 2π ×64 MHz at a height of 100 nm above the resonator. Our simulation result indicates that miniature atomic beams with a longitudinal speed of 0.2m/s to 30m/s will strongly interact with our resonator, allowing for the detection of single-atom transits and the realization of scalable single-atom photonic devices. Racetrack resonators with a similar Q can be used to detect thermal atomic beams with velocities around 300 m/s.

Multiport Swept-Wavelength Interferometer with Laser Phase Noise Mitigation Employing Broadband Ultra-Weak FBG Array

Haoshuo Chen, Nicolas Fontaine, Juan Carlos Alvarado Zacarias, Cheng Cheng, Marianne Bigot, Pierre Sillard, Roland Ryf, Mikael Mazur, David Neilson, Rodrigo Amezcua Correa, and Minghong Yang

DOI: 10.1364/OL.402388 Received 14 Jul 2020; Accepted 21 Sep 2020; Posted 21 Sep 2020  View: PDF

Abstract: Optical vector network analyzers (OVNAs) based on swept-wavelength interferometry are widely applied in optical metrology and sensing to measure the complex transfer functions of optical components, devices, and fibers.Phase noise from laser sweep nonlinearities degrade the measurement quality as the distance increases and limit the usage of the OVNA in characterizing systems with long impulse responses as required in space-division multiplexing (SDM) links with a high mode count or in the presence of large modal differential group delay (DGD).In the paper, we use a densely-distributed broadband ultra-weak fiber Bragg grating (FBG) arrays to directly measure the distortion due to phase noise at a 5-m increment up to 400~m and use this measured data to directly eliminate the distortion.We experimentally extend the measurement range of the swept-wavelength OVNA over 400~m and successfully characterize a 2-km 6-mode multimode fiber (MMF) link with an accumulated impulse response as wide as 20~ns.Due to the ultra-low transmission loss and broad wavelength range, the number of the FBGs can be easily scaled up to further increase the measurement range of the swept-wavelength OVNA to tens of kilometers for long-range optical sensing while providing sub-ps temporal resolution employing FBGs with a bandwidth >8~nm.

Information sufficient segmentation and signal to noise ratio in stochastic optical localization nanoscopy

Yi Sun

DOI: 10.1364/OL.405903 Received 19 Aug 2020; Accepted 21 Sep 2020; Posted 24 Sep 2020  View: PDF

Abstract: In stochastic optical localization nanoscopy it is a common practice that a localization algorithm segments the power-effective pixels, which are brighter than a threshold, and discards the rest of a data frame. In this scenario we investigate the power-effective Fisher information and the power-effective signal to noise ratio (SNR) with respect to an index ρ < 1, indicting that ρ fraction of emitter power is utilized. The ρ-power effective Fisher information and the ρ-power effective SNR are derived for the Airy and Gaussian point spread functions (PSFs). It is shown that as ρ increases, the root mean square error of Fisher information sharply drops to its lower bound about ρ = 0.8 for both PSFs. The result suggests that the 80%-power effective data in emitter localization is information sufficient and the 80%-power effective SNR is appropriate to indicate the quality of a data frame in the presence of noise.

A fiber laser system for standoff coherent Raman spectroscopy

Edoardo Vicentini, Alessio Gambetta, Gianluca Galzerano, Paolo Laporta, Kelly Curtis, Kenneth McEwan, Christopher Howle, and Nicola Coluccelli

DOI: 10.1364/OL.404832 Received 06 Aug 2020; Accepted 21 Sep 2020; Posted 21 Sep 2020  View: PDF

Abstract: A fiber laser system for standoff detection of chemical and biological species by coherent anti-Stokes Raman scattering is presented. The system is based on an ytterbium fiber laser and a hollow-core photonic crystal fiber for generation of broadband pump/Stokes pulses. High-resolution Raman spectra encompassing the fingerprint region (600-1600 cm-1) are obtained for toluene, and two simulants of chemical and biological warfare agents, specifically dimethyl methylphosphonate, and sodium dipicolinate. The system is operated at standoff distances of 2 m and integration times of 8 ms. The fiber technology makes the approach suitable for implementation as a compact stand-off detection and identification system.

Spectral compression using time-varying cavities

Karthik Myilswamy and Andrew Weiner

DOI: 10.1364/OL.404891 Received 07 Aug 2020; Accepted 21 Sep 2020; Posted 21 Sep 2020  View: PDF

Abstract: Spectral compression will be needed for efficient interfacing of broadband photons with narrowband quantum memories for applications in quantum information and networking. In this paper we propose spectral compression via a time-varying, linear optical cavity. Unlike other recent works on time-varying cavities based on modulation of the intracavity phase, our spectral compression concept is based on rapid switching of coupling into the cavity. We analyze spectral compression performance metrics as a function of mirror reflectivity, cavity loss and switching speed and discuss potential implementation in integrated photonics.

Phase-stable optical activity measurement by common-path spectral interferometry

Yoshio Nishiyama, Shoichi Ishikawa, and Hirohisa Nagatani

DOI: 10.1364/OL.405066 Received 19 Aug 2020; Accepted 21 Sep 2020; Posted 21 Sep 2020  View: PDF

Abstract: A robust optical activity (OA) spectrometer covering the visible and near-infrared regimes was designed and built via a combination of a linear polarizer and a birefringent plate. The OA spectrometer relies on common-path spectral interferometry, where the two interfering fields travel common optical paths, and ensures signal reproducibility over several hours. By detecting OA without polarization switching, the data acquisition time is shortened to 1 s, enabling real-time monitoring of the chiral complex formation. The present configuration also allows OA measurement with broadband pulses, which is promising for probing ultrafast circular dichroism and optical rotatory dispersion.

Resolution Enhanced Photothermal Imaging By High-order Correlation

Xiaobing Sunian, Wei Liu, Lei Chen, Zhihao Zhou, Dongyi Shen, Yuehan Liu, and Wenjie Wan

DOI: 10.1364/OL.396780 Received 04 May 2020; Accepted 21 Sep 2020; Posted 21 Sep 2020  View: PDF

Abstract: Laser scanning photothermal imaging offers a powerful non-destructive testing tool to visualize subsurface structures of opaque materials, but it suffers the resolution limit imposed by thermal diffusion. To address this problem, we have theoretically and experimentally demonstrated that the resolution of photothermal imaging could be enhanced using the high-order correlation imaging method inspired by correlated optical imaging. By carefully designing the laser scanning and modulation behavior, we can individually control the statistical properties of isolated hotspots induced by laser. Imaging reconstructions of subsurface structures are performed afterward by reading out time-fluctuated thermal images. Moreover, the resolution can be further enhanced by using the high-order correlation, which enables a new way for high-resolved thermal imaging and metrology applications.

Watt-level efficient 2.3 μm thulium fluoride fiber laser

Aleksey Tyazhev, Florent Starecki, Solenn Cozic, Pavel Loiko, Lauren Guillemot, Alain BRAUD, Franck Joulain, mincheng tang, Thomas Godin, Ammar Hideur, and Patrice Camy

DOI: 10.1364/OL.403450 Received 22 Jul 2020; Accepted 21 Sep 2020; Posted 21 Sep 2020  View: PDF

Abstract: We report on an efficient mid-infrared Thulium (Tm) fiber laser operating on the 3H4 → 3H5 transition and featuring an upconversion (UC) pumping scheme. This laser comprises a heavily Tm3+-doped (2.50 mol%) zirconium fluoride glass fiber pumped by a tunable Yb fiber laser around 1.05 μm corresponding to the 3F4 → 3F2,3 excited-state absorption (ESA) transition. The laser generates 1.24 W at 2269-2282 nm with a slope efficiency of 37% in the quasi-continuous-wave regime. The Tm-glass fiber exhibits a broadband 3H4 → 3H5 emission with a bandwidth of 173 nm making it very promising for femtosecond fiber oscillators building at ~2.3 μm.

Simutaneous scattering-absorption dual-modal cell imaging with a single-shot by using an inverted photoacoustic microscope

Shiqing Wu, Chao Tao, Xiang Zhang, Feng Lu, and XiaoJun Liu

DOI: 10.1364/OL.403537 Received 24 Jul 2020; Accepted 20 Sep 2020; Posted 21 Sep 2020  View: PDF

Abstract: A microscopy scheme is proposed to simultaneously achieve optical scattering-absorption dual contrast images of the specimen. This scheme is developed based on an inverted photoacoustic microscope. We reveal that two peaks exist in each detected photoacoustic signal. One is caused by the optical absorption of the specimen, and the other is related to the optical scattering of the specimen. Therefore, the microscope can simultaneously extract absorption and scattering, through analyzing the same photoacoustic signal excited by a single-shot laser pulse. After the microscope was validated by imaging a binary mixture consisting of particles with different optical properties, it was successfully applied to achieve two images of red blood cells (RBCs) with different contrasts. Quantitative analysis revealed that the two images reflect the optical absorption and scattering properties of the specimen, respectively. The proposed dual-modal imaging method would be useful in revealing the structural and functional properties of tissue in cell level or assessing pathological section in clinics.

Low-noise high-order Raman fiber laser pumped by random lasing

Bing Han, Yun-Jiang Rao, Han Wu, Jiazhen Yao, Hongjian Guan, Rui Ma, and Zinan Wang

DOI: 10.1364/OL.405899 Received 19 Aug 2020; Accepted 19 Sep 2020; Posted 21 Sep 2020  View: PDF

Abstract: Raman fiber lasers (RFLs) have been widely utilized in long-haul optical transmission systems as pump sources for distributed Raman amplification (DRA) to increase transmission distance and capacity. However, RFLs with relatively large temporal intensity fluctuations would deteriorate signal quality due to the transfer of relative intensity noise (RIN). In this letter, a low-noise high-order RFL common-cavity pumped by an ytterbium-doped random fiber laser (YRFL) is proposed and demonstrated, for the first time. Stable 4th-order random Raman lasing operating at 1365 nm is generated with 8.9 W of output power, without use of a multi-stage master oscillation power amplification (MOPA) system. Thanks to the YRFL common-cavity pumping where a wavelength division multiplexer (WDM)-assisted fiber-loop mirror is used to generate stable 1090 nm ytterbium-doped random lasing and cascaded random Raman lasing simultaneously, the RIN of the 1365 nm RFL is suppressed as low as -120 dB/Hz without any peak over 0-100 MHz span. Such a low-noise high-order RFL may pave a way for development of novel RFLs with stable temporal output, leading to potential replacement of conventional RFLs for DRA in long-haul optical transmission systems to achieve better performances.

Dissipative soliton generation from a large anomalous dispersion Yb-doped fiber laser

Jiaqi Zhou, Weiao Qi, Weiwei Pan, and Yan Feng

DOI: 10.1364/OL.406104 Received 21 Aug 2020; Accepted 19 Sep 2020; Posted 21 Sep 2020  View: PDF

Abstract: Dissipative soliton (DS) is demonstrated in a large anomalous dispersion Yb-doped fiber laser. A chirped fiber Bragg grating is implemented into a figure-of-9 cavity to provide large anomalous dispersion. Self-starting and stable DS operation is achieved with 3.5 nJ pulse energy and 12.9 ps pulse duration under 26.4 MHz repetition rate. Numerical simulations reveal that the DSs presented here experience strong temporal, spectral and chirp breathing in the cavity. Thanks to the characteristic chirp breathing, the output DS pulses can maintain a narrow spectral bandwidth of 0.17 nm. We believe that the laser design presented here has a lot of potentials and could be an outstanding ultrafast seed laser solution for industrial applications including micromachining and hard-brittle material processing.

Fuseless side-pump combiner for efficient fluoride-based double-clad fiber pumping

Sébastien Magnan-Saucier, Simon Duval, Charles Matte-Breton, Yigit Aydin, Vincent Fortin, Sophie LaRochelle, Martin Bernier, and Real Vallee

DOI: 10.1364/OL.409174 Received 02 Sep 2020; Accepted 19 Sep 2020; Posted 21 Sep 2020  View: PDF

Abstract: We report a novel technique for side-pumping fluoride-based double clad fibers allowing a record coupling efficiency of 93 % and a maximum power handling near 100 W at 981 nm. Our simple technique is based on wrapping a silica taper around a fluoride fiber and therefore does not require any complex fusion between these two dissimilar fibers. Under passive cooling, pump combiners made of undoped and erbium-doped fluoride fibers were successfully operated during several hours at respective incident powers of 91 W and 44 W. Heat management issues and active cooling strategies are also discussed. This innovative combiner is a keystone towards the development of compact and robust high-power mid infrared fiber lasers and amplifiers.

A Lidar System Based on Lens Assisted Integrated Beam steering

Xianyi Cao, Gaofeng Qiu, Kan Wu, Chao Li, and Jianping Chen

DOI: 10.1364/OL.401486 Received 01 Jul 2020; Accepted 19 Sep 2020; Posted 23 Sep 2020  View: PDF

Abstract: We present a demonstration of solid-state light detection and ranging (Lidar) at 1550 nm by applying integrated two-dimensional (2D) lens assisted beam steering (LABS) technology. LABS has O(logN) power consumption for N antennas and allows a simple control complexity with digital signal input. A time-of-flight (ToF) coaxial Lidar is demonstrated with this beam-steering technology. The integrated beam-steering chip and lens both transmit and receive the light. The Lidar has 16 scanning angles, 19.5 m ranging distance and 3 cm ranging error. This work proves the potential application of 2D LABS in Lidar and paves the way for a fully integrated Lidar system.

Three-dimensional extension of effective calculation range of angular spectrum based on matrix product

Wanli Zhao, Chenlu Wei, Caojin Yuan, Chenliang Chang, Jun Ma, and Rihong Zhu

DOI: 10.1364/OL.405257 Received 14 Aug 2020; Accepted 18 Sep 2020; Posted 21 Sep 2020  View: PDF

Abstract: Fast Fourier Transform (FFT) is the most commonly used mathematical method in numerical calculation, and the FFT-based angular spectrum method (ASM) is also used widely in diffraction calculation. However, the frequency and spatial sampling rules in FFT limit the effective propagation distance and the observation window range of ASM. A novel method for calculating the angular spectrum based on the matrix product is proposed in this letter. This method realizes the fast calculation of discrete Fourier transform (DFT) based on matrix product, in which, the sampling matrix is orthogonally decomposed into two vectors. Instead of FFT, angular spectrum diffraction calculation is carried out based on the matrix product, which is named MPASM for short. The method in this letter uses a simple mathematical transformation to achieve maximum compression of the sampling interval in the frequency domain, which significantly increases the effective propagation distance of the angular spectrum. And the size of the observation window can be enlarged to obtain a wider calculation range by changing the spatial sampling of the output plane.

