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3D trapping of microbubbles by the Marangoni force

Julio Sarabia Alonso, Ruben Ramos-Garcia, Svetlana Mansurova, Jose Ortega, and F. M. Muñoz-Pérez

DOI: 10.1364/OL.440290 Received 13 Aug 2021; Accepted 25 Oct 2021; Posted 26 Oct 2021  View: PDF

Abstract: In this letter, we show 3D steady-state trapping and manipulation of vapor bubbles in liquids employing a low-power continuous-wave laser using the Marangoni effect. Light absorption from photodeposited silver nanoparticles on the distal end of multi-mode optical fiber is used to produce bubbles of different diameters. The thermal effects produced by either nanoparticle on the fiber tip or light bulk absorption modulate the surface tension of the bubble wall creating both longitudinal and transversal forces just like optical forces, effectively creating a 3D potential well. Using numerical simulations, we obtain expressions for the temperature profiles and present analytical expressions for the Marangoni force. In addition, using an array of three fibers with photodeposited nanoparticles is used to demonstrate the transfer of bubbles from one fiber to another by switching on and off sequentially the lasers.

Laser Cooling of a Yb Doped Silica Fiber by 18 Kelvin From Room Temperature

Brian Topper, Mostafa Peysokhan, Alexander Albrecht, Angel Flores, Stefan Kuhn, Denny Haessner, Sigrun Hein, Christian Hupel, Johannes Nold, Nicoletta Haarlammert, Thomas Schreiber, Mansoor Sheik-Bahae, and Arash Mafi

DOI: 10.1364/OL.444709 Received 01 Oct 2021; Accepted 25 Oct 2021; Posted 26 Oct 2021  View: PDF

Abstract: A ytterbium doped silica optical fiber has been cooled by 18.4 K below ambient temperature by pumping with 20 W of 1035 nm light in vacuum. In air, cooling by 3.6 K below ambient was observed with the same 20 W pump. The temperatures were measured with a thermal imaging camera and differential luminescence thermometry. The cooling efficiency is calculated to be 1.2+- 0.1%. The core of the fiber was codoped with Al3+ for an Al to Yb ratio of 6:1, to allow for a larger Yb concentration and enhanced laser cooling.

Large-range phase-difference sensing technology for low-frequency strain interrogation

Jinhui Shi, Dong Guang, Shili Li, Xuqiang Wu, Cheng Zuo, Lei Gui, Wujun Zhang, Guosheng Zhang, Qiang Ge, Gang Zhang, Rui Wang, and Benli Yu

DOI: 10.1364/OL.444091 Received 23 Sep 2021; Accepted 25 Oct 2021; Posted 26 Oct 2021  View: PDF

Abstract: Phase-difference sensing technology (PDST) has been applied to strain measurement, but its completeness is destroyed by the phase-difference measurement range. A scheme that can realize the completeness of the PDST for low-frequency strain interrogation is proposed. It is built on dual-interferometers (DIs) and the elliptic-fitting algorithm (EFA). To break the measurement range limitation (0, π), a phase compensation setting is applied. Experimental results demonstrate that the method can obtain low-frequency strain signals, and the low-frequency signal whose phase amplitude is greater than π is recovered. The scheme is an efficient and complete method for measuring the strain of low-frequency optical fiber length, which could be applied to low-frequency seismic wave monitoring and rock deformation detection.

Algebraic cancellation of inter-channel crosstalk in multiplexed heterodyne interferometry

Anneshwa Dey, Ya Zhang, Justin Wong, Paul Sibley, Chathura Bandutunga, Malcolm Gray, and Jong Chow

DOI: 10.1364/OL.444116 Received 28 Sep 2021; Accepted 25 Oct 2021; Posted 26 Oct 2021  View: PDF

Abstract: We demonstrate the algebraic cancellation of residual phase cross-talk in digitally enhanced heterodyne interferometry (DEHeI), a code division multiplexing technique for interferometric sensing. By using linear combinations of parallel decoding operations at multiple delays, we synthesize a zero-correlation for spurious signals and removes phase cross-talk: a method we call Offset Decoding. We experimentally demonstrate 70 dB of signal isolation and over 40 dB greater isolation than the equivalent standard implementation of DEHeI.

Fundamental and vortex gap solitons in quasiperiodic optical lattices

Changming Huang, Liangwei Dong, Hanying Deng, Zhang Xiao, and Penghui Gao

DOI: 10.1364/OL.443051 Received 10 Sep 2021; Accepted 24 Oct 2021; Posted 26 Oct 2021  View: PDF

Abstract: We address the existence and stability of fundamental, single-charged vortex, and double-charged vortex gap solitons in two-dimensional ($2$D) quasiperiodic optical lattices. Fundamental and vortex gap solitons can bifurcate from linear localized states or their combination supported by quasiperiodic lattices for both defocusing and focusing nonlinearities. We find that the three types of solitons mentioned above are stable in the entire existence domain for defocusing nonlinearities, and that they can also be stable at a lower power level for focusing nonlinearities. At higher power, unstable solitons are characterized by a ring-shaped symmetry-breaking distribution, and the unique spot profile formed is repeatedly observed with change in the propagation distance.

Parallel emitted silicon nitride nanophotonic phased arrays for efficient 2D beam steering

Caiming Sun, Lesi Yang, Binghui Li, Wu Shi, Hongjie Wang, Zhenmin Chen, Xiaomin Nie, Shupeng Deng, Ning Ding, and Aidong Zhang

DOI: 10.1364/OL.443414 Received 15 Sep 2021; Accepted 23 Oct 2021; Posted 26 Oct 2021  View: PDF

Abstract: In this Letter, a two-dimensional beam steering on silicon nitride (SiNx) nanophotonic phased array from visible to near-infrared wavelengths is reported for the first time. In order to implement beam steering along transversal direction for 1D waveguide surface grating arrays, wavelengths of 650 nm to 980 nm provided by the supercontinuum (SC) laser are used to excite the phased array, generate massively parallel radiated beams with steering angles in sequence of 26.84° to -16.54° along transversal direction, and eventually, produce a continuous line in far field consisting of parallel emitted spots with total view angle of 43.38°. Then, this continuous far-field line is steered along longitudinal direction, with massive wavelengths simultaneously tuned by phase shifting from –π/2 to over +π/2. To the best of our knowledge, this 2D steering with massive wavelengths parallel emitted is the most efficient solution so far on silicon nitride nanophotonic phased array, compared to others with single wavelength.

Amplification of 14 orbital angular momentum modes in ring-core erbium-doped fiber with high modal gain

Xi Zhang, Jun Liu, Shi Chen, Cheng Du, Wei Li, and Jian Wang

DOI: 10.1364/OL.440098 Received 09 Aug 2021; Accepted 23 Oct 2021; Posted 26 Oct 2021  View: PDF

Abstract: We theoretically propose and experimentally demonstrate an orbital angular momentum (OAM) fiber amplifier supporting 14 OAM modes based on a fabricated ring-core erbium-doped fiber (RC-EDF) with tailored erbium-doped profile. Theoretical analyses and numerical simulations suggest that the proposed RC-EDF provides relatively uniform gain larger than 20 dB for all 14 modes. With a core pump configuration, we experimentally characterize the performance of the RC-EDF-assisted OAM fiber amplifier, which can acquire a high modal gain up to 33.4 dB and low differential modal gain (DMG) less than 1.8 dB at the wavelength of 1550 nm. The obtained results indicate successful implementation of the RC-EDF-assisted OAM fiber amplifier for 14 OAM modes with favorable performance.

A PVA-SM microstructure enhanced ratiometric fluorescence probe for formaldehyde detection in solution and gas

Tianci Wang, Haiyan Wang, Pengfei Xia, Sheng Xue, Guanjie Yang, Heng Li, Zongbao Li, Xiaofang Jiang, and Xiaobo Xing

DOI: 10.1364/OL.441296 Received 23 Aug 2021; Accepted 23 Oct 2021; Posted 26 Oct 2021  View: PDF

Abstract: Formaldehyde (FA) as one of the most common pollu-tants has tremendous harm to humans and the envi-ronment. In this work, 4-amino-3-pentene-2-one (Fluoral-p) and SiO₂ coated quantum dot (QD@SiO₂) were combined to implement a new ratiometric fluorescence probe QD@SiO₂-Fluoral-p for FA detection. In addition, by utilization of polyvinyl alcohol (PVA) and SiO₂ microsphere (SM), a kind of PVA-SM microstructure was assembled with QD@SiO₂-Fluoral-p to composite a signal enhanced sensing film. The QD@SiO₂-Fluoral-p exhibited good response to 0-400 mg/L FA solution and an en-hancement around 15 folds was realized after introducing PVA-SM microstructure. In both situations, the probe showed linear relationship to FA concentration (CFA), with detection limit of 14 and 0.5 mg/L, respectively. Also, the sensing film showed a good linear response to FA gas in the range of 0 to 2 ppm, with a detection limit 0.03 ppm. As a result, the microstructure enhanced ratiometric fluorescence probe features high sensitivity, low detection limit, good selectivity, as well as portable, which can serve as a useful tool for investigating FA in solution and gas at room temperature.