Measurement of absolute timing jitter of SESAM-mode-locked lasers with yoctosecond sensitivity

Alexis Casanova, Trophème Benoît, Antoine Courjaud, and GIORGIO SANTARELLI

DOI: 10.1364/OL.405761 Received 19 Aug 2020; Accepted 18 Sep 2020; Posted 22 Sep 2020  View: PDF

Abstract: Absolute timing jitter from a mode-locked laser is characterized with a record low noise floor of 122ys/Hz^(½) (1.5x10^(-14) fs²/Hz). We develop a novel measurement technique using cross-spectrum methods in combination with a dual balanced optical cross-correlator system, with three independent low-noise mode-locked lasers. The obtained performances are promising for further in-depth investigations and improvements of ultrafast laser timing jitter

Single-Photon Stored-Light Interferometry

Yuechun Jiao, Nicholas Spong, Oliver Hughes, Chloe So, T Ilieva, Kevin Weatherill, and Charles Adams

DOI: 10.1364/OL.405143 Received 12 Aug 2020; Accepted 18 Sep 2020; Posted 21 Sep 2020  View: PDF

Abstract: We demonstrate a single-photon stored-light interferometer, where a photon is stored in a laser-cooled atomic ensemble in the form of a Rydberg polariton with a spatial extent of 10×1×1 μm³. The photon is subject to a Ramsey sequence, i.e. `split' into a superposition of two paths. After a delay of up to 450 ns, the two paths are recombined to give an output dependent on their relative phase. The superposition time of 450 ns is equivalent to a free-space propagation distance of 135 m. We show that the interferometer fringes are sensitive to external fields, and suggest that stored-light interferometry could be useful for localized sensing applications.

Increased Sensitivity of Higher-Order Laser Beams to Mode Mismatches

Aaron Jones and Andreas Freise

DOI: 10.1364/OL.403802 Received 26 Aug 2020; Accepted 18 Sep 2020; Posted 18 Sep 2020  View: PDF

Abstract: This manuscript derives mode-mismatch-induced power losses, as a function of mode order, for Hermite-Gauss optical modes. Explicit factors are derived linking mode mismatch losses in a higher order mode to mismatch losses of the fundamental mode. This is particularly relevant for gravitational-wave detectors, where lasers employing higher-order optical modes have been proposed to mitigate thermal noise and quantum-enhanced detectors are very susceptible to losses. This work should inform mode matching and squeezing requirements for \textit{Advanced+} and \textit{Third Generation} detectors.

Liquid crystal bifocal lens with adjustable intensities through polarization controls

Yingjie Zhou, Yide Yuan, Tibin Zeng, Xiangru Wang, Dongliang Tang, Fan Fan, and Shuangchun Wen

DOI: 10.1364/OL.405722 Received 19 Aug 2020; Accepted 18 Sep 2020; Posted 18 Sep 2020  View: PDF

Abstract: In this Letter, transverse and longitudinal liquid crystal bifocal lenses (LCBL) are proposed to continuously control the relative intensity of two foci through a simple polarization control. The modulation of LCBL comes from the geometric phase control and is designed through the principle of holography, where the object wave is light field from two foci respectively formed by the left-circular polarized (LCP) and right-circular polarized (RCP) light and the reference wave is the plane wave. The constructed millimeter-scale LCBLs are experimentally verified and the foci are precisely formed at the preset plane. Besides, the relative intensity can be easily controlled under different weights of LCP and RCP light. We expect that our LCBLs with the non-invasively adjustable intensity may find potential applications in parallel optical processing and optical interconnections.

Compensation of nonlinear distortion in coherent optical OFDM systems using a MIMO deep neural network-based equalizer

Ivan Aldaya, Elias Giacoumidis, Athanasios Tsokanos, Mutsam Jarajreh, YANNUO WEN, Jinlong Wei, Gabriel Campuzano, Marcelo Abbade, and Liam Barry

DOI: 10.1364/OL.403778 Received 06 Aug 2020; Accepted 17 Sep 2020; Posted 18 Sep 2020  View: PDF

Abstract: A novel nonlinear equalizer based on a multiple-input multiple-output (MIMO) deep neural network (DNN) is proposed and experimentally demonstrated for compensation of inter-subcarrier nonlinearities in a 40-Gb/s coherent optical orthogonal frequency division multiplexing (CO-OFDM) system. Experimental results reveal that MIMO-DNN can extend the power margin by 4 dB at 2000 km of standard single-mode fiber transmission when compared to linear compensation or conventional single-input single-output (SISO)-DNN. It is also found that MIMO-DNN outperforms Digital Back Propagation by increasing up to 2 dB the effective Q-factor and reducing by a factor of 3 the computationalcost.

Electronically tunable picosecond pulse generation from Ho3+-doped fluoride fiber laser using frequency shifted feedback

Matthew Majewski, Maria Pawliszewska, and Stuart Jackson

DOI: 10.1364/OL.408609 Received 28 Aug 2020; Accepted 17 Sep 2020; Posted 18 Sep 2020  View: PDF

Abstract: We demonstrate electronically tunable picosecond pulse emission from the 5I6-5I7 transition of the Ho3+ ion using an acousto-optic tunable filter. The holmium and praseodymium co-doped ZBLAN fiber laser produced sub-50 ps pulses over a 100 nm tuning range, critically reaching a longest wavelength of 2.94 μm which overlaps with the peak absorption of liquid water. Measured pulse energies of 8.1 nJ well exceed that expected from picosecond solitonic operation, suggesting possible application in ablative medicine. Furthermore,we present harmonically mode-locked operation of the oscillator, which indicate the possibility of expanding the capabilities of mid-infrared frequency shifted feed-back lasers through the ability to achieve higher pulse repetition rates.

All-Fiber Online Raman Biosensor with Enhancement via FP Cavity

Xingtao Yu, Caoxin Li, Dora Juan Juan Hu, Karolina Milenko, Guanghui Wang, Ping Shum, Fei Xu, Yanqing Lu, and Xuping Zhang

DOI: 10.1364/OL.404404 Received 14 Aug 2020; Accepted 17 Sep 2020; Posted 17 Sep 2020  View: PDF

Abstract: In this paper, a novel all-fiber online Raman biosensor with significant signal enhancement via FP cavity is proposed and demonstrated. The FP cavity structure is formed by inserting a long-pass coated fiber and a gold-plated capillary into a silver-lined capillary with a gap. A corroded single mode fiber is inserted into the gold-plated capillary to guide the excitation light into the FP cavity. The multiple reflections of excitation light in the FP cavity has significantly increased the interaction volume between the light and the sample. Experiment results have demonstrated an enhancement factor of 5 times in the detected Raman signal for ethanol compared to that is measured using the silver-lined hollow-core fiber based Raman cell without FP cavity, or 86 times compared with direct detection using bare fiber tip. The measurement results are in good agreements with theoretical analyses. This Raman biosensor with signal enhancement via FP cavity has the potential to realize rapid sample replacement and online detection with high sensitivity and high accuracy.

Customizing twisted Schell-model beams

Cong Tian, Shijun Zhu, Hongkun Huang, Yangjian Cai, and Zhen-hua Li

DOI: 10.1364/OL.405149 Received 11 Aug 2020; Accepted 17 Sep 2020; Posted 17 Sep 2020  View: PDF

Abstract: Since the celebrated twist phase was proposed by Simon and Mukunda, despite the tremendous progress made in theory over the past decades, developing a simple and flexible experimental method to customize this novel phase has long been a tricky challenge. In this letter, we demonstrate a convenient experimental method for generating TGSM beams by implementing the continuous coherent beam integral function in a discrete form. The experimental results based on rigorous modal expansion synthesis are also demonstrated, indicating that our method is more convenient and of higher quality. The twist factor is also measured using the rotation characteristics during propagation, and the results agree well with the theoretical prediction. The method could serve as a general way for customizing bona fide twisted cross-spectral densities while facilitating certain applications.

Fast phase cycling in non-collinear optical two-dimensional coherent spectroscopy

Maria Munoz, Adam Medina, Travis Autry, Galan Moody, Mark Siemens, Alan Bristow, Steven Cundiff, and Hebin Li

DOI: 10.1364/OL.405196 Received 12 Aug 2020; Accepted 17 Sep 2020; Posted 17 Sep 2020  View: PDF

Abstract: As optical two-dimensional coherent spectroscopy (2DCS) is extended to a broader range of applications, it is critical to improve the detection sensitivity of optical 2DCS. We developed a fast phase-cycling scheme in a non-collinear optical 2DCS implementation by using liquid crystal phase retarders to modulate the phases of two excitation pulses. The background in the signalcan be eliminated by combining either two or four interferograms measured with a proper phase configuration. The effectiveness of this method was validated in optical 2DCS measurements of an atomic vapor. This fast phase-cycling scheme will enable optical 2DCS in novel emerging applications that require enhanced detection sensitivity.

Chiral condensates in a polariton hexagonal ring

Xuekai Ma, Yaroslav Kartashov, Alexey Kavokin, and Stefan Schumacher

DOI: 10.1364/OL.405400 Received 14 Aug 2020; Accepted 17 Sep 2020; Posted 17 Sep 2020  View: PDF

Abstract: We model generation of vortex modes in exciton-polariton condensates in semiconductor micropillars, arranged into a hexagonal ring molecule, in the presence of TE-TM splitting. This splitting lifts the degeneracy of azimuthally modulated vortex modes with opposite topological charges supported by this structure, so that a number of non-degenerate vortex states characterized by different combinations of topological charges in two polarization components appears. We present a full bifurcation picture for such vortex modes and show that because they have different energies, they can be selectively excited by coherent pump beams with specific frequencies and spatial configurations. At high pumping intensity, polariton-polariton interactions give rise to the coupling of different vortex resonances and a bistable regime is achieved.

Boosting the figure of merit of refractive index sensing via magnetoplasmon in H-shaped magnetoplasmonic crystals

Ruxian Zhu, Leyi Chen, SIHAO WANG, Shaolong Tang, and Youwei Du

DOI: 10.1364/OL.403864 Received 07 Aug 2020; Accepted 16 Sep 2020; Posted 17 Sep 2020  View: PDF

Abstract: Nanoscale refractive index (RI) sensors based on plasmonic structures usually suffer from a low figure of merit (FoM) due to the broad linewidth of the resonance peaks. Here, we report a magnetoplasmon-based RI sensing method with high FoM in the designed H-shaped magnetoplasmonic crystals. Instead of the light intensity spectrum, the Faraday signal is detected to analyze the changes of the surrounding RI. Sharp resonance with extremely narrow linewidth is obtained by plotting the reciprocal Faraday rotation near the null point region. Therefore, the FoM is hugely enhanced and a theoretical value exceeding 1775/RIU is achievedafter optimizing the structure, which is one order of magnitude higher than the results ever reported. The Faraday reversal and the enhanced FoM arise from the Fano resonance. These findings are of potential valuesfor practical high performance biochemical sensors.

Implementation of the toroidal absorption cell with multi-layer patterns by a single ring surface

Hong Chang, shiling feng, Xuanbing Qiu, huiyan meng, guqing guo, Xiaohu He, Qiusheng He, Xiaohua Yang, Weiguang Ma, Ruifeng Kan, Christa Fittschen, and chuanliang Li

DOI: 10.1364/OL.404198 Received 31 Jul 2020; Accepted 16 Sep 2020; Posted 17 Sep 2020  View: PDF

Abstract: We developed a type of toroidal multi-pass cell with multi-layer patterns based on the off-axis model. The effective path-length of the original toroidal multi-pass cell is extended several roundtrips in comparison with the single-layer pattern, since the inner surface of the toroidal multi-pass cell is more efficiently utilized. The light pattern has been achieved by using the simple ring surface which is easy to fabricate. The exact analytical equations for the design of the toroidal multi-pass cell were derived based on analytical vector calculations. A series of numerical ray tracing simulations is presented, and the maximum theoretical optical path length that can be reached is 30 m with a setup of 5 cm column radius. Furthermore, two practical spot patterns are demonstrated with a path length of 8.3 m for 2-layer pattern and 10 m for 3-layer pattern with respective volumes of 71 mL and 110 mL. Furthermore, the fringe effect is substantially reduced to less than 0.5 % by the usage of our designed mask.

Light propagation through metamaterial temporal slabs: reflection, refraction, and special cases

Davide Ramaccia, Alessandro Toscano, and Filiberto Bilotti

DOI: 10.1364/OL.402856 Received 15 Jul 2020; Accepted 16 Sep 2020; Posted 18 Sep 2020  View: PDF

Abstract: Time-varying metamaterials are artificial materials whose electromagnetic properties change over time. Similar to a spatial medium discontinuity, a sudden change in time of the metamaterial refractive index induces the generation of a reflected and refracted light waves. The relationship between the incident and emerging fields at one temporal interface has been subject of investigation in earlier studies. Here, we extend the study to a temporal slab, i.e., a uniform homogeneous medium that is present in the whole space for a limited time. The scattering coefficients have been derived as a function of the refractive indices and application time, demonstrating that the response of the temporal slab can be controlled through the application time, which acts similarly to the electrical thickness of conventional spatial slabs. The results reported in this letter pave the way to novel devices based on temporal discontinuities, such as temporal matching networks, Bragg grating, dielectric mirrors, which exhibit zero space occupancy by exploiting the time dimension, instead of the spatial one.

Adaptive-glasses wavefront sensorless full-field OCT for high-resolution retinal imaging over a wide field-of-view

Jules Scholler, Kassandra Groux, Kate Grieve, Claude Boccara, and Pedro Mece

DOI: 10.1364/OL.403135 Received 20 Jul 2020; Accepted 15 Sep 2020; Posted 17 Sep 2020  View: PDF

Abstract: The highest three-dimensional (3D) resolution possible in in-vivo retinal imaging is achieved by combining optical coherence tomography (OCT) and adaptive optics (AO). However, this combination brings important limitations, such as small field-of-view and complex, cumbersome systems, preventing so far the translation of this technology from the research lab to clinics. In this Letter, we introduce an approach that avoids these limitations by using a multi-actuator adaptive lens just in front of the eye, in a technique we call the adaptive-glasses wavefront sensorless approach. We implemented this approach on our compact full-field OCT (FFOCT) retinal imager without increasing its footprint or optical complexity. The correction of ocular aberrations through the adaptive-glasses approach increased the FFOCT signal-to-noise ratio and enabled us to image different retinal layers with a 3D cellular resolution in a 5° x 5° field-of-view, without apparent anisoplanatism.

Mid-wave infrared polarization imaging system for detecting moving scene

Min Yang, Wenbin Xu, Zhenyuan Sun, Hao Wu, Yuze Tian, and Longting Li

DOI: 10.1364/OL.400872 Received 26 Jun 2020; Accepted 15 Sep 2020; Posted 22 Sep 2020  View: PDF

Abstract: In this paper, a polarimetric analyzer is designed for mid-wave infrared camera which makes a kind of infrared camera is transformed into the mid-wave infrared polarization imaging system to measure the infrared polarization characteristics of the object with the moving scene. The polarimetric analyzer is designed by using the ultra-high-speed and high-position method to drive the polarizer to rotate uniformly with the speed of 900 rpm, the polarization state of the object scene is changed, and the mid-wave infrared camera synchronously acquires the infrared intensity image in different polarized direction, that of 0°, 120°, and 240°. Then, a Stokes vector model is established with the basic of the rotation angles, and a sort iteration method is proposed to process the original infrared intensity image that three continuously neighboring infrared intensity images are used to calculate the degree of linear polarization (DoLP) and the angle of polarization (AoP), which makes the infrared polarization image has the same imaging frame of infrared intensity image. Test results show that the mid-wave infrared polarization imaging system can complete the acquisition of DoLP and AoP image well with the frame frequency of 45 fps, which is suitable for the real-time infrared polarization detection of the moving scenes. The study has a great potential for polarization remote sensing and marine object detection.

Low dark current and high-performance hybrid perovskite photodetectors with PBDB-T:IHIC ultrathin passivation layer

Bowen Pan, Mengge Wu, Genjie Yang, Dan Zhao, and Junsheng Yu

DOI: 10.1364/OL.402834 Received 22 Jul 2020; Accepted 14 Sep 2020; Posted 18 Sep 2020  View: PDF

Abstract: In this letter, the reduction of undesired high dark current caused by defect-states in solution-processed perovskite for photodetectors is realized with the introduction of ultrathin buffer layer of PBDB-T:IHIC bulk heterojunction (BHJ). By controlling the concentration of BHJ precisely during solution process, a low dark current density (Jd) of 1.01 × 10-4 mA/cm2 and a high detectivity of 2.61 × 1012 Jones were achieved. It was found that low Jd is attributed to the passivation effect of BHJ on defect-states, where BHJ acts as Lewis base and interacts with unbonded Pb2+ in perovskite. This work demonstrates that the application of ultrathin organic BHJ has significant potential for the fabrication of high-performance optoelectronic devices.

Efficient, sub-4-cycle, 1-µm-pumped optical parametric amplifier at 10 µm based on BaGa₄S₇

Zsuzsanna Heiner, Valentin Petrov, and Mark Mero

DOI: 10.1364/OL.403856 Received 27 Jul 2020; Accepted 14 Sep 2020; Posted 14 Sep 2020  View: PDF

Abstract: We report on a µJ-scale mid-infrared optical parametric amplifier (OPA) based on the recently developed wide-bandgap orthorhombic crystal, BaGa4S7 (BGS), and directly compare its performance to that of LiGaS2 (LGS) in the same OPA setup. The source is based on a single OPA stage amplifying supercontinuum seed pulses with a quantum efficiency of 29% at an idler wavelength of 10 µm, featuring nominally carrier-envelope phase-stable pulses. As a result of pumping the OPA directly at 1 µm, the overall conversion efficiency far exceeds that of traditional schemes based on OPA’s followed by difference frequency generation. Chirp compensation using bulk germanium resulted in 126-fs pulses covering the 7.6-11.5-µm spectral range. BGS holds great promise for power scaling due to its availability in larger single-crystal sizes than LGS.

Nonlinearity and ionization in Xe: Experiment-based calibration of a numerical model

Jared Tolliver, Sina Zahedpour Anaraki, Jared Wahlstrand, Howard Milchberg, and Miroslav Kolesik

DOI: 10.1364/OL.408403 Received 25 Aug 2020; Accepted 14 Sep 2020; Posted 14 Sep 2020  View: PDF

Abstract: Recently proposed universality of the nonlinear response is put to the test and used to improve a previously designed model for Xenon.Utilizing accurate measurements resolving the nonlinear polarization and ionizationin time and space, we calibrate the scaling parameters of the model, and demonstrate agreement with severalexperiments spanning the intensity range relevant for applications in nonlinear optics at near-infrared andmid-infrared wavelengths. Applications to other species including small molecules are discussed,suggesting a self-consistent way to calibrate light-matter interaction models.

Dynamic tuning of photon-plasmon interaction based on three-dimensionally confined microtube cavities

Ehsan Ghareh Naz, YIN YIN, Jiawei Wang, Abbas Madani, Libo Ma, and Oliver Schmidt

DOI: 10.1364/OL.406292 Received 25 Aug 2020; Accepted 14 Sep 2020; Posted 18 Sep 2020  View: PDF

Abstract: We present tunable coupling between surface plasmon resonances supported by a metal-nanoparticle-coated tip and three-dimensionally confined optical modes supported by a microtube cavity. The competition and transition between two types of coupling mechanisms, i.e. dielectric-dielectric and plasmon-dielectric coupling, are observed in the tunable system. Owing to the competition between the two coupling mechanisms, the resonant modes can be dynamically tuned to firstly shift from higher to lower energies and revert to higher energy. Moreover, the unique spatial field distribution of 3D confined modes allows selective coupling of different order axial modes with surface plasmon resonances.

Designing high-performance night-time thermoradiative systems for harvesting energy from outer space

Xin Zhang, Jianying Du, Fen Qiao, Lay Kee Ang, and Yee Sin Ang

DOI: 10.1364/OL.400349 Received 16 Jun 2020; Accepted 14 Sep 2020; Posted 14 Sep 2020  View: PDF

Abstract: Energy harvesting using thermoradiative systems has been extensively explored in recent years as a novel strategy for further reducing our energy footprint. However, the night-time application, thermodynamic limit, and optimal design of such a system during nighttime remain largely unaddressed so far. Here we propose an improved night-time thermoradiative system (NTS) based on mercury cadmium telluride for electrical power generation by optically coupling Earth surface with outer space. Our rigorously theoretical model, considering irreversible loss mechanisms, predicts that the NTS operating with Earth (deep space) at 300 K (3 K) yields a maximum power density of 12.3 Wm-2 with a conversion efficiency limit of 18.5%. We find that optimizing the thickness of the active layer, enhancing thermal infrared emission, and employing a silver back reflector for photons recycling are crucially important in achieving improved system performance. This letter provides new insights for the optimal designs of NTSs and paves the way towards practical night-time power generation.

Fully reconfigurable coherent optical vector-matrix multiplication

James Spall, Xianxin Guo, Thomas Barrett, and Alexander Lvovsky

DOI: 10.1364/OL.401675 Received 02 Jul 2020; Accepted 14 Sep 2020; Posted 14 Sep 2020  View: PDF

Abstract: Optics is a promising platform in which to help realise the next generation of fast, parallel and energy-efficient computation. We demonstrate a reconfigurable free-space optical multiplier that is capable of over 3000 computations in parallel, using spatial light modulators with a pixel resolution of only 340×340. This enables vector-matrix multiplication and parallel vector-vector multiplication with vector size of up to 56. Our design is the first to simultaneously support optical implementation of reconfigurable, large-size and real-valued linear algebraic operations. Such an optical multiplier can serve as a building block of special-purpose optical processors such as optical neural networks and optical Ising machines.

Extreme Local Field Enhancement by Hybrid Epsilon-near-zero/Plasmon Mode in Thin Films of Transparent Conductive Oxides

Innem Venkata Reddy, Josep Jornet, Alexander Baev, and Paras Prasad

DOI: 10.1364/OL.402647 Received 13 Jul 2020; Accepted 14 Sep 2020; Posted 14 Sep 2020  View: PDF

Abstract: Epsilon-near-zero (ENZ) materials display unique properties, among them large local field enhancement at ENZ frequency is of particular interest for many potential applications. In this paper, we introduce a concept that a combination of epsilon-near-zero- and surface plasmon-polariton modes can be excited over an interface between a dielectric and a single ENZ layer in a specific frequency region which can lead to extreme enhancement of local electric field. We demonstrate it by a systematic numerical simulation using finite element analysis and consider two configurations (Kretschmann configuration and a grating configuration), where an indium tin oxide (ITO) layer is sandwiched between two dielectric slabs. We confirm the formation of a hybrid mode at the ITO/dielectric interface at the wavelength of ENZ, as the ITO layer thickness reduces. The hybrid mode provides both high confinement and long propagation distance, which makes it more attractive for many applications than just a pure ENZ mode.

MHz-rate OH planar laser-induced fluorescence imaging in a rotating detonation combustor

Paul Hsu, Mikhail Slipchenko, Naibo Jiang, Christopher Fugger, Austin Webb, Venkat Athmanathan, Terrence Meyer, and Sukesh Roy

DOI: 10.1364/OL.403199 Received 21 Jul 2020; Accepted 13 Sep 2020; Posted 14 Sep 2020  View: PDF

Abstract: MHz-rate hydroxyl radical planar laser-induced fluorescence (OH-PLIF) was demonstrated in a hydrogen/air rotating detonation combustor (RDC) for the first time. A custom injection-seeded optical parametric oscillator (OPO) pumped by the 355-nm output of a high-energy burst-mode laser produced narrowband pulses near 284 nm for OH excitation. The system generated sequences of more than 150 ultraviolet pulses with 400 J/pulse at 1 MHz and 150 J/pulse at 2 MHz. The order of magnitude improvement in the repetition rate over prior OH PLIF measurements and in the number of pulses over previous MHz burst-mode OPOs enables spatiotemporal analysis of complex detonation combustion dynamics.

Electro-optically spectrum switchable, multi-wavelength optical parametric oscillators based on aperiodically poled lithium niobate

Tai-Jie Wang, Lin-Ming Deng, Hung-Pin Chung, Wei Kun Chang, Tien-Dat Pham, Quan-Hsiang Tseng, Reinhard Geiss, Thomas Pertsch, and Yen-Hung Chen

DOI: 10.1364/OL.404742 Received 06 Aug 2020; Accepted 13 Sep 2020; Posted 14 Sep 2020  View: PDF

Abstract: We report the first fast switchable multi-wavelength optical parametric oscillator (OPO) based on domain engineered lithium niobate which is realized by the aperiodic optical superlattice technology. The constructed aperiodically poled lithium niobate (APPLN) integrates the functionalities of two quasi-phase-matching devices on a chip to work simultaneously as an electro-optic (EO) switchable notch-like filter and a multi-wavelength optical parametric down converter. When such an APPLN device is built in a 1064-nm pumped optical resonator system, we achieve the oscillation of dual signal lines at 1540 and 1550 nm, for a single signal line at 1540 nm, and a single signal line at 1550 nm in the system when the 3-cm long APPLN chip is driven by 0 V, 354 V, and 805 V, respectively. The switching among the three interesting signal spectra is operationally simple and electro-optically fast. We also observed the electro-optically switched signals feature enhanced power spectral density due to the unique EO gain-spectrum filtering mechanism employed in this work.

Achromatic super-oscillatory metasurface through optimized multiwavelength functions for sub-diffraction focusing

Long Chen, jia liu, xiaohu zhang, and Dongliang Tang

DOI: 10.1364/OL.404764 Received 06 Aug 2020; Accepted 13 Sep 2020; Posted 14 Sep 2020  View: PDF

Abstract: Optical super-oscillatory lenses (SOLs) based on the planar micro/nano structures have been demonstrated as promising alternatives for shaping wavefronts of light and realizing super-resolution images in a NA-limited optical system. However, as the super-oscillatory foci originated from the delicate interference of the light, the slight change of the parameters might destroy the hotspots, such as the working wavelength, incident angle and fabrication error. Here, a multiwavelength achromatic super-oscillatory metasurface (ASOM) is proposed through simultaneously controlling distinct wavelength-dependent wavefronts. The constructed multiwavelength ASOM is then verified numerically and the foci are precisely formed at the same axial plane for the design wavelengths with resolution beyond the diffraction limit. We expect that our proposed multiwavelength controllable method will give more freedom for the designs of planar and lightweight components, which would be useful in optical applications, such as data storage, super-resolution imaging, holography, etc.

Hybrid autofluorescence and photoacoustic label-free microscopy for the investigation and identification of malignancies in ocular biopsies

George Tserevelakis, Kostas Mavrakis, Danai Pantazopoulou, Eleni Lagoudaki, Efstathios Detorakis, and Giannis Zacharakis

DOI: 10.1364/OL.403435 Received 21 Jul 2020; Accepted 13 Sep 2020; Posted 15 Sep 2020  View: PDF

Abstract: We demonstrate the development and application of a prototype hybrid microscopy system integrating autofluorescence (AF) and photoacoustic (PA) label-free contrast modes, for the differentiation of ocular tumors in human surgical biopsies. Hybrid imaging was performed in conjunctival nevi and choroidal melanomas tissue sections to acquire quantified data for each molecular background. The AF and PA signals were spatially correlated to establish a novel malignancy indicator which could detect melanomas with high accuracy. The proposed methodology has the potential to simplify relevant diagnostic procedures and paves the way for the development of novel ophthalmoscopes aiming to the early diagnosis of ocular malignancies in a clinical setting.

Photonic compressive sensing of sparse radio frequency signals with a single dual-electrode Mach-Zehnder modulator

Bo Yang, Shuna Yang, Zizheng Cao, Jun Ou, Yanrong Zhai, and Hao Chi

DOI: 10.1364/OL.403782 Received 28 Jul 2020; Accepted 12 Sep 2020; Posted 15 Sep 2020  View: PDF

Abstract: A novel approach to realizing compressive sensing (CS) of sparse radio frequency (RF) signals based on photonic random demodulation (RD) is proposed. The key function of mixing the RF signal under test and the bipolar pseudo-random binary sequence (PRBS) in photonic RD is implemented with a single dual-electrode Mach-Zehnder modulator (DEMZM). By properly setting the DC bias of the DEMZM at Vπ and the voltages of the PRBS at ±Vπ/2, a pure desired multiplication term between the signal and the bipolar PRBS is obtained after an AC-coupled photodetector (PD), which not only simplifies the modeling of the CS link but also improves the recovery performance. A proof-of-concept experiment is demonstrated where a sparse signal with spectral components of 500 MHz and 950 MHz is successfully identified with a compression ratio of 20. Simulation results are also given to show the advantage of the given photonic CS scheme with bipolar random mixing.

High Resolution Phase-Sensitive Sum Frequency Generation Spectroscopy by Time-Domain Ptychography

Tobias Schweizer, Bruno Nicolau, Priscila Cavassin, Thomas Feurer, Natalie Banerji, and Julien Réhault

DOI: 10.1364/OL.403339 Received 22 Jul 2020; Accepted 12 Sep 2020; Posted 17 Sep 2020  View: PDF

Abstract: We demonstrate that time-domain ptychography, when applied to a set of broadband vibrational sum frequency spectra, reconstructs amplitude and phase of the vibrational free induction decay from an interfacial sample with a resolution that is independent of up-converting pulse bandwidth and spectrometer resolution. These important improvements require no modifications to most standard homodyne setups and the method is applicable to other coherent homodyne spectroscopies like Coherent Anti-Stokes Raman Spectroscopy (CARS) or Transient Grating Spectroscopy (TG).

Steady-state and time-resolved upconversion photoluminescence in Yb3+-Er3+ co-doped transparent ceramic of YAG

Fei Tang, Haoqi Li, Kangzhen Tian, J. Q. Ning, H. G. Ye, and SJ Xu

DOI: 10.1364/OL.408308 Received 24 Aug 2020; Accepted 11 Sep 2020; Posted 17 Sep 2020  View: PDF

Abstract: Transparent ceramics (TCs) represent a new family of functional hard materials. In this Letter, steady-state and time-resolved upconversion photoluminescence in Yb3+-Er3+ co-doped transparent ceramic of YAG (TC-YAG) are reported for the first time. Under the excitation of near-infrared 940 nm laser at room temperature, the Yb3+-Er3+ co-doped TC-YAG emits intense warm-white luminescence consisting of cyan, green and red groups of sharp lines. More excitingly, the green group of luminescence due to the transitions from 4S3/2 to 4I15/2 states of Er3+ are the prominent components with the average lifetime of ~0.3 ms. The internal quantum efficiency of the green luminescence is estimated to be 32.8%. A unique dual-resonance energy transfer from Yb3+ to Er3+ via the excited-state vibronic transitions is proposed as the principal mechanism of the strongest green luminescence of Er3+ ions in TC-YAG.

Unitary matrix approach for a precise voltage dependent characterization of reflective liquid crystal devices by average Stokes polarimetry

Andrés Márquez, Francisco Martínez, Jorge Francés Monllor, Eva Estepa, Dan Puerto, Sergi Gallego, Inmaculada Pascual, and Augusto Belendez

DOI: 10.1364/OL.403394 Received 21 Jul 2020; Accepted 11 Sep 2020; Posted 14 Sep 2020  View: PDF

Abstract: Precise characterization of parallel-aligned liquid crystal on silicon (PA-LCoS) microdisplays has an important impact in many advanced photonics applications. We show the LCoS modelled as a non-absorbent reciprocal device which, combined with time-average Stokes polarimetry, enables to demonstrate robust measurements across the whole applied voltage range for the retardance and its flicker, and also as a novelty for the director orientation. We obtain that the director orientation changes across the voltage range, especially at larger applied voltages. This is a small effect but it may provide a deeper insight into the internal dynamics in the LC layer, and in sensitive phase-only applications will produce a coupling between amplitude and phase.