Transient soliton dynamics in a mode-locked fiber laser: from stationary to pulsation

Qianqian Huang, Zinan Huang, and Chengbo Mou

DOI: 10.1364/OL.442740 Received 08 Sep 2021; Accepted 23 Oct 2021; Posted 26 Oct 2021  View: PDF

Abstract: We report the experimental observation of the transformation process from a stationary soliton to a pulsating soliton in a mode-locked fiber laser. The detailed soliton dynamics including soliton parameter changings and sidebands variation in the transition are analyzed by dispersive Fourier transformation (DFT) technique. Our work unveils that it is the great enhancement of intracavity periodic perturbations resulting from the slight increase of soliton intensity induced soliton modulation instability (MI), leading to the occurrence of parametric sidebands (PSs) and multi-periodic pulsing instability. Our experimental results can shed some light on the intrinsic mechanism of soliton pulsation as well as contribute to the study of soliton stability and laser dynamics.

Omnidirectional ghost imaging system and unwrapping-free panoramic ghost imaging

Huan Cui, jie CAO, Qun Hao, Dong Zhou, Mingyuan Tang, kaiyu zhang, and Yingqiang Zhang

DOI: 10.1364/OL.440660 Received 16 Aug 2021; Accepted 22 Oct 2021; Posted 26 Oct 2021  View: PDF

Abstract: Ghost imaging (GI) is a novel imaging method that can reconstruct object information via light-intensity correlation measurements. However, at present, the field of view (FOV) of this method is limited to the illumination range of light patterns. To enlarge the FOV of GI efficiently, we propose an omnidirectional GI system (OGIS) that can achieve a 360° omnidirectional FOV in only one shot via the addition of a curved mirror. The OGIS features retina-like annular patterns designed as a log-polar structure, and the system can obtain unwrapping-free undistorted panoramic images with uniform resolution. This research presents a new perspective for the applications of GI.

Femtosecond laser line-by-line inscription of apodized fiber Bragg gratings

Jun He, Ziyong Chen, Xizhen Xu, Jia He, Baijie Xu, Bin Du, Kuikui Guo, Runxiao Chen, and Yiping Wang

DOI: 10.1364/OL.441888 Received 30 Aug 2021; Accepted 22 Oct 2021; Posted 26 Oct 2021  View: PDF

Abstract: The reflection spectra of conventional fiber Bragg gratings (FBGs) with uniform index modulation profile typically have strong sidelobes, which hamper the performance of FBG-based optical filters, fiber lasers and sensors. Here, we propose and demonstrate a femtosecond laser line-by-line (LbL) scanning technique for fabricating apodized FBGs with suppressed sidelobes. This approach can flexibly achieve various apodized modulation profiles via precise control over the length and/or transverse position of each laser-inscribed index modification track. We theoretically and experimentally studied the influences of apodization function on the side-mode suppression ratio (SMSR) in the fabricated apodized FBG, and the results show that a maximum SMSR of 20.6 dB was achieved in a Gaussian-apodized FBG. Subsequently, we used this method to fabricate various apodized FBGs, and the SMSRs in these FBGs have be reduced effectively. Specifically, a dense-wavelength-division-multiplexed (DWDM) Gaussian-apodized FBG array with a wavelength interval of 1.50 nm was successfully fabricated, and the SMSR in such an array is 14 dB. Moreover, Gaussian-apodized phase-shifted FBG and chirped FBG were also demonstrated with a high SMSR of 14 and 16 dB, respectively. Therefore, such an apodization method based on modified femtosecond laser LbL scanning technique is an effective and flexible way to fabricate various FBGs with high SMSR, which is promising to improve the performance of optical filters, fiber lasers and sensors.

Polarization-guided road detection network for LWIR DoFP camera

Ning Li, Yongqiang Zhao, Rongyuan Wu, and Quan Pan

DOI: 10.1364/OL.441817 Received 30 Aug 2021; Accepted 21 Oct 2021; Posted 26 Oct 2021  View: PDF

Abstract: Long-wave infrared division-of-focal-plane polarization imaging technique recently has been applied in full-time road detection. However, the existing heuristic method has the limitation of fully using the polarization information of the road. In this letter, we propose a polarization-guided road detection network (PolarNet) collaborating with the distinguishable polarization characteristics of the road. A coarse road map obtained by the polarization priors of the road guides the network to focus on the road regions through a polarization-guided branch. A spatial-attention feature fusion module is designed to perform road-region-aware feature fusion. This customized design of the network gives full play to the advantages of deep-learning networks and polarization information. Experiments on the LDDRS dataset demonstrate that the proposed PolarNet outperforms state-of-the-art real-time segmentation networks with fewer parameters and faster speed.