Theory of slow-light semiconductor optical amplifiers

Marco Saldutti, Thorsten Rasmussen, Mariangela Gioannini, and Jesper Mork

DOI: 10.1364/OL.403446 Received 23 Jul 2020; Accepted 11 Sep 2020; Posted 14 Sep 2020  View: PDF

Abstract: We have developed an efficient framework for analyzing the reflection and transmission properties of semiconductor photonic crystal optical amplifiers. Specifically, we have investigated the use of slow-light to enhance the gain of short integrated amplifiers. We find that the expected enhancement in transmission is limited by distributed feedback induced by the material gain itself. Such back-scattering is further enhanced by the refractive index variation associated with the linewidth enhancement factor. The inclusion of this effect reveals that for a given material gain, devices with smaller linewidth enhancement factor may offer better performance.

Graphene-based Mid-Infrared Plasmonic Isolatorwith Multimode Interferometer

Mohsen Heidari and Vahid Ahmadi

DOI: 10.1364/OL.402695 Received 17 Jul 2020; Accepted 11 Sep 2020; Posted 14 Sep 2020  View: PDF

Abstract: This Letter presents an extremely compactgraphene-based plasmonic isolator with a two-port multimode interference (MMI) structure. Theproposed structure utilizes the non-reciprocalphase shift (NRPS) effect to provide opticalisolation. A new approach is used to MMI-based isolator design in terms of m-th overallself-imaging (OSI). The influence of the devicegeometries and m parameter on the isolation ratio(ISR), insertion loss (IL), operation bandwidth(BW), required magnetic field strength B and itsappropriate direction are investigated. The devicehas an ultra-small footprint compared to the operating wavelength of λ=8 µm with high ISR. AnMMI isolator with a size of 0.5 µm × 4.02 µm(≈ λ^2 /32) and ISR=18.35 dB and an MMI isolatorwith a size of 0.7 µm × 7.44 µm (≈ λ^2 /12) withISR=27.1 dB is achieved. Moreover, the non-reciprocal MMI shows ultra-wide 20 dB-isolationBW of 0.75–2.34 THz.

A colour discrimination metric based on the neutrality of lighting and hue transposition quantification

qiang liu, Ying Liu, Michael Pointer, zheng huang, Xinwei Wu, Zhiyu Chen, and Ming Luo

DOI: 10.1364/OL.400422 Received 17 Jun 2020; Accepted 11 Sep 2020; Posted 14 Sep 2020  View: PDF

Abstract: In this letter, we proposed a combined metric to quantify the colour discrimination capability of white light sources. This metric is based on considerations of the human visual adaptation to daylight chromaticities, as well as on the concerns of the huge spectral diversity of modern light sources. Two existing metrics, Sneutral (degree of neutrality) by Smet et al. and Rd (hue transposition among colour samples of Farnsworth-Munsell 100 Hue colour vision test) by Esposito et al. were adopted, and their weights were determined by a meta-analysis of 5 groups of psychophysical data on colour discrimination. The superiority of the proposed metric was comprehensively demonstrated by 16 groups of psychophysical data from 8 colour discrimination studies, as well as by a comparison with 29 typical colour quality metrics and their linear combinations.

Zerinke phase retrieval: a bridge between qualitative phase contrast and quantitative phase imaging by phase retrieval algorithms

Nathaniel Hai and Joseph Rosen

DOI: 10.1364/OL.403020 Received 17 Jul 2020; Accepted 10 Sep 2020; Posted 10 Sep 2020  View: PDF

Abstract: In the last five decades, iterative phase retrieval methods draw large amount of interest across the research community as a non-interferometric approach to recover quantitative phase distributions from one (or more) intensity measurement. However, in cases where a unique solution does exist, these methods often require oversampling and high computational resources, which limits the use of this approach in important applications. On the other hand, phase contrast methods are based on a single camera exposure but provides only a qualitative description of the phase, thus are not useful for applications in which the quantitative phase description is needed. In this study we adopt a combined approach of the two above-mentioned methods to overcome their respective drawbacks. We show that a modified phase retrieval algorithm easily converges to the correct solution by initializing the algorithm with a phase-induced intensity measurement, namely with a phase contrast image of the examined object. Accurate quantitative phase measurements for both binary and continuously varying phase objects are demonstrated to support the suggested system as a single-shot quantitative phase contrast microscope.

Spin-independent metalens for helicity-multiplexing of converged vortices and cylindrical vector beams

Tao Zhou, Qian Liu, Yongsheng Liu, and XiaoFei Zang

DOI: 10.1364/OL.404436 Received 03 Aug 2020; Accepted 10 Sep 2020; Posted 10 Sep 2020  View: PDF

Abstract: Converged vortex beam with a well-defined focal plane is an essential ingredient for trapping and rotating microparticles. Metasurfaces, two-dimensional metamaterials, provide an ultra-compact and flexible platform for designing converged vortices by integrating the functions of a lens and a vortex plate. However, traditional geometric metalenses can only focus one specific spin state of electromagnetic waves, hindering the further applications. Here, we propose an approach to realizing spin-independent metalens that can simultaneously focus terahertz waves with orthogonal spin states into helicity-dependent vortices. Under the illumination of linearly-polarized terahertz waves, all of the helicity-dependent vortices are observed, leading to helicity-multiplexing of converged vortices. Furthermore, the longitudinal multiplexing of converged cylindrical vector beams is demonstrated by superposition of the helicity-dependent vortices. This unique approach for multiplexing converged vortices and cylindrical vector beams may open a window for designing future ultra-compact and multifunctional devices with potential applications in communications, optical trapping and focusing.

Ultra-fast Stokes parameter correlations of true unpolarized thermal light: type-I unpolarized light

Florian Kroh, Markus Rosskopf, and Wolfgang Elsaesser

DOI: 10.1364/OL.409322 Received 02 Sep 2020; Accepted 10 Sep 2020; Posted 14 Sep 2020  View: PDF

Abstract: We measure Stokes parameter correlations in analogy to the intensity correlation measurements in the original Hanbury-Brown Twiss configuration by realizing an experimental set-up by combining a Schaefer-Collett or Berry-Gabrielse-Livingston polarimeter with a Hanbury-Brown Twiss intensity interferometer. We investigate true unpolarized light emitted from a broad-band thermal light source, which we realize by an erbium-doped fiber amplifier, thus being an ideal source of true unpolarized light. We find that all Stokes parameter correlations < S_n S_n >, n=1,2,3 are equal to 0.5 < I >^2. The proven invariance of the Stokes parameter correlations against retardation by wave-plates clearly shows for the first time that our true unpolarized thermal light represents type I unpolarized light in accordance with a theoretical prediction for a classification of unpolarized light postulated more than 20 years ago.

A near-single-mode 3 kW monolithic fiber oscillator based on longitudinally spindle-shaped Yb-doped fiber

zeng lingfa, Xiaoming Xi, Yun Ye, hanwei zhang, Xiaolin Wang, Zhiyong Pan, Zefeng Wang, and Xiaojun Xu

DOI: 10.1364/OL.404893 Received 06 Aug 2020; Accepted 10 Sep 2020; Posted 14 Sep 2020  View: PDF

Abstract: In order to balance the mitigation of transverse mode instability and stimulated Raman scattering effect in the high power fiber lasers, a specially designed Yb-doped fiber, named spindle-shaped Yb-doped fiber, was fabricated with a core/cladding diameter of 20/400 μm at both ends and 30/600 μm in the middle. Based on this fiber, an all-fiber laser oscillator was built and over 3 kW near-single-mode (M² factor ~1.3) laser was achieved with optical-to-optical conversion efficiency of 78.4%. While operating at the maximum power, the transverse mode instability is well mitigated and the stimulated Raman scattering effect is well suppressed (>34 dB lower than signal laser). Further power scaling is promising by optimizing the structure of the Yb-doped fiber.

Large Rabi splitting of mixed plasmon-exciton states in small plasmonic Moiré cavities

Atilla Aydinli, Simge Ates, SINAN BALCI, Coskun Kocabas, and Ertugrul Karademir

DOI: 10.1364/OL.405278 Received 12 Aug 2020; Accepted 10 Sep 2020; Posted 14 Sep 2020  View: PDF

Abstract: We report on exciton-plasmon coupling in metallic Moiré cavities both numerically and experimentally. Moiré cavities fabricated using double exposure laser interference lithography were filled with a molecular dye, J-aggregate. Polarization dependent spectroscopic reflection measurements supported by simulations reveal strong coupling of organic dye excitons with cavity modes of the plasmonic Moiré cavities. Anti-crossing at zero detuning, a clear indication of strong coupling, has been observed when the excitonic absorption band resonates with the cavity mode. Large Rabi splitting energies owing to the strong coupling of plasmons and excitons are clearly observed

Phenethylamine Ligand Engineering of Red InP Quantum Dots for Improving the Efficiency of Quantum Dot Light-Emitting Diodes

WEI JIANG, BO KIM, and Heeyeop Chae

DOI: 10.1364/OL.405520 Received 18 Aug 2020; Accepted 10 Sep 2020; Posted 14 Sep 2020  View: PDF

Abstract: In this study, red-emitting multi-shelled indium phosphide (InP) quantum dots (QDs) were synthesized using the safe phosphorus precursor tris(dimethylamino)phosphine ((DMA)₃P). The long-chain ligands of oleylamine (OAm) in the (DMA)₃P phosphide source-based InP QDs were partially exchanged with short-chain ligands of phenethylamine (PEA) in the core formation process, and the resulting InP QDs were applied to quantum dot light-emitting diodes (QLEDs). The short-chain ligands of PEA with the π-conjugated benzene ring improved the charge transport and electrical conduction of the QLEDs with (DMA)3P phosphide source-based InP QDs. The PEA-engineering of InP QDs improved their maximum quantum yield from 71% to 85.5% with the full-width at half maximum of 62 nm. Furthermore, the maximum external quantum efficiency(EQE) of QLEDs with the PEA-engineered InP QDs improved from 1.9% to 3.5%, and their maximum power efficiency increased from 2.8 to 6.0 lm/W. This study demonstrates that engineering the core formation process with the short-chain ligands of PEA provides an efficient and facile way to improve the charge transport and electrical conduction in (DMA)₃P phosphide source-based InP QLEDs for electroluminescent devices.

Few-cycle pulses tunable from 3 to 7 μm via intrapulse difference-frequency generation in oxide LGN crystals

Jinsheng Liu, Jingui Ma, Dazhi Lu, Xingbin Gu, ZIRUO CUI, Peng Yuan, Jing Wang, Guoqiang Xie, Haohai Yu, Huaijin Zhang, and Liejia Qian

DOI: 10.1364/OL.406025 Received 20 Aug 2020; Accepted 09 Sep 2020; Posted 14 Sep 2020  View: PDF

Abstract: Ultrashort mid-infrared (mid-IR) source beyond 5 μm is crucial for a plethora of existing and emerging applications in spectroscopy, medical diagnostics and high-field physics. Nonlinear generation of such sources from well-developed near-IR lasers, however, remains a challenge due to the limitation of mid-IR crystals. Based on oxide La₃Ga₅.₅Nb₀.₅O₁₄ (LGN) crystals, here we report the generation of femtosecond pulses tunable from 3 to 7 μm by intrapulse difference-frequency generation of 7.5 fs, 800 nm pulses. The efficiency and bandwidth dependences on pump polarization and crystal length are studied for both Type-I and Type-II phase-matching configurations. A maximum pulse energy of ~10 nJ is generated at 5.2 μm with a conversion efficiency of ~0.14%. Because of the few-cycle pump pulse duration, the generated mid-IR pulses are as short as about three cycles. These results, to the best of our knowledge, represent the first experimental demonstration of LGN in generating mid-IR ultrashort pulses.

A highly efficient optical antenna with small beam divergence in silicon waveguides

Pavel Cheben, Daniel Pereira-Martín, Abdelfettah Hadij-ElHouati, Winnie Ye, Daniele Melati, Dan-Xia Xu, Siegfried Janz, Alejandro Ortega-Moñux, J. Gonzalo Wangüemert-Pérez, Robert Halir, I. Molina-Fernández, Jens Schmid, and Pablo Guinel-Moreno

DOI: 10.1364/OL.404012 Received 29 Jul 2020; Accepted 09 Sep 2020; Posted 10 Sep 2020  View: PDF

Abstract: Optical antennas are key components in optical phased arrays (OPAs) for light detection and ranging (LIDAR) technology requiring long sensing range and high scanning resolution. To achieve a narrow beam-width in the far-field region, antenna lengths of several millimeters or more are required. To date, such long antennas have been impossible to realize in silicon waveguides since currently demonstrated technologies do not allow accurate control of grating strength. Here we report on a new type of surface emitting silicon waveguide with dramatically increased antenna length of L = 3.65 mm. This is achieved by using a subwavelength metamaterial waveguide core evanescently coupled with radiative segments laterally separated from the core. This results in a far-field diffracted beam width of 0.025°, which is a record small beam divergence for a silicon photonics surface emitting device. We also demonstrate that by using a design with L-shaped surface-emitting segments the radiation efficiency of the antenna can be substantially increased compared to a conventional design, with an efficiency of 72%.

Multi-Source Aliasing Suppression for Distributed Fiber Acoustic Sensing with Directionally Coherent Enhancement Technology

Zhaoyong Wang, Junqi Yang, jinfeng gu, Bin Lu, Lei Ye, Kang Ying, qing ye, Ronghui Qu, and Haiwen Cai

DOI: 10.1364/OL.404736 Received 10 Aug 2020; Accepted 09 Sep 2020; Posted 10 Sep 2020  View: PDF

Abstract: Distributed fiber acoustic sensing can detect almost all of disturbances along the sensing fiber and is widely applied. However, the signals from multiple adjacent disturbance sources are superimposed, according to the sensing principle. A directionally coherent enhancement technology is demonstrated for DAS to suppress multi-source aliasing in air. In preliminary works, the submerged weak target signal is effectively extracted from strong broadband noise with 21 dB SNR enhancement, and the two different same-frequency signals from two sources are separately rebuilt with the same detected signal. As far as we know, this is the first time that the directionally coherent enhancement is proposed for DAS and the multi-source aliasing is suppressed. This technique will help DAS find new important foreground in acoustic detection of large-scale plant with many similar noisy devices, including discharge detection in high voltage substation, acoustic emission flaw detection in mechanical factory, etc.

Deep learning based hyperspectral recovery from a single RGB image

Junchao Zhang, YUANYUAN SUN, Jianlai Chen, Degui Yang, and Rongguang Liang

DOI: 10.1364/OL.405061 Received 13 Aug 2020; Accepted 09 Sep 2020; Posted 10 Sep 2020  View: PDF

Abstract: Commercial hyperspectral imaging devices are expensive and tend to suffer from the degradation of spatial, spectral or temporal resolution. To address these problems, we propose a deep learning-based method to recover hyperspectral images from a single RGB image. The proposed method learns an end-to-end mapping between an RGB image and corresponding hyperspectral images. Moreover, a customized loss function is proposed to boost the performance. Experimental results on a variety of hyperspectral dataset demonstrate that our proposed method outperforms several state-of-the-art methods in terms of both quantitative measurements and perceptual quality.

Extrinsic optical activity in all-dielectric terahertz metamaterial

Shijun Yang, Yanfeng Li, Xieyu Chen, Quanlong Yang, Jiaguang Han, and Weili Zhang

DOI: 10.1364/OL.403377 Received 21 Jul 2020; Accepted 09 Sep 2020; Posted 14 Sep 2020  View: PDF

Abstract: Chiral metamaterials have attracted wide interest because strong optical activity at designed frequencies could be achieved with them beyond those in natural materials. Here we propose an all-dielectric metamaterial with strong extrinsic circular dichroism and circular birefringence by periodically arranging symmetry-broken dielectric Mie resonators at terahertz frequencies. The excited Mie resonances from circularly polarized incident waves with opposite handedness dominate the performance of the all dielectric metamaterial, which exhibits a 60% circular dichroism in transmission and a polarization rotation angle of 60° at maximum, respectively. Additionally, the spectral range of circular dichroism with preserved amplitude can be adjusted continuously in the frequency range from 0.67 to 0.79 THz by tuning the tilt angle of the incident wave. Our findings provide an alternative for designing and improving the chirality and circular dichroism based on all-dielectric metamaterials.

A VECSEL-based virtually imaged phased array spectrometer for rapid gas phase detection in the mid-infrared

Robert Rockmore, Ricky Gibson, Jerome Moloney, and R. Jason Jones

DOI: 10.1364/OL.405192 Received 12 Aug 2020; Accepted 08 Sep 2020; Posted 11 Sep 2020  View: PDF

Abstract: We present a novel system for high-resolution, time-resolved spectroscopy in the mid-wave infrared based on a modelocked vertical external cavity surface emitting laser (VECSEL) frequency comb coupled to a virtually imaged phased array (VIPA) spectrometer. The GHz level repetition rate of VECSEL based systems coupled to VIPA spectrometers enables comb-tooth resolved spectra without the use of additional filter cavities often required to increase comb tooth spacing. We demonstrate absorption spectroscopy on a methane (CH₄) gas mixture at 2900 cm-1 (3.4 μm) with over 35 cm−1 spectral bandwidth in a single image. Rapid time-resolved measurements were made using a 300 μs exposure time with an acquisition rate limited to 125 Hz by the available camera. High-resolution absolute frequency measurements were performed by scanning the repetition rate of the VECSEL frequency comb.

Generation of tunable linearly chirped signals with long temporal duration in the photonic time-stretched coherent radar

Xing Li, Siteng Zhang, Shuguang Li, Jianping Chen, and Weiwen Zou

DOI: 10.1364/OL.405640 Received 18 Aug 2020; Accepted 08 Sep 2020; Posted 10 Sep 2020  View: PDF

Abstract: Linearly chirped signals with long temporal duration are generated by adding a dispersive loop in the photonic time-stretched coherent radar. The signal-to-noise ratio (SNR) variation of the optical pulse recirculation is analyzed and simulated to evaluate the multiple extension of temporal duration. The achievable temporal duration is confined by the dispersion and the amplified spontaneous emission noise accumulation. Experimental results verify the SNR variation of the optical pulse recirculation. Moreover, tunable linearly chirped signals with temporal duration of 100 ns and bandwidth of 4 GHz at X or Ku band are generated. With the improvement of the time-bandwidth product, a drone can be successfully observed.

Handheld swept-source optical coherence tomography guided by smartphone-enabled wide-field autofluorescence photography for imaging facial sebaceous glands

QINGHUA HE, Ruikang Wang, and Teng Liu

DOI: 10.1364/OL.405765 Received 18 Aug 2020; Accepted 08 Sep 2020; Posted 14 Sep 2020  View: PDF

Abstract: We report a method to image facial sebaceous glands (SG) using smartphone-enabled wide-field autofluorescence photography (AFP) and handheld swept-source optical coherence tomography (SS-OCT). Smartphone-enabled AFP provides 2D wide-field fluorescence image that is used both as a functional mapping of the sebum and a positioning guidance for OCT imaging of the SG. Following the guidance, handheld SS-OCT conducts the volume scan to investigate depth-resolved conditions of the SG in the selected regions of interest. We show the results from smartphone-enabled AFP and handheld SS-OCT to demonstrate the ability of our method to image facial SGs, potentially useful for the assessment of skin conditions in dermatology and cosmetology.

Angle-sensitive transmission grating for grating lobe suppression of optical phased arrays

Guangzhu Zhou, Shi-Wei Qu, and JieYun Wu

DOI: 10.1364/OL.401484 Received 29 Jun 2020; Accepted 07 Sep 2020; Posted 10 Sep 2020  View: PDF

Abstract: Optical phased arrays based on optical waveguides are compelling components enabling efficient and accurate beam steering. However, to avoid crosstalk between the waveguides, the element pitch is typically larger than one wavelength, which will give rise to grating lobes in the real space. In this Letter, we report that near-wavelength gratings can be employed to suppress the grating lobes by utilizing the angular low-pass-filter characteristics. The properly designed near-wavelength grating acts as an angle-sensitive transmission structure. Nearly 100% transmissivity can be realized at small incident angles. However, it quickly declines to a low level when the incident angle is over the critical one. Then, a simple line current array is utilized to demonstrate the grating lobe suppression effect with the grating designed for the TE-polarized incidence. Finally, we demonstrate that by loading the proposed grating designed for the TM-polarized incidence upon a waveguide grating array with a 2.4 μm pitch, the grating lobe suppression of 10 dB can be achieved when scanning up to ±14°.

On-chip miniature metalenses based on one-dimensional gradient trench architecture in the broadband visible regime

Rui Yang, Yangyang Shi, Chenjie Dai, chengwei wan, shuai wan, and Zhongyang Li

DOI: 10.1364/OL.405446 Received 20 Aug 2020; Accepted 07 Sep 2020; Posted 09 Sep 2020  View: PDF

Abstract: Metasurfaces are composed of fat, ultrathin subwavelength nanoantennas with strong capability in manipulating the light propagation by modulations on its phase, amplitude, and polarization. For instance, the invention of two-dimensional (2D) metalenses has enabled the light focusing and imaging in three-dimensional (3D) free-space with miniaturized thickness and device size at a planar surface. However, such inherent form of 2D arrays and focusing functionality at 3D optical free-space limits the degree of freedom for light propagation and manipulation along a 2D planar surface and eventually the possibility of on-chip photonic system integration. Here, we theoretically study and demonstrate a new type of planar on-chip metalens, which enables light focusing and strongly localization at a 2D surface. The planar on-chip architecture design is based on the one-dimensional (1D) length or width gradient trench metalens (L_GTM or W_GTM), which could yield the elaborately engineered phase shift for the propagating light within the on-chip waveguide at the visible wavelength of 500 nm. By generating 1D phase arrangement at the nanoscale, a miniature on-chip metalens with ~ 3×0.5 μm dimension could achieve light focusing on a 2D waveguide surface with the flexibility to design scalable focal lengths and ultra-high numerical aperture (NA) of up to ~ 0.99. Additionally, GTM metalens designs could also exhibit overlapped high depth-of-focus (DOF), which consequently could behave as achromatic-like lensing at the selected focal plane. Furthermore, we manifest that the focusing functionality can also be subject to dynamically tuning and switching on-and-off with TE/TM polarization change or waveguide index alteration. We believe this new form of on-chip 1D metalens holds potential applications including on-chip light manipulation functionality of focusing and diverging, optical on-chip sensing, next-generation on-chip optical communication, signal processing as well as imaging devices, etc.

Multi-element microscope optimization by a learned sensing network with composite physical layers

Kanghyun Kim, Pavan Chandra Konda, Colin Cooke, Ron Appel, and Roarke Horstmeyer

DOI: 10.1364/OL.401105 Received 29 Jun 2020; Accepted 07 Sep 2020; Posted 08 Sep 2020  View: PDF

Abstract: Standard microscopes offer a variety of settings to help improve the visibility of different specimens to the end microscope user. Increasingly, however, digital microscopes are used to capture images for automated interpretation by computer algorithms (e.g., for feature classification, detection or segmentation), often without any human involvement. In this work, we investigate an approach to jointly optimize multiple microscope settings, together with a classification network, for improved performance with such automated tasks. We explore the interplay between optimization of programmable illumination and pupil transmission, using experimentally imaged blood smears for automated malaria parasite detection, to show that multi-element "learned sensing" outperforms its single-element counterpart. While not necessarily ideal for human interpretation, the network’s resulting low-resolution microscope images (20X-comparable) offer a machine learning network sufficient contrast to match the classification performance of corresponding high-resolution imagery (100X-comparable), pointing a path towards accurate automation over large fields-of-view.

Admissible surfaces in progressive addition lenses

Sergio Barbero and María del Mar González

DOI: 10.1364/OL.401927 Received 02 Jul 2020; Accepted 07 Sep 2020; Posted 08 Sep 2020  View: PDF

Abstract: Progressive addition lenses contain a surface of spatially-varying curvature, which provides variable optical power for different viewing areas over the lens. We derive complete compatibility equations that provide the exact magnitude of cylinder along lines of curvatures on any arbitrary PAL smooth surface. These equations reveal that, contrary to current knowledge, cylinder, and its derivative, does not only depend on principal curvature and its derivatives along the principal line but also on the geodesic curvature and its derivatives along the line orthogonal to the principal line.We quantify the relevance of the geodesic curvature through numerical computations. We also derive an extended and exact Minkwitz theorem only restricted to be applied along lines of curvatures, but excluding umbilical points.

Advancing the sensitivity of integrated epoxy-based Bragg grating refractometry by high-index nanolayers

Steffen Hessler, Stefan Knopf, Mathias Rommel, Maiko Girschikofsky, Bernhard Schmauss, and Ralf Hellmann

DOI: 10.1364/OL.402768 Received 14 Jul 2020; Accepted 06 Sep 2020; Posted 08 Sep 2020  View: PDF

Abstract: In this letter, we report on significantly improved surrounding refractive index sensitivity of epoxy polymer waveguide Bragg grating sensors. Uniform Bragg gratings were generated inside flat rectangular epoxy waveguides near cut-off-regime using standard phase-mask excimer laser writing. Thickness controlled nanolayers of high-index titanium dioxide were deposited homogeneously on the waveguide sensor’s surface area by repeated reactive sputter processing. Maximum Bragg wavelength shifts as high as 74.22 nm as well as maximum sensitivities around 5 nm/RIU corresponding to a minimum refractive index resolution of 1.9 · 10-6 could be obtained by employing a ~75 nm thick titanium dioxide coating.

20 kHz Dual-Plane Stereo-PIV Measurements on aSwirling Flame Using a Two-legged Burst-Mode Laser

Zifeng Yang, Sirui Wang, Jianyi Zheng, Lei Li, Xunchen Liu, Yi Gao, and Fei Qi

DOI: 10.1364/OL.404002 Received 28 Jul 2020; Accepted 06 Sep 2020; Posted 09 Sep 2020  View: PDF

Abstract: Dual-plane stereoscopic particle image velocimetry (PIV) is capable of quantifying the flow field in terms of three-component (3C) flow vectors and 3C vorticity vectors simultaneously. Here, we present a test rig to carry out the 20 kHz dual-plane stereo PIV measurements on a premixed swirling flame by using a high-frequency burst-mode laser. Other than the traditional methods including the polarization direction separation method and the two-color separation method, a short delay of 100 ns between the two same-color laser sheets was adopted to separate the imaging processes for the two pairs of cameras. Each laser sheet with the same wavelength of 532 nm is generated by the burst-mode high-frequency laser with a pulse cyclic frequency of 20 kHz within each burst. 3C velocity vectors for a swirling flame were obtained based on the sequential particle images for each laser sheet. In spite of non-perfect simultaneous flow measurements on the two spatially separated laser sheets, the velocity error caused by the 100 ns delay on top of a 50 μs duration, which was used for the velocity vector calculation, is negligible. This short-delay separation method significantly simplifies the experimental setup for dual-plane stereo PIV measurements, especially for low-speed flows.

Enhanced resonant vibrational Raman scattering of N₂^{+} induced by self-seeding ionic lasers created in polarization-modulated intense laser fields

Guihua Li, Hongqiang Xie, Zhang Qian, Lei Hongbin, Xingyu Zhou, zhiming chen, Xiaowei Wang, and Zengxiu Zhao

DOI: 10.1364/OL.403110 Received 20 Jul 2020; Accepted 05 Sep 2020; Posted 08 Sep 2020  View: PDF

Abstract: We report on an experimental investigation of the five vibrational Raman lines at 358 nm, 388 nm, 391 nm, 428 nm and 471 nm of N₂^{+} resonantly driven by the self-seeding ionic lasers generated by a polarization-modulated (PM) or alternatively a linearly-polarized (LP) femtosecond laser. It was found that the spectral intensities of several Raman lines can be dramatically enhanced by exploiting the PM laser pulses in comparison to the LP laser pulses. The evaluated Raman conversion efficiency reaches ~10^{-2} for some lasing lines at suitable pressures. Moreover, the role of interplay between the seed amplification and the resonant vibrational Raman scattering processes in inducing the gain of N₂^{+} lasing is characterized for the first time. The developed vibrational Raman spectroscopy with intense ultrafast lasers provides an additional approach to interrogate the products in a femtosecond filament and it therefore can be a powerful tool for identifying chemical species at remote distances in the atmosphere.

Frequency selective topological edge wave routing in meta-structures made of cylinders

Guochao Wei, Zhenzhen Liu, Yuchen Liu, Dasen Zhang, and Jun Jun Xiao

DOI: 10.1364/OL.405512 Received 14 Aug 2020; Accepted 05 Sep 2020; Posted 08 Sep 2020  View: PDF

Abstract: Valley degree of freedom provides a convenient way to manipulate classical waves. The propagation direction of edge states is essentially related to the band valley and the orbit angular momenta (OAM) of excitation source. However, it is difficult to control the propagation path when the chirality of the excitation source and the boundary structures are determined. Here, we study a frequency selective photonic crystal waveguide structure based on topological photonic insulators with different bulk polarization. By designing different type of interface made from spatially arranged dielectric rods, distinct topological edge states could be realized at different frequencies in the band gap. Therefore, we can construct a meta-structure in which the wave guiding path can be switched by excitation frequency. Our study provides an alternative approach to designing topological devices such as frequency dependent optical waveguides and frequency division devices.

Trains of attosecond pulses structured with time-ordered polarization states

Laura Rego Cabezas, Julio San Roman, Luis Plaja, and Carlos Hernandez-Garcia

DOI: 10.1364/OL.404402 Received 17 Aug 2020; Accepted 05 Sep 2020; Posted 08 Sep 2020  View: PDF

Abstract: Ultrafast laser pulses generated at the attosecond timescale represent a unique tool to explore the fastest dynamics in matter. An accurate control of their properties, such as the polarization, is fundamental to shape three-dimensional laser-driven dynamics. We introduce a technique to generate attosecond pulse trains whose polarization state varies from pulse to pulse. This is accomplished by driving high harmonic generation with two time-delayed bichromatic counter-rotating fields with proper orbital angular momentum (OAM) content. Our simulations show that the evolution of the polarization state along the train can be controlled via the OAM, pulse duration and time delay of the driving fields. We thus introduce an additional control into structured attosecond pulses that provides an alternative route to explore ultrafast dynamics with potential applications in chiral and magnetic materials.