Frequency manipulation of topological surface states by Weyl phase transitions

Bing Wang, Zhuoxiong Liu, Chengzhi Qin, Weiwei Liu, ling zhi zheng, Shuaifei Ren, and Peixiang Lu

DOI: 10.1364/OL.442890 Received 10 Sep 2021; Accepted 21 Oct 2021; Posted 26 Oct 2021  View: PDF

Abstract: By creating a synthetic frequency dimension with dynamic modulation in a 2D honeycomb waveguide array, we construct both Type-I and Type-II Weyl semimetals (WSMs) and utilize the WSM phase transition to control the frequency evolutions of topological surface states. We show that Type-I and Type-II WSMs manifest opposite and same band slopes for the two surface states, which give rise to the bidirectional and unidirectional frequency shifts, respectively. Moreover, by cascading Type-I and Type-II Weyl lattices together, we also achieve time-reversed evolution of frequency, such as frequency negative refraction, bandwidth expansion-compression, and perfect imaging. The study may find applications in robust signal transmission and processing with synthetized topological states.

Spatiotemporal couplings through nonlinear phase in broadband optical parametric amplification

Wang Yirui, Jing Wang, Binjie Zhou, Jingui Ma, Peng Yuan, and Liejia Qian

DOI: 10.1364/OL.440145 Received 09 Aug 2021; Accepted 20 Oct 2021; Posted 21 Oct 2021  View: PDF

Abstract: Optical parametric chirped-pulse amplification (OPCPA) is prone to undesired spatiotemporal couplings. This Letter studies a family of OPCPA couplings resulting from the nonlinear phase shift induced by frequency-dependent phase-mismatch. These OPCPA couplings manifest as pulse-front deformation, transversely-varying pulse duration and spectrally-varying wavefront curving, which are directly linked with the phase-mismatch dispersion terms. The numerical study also reveals that the focused signal intensity severely degrades with increasing signal bandwidth and pump depletion.

120 mJ, 1 kHz, picosecond laser at 515 nm

Zbynek Hubka, Roman Antipenkov, Robert Boge, Emily Erdman, Michael Greco, Jonathan Green, Martin Horáček, Karel Majer, Tomáš Mazanec, Petr Mazurek, Jack Naylon, Jakub Novák, Vaclav Sobr, Petr Strkula, Murat Torun, Boguslaw Tykalewicz, Pavel Bakule, and Bedrich Rus

DOI: 10.1364/OL.440448 Received 18 Aug 2021; Accepted 20 Oct 2021; Posted 21 Oct 2021  View: PDF

Abstract: We report on a 1 kHz, 515 nm laser system, based on a commercially available 0 W average power Yb:YAG thin-disk regenerative amplifier, developed for pumping one of the last OPCPA stages of the Allegra laser system at ELI Beamlines. To avoid problems with self-focusing of picosecond pulses, the 1030 nm output pulses are compressed and frequency doubled with an LBO crystal in vacuum. Additionally, development of a thermal management system was needed to ensure stable phase matching conditions at high average power. The resulting 515 nm pulses have an energy of more than 120 mJ with SHG efficiency of 60% and an average RMS stability of 1.1% for more than 8 h.

Intense ~4 μm emission from Pr3+/Yb3+ co-doped fluoroindate glass

Huiyu He, Zhixu Jia, Yasutake Ohishi, WP Qin, and Guanshi Qin

DOI: 10.1364/OL.440635 Received 16 Aug 2021; Accepted 20 Oct 2021; Posted 21 Oct 2021  View: PDF

Abstract: Pr3+/Yb3+ co-doped fluoroindate (InF3) glasses were prepared by using a traditional melt-quenching method in dry N2 atmosphere. Pumped by a 976 or 1570 nm laser diode, intense emissions at ~4 μm were obtained from the Pr3+/Yb3+ co-doped glasses, which was attributed to the transition 3F4→3H6 of Pr3+ ions. The relative stimulated emission cross section was calculated to be ~1.44×10-24 m2 at 4 μm, which was ~4.2 times larger than that of transition Ho3+:5I5→5I6 (3.4×10-25 m2). In addition, combined with transitions 1G4→3F3 and 1G4→3F4 of Pr3+ ions, ultra-broadband emission ranging from 2.7 to 4.2 μm was also obtained. Our results indicated that Pr3+/Yb3+: InF3 glasses could be used to develop efficient ~4 μm lasers and widely tunable mid-infrared lasers.

Pulse photothermal optical coherence tomography for multimodal hemodynamic imaging

Jiayi Wu, nanshou wu, Peijun Tang, Jiayi LIN, Yi Lian, and Zhilie Tang

DOI: 10.1364/OL.442552 Received 07 Sep 2021; Accepted 20 Oct 2021; Posted 21 Oct 2021  View: PDF

Abstract: To realize multimodal hemodynamic imaging, pulse photothermal optical coherence tomography(P-PTOCT) is proposed in this paper to solve the separation problem of photothermal phase and Doppler phase which is difficult to be solved in traditional PTOCT. This technique can obtain blood flow distribution, light absorption distribution and concentration images simultaneously. Based on the difference between pulse photothermal phase and Doppler phase, we proposed an even number differential demodulation algorithm(ENDDA), which can separate the photothermal phase and Doppler phase from the same scanning data set. The separated photothermal phase can characterize the trend of drug concentration, which provides the possibility for quantitative measurement of plasma concentration. The combination of photothermal phase and Doppler phase is helpful for the potential clinical research on hemodynamics of cerebral ischemia, and provides a technical reference for the rapid acquisition of perfusion volume and plasma concentration at one time.

On-chip multi-color microdisk laser on Yb3+-doped thin-film lithium niobate

Yuan Zhou, Zhe Wang, Zhiwei Fang, zhaoxiang liu, Haisu Zhang, Difeng Yin, Youting Liang, Zhihao Zhang, jian Liu, Ting Huang, Rui Bao, Rongbo Wu, Jintian Lin, Min Wang, and Ya Cheng

DOI: 10.1364/OL.440379 Received 13 Aug 2021; Accepted 19 Oct 2021; Posted 21 Oct 2021  View: PDF

Abstract: We demonstrate an on-chip Yb3+-doped lithium niobate (LN) microdisk laser. The intrinsic quality factors of the fabricated Yb3+-doped LN microdisk resonator are measured up to 3.79 × 105 at 976 nm wavelength and 1.1 × 106 at 1514 nm wavelength. The multi-mode laser emissions are obtained in a band from 1020 nm to 1070 nm pumped by 984 nm laser and with the low threshold of 103 μW, resulting in a slope efficiency of 0.53% at room temperature. Furthermore, the second-harmonic frequency of pump light and the sum-frequency of the pump light and laser emissions are both generated in the on-chip Yb3+-doped LN microdisk benefited from the strong χ(2) nonlinearity of LN. These microdisk lasers are expected to contribute to the high-density integration of LNOI-based photonic chip.

Structural modulation designed a thermally robust blue-cyan emitting phosphor Y2Mg0.8Sr0.2Al4SiO12: Eu2+ for the high color rendering index white LEDs

Panlai Li, Huifang Yang, Zejun Ye, Xiaoxue Huo, Qian Wu, Yu Wang, and Zhijun Wang

DOI: 10.1364/OL.442627 Received 06 Sep 2021; Accepted 19 Oct 2021; Posted 21 Oct 2021  View: PDF

Abstract: In this work, the novel blue-cyan Y2Mg0.8Sr0.2Al4SiO12:Eu2+ (YM0.8S0.2AS:Eu2+) phosphors were synthesized by the solid-state method. At 150 ℃, the emission intensity of Y2Mg0.8Sr0.2Al4SiO12: 0.005Eu2+ can retain 96.38% of the relative intensity, which means that this phosphor shows the high thermal stability. A white light-emitting diode (WLED) device is fabricated by combining 370nm near ultraviolet LED chip and commercial phosphors (Green: (Ba, Sr)2SiO4:Eu, Red: CaSiAlN3:Eu). The WLED has excellent property with the correlated color temperature CCT=5 6 K and superhigh color rendering index Ra=96.1, which indicates the potential application in WLED fields.

Secure transmission of W-band millimeter-wave based on CNN and dynamic resource allocation

Yaoqiang Xiao, Yating Chen, and Zhiyi Wang

DOI: 10.1364/OL.436366 Received 12 Jul 2021; Accepted 19 Oct 2021; Posted 21 Oct 2021  View: PDF

Abstract: A secure transmission scheme of radio-over-fiber (RoF) system based on cellular neural network (CNN) and dynamic resource allocation is proposed. A 7-D CNN chaotic system is constructed to ensure the security of the system, and the spectrum resources are optimized by dynamic allocation. The analyses show that the proposed scheme can achieve a key space of 10¹¹² . The encrypted W-band millimeter-wave signal is successfully transmitted in the filter-OFDM RoF system of 50-km SSMF and 5m wireless channel, and the experimental results show that the proposed scheme can improve the bit error ratio (BER) performance by approximately 0.3 dB compared with the traditional one.