Gas sensing with mode-phase-difference photothermal spectroscopy assisted by a long period grating in a dual-mode negative-curvature hollow-core optical fiber

Pengcheng Zhao, HOI LUT HO, Wei Jin, Shangchun Fan, Shoufei Gao, Yingying Wang, and Pu Wang

DOI: 10.1364/OL.404323 Received 07 Aug 2020; Accepted 04 Sep 2020; Posted 08 Sep 2020  View: PDF

Abstract: We demonstrate sensitive gas detection with mode-phase-difference photothermal spectroscopy assisted by a long period grating (LPG) inscribed on a dual-mode negative-curvature hollow-core fiber (NC-HCF). The LPG is inscribed using a pulsed CO2 laser, which enables pump propagation in the fundamental LP01 mode to achieve maximum photothermal phase modulation while excites both the LP01 and LP11 modes at the probe wavelength to form an dual-mode interferometer for the detection of the phase difference. With a 1533 nm pump and a 1620 nm probe, a noise-equivalent-concentration of ~2.2 ppb acetylene is achieved with an 85-cm-long NC-HCF gas cell and 1 s lock-in time constant.

Efficient solid-state Raman yellow laser at 579.5 nm

Yung-Fu Chen, C. M. Chen, C. C. Lee, H. Y. Huang, Di Li, J. Q. Hsiao, C. H. Tsou, and Hsing-Chih Liang

DOI: 10.1364/OL.405970 Received 19 Aug 2020; Accepted 04 Sep 2020; Posted 08 Sep 2020  View: PDF

Abstract: A highly efficient diode-pumped Nd:YVO4/KGW Raman yellow laser is developed to produce 6.8 W yellow light at 579.5 nm accompanied by 3.2 W Stokes wave at 1159 nm under an incident pump power of 30 W. The intracavity stimulated Raman scattering (SRS) with the shift of 768 cm-1 is generated by setting the polarization of the fundamental wave along the Ng direction of a Np-cut KGW crystal. The Nd:YVO4 gain medium is coated as a cavity mirror to reduce the cavity losses for the fundamental wave. More importantly, the KGW crystal is specially coated to prevent the Stokes wave from propagating through the gain medium to minimize the cavity losses for the Stokes wave.

High-power diode-pumped Nd:GdVO4/KGW Raman laser at 578 nm

Yung-Fu Chen, H. Y. Huang, C. C. Lee, J. Q. Hsiao, C. H. Tsou, and Hsing-Chih Liang

DOI: 10.1364/OL.406173 Received 21 Aug 2020; Accepted 04 Sep 2020; Posted 08 Sep 2020  View: PDF

Abstract: A diode-pumped Nd:GdVO4 laser is developed as a compact efficient yellow light at 578 nm by means of the intracavity stimulated Raman scattering (SRS) in a potassium gadolinium tungstate (KGW) and the second harmonic generation in a lithium triborate (LBO). The SRS process with the shift of 768 cm-1 is realized by setting the polarization of the fundamental wave along the Ng axis of KGW crystal. The self-Raman effect arising from the Nd:GdVO4 crystal is systematically explored by employing two kinds of coating specification for the output coupler. With a specific coating on the output coupler to suppress the self-Raman effect, the maximum output power at 578 nm can reach 3.1 W at a pump power of 32 W. Moreover, two different lengths for the Nd:GdVO4 crystal are individually used to verify the influence of the self-Raman effect on the lasing efficiency.

Interface states and bound states in the continuum in photonic crystals with different lattice constants

Jiwang Chai, Liang Liu, Peng Hu, Hong Xiang, and Dezhuan Han

DOI: 10.1364/OL.404035 Received 30 Jul 2020; Accepted 04 Sep 2020; Posted 08 Sep 2020  View: PDF

Abstract: The existence of interface states at the boundary of two semi-infinite photonic crystals (PhCs) with different lattice constants are investigated systematically. Compared to the interface states in the two PhCs with the same period, a band folding effect is observed for the interface states inside the common band gap of the two PhCs with different lattice constants. We demonstrate that these interface states can be predicted by the surface impedance of the two PhCs. The dispersion of interface states can be determined by the condition of impedance matching combined with the band folding effect. Moreover, some part of the folded interface states penetrates the region of projected bulk bands, and they usually leak to the bulk and form resonant states. However, the interface state at the Γ point can be perfectly localized and becomes a bound state in the continuum (BIC) due to the symmetry mismatch. These findings may provide another way for designing the PhC structures that support interface states and BICs.

Enabling time-resolved 2D spatial-coherence measurements using the Fourier-analysis method with an integrated curved-grating beam monitor

Kai Bagschik, Michael Schneider, Jochen Wagner, Ralph Buß, Matthias Riepp, Andre Philippi-Kobs, Leonard Mueller, Wojciech Roseker, Florian Trinter, Moritz Hoesch, Jens Viefhaus, Stefan Eisebitt, Gerhard Gruebel, Hans Peter Oepen, and Robert Frömter

DOI: 10.1364/OL.402264 Received 08 Jul 2020; Accepted 04 Sep 2020; Posted 08 Sep 2020  View: PDF

Abstract: Direct 2D spatial-coherence measurements are increasingly gaining importance at synchrotron beamlines, especially due to present and future upgrades of synchrotron facilities to diffraction-limited storage rings. We present a method to determine the 2D spatial coherence of synchrotron radiation in a direct and particularly simple way by using the Fourier-analysis method in conjunction with curved gratings. Direct photon-beam monitoring provided by a curved grating circumvents the otherwise necessary separate determination of the illuminating intensity distribution required for the Fourier-analysis method. Hence, combining these two methods allows for time-resolved spatial-coherence measurements. As a consequence, spatial-coherence degradation effects caused by beamline optics vibrations, which is one of the key issues of state-of-the-art X-ray imaging and scattering beamlines, can be identified and analyzed.

Single Pixel Polarimetric Imaging through Scattering Media

Cheryl Seow, Peter Torok, and Matthew Foreman

DOI: 10.1364/OL.399554 Received 04 Jun 2020; Accepted 04 Sep 2020; Posted 08 Sep 2020  View: PDF

Abstract: Polarimetric imaging can provide valuable information about biological samples in a wide range of applications. Detrimental scattering however currently limits the imaging depth of in-vivo imaging to ~1 transport mean free path. In this work, single pixel imaging is investigated as a means of reconstructing polarimetric images through scattering media. A theoretical imaging model is presented and the recovery of the spatially resolved Mueller matrix of a hidden test object is demonstrated experimentally for scattering phantoms with thicknesses up to twice the transport mean free path.

Generation of broadband circularly polarized deep-ultraviolet pulses in hollow capillary fibers

Athanasios Lekosiotis, Federico Belli, Christian Brahms, and John Travers

DOI: 10.1364/OL.400362 Received 23 Jun 2020; Accepted 04 Sep 2020; Posted 08 Sep 2020  View: PDF

Abstract: We demonstrate an efficient scheme for the generation of broadband, high-energy, circularly polarized femtosecond laser pulses in the deep ultraviolet through seeded degenerate four-wave mixing in stretched gas-filled hollow capillary fibers. Pumping and seeding with circularly polarized 35 fs pulses centered at 400 nm and 800 nm, respectively, we generate idler pulses centered at 266 nm with more than 25 μJ of energy and over 95% spectrally averaged ellipticity. Even higher idler energies and broad spectra (27 nm bandwidth) can be obtained at the cost of reduced ellipticity. Our system can be scaled in average power and used in different spectral regions, including the vacuum ultraviolet.

Computational adaptive optics in optical coherence tomography with phase unstable systems

Sebastian Ruiz Lopera, René Restrepo, Carlos Cuartas-Vélez, Brett Bouma, and Néstor Uribe-Patarroyo

DOI: 10.1364/OL.401283 Received 03 Jul 2020; Accepted 03 Sep 2020; Posted 08 Sep 2020  View: PDF

Abstract: We present a scheme for computational aberration correction in optical coherence tomography (OCT) designed to work with phase unstable systems with no hardware modifications. Our approach, termed SHARP, is based on computational adaptive optics and numerical phase correction and follows from the fact that local phase stability is sufficient for the deconvolutionof optical aberrations. We demonstrate its applicability in a raster-scan polygon-laser OCT system with strong phase-jitter noise, achieving successful refocusing at depths up to four times the Rayleigh range. We also present in vivo endoscopic and ex vivo anterior segment OCT data, showing significant enhancement of image quality, particularly when combining SHARP results with a resolution-preserving despeckling technique like TNode.

Higher-order dispersion and the spectral behaviour in a doubly resonant optical parametric oscillator

Christian Dietrich, Ihar Babushkin, José Ricardo Cardoso de Andrade, Han Rao, Ayhan Demircan, and Uwe Morgner

DOI: 10.1364/OL.405483 Received 13 Aug 2020; Accepted 03 Sep 2020; Posted 08 Sep 2020  View: PDF

Abstract: In doubly resonant optical parametric oscillators(DROPOs) it is possible to generate, enhance, andphase lock two frequencies at once. Following intracavity phase conditions, a complex tuning behaviour of the signal and idler spectra takes placein DROPOs, cumulating into degeneracy withphase self-locking and coherent wavelength doubling. In this work we identify the group delaymatching as the important parameter determiningthe global tuning behaviour and demonstrate thekey role of higher order dispersion in the spectraldependencies. Applicationwise, we suggest a simple way to control the phase self-locking region byvarying the intracavity third-order dispersion.

Experimental observation of tunable Wood type resonances in all-ferrodielectric periodical metasurface

Liubov Ivzhenko, SERGEY POLEVOY, Sergey Tarapov, Vladimir Yachin, Kestutis Kurselis, Roman Kiyan, and Boris Chichkov

DOI: 10.1364/OL.402936 Received 27 Jul 2020; Accepted 03 Sep 2020; Posted 03 Sep 2020  View: PDF

Abstract: In this letter, a completely ferrodielectric metasurface consisting of an array of cylinders on a substrate is studied. All structural elements are made of ferrodielectric. The conditions for the excitation of Wood's anomaly mode, obtained for different geometric parameters of the metasurface, are revealed. By continuously changing the structure parameters, we can change the position of the resonance at the Wood anomaly, thereby setting the position of the resonance at the frequency we need. It is shown that there is a resonant increase in the polarization plane rotation of the transmitted waves at the corresponding resonant frequency of the lattice mode excitation. Such polarization rotation is demonstrated both experimentally and theoretically.

Exploiting redundancy in color-polarization filter arrayimages for dynamic range enhancement

Pierre-Jean Lapray

DOI: 10.1364/OL.398258 Received 27 May 2020; Accepted 02 Sep 2020; Posted 02 Sep 2020  View: PDF

Abstract: Color-Polarization Filter Array (CPFA) sensors are ableto capture polarization and color information in a singleshot. For a scene that contains high dynamic rangeof joint irradiance and polarization signatures, somepixel values approach the saturation and noise level ofthe sensor. The most common CPFA configuration isover-determined, and contains four different linear polarizationanalyzers, whereas only three are sufficientto estimate the first three Stokes elements. Assumingthat not all the pixel responses are equally reliable inCPFA channels, and one can apply the High DynamicRange Imaging (HDRI) scheme to enhance the Stokesestimation from a single CPFA image. Here I presentthis alternative methodology, and show qualitative resultson visualized data.

High-conversion-gain and deep-image-rejection Brillouin chip-based photonic RF mixer

zihang zhu, Duk-Yong Choi, Stephen Madden, Benjamin Eggleton, and Moritz Merklein

DOI: 10.1364/OL.400511 Received 17 Jun 2020; Accepted 02 Sep 2020; Posted 03 Sep 2020  View: PDF

Abstract: In this Letter, we report a chip-based photonic radio-frequency (RF) mixer with a maximum conversion gain (CG) of -9 dB and image rejection ratio (IRR) of 50 dB for 3.2 GHz to 13.2 GHz RF frequency range. This is achieved by the combined use of optical carrier suppression (OCS) modulation and on chip stimulated Brillouin scattering. These results will stimulate future implementations of integrated photonic RF mixer in complicated electromagnetic environments.

Partially coherent Pearcey-Gauss beams

Xiaoyan Zhou, Zihao Pang, and Daomu Zhao

DOI: 10.1364/OL.404277 Received 31 Jul 2020; Accepted 02 Sep 2020; Posted 02 Sep 2020  View: PDF

Abstract: We investigate the dynamics of partially coherent Pearcey-Gauss beams propagating in free space, theoretically and experimentally. They are produced by introducing the degree of coherence (DOC) function with Gaussian Schell-model correlation into the light source in the frequency domain. Under a nearly incoherent state, the oscillation of the side lobe turns smooth and the intensity distribution concentrates on the main lobe. Particularly, partially coherent Pearcey-Gauss beams would maintain the inherent properties of auto-focusing performance and inversion effect without diminishing the autofocusing distance and form-invariable propagation. Moreover, the opening angle and the shift of peak intensity of the beams can be controlled by the binary parabola in the spectrum distribution of the Pearcey function. Our experimental results are in great agreement with the theoretical analysis.

Watt-level ultrafast bulk laser with Graphdiyne saturable absorber mirror

Qianqian Hao, Jia Guo, Luying Yin, Tingyin Ning, Yanqi Ge, and Jie Liu

DOI: 10.1364/OL.404540 Received 03 Aug 2020; Accepted 02 Sep 2020; Posted 02 Sep 2020  View: PDF

Abstract: Few-layered Graphdiyne (GDY) was successfully fabricated and applied as a saturable absorber to generate a watt-level ultrafast solid-state bulk laser. The maximum output power of up to 1.27 W was obtained with a pulse width of ps and a repetition rate of 92.9 MHz, using Nd:YVO₄ crystal as gain medium. To the best of our knowledge, this is the first application of GDY as a mode-locker in all-solid-state bulk lasers. These results indicate the promising potential of GDY for producing high-power ultrafast lasers.

Compact deep ultraviolet frequency-doubled Tb:LiYF₄ lasers at 272 nm

Hengjun Chen, Hiyori Uehara, and Ryo Yasuhara

DOI: 10.1364/OL.404949 Received 10 Aug 2020; Accepted 02 Sep 2020; Posted 02 Sep 2020  View: PDF

Abstract: A new laser system has been developed to generate coherent deep ultraviolet (DUV) radiation at 272 nm. The DUV lasers were produced via intra-cavity frequency-doubling of the Tb3+:LiYF4 lasers emitting fundamentally at 544 nm. Continuous-wave (cw) and Q-switched operations were performed with a Type-I phase-matched BBO nonlinear crystal. The cw operation produces 127 mW of averaged DUV output power. Passive Q-switched operation was realized by using Co2+:MgAl2O4 saturable absorbers. At an initial transmittance of 99.2% at 544 nm, stable pulsed output at 272 nm with a maximum single-pulse energy of 7.6 µJ and peak power of 7.2 W was obtained. Furthermore, by employing a smaller initial transmittance of 96.4%, we achieved maximum averaged DUV output power of 277 mW. The statistically averaged single-pulse DUV energy and peak power were estimated to be around 100 µJ and 480 W, respectively, which indicates the great potential for this DUV laser system toward high energy and peak power.

Compact and low-loss 1×3 polarization-insensitive optical power splitter using cascaded tapered silicon waveguides

hongxiang Li, Weiwei Chen, Pengjun Wang, Shixun Dai, yuxiao Liu, qiang fu, Jun Li, Yan Li, Tingge Dai, hui Yu, and Jianyi Yang

DOI: 10.1364/OL.401036 Received 26 Jun 2020; Accepted 02 Sep 2020; Posted 02 Sep 2020  View: PDF

Abstract: In this paper, a 1×3 polarization-insensitive optical power splitter based on cascaded tapered silicon waveguides is proposed and experimentally demonstrated on silicon-on-insulator platform. By utilizing the particle swarm optimization algorithm and finite difference time domain method, structural parameters of the coupling regions are carefully designed to achieve polarization-insensitive property, compact size, low insertion loss, high uniformity and broad bandwidth. The coupling length can be as short as 7.3 μm. Our measurement results show that, at 1550 nm, the insertion losses of the fabricated device operating in TE and TM polarizations are respectively 0.068 dB and 0.62 dB. Within a bandwidth from 1525 to 1575 nm, the insertion loss is lower than 0.82 dB and the uniformity is less than 1 dB for the fabricated device operating in TE polarization, while the fabricated device operating in TM polarization can have an insertion loss smaller than 1.50 dB and an uniformity lower than 1 dB from 1528 to 1582 nm.

Phase Tristability in Parametric Three-Photon Down-Conversion

Mohammad-Ali Miri

DOI: 10.1364/OL.402122 Received 07 Jul 2020; Accepted 01 Sep 2020; Posted 02 Sep 2020  View: PDF

Abstract: It is shown that the parametric processes of spontaneous three-photon down-conversion is accompanied by phase tristability of the sub-harmonic signal. The oscillations of the signal in a resonant cavity is modeled through an analytically solvable second-order nonlinear oscillator. Self-sustained oscillations of the signal at a finite amplitude is found to be equally probable in three states with uniform phase contrasts. The onset of oscillations is a case of bifurcation from infinity. The stability of the ternary states is proven through an energy landscape function that identifies the attractor basins of the three states. An analogy is drawn between the oscillation threshold of a three-photon down-conversion oscillator and a first-order phase transition. The investigated phase-tristable oscillator can serve as a classical ternary bit for unconventional computing applications.

ULTRAFAST GREEN THIN-DISK LASER EXCEEDING 1.4 KW OF AVERAGE POWER

Christoph Röcker, André Loescher, Florian Bienert, Phillipe Villeval, Dominique Lupinski, Dominik Bauer, Alexander Killi, Thomas Graf, and Marwan Abdou Ahmed

DOI: 10.1364/OL.403781 Received 27 Jul 2020; Accepted 01 Sep 2020; Posted 02 Sep 2020  View: PDF

Abstract: We present an ultrafast laser with a near-diffraction-limited beam quality delivering more than 1.4 kW of average power in the visible spectral range. The laser is based on second harmonic generation in a lithium triborate crystal of a Yb:YAG thin-disk multipass amplifier emitting more than 2 kW of average power in the infrared.

Sub-200 fs, 344 MHz mode-locked Tm-doped fiber laser

weiyu lai, Hui Zhang, Zexiu Zhu, Peiguang Yan, Shuangchen Ruan, Zhipei Sun, and Jinzhang Wang

DOI: 10.1364/OL.403855 Received 27 Jul 2020; Accepted 01 Sep 2020; Posted 02 Sep 2020  View: PDF

Abstract: We demonstrate a compact, self-starting mode-locked thulium-doped fiber laser based on nonlinear polarization evolution (NPE), with a fundamental repetition rate of 344 MHz and a pulse duration of ~160 fs. The generated pulses centered at 1975 nm have a maximum output power of 560 mW, corresponding to a pulse energy of 1.63 nJ. To the best of our knowledge, the achieved repetition rate represents the highest value of fundamentally NPE mode-locked fiber lasers around 2 µm, while the average output power is also higher than the previously-reported 2 µm ultrafast fiber oscillators based on single mode fiber. The timing jitter in the integrated range [5 kHz, 10 MHz] and the integrated relative intensity noise in the range [10 Hz, 10 MHz] reach 35 fs and 0.009%, respectively. Our high-performance laser is an ideal candidate for various applications, including mid-IR frequency metrology and high-speed optical sampling.

Picosecond stimulated Raman scattering at 3000 and 3430 cm-1 OH vibrations without optical breakdown

Sergey Pershin, Alexander Vodchits, Inna Khodasevich, Mikhail Grishin, Vasily Lednev, Valentin Orlovich, and Pavel Chizhov

DOI: 10.1364/OL.402358 Received 09 Jul 2020; Accepted 01 Sep 2020; Posted 02 Sep 2020  View: PDF

Abstract: For the first time, stimulated Raman scattering (SRS) of picosecond laser pulses without optical breakdown has been detected simultaneously (as the first Stokes and anti-Stokes paired components) at ~3430 and ~3000 cm-1 vibrations of water OH band. These components were generated coaxially to the pump beam in the forward direction as axial and conical ring beams, respectively, when the pump beam was focused at the water-air interface. We suggest an explanation of these new SRS phenomena by non-collinear four-wave parametric interaction.

Line-scan Compressive Raman imaging with spatio-spectral encoding

Camille Scotté, Siddharth Sivankutty, Randy Bartels, and Herve Rigneault

DOI: 10.1364/OL.400151 Received 16 Jun 2020; Accepted 01 Sep 2020; Posted 02 Sep 2020  View: PDF

Abstract: We report a line-scanning imaging modality of Compressive Raman technology with a single-pixel detector. The spatial information along the illumination line is encoded onto one axis of a digital micromirror device, while spectral coding masks are applied along the orthogonal direction. We demonstrate imaging and classification of three different chemical species.

High performance of a semipolar InGaN laser with a phase-shifted embedded HSQ grating

HAOJUN ZHANG, Dan Cohen, Philip Chan, Matthew Wong, Panpan Li, HONGJIAN LI, Shuji Nakamura, and Steven P. DenBaars

DOI: 10.1364/OL.403679 Received 24 Jul 2020; Accepted 01 Sep 2020; Posted 08 Sep 2020  View: PDF

Abstract: Single-frequency blue laser sources are of interest for an increasing number of emerging applications, but are still difficult to implement, expensive to fabricate and suffer from poor robustness. Here a novel and universal grating design to realize distributed optical feedback was demonstrated on a semipolar InGaN laser diode and its unique effect on the laser performance was investigated. For the first time, a low threshold voltage, record-high power output and ultra-narrow single mode lasing were simultaneously obtained on the new laser structure with a thinner p-GaN layer and a third-order phase-shifted embedded dielectric grating. Under continuous-wave operation, such lasers achieved 35 dB SMSR, less than 2 pm FWHM and near 400 mW total output power at room temperature.

Generation of a high-resolution 3D-printed freeform collimator for VCSEL based 3D-depth sensing

Bo Chen, Daniel Claus, Detlef Russ, and Moaaz Nizami

DOI: 10.1364/OL.401160 Received 24 Jun 2020; Accepted 01 Sep 2020; Posted 08 Sep 2020  View: PDF

Abstract: This paper discusses the generation of 3D-printed micro-optics to obtain the desired beam profile from a multimode vertical-cavity surface-emitting laser (VCSEL) with a significantly reduced divergence angle via the usage of high-resolution two-photon polymerization. Due to the low cost and compact packaging, VCSEL array is a novel light source for structured-light projection. For 3D sensing applications, a significantly reduced divergence angle ensures that a good signal with a sufficiently large number of photons can be recorded and that the projected illumination spots do not overlap. Furthermore, elliptical beam profiles with different orientations solves the correspondence problem in structured-light 3D sensing. To generate this special type of beam profile and verify the optical design process, this paper describes thoroughly the optical process starting from the beam characterization, the optical design to the production of the two-photon polymerized optics, and its evaluation. The test of beam profile and divergence confirm a good match of the produced optics with the physical optical simulation in Zemax. The collimator transforms the input laser beam divergence angle of 324mrad to an output angle of 20mrad only.

Complementary Photonic Crystals Integrated Logic Devices

Luis Pedraza, Michelle Povinelli, JHONATTAN CORDOBA RAMIREZ, Paulo Soares Guimaraes, and Omar Vilela Neto

DOI: 10.1364/OL.393846 Received 30 Mar 2020; Accepted 31 Aug 2020; Posted 31 Aug 2020  View: PDF

Abstract: We theoretically propose and demonstrate trough FDTD simulations the operation of photonic crystal (PhC) equivalents of logic CMOS transistor switches N and P. These switches together stand for complementary photonic crystal integrated logic (CPCL). They are the core hardware components to support the development of large scale PhC integrated circuits. We design them in a two-dimensional PhC slab composed of a triangular lattice of holes embedded in a GaAs/AlGaAs heterostructure. Simulation results prove that they operate with the same input and output wavelength and have well-defined output power values to represent the two logic states, allowing their adjacent connection.

Silicon subwavelength modal Bragg grating filters with narrow bandwidth and high optical rejection

Dorian Oser, Diego Pérez Galacho, Xavier Le Roux, Sebastien Tanzilli, Laurent Vivien, laurent labonte, Eric Cassan, and Carlos Alonso Ramos

DOI: 10.1364/OL.394455 Received 03 Apr 2020; Accepted 31 Aug 2020; Posted 31 Aug 2020  View: PDF

Abstract: Waveguide Bragg grating filters with narrow bandwidth and high optical rejection are key functions for several advanced silicon photonics circuits. However, the performance of state-of-the-art silicon Bragg filters are limited by a trade-off between rejection level and bandwidth. Here, we propose and demonstrate a new Bragg grating geometry that provides narrowband and high rejection response. It combines the advantages of subwavelength and modal engineering to overcome this limitation. Subwavelength index engineering enables a narrowband response while multimodal-based non-coherent cascading provides a strong rejection. As a proof-of-concept demonstration, we implement the proposed Bragg filters in the 220-nm-thick Si technology with a single etch step. We experimentally show flexible control of the filter selectivity, with measured bandwidths below 2~nm, and strength of 60 dB rejection with a null-to-null bandwidth of 1.8~nm.

A multifunctional mid-infrared photonic switch using MEMS-based tunable waveguide coupler

Qifeng Qiao, Mahmut Sami Yazici, Bowei Dong, Xinmiao Liu, Chengkuo Lee, and Guangya Zhou

DOI: 10.1364/OL.400132 Received 11 Jun 2020; Accepted 31 Aug 2020; Posted 31 Aug 2020  View: PDF

Abstract: We demonstrate a multifunctional photonic switch on silicon-on-insulator platform operating at the mid-infrared wavelength range (3.85-4.05 μm) using suspended waveguides with sub-wavelength cladding and MEMS tunable waveguide coupler. Leveraging the flip-chip bonding technology, a top wafer acting as the electrode is assembled above the SOI wafer to enable the electrostatic actuation. Experimental characterizations for the functions of the proposed device include (I) an optical attenuator with 25 dB depth using DC voltage actuation, (II) a 1×2 optical switch with response time of 8.9 μs and -3 dB bandwidth up to 127 kHz using AC voltage actuation, (III) an on-chip integrated light chopper with comparable performance of a commercial rotating disc light chopper.

Demonstration of 0.67 mJ and 10 ns high energy pulses at 2.72 μm from large core Er:ZBLAN fiber amplifiers

Weizhi Du, Xuan Xiao, Yifan Cui, John Nees, Igor Jovanovic, and Almantas Galvanauskas

DOI: 10.1364/OL.400065 Received 16 Jun 2020; Accepted 31 Aug 2020; Posted 01 Sep 2020  View: PDF

Abstract: We explored generation of high energy ns-short pulses in the mid-IR wavelength range using 30μm – 70μm core Er:ZBLAN fiber amplifiers. The highest energies achieved are ~0.7-mJ at 2.72-μm in 11.5-ns long pulses, with the corresponding peak power of 60.3 kW, obtained with a 70-µm diameter core fiber amplifier pumped at 976 nm and seeded by an KTiOAsO4-based optical parametric oscillator/optical parametric amplifier system. To the best of our knowledge, these pulse energies and peak powers are the highest achieved to date from mid-IR fiber lasers at longer than 2µm wavelengths with nanosecond pulses. The achieved highest pulse energies were limited by the surface damage of unprotected fiber output facets.

Effects of Transceiver Jitter On the Performance of Optical Scattering Communication Systems

Zanqiu shen, Jianshe Ma, Serge Provost, and Su Ping

DOI: 10.1364/OL.400969 Received 24 Jun 2020; Accepted 27 Aug 2020; Posted 28 Aug 2020  View: PDF

Abstract: In ultraviolet communications, the transceiver jitter effects have been ignored in previous studies, which can result in non-negligible performance degradation especially in vibration states or in mobile scenes. To address this issue, we model the relationship between the received power and transceiver jitter by making use of a moment-based density function approximation method. Based on this relationship, we incorporate the transceiver jitter effects in combination with Poisson distribution. The error rate results are obtained assuming on-off key modulation with optimal threshold based detection. We validate the error rate expressions by comparing the analytical results with Monte-Carlo simulation results. The results show that the transceiver jitter effects cause performance degradation especially in smaller transceiver elevation angles, which are often adopted in short-range ultraviolet communications. The results also show that larger elevation angle cases have a better performance with respect to anti-jitter and may perform better compared to smaller elevation angle situations in the case of larger standard deviation of jitter. This work studies for the first time the transceiver jitter effects in ultraviolet communications and provides guidelines for experimental system design.

Nanoparticle trapping and manipulation using a silicon nanotrimer with polarized light

ying guo, Yi Liao, Yefeng Yu, Yuzhi Shi, and Sha Xiong

DOI: 10.1364/OL.403754 Received 29 Jul 2020; Accepted 26 Aug 2020; Posted 26 Aug 2020  View: PDF

Abstract: Optical tweezers based on plasmonics experience a tremendous development on manipulating nanoparticles, but unable to avoid the problem of Joule heating. In this letter, we report a silicon nanotrimer to optically trap and manipulate nanoparticles with negligible local heating. The optical forces and trapping potential of the nanotrimer are investigated using the finite-difference time-domain method. The results indicate that the trapping position can be shifted by tuning the polarization of the incident light. Furthermore, the silicon nanotrimer enables simultaneous trapping of multiple nanoparticles using circularly polarized illumination. Our work provides a promising building block for integrated all-dielectric platform to realize optically driven nano-manipulation, which offers new possibilities for on-chip optofluidic applications.

Guided Filtering based Nonlocal Means Despeckling of Optical Coherence Tomography Images

Quan Zhou, JingMin Guo, Mingyue DING, and Xuming Zhang

DOI: 10.1364/OL.400926 Received 23 Jun 2020; Accepted 26 Aug 2020; Posted 26 Aug 2020  View: PDF

Abstract: This letter has presented a guided filtering (GF) based nonlocal means (NLM) method for despeckling of optical coherence tomography (OCT) images. Unlike existing NLM methods which determine the weights only using image intensities or features, the proposed method firstly uses the GF to capture both grayscale information and features of the input image and then introduces them into the NLM for accurate weight computation. The boosting and iterative strategies are further incorporated to ensure despeckling performance. Experiments on the real OCT images demonstrate that our method outperforms the compared methods by delivering sufficient noise reduction and preserving image details well.