Deep learning assisted Fiber Bragg Grating interrogation by random speckles

Tianliang Wang, Yi Li, Jinchao Tao, Xu Wang, Yan-Qing Qiu, Bangning Mao, Miao-Gen Chen, Yanlong Meng, Chunliu Zhao, Juan Kang, Yong Guo, and Yu Shen

DOI: 10.1364/OL.445159 Received 06 Oct 2021; Accepted 19 Oct 2021; Posted 26 Oct 2021  View: PDF

Abstract: Fiber Bragg Gratings (FBGs) have been widely employed as a sensor for temperature, vibration, strain, and etc. measurements. However, extant methods for FBG interrogation still face challenges in the aspects of sensitivity, measurement speed, and cost. In this Letter, we introduced random speckles as the FBG’s reflection spectrum information carrier for demodulation. Instead of commonly used InGaAs cameras, a quadrant detector (QD) was firstly utilized to record speckle patterns in experiments. Although the speckle images were severely compressed into four channel signals by the QD, the spectral features of FBGs still can be precisely extracted with the assistance of a deep convolution neural network (CNN). The temperature and vibration experiments were demonstrated with a resolution of 1.2 pm. These results show that the new speckle based demodulation scheme can satisfy the requirements of both high-resolution and high-speed measurements, which should pave a new way for the optical fiber sensors.

Millimeter-level resolution through-the-wall radar imaging enabled by an optically injected semiconductor laser

Guanqun Sun, Fangzheng Zhang, and Shilong Pan

DOI: 10.1364/OL.441803 Received 30 Aug 2021; Accepted 19 Oct 2021; Posted 26 Oct 2021  View: PDF

Abstract: Millimeter-level resolution through-the-wall radar (TWR) imaging is demonstrated using a broadband nonlinear frequency modulated (NLFM) signal that is generated by an optically injected semiconductor laser. The proposed system uses period-one dynamics of a semiconductor laser together with optical frequency down conversion technique to generate NLFM signals, which addresses the problem that traditional period-one oscillation cannot generate broadband signals in the low-frequency region. In the experiment, an NLFM signal having a broad bandwidth of 18.5 GHz (1.5-20GHz) is generated with a corresponding radar range resolution of 8.1 mm. Using this signal, TWR imaging is demonstrated, in which the use of NLFM signal achieves good side-lobe suppression during pulse compression and a modified back projection imaging algorithm with sub-aperture weighting is proposed to improve the imaging quality.

Silicon photonic MEMS phase shifters for scalable programmable photonic circuits

Pierre Edinger, Alain Takabayashi, Carlos Errando-Herranz, Umar Khan, Hamed Sattari, Peter Verheyen, Wim Bogaerts, Niels Quack, and Kristinn Gylfason

DOI: 10.1364/OL.436288 Received 07 Jul 2021; Accepted 19 Oct 2021; Posted 19 Oct 2021  View: PDF

Abstract: Programmable photonic integrated circuits are emerging as an attractive platform for applications such as quantum information processing and artificial neural networks. However, current programmable circuits are limited in scalability by the lack of low-power and low-loss phase shifters in commercial foundries. Here, we demonstrate a compact phase shifter with low-power photonic MEMS (micro-electro-mechanical systems) actuation on a silicon photonics foundry platform (IMEC’s iSiPP50G). The device typically attains 2.9π phase shift at 1550 nm, with an insertion loss of 0.33 dB, a Vπ of 10.7 V, and an Lπ of 17.2 μm. We also measured an actuation bandwidth f-3dB of 1.03 MHz. We believe that our demonstration of a low-loss and low-power photonic MEMS phase shifter implemented in silicon photonics foundry compatible technology lifts a main roadblock towards the scale-up of programmable photonic integrated circuits.

Long distance twin-field quantum key distribution with entangled sources

Bing-Hong Li, Yuan-Mei Xie, Zhao Li, Chen-Xun Weng, Chen-Long Li, Hua-Lei Yin, and Zeng-Bing Chen

DOI: 10.1364/OL.443099 Received 10 Sep 2021; Accepted 19 Oct 2021; Posted 19 Oct 2021  View: PDF

Abstract: Twin-field quantum key distribution (TFQKD), using single-photon-type interference, offers a way to exceed the rate-distance limit without quantum repeaters. However, it still suffers from the photon losses and dark counts, which impose an ultimate limit on its transmission distance. In this letter, we propose a scheme to implement TFQKD with an entangled coherent state source in the middle to increase its range, as well as comparing its performance under coherent attacks with that of TFQKD variants. The simulation results show that our protocol can achieve a maximum transmission distance nearly 400 km longer than that of other protocols with current detectors and fibers. Moreover, the scheme has great robustness against the misalignment error and finite-size effects. Our work is a promising step toward long-distance secure communication and is greatly compatible with future global quantum network.

Deep learning based brightfield image generation from a single hologram using unpaired dataset

Dániel Terbe, Laszlo Orzó, and Ákos Zarándy

DOI: 10.1364/OL.440900 Received 18 Aug 2021; Accepted 18 Oct 2021; Posted 19 Oct 2021  View: PDF

Abstract: We applied an unpaired (label-free) neural network training technique, namely CycleGAN, to generate brightfield microscope-like images from hologram reconstructions. The motivation for unpaired training is that the construction of paired/parallel datasets is cumbersome, or sometimes it is not even feasible (e.g. lensless or flow through holographic measuring setups). Our results show that the proposed method is applicable in these cases and provides comparable results that of the paired training. Furthermore, it has some additional favorable properties even though its metric scores are lower. The introduced unpaired training results in sharper -- from this point of view -- more realistic object reconstructions compared to the baseline paired setting. Finally, we show that the lower SSIM score of the unpaired training does not necessarily imply a worse image generation, but a correct object synthesis yet with a different focal representation.