Localized surface plasmon modes enhanced spectrum tunable radiation in electrically driven plasmonic antennas

Yingjian Liu, Zhongjun Jiang, Jin Qin, and Liang Wang

DOI: 10.1364/OL.402163 Received 09 Jul 2020; Accepted 26 Aug 2020; Posted 26 Aug 2020  View: PDF

Abstract: A spectrum tunable source with ultra-small volume is highly desired by on-chip information processing technologies. As a promising candidate, light emission from electrically driven tunnel junctions has gained much interest. In this work, using a gap bowtie-antenna based metal-insulator-metal junction as the source, multiple peaks are found in electroluminescence spectrum of the antenna system. We attribute the peaks observed in the experimental emission spectrum to resonant plasmon modes that are supported by the antennas. This explanation is confirmed numerically by finite difference time domain (FDTD) calculations and analytically by using a theory imitated from scanning tunneling microscopy (STM). Our results show that the localized surface plasmon modes can be finely tuned by varying the gap distances and the geometries of the antennas, which eventually contributes to a spectrum tunable light source. This work may provide a path for spectrum tunable electrically driven light sources on photonic devices.

Statistical Properties of Nonlinear Distortion of Polarization-Multiplexed OFDM-Signal in Long-Haul Fiber Links

Anton Skidin, Oleg Sidelnikov, and Mikhail Fedoruk

DOI: 10.1364/OL.402401 Received 09 Jul 2020; Accepted 25 Aug 2020; Posted 26 Aug 2020  View: PDF

Abstract: We investigate nonlinear transmission regimes ofa polarization-multiplexed 16-QAM (PDM-16-QAM) OFDM-signal in a long-haul optical link. We study the dependence between the strength of nonlinear distortion and statistical properties of a PDM-OFDM-signal. We also consider the constellation shaping-based solutions that allow to significantly reduce the bit error rate (up to 3-12 times at the cost of 5-20% signal redundancy), and propose the method to analytically optimize the symbol distribution for higher-order modulation formats using only the initial signal.

Nested-ring doping for highly-efficient 1907 nm short wavelength cladding-pumped thulium fiber lasers

Matthew Barber, Peter Shardlow, Pranabesh Barua, Jayanta Sahu, and W. Clarkson

DOI: 10.1364/OL.401228 Received 14 Jul 2020; Accepted 25 Aug 2020; Posted 26 Aug 2020  View: PDF

Abstract: Cladding-pumped Tm-doped fiber lasers operating below 1950 nm have difficulty matching the high efficiency, power-scalable output that can be achieved at longer wavelengths. This challenge arises due to the strong three-level behavior at short wavelengths and strong competition from higher-gain long wavelength emission. In this paper, we demonstrate a nested-ring fiber design in which a highly-doped Tm ring is embedded within a larger undoped core. The fiber is specifically tailored for highly efficient and high power short wavelength operation (<1950 nm). The nested-ring Tm fiber laser has generated 62 W of single-mode 1907 nm output with up to 65% (70%) slope efficiency with respect to launched (absorbed) pump power.

Spatial-diversity detection of optical vortices for OAM signal modulation

Jaime Anguita and Jaime Cisternas

DOI: 10.1364/OL.397771 Received 15 May 2020; Accepted 25 Aug 2020; Posted 26 Aug 2020  View: PDF

Abstract: We propose a method for identifying orbital angular momentum (OAM) states within a vortex superposition using a Shack-Hartmann (SH) sensor as a spatial-diversity detector. We define a local OAM at every pixel of the SH image, from which we construct an OAM spectrum. The topological charges are determined from the OAM spectrum using a low-complexity algorithm, resulting in estimates that are robust to beam wandering, producing relatively narrow OAM crosstalk in turbulence. Data from a 200-m experimental transmission is successfully tested using the proposed technique.

Terahertz Nonlinear Index Extraction via Full-Phase Analysis

Kareem Garriga Francis, Mervin Lim Pac Chong, Yiwen E, and Xi-Cheng Zhang

DOI: 10.1364/OL.399999 Received 10 Jun 2020; Accepted 24 Aug 2020; Posted 24 Aug 2020  View: PDF

Abstract: We experimentally show the spectrally averaged nonlinear refractive index and absorption coefficient for liquid water, water vapor, α-pinene, and Si using a full-phase analysis in the terahertz regime through a standard time-domain spectrometer. Our results confirm that the nonlinear index of refraction of the liquid samples in this regime exceeds the near-infrared optical nonlinear index by six orders of magnitude. In the case of liquid water and water vapor at atmospheric pressure, we find a nonlinear index of 7.8x10-10 cm2/W and 6x10-11 cm2/W respectively, which are both much larger than expected.

Simultaneous measurements of specimen quantitative-phase signal and surrounding medium refractive index using digital holographic microscopy

Bertrand de Dorlodot, Erik Bélanger, Émile Rioux-Pellerin, and Pierre Marquet

DOI: 10.1364/OL.391641 Received 03 Mar 2020; Accepted 24 Aug 2020; Posted 25 Aug 2020  View: PDF

Abstract: The advance presented in this paper demonstrates a method to simultaneously measure the quantitative phase signal (QPS) of the observed specimen and the refractive index of its surrounding medium (n_m) in a time-resolved manner based on a micro-structured cell imaging substrate integrated with digital holographic microscopy (DHM). QPS is highly dependent on n_m, therefore knowing its value provides a referenced QPS enabling for n_m artifacts control and well-suited for the determination of cell-specific biophysical parameters. The proposed approach easily allows others quantitative-phase imaging techniques to measure a referenced QPS, while not being restricted to DHM.

DFG based MID-IR tunable source with 0.5 mJ energy and 30 pm linewidth

Lyubomir I. STOYCHEV, Humberto Cabrera, Jose Suarez-Vargas, Marco Baruzzo, Komlan Segbéya GADEDJISSO-TOSSOU, Ivaylo Nikolov, Paolo Sigalotti, Alexander Demidovich, Emiliano Mocchiutti, CECILIA PIZZOLOTTO, Joseph Niemela, Guido Toci, M Danailov, and Andrea Vacchi

DOI: 10.1364/OL.405272 Received 12 Aug 2020; Accepted 21 Aug 2020; Posted 26 Aug 2020  View: PDF

Abstract: We report on a laser system based on difference frequency generation (DFG) to produce tunable, narrow-linewidth (<30 pm), and comparatively high energy mid-IR radiation in the 6.8 µm region. The system exploits a lithium thioindate (LiInS2) non-linear crystal and nanosecond pulses generated by single frequency Nd:YAG and Cr:forsterite lasers at 1064 nm and 1262 nm, respectively. Two experimental configurations are used: In the first one, single-pass, the mid-IR energy achieved is 205 µJ. Additional increments, up to 540 µJ, are obtained by performing double-pass through the non-linear crystal. This laser has been developed for high-resolution photon-hungry spectroscopy in the mid-IR.

Complex modes in an open lossless periodic waveguide

Amgad Abdrabou and Ya Yan Lu

DOI: 10.1364/OL.403204 Received 20 Jul 2020; Accepted 21 Aug 2020; Posted 21 Aug 2020  View: PDF

Abstract: Guided modes of an open periodic waveguide, with a periodicity in the main propagation direction, are Bloch modes confined around the waveguide core with no radiation loss in the transverse directions. Some guided modes can have a complex propagation constant, i.e. a complex Bloch wavenumber, even when the periodic waveguide is lossless (no absorption loss). These so-called complex modes are physical solutions that can be excited by incident waves whenever the waveguide has discontinuities and defects. We show that the complex modes in an open dielectric periodic waveguide form bands, and the endpoints of the bands can be classified to a small number of cases, including extrema on dispersion curves of the regular guided modes, bound states in the continuum, degenerate complex modes, and special diffraction solutions with blazing properties. Our study provides an improved theoretical understanding on periodic waveguides and a useful guidance to their practical applications.

Miniature all-optical flexible forward-viewing photoacoustic endoscopy probe for surgical guidance

Rehman Ansari, Edward Zhang, Adrien Desjardins, and Paul Beard

DOI: 10.1364/OL.400295 Received 15 Jun 2020; Accepted 18 Aug 2020; Posted 26 Aug 2020  View: PDF

Abstract: A miniature flexible photoacoustic endoscopy probe that provides high-resolution 3D images of vascular structures in the forward-viewing configuration is described. A planar Fabry-Perot ultrasound sensor with a -3dB bandwidth of 53MHz located at the tip of the probe is interrogated via a fibre bundle and a miniature optical relay system to realise an all-optical probe measuring 7.4 mm in outer diameter at the tip. This approach to photoacoustic endoscopy offers significant advantages over previous piezoelectric based distal-end scanning probes. These include forward-viewing configuration, finer spatial sampling (87 µm pitch) and wider detection bandwidth (53 MHz) than achievable with conventional piezoelectric receivers and an all-optical passive imaging head for safe endoscopic use. The lateral resolution of the probe varies from 60 μm at 1 mm depth to 166 μm at 8 mm depth. High-resolution photoacoustic imaging capability in tomography mode is demonstrated by visualising sub-surface vascular structures of an ex vivo human placenta.

Ultrafast electron and proton bunches correlation in laser-solid matter experiments

Fabrizio Bisesto, Mario Galletti, Maria Pia Anania, Gemma costa, Massimo Ferrario, Riccardo Pompili, Alexandre Poye, Fabrizio Consoli, martina salvadori, mattia cipriani, claudio verona, and Arie Zigler

DOI: 10.1364/OL.402938 Received 16 Jul 2020; Accepted 17 Aug 2020; Posted 17 Aug 2020  View: PDF

Abstract: The interaction of an ultra-intense laser with a solid state target allows the production of multi-MeV proton and ion beams. This process is explained by the TNSA model, predicting the creation of an electric field on the target rear side, due to an unbalanced positive charge. This process is related to the emission of relativistic ultrafast electrons, occurring at earlier time. In this work, we highlight the correlations between the ultrafast electron component and the protons by their simultaneous detection by means of an Electro-Optical Sampling and a TOF diagnostics, respectively, supported by numerical simulations showing an excellent agreement.

The potassium tantalate niobate (KTN) crystal-based polarization-modulated 3D ladar with a large field of view

song yishuo, Jiguang Zhao, Bin Wang, Xuping Wang, Bing Liu, Xiaolei Lv, and jianwei Zhang

DOI: 10.1364/OL.398225 Received 21 May 2020; Accepted 14 Aug 2020; Posted 26 Aug 2020  View: PDF

Abstract: An innovative 3D ladar that utilizes potassium tantalate niobate (KTN) crystal as a polarization modulator is proposed in this paper. The optical isotropy of KTN in cubic phase can effectively suppress the range errors induced by the incident angles of collected beams in 3D imaging. The giant quadratic electro-optic coefficient can dramatically lower the voltage that is required to modulate the polarization so that a high voltage amplifier with less noise can be used to improve the ranging performances. By virtue of these two advantages, a range error of 4.8cm and a range precision of 4.4cm at 15m have been achieved under a large field of view (FOV) of 20° (about 0.35rad) and a wide range gate of 60m.

Rectifying the phase-matching condition for fiber mode converters using two-mode interference

Soham Basu

DOI: 10.1364/OL.402444 Received 10 Jul 2020; Accepted 12 Aug 2020; Posted 13 Aug 2020  View: PDF

Abstract: The resonance wavelength, where a fiber grating mode converter written using periodic external perturbations achieves phase-matching, is both a critical design parameter and a device parameter. However, a method to precisely predict the resonance wavelength for any new fiber and grating writing apparatus has been missing so far. The missing link was the lack of direct experimental methods to estimate the modified intermodal phase after writing with external perturbations. The presented method can make this estimation from a single experiment, over a broad wavelength range, based on a novel mathematical connection between two-mode interference (TMI) and mode conversion. Using the novel methods, experimentally measured resonance wavelengths for different pitch and irradiation conditions have been predicted within relative errors of 0.004.

Study of omni-directional ultraviolet communication based on Monte-Carlo method

Tao Shan, Jianshe Ma, Tianfeng Wu, Zanqiu shen, and Su Ping

DOI: 10.1364/OL.400028 Received 11 Jun 2020; Accepted 12 Aug 2020; Posted 14 Aug 2020  View: PDF

Abstract: The existing study of the ultraviolet (UV) communication channel model mainly focuses on the point-to-point scenarios. To analyze the UV channel characteristics in omni-directional scenarios, we proposed a multiple scattering omni-directional channel model based on Monte-Carlo simulation in this letter, where we assume that the receiver can be anywhere in a certain area and all the receivers point to the vertical axis of the transmitter. Meanwhile, the proposed model is validated by comparing with existing point-to-point Monte-Carlo simulation model. Simulation results indicate the path loss difference between the single scattering model and the multiple scattering model, which implies that the single scattering model is not applicable to omni-directional analysis. Furthermore, the transceivers configurations can affect the coverage area of omni-directional UV communication system significantly. This work presents a new way to obtain the path loss distribution and provides guidelines for the omni-directional communication system design.

Evaluation of the Tenti S6 model for hydrocarbon fuels at elevated temperatures using filtered Rayleigh scattering measurements

Jinpeng Pu and Jeffrey Sutton

DOI: 10.1364/OL.403391 Received 22 Jul 2020; Accepted 12 Aug 2020; Posted 17 Aug 2020  View: PDF

Abstract: The current Letter targets the assessment of the well-known Tenti S6 model for predicting the Rayleigh-Brillouin scattering (RBS) spectra of select gas-phase hydrocarbon fuels (CH4, C2H2, C2H4, C3H8, and C4H10) over a temperature range of 300 K to 700K. The Tenti S6 model is evaluated by comparing filtered Rayleigh scattering (FRS) measurements to synthetic FRS signals generated from the combination of the Tenti S6 output and an accurate iodine absorption filter model. The experimental and synthetic FRS results agree very well over the full temperature range for CH4, C2H2 and C2H4, indicating accurate calculation of the RBS spectra. For C3H8 and C4H10 there are some large differences between the experimental and synthetic FRS results which cannot be resolved through tuning of bulk viscosity, internal heat capacity, nor inclusion of vibrational degrees of freedom, suggesting the need for detailed measurements of the Rayleigh-Brillouin spectra and potentially new modeling approaches for more complex hydrocarbons.

Electrically-pumped continuous-wave O-band quantum-dot superluminescent diode on silicon

ying lu, Victoria Cao, Mengya liao, Wei Li, Mingchu Tang, Ang Li, Peter Smowton, Alwyn Seeds, Huiyun Liu, and Siming Chen

DOI: 10.1364/OL.401042 Received 25 Jun 2020; Accepted 03 Aug 2020; Posted 03 Aug 2020  View: PDF

Abstract: High-power, broadband quantum dot (QD) superluminescent diodes (SLDs) are ideal light sources for optical coherence tomography (OCT) imaging systems but have previously mainly been fabricated on native GaAs- or InP-based substrates. Recently, significant progress has been made to emigrate QD SLDs from native substrates to silicon substrates. Here, we demonstrate electrically pumped continuous-wave (CW) InAs QD SLDs monolithically grown on silicon substrates with significantly improved performance thanks to the achievement of a low density of defects in the III-V epilayers. The fabricated narrow ridge-waveguide device exhibits a maximum 3-dB bandwidth of 103 nm emission spectrum centred at O-band together with a maximum single-facet output power of 3.8 mW at room temperature. The silicon based SLD has been assessed for application in an OCT system. Under optimised conditions, a predicted axial resolution of ~ 5.3 µm is achieved with a corresponding output power of 0.66 mW/facet. The realisation of high-performance III-V SLDs on silicon substrates will be the enabling technology for low-cost, large-scale deployment of fully integrated silicon photonic OCT systems.

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