Compact dark-field confocal microscopy based on an annular beam with orbital angular momentum

Jian Liu, Zijie Hua, and Liu Chenguang

DOI: 10.1364/OL.439711 Received 05 Aug 2021; Accepted 18 Oct 2021; Posted 21 Oct 2021  View: PDF

Abstract: This study proposes a dark-field confocal microscopy (DFCM) based on fiber-mode excitation-assisted orbital angular momentum (OAM) annular beam generation with a two-mode fiber (TFM) to avoid diffraction distortion. The application of optical fibers compresses the DFCM volume, providing new ideas for system miniaturization. System adjustment difficulty is reduced and feasibility is verified by imaging 2D and 3D samples. High imaging contrast is achieved by fully blocking the reflected light and annular illumination without diffraction within a propagation distance of 3 m. The application of our scheme can be further extended to detect subsurface defects in optical components and high-contrast biological imaging.

Poincare sphere representation of scalar two-beam interference under spatial unitary transformations

Atri Halder, Andreas Norrman, and Ari Tapio Friberg

DOI: 10.1364/OL.443741 Received 20 Sep 2021; Accepted 18 Oct 2021; Posted 18 Oct 2021  View: PDF

Abstract: We consider two partially correlated scalar light beams in a spatially unitary interference setup. We introduce a state vector in a Poincaré sphere like geometrical configuration which fully specifies such an optical system and its evolution under spatial unitary transformations. We also identify three particular unitary operations together with their geometrical representations that can be optically implemented to realize an arbitrary spatial unitary transformation. Our work forms an advantageous geometrical platform to characterize distinguishability, visibility, degree of coherence, and classical entanglement, as well as their spatial unitary evolutions, in scalar two-beam light interference.

Computational fiducial using G and C vector polynomials for alignment of deflectometry system

Hyukmo Kang, Henry Quach, Joel Berkson, Maham Aftab, George Smith, Heejoo Choi, and Daewook Kim

DOI: 10.1364/OL.442223 Received 03 Sep 2021; Accepted 17 Oct 2021; Posted 18 Oct 2021  View: PDF

Abstract: One of deflectometry’s cardinal strengths is its ability to measure highly dynamically-sloped optics without needing physical null references. Accurate surface measurements using deflectometry, however, require precise calibration processes. In this letter, we introduce an alignment technique using a computational fiducial to align a deflectometry system without additional hardware equipment (i.e., algorithmic innovation). Using the ray tracing program, we build relationships between the plane of the screen and detector and algorithmically generate a fiducial pattern for the deflectometry configuration. Since the fiducial pattern is based on the ideal system geometry, misalignment of the unit under test with its target position causes a discrepancy between the actual image on the camera detector and the ideal fiducial image. We leverage G and C vector polynomials to quantify misalignment and estimate the alignment status through a reverse optimization method. Simulation and experimental results demonstrate that the proposed algorithm can align the 195 mm × 80 mm of rectangular aperture freeform optic within 10 μm of peak-to-valley accuracy. The computational fiducial-based alignment algorithm is simple to apply and can be an essential procedure to conventional methods of deflectometry system alignment.

Metallic nanosphere-assisted coupling ultrafast surface plasmon polaritons background-free tip nanofocusing

Chao Meng, Weijian Li, zhonglin xie, Lu Zhang, Lei Xu, Feng Gao, Wending Zhang, Ting Mei, and Jianlin Zhao

DOI: 10.1364/OL.443079 Received 10 Sep 2021; Accepted 17 Oct 2021; Posted 18 Oct 2021  View: PDF

Abstract: Plasmonic tip nanofocusing has gained much attention owing to its wide application in the field of nanospectroscopy. Here, we present the gold nanosphere (AuNS) -assisted coupling ultrafast surface plasmon polaritons (SPP) background-free tip nanofocusing. The plasmonic tip was prepared by attaching an AuNS on the shaft of an Au conical tip fabricated by electrochemical etching. The AuNS was adopted as an antenna to couple the far-field excitation light to the propagating SPP along the shaft to the tip apex for achieving power compression. Importantly, we experimentally and theoretically demonstrate that such a plasmonic tip can realize background-free ultrafast SPP tip nanofocusing with radially-polarized feature in a wide spectral range based on the localized surface plasmon polaritons resonance effect supported by AuNS. Furthermore, the intensity of the tip nanofocusing light field has a strong polarization-dependent under linearly polarized light excitation, providing a powerful platform for spatiotemporal light control on the nanoscale. Our technique realizes remote excitation of background-free tip nanofocusing with structured light feature, it holds promising potential for tip-enhanced nanospectroscopies and nonlinear nanophotonics, etc.

Unscrambling OAM mode using digital phase-shifting in the Stokes fluctuations correlation

TUSHAR SARKAR, REAJMINA PARVIN, Maruthi Brundavanam, and Rakesh Singh

DOI: 10.1364/OL.440160 Received 10 Aug 2021; Accepted 16 Oct 2021; Posted 18 Oct 2021  View: PDF

Abstract: In this paper, we propose and experimentally demonstrate a new non-interferometric and highly stable technique to unscramble the incident orbital angular momentum (OAM) state and quantitatively measure the phase structure from the non-imaged random light. A new theoretical basis is developed and also verified by numerical simulation and experimental demonstration. We also quantitatively investigated the OAM modes of the incident light using orthogonal projection.

Periodic attraction and repulsion within the tight-bound π-phase soliton molecule

Du Yueqing, Qun Gao, Jingyi Li, Chao Zeng, Dong Mao, and Jianlin Zhao

DOI: 10.1364/OL.440478 Received 13 Aug 2021; Accepted 16 Oct 2021; Posted 19 Oct 2021  View: PDF

Abstract: In the over-pumped dissipative system, the single pulse is prone to split into multi-soliton modes, among which the soliton molecule (SM) comprised of two pulses attracts lots of interest recently. In this letter, the tight-bound SM with the π-phase-difference, a soliton-pair predicted to be unstable in fiber lasers, is found to have oscillating separation with excellent stability. During the periodic propagation, the destructive interference between solitons produces the repulsion while the dispersive waves give rise to the attractive force, leading to the dynamic oscillating behavior of the SM. The numerical simulation reproduces the experimental observation and offers panoramic insights into the nonlinear interactions between multiple components in dissipative systems.

Conjugate wavefront encoding: an efficient eyebox extension approach for holographic Maxwellian near-eye display

Zi Wang, Xu Zhang, Guoqiang Lv, Qibin Feng, Anting Wang, and Hai Ming

DOI: 10.1364/OL.444594 Received 28 Sep 2021; Accepted 16 Oct 2021; Posted 21 Oct 2021  View: PDF

Abstract: Conventional holographic display suffers from the conjugate light issue. In this paper, we propose to efficiently extend the eyebox of holographic Maxwellian near-eye display by encoding the conjugate wavefront as the multiplication of plane wave phase with the target image. It is interesting that after focused by the lens, the generated conjugate viewpoints also present erect virtual image with the same image quality as the signal viewpoints. Multiple plane waves encoding is used for eyebox extension, and because of the utilization of conjugate light, the effect of eyebox extension is doubled. That is, the space bandwidth of the amplitude-type hologram is fully used. A speckless holographic image is produced in the mid-air with high quality within a large depth range. The proposed display is compact and promising for the augmented reality near-eye display. Furthermore, it may inspire better solutions for the conjugate light issue of amplitude-type holography.

Raman-scattering-assistant large energy dissipative soliton and multi-color coherent noise-like pulse complex in an Yb-doped fiber laser

shuo chang, Yameng Zheng, Wang Zhaokun, and Changyu Shen

DOI: 10.1364/OL.443319 Received 17 Sep 2021; Accepted 16 Oct 2021; Posted 21 Oct 2021  View: PDF

Abstract: In this letter, we have demonstrated the generation of the dissipative solitons (DSs) or the multi-wavelength noise-like pulses (NLPs) directly from a common linear Yb-doped fiber laser in the presence of the stimulated Raman scattering (SRS). For the DSs, the pulse energy of the solitons with the pulse width of 74.2 ps reaches to be 21.2 nJ. For the NLPs, the generation of the main NLP (1032 nm) together with the first-order Raman NLP (1080 nm) is realized. The narrow peak of the double-scale autocorrelation trace is characterized by quasi-periodic beat pulses with a pulse beating of 40.6 fs and a pulse separation of 79 fs, indicating that the generated solitons at dual wavelengths are mutually coherent. Furthermore, a three-color stable NLP complex with a broader spectrum is also obtained. The results contribute to an in-depth understanding of nonlinear dynamics and ultrafast physics.

Mid-infrared soliton self-frequency shift in chalcogenide glass

Imtiaz Alamgir, Md Hosne Shamim, Wagner Correr, Younes Messaddeq, and Martin Rochette

DOI: 10.1364/OL.443848 Received 20 Sep 2021; Accepted 16 Oct 2021; Posted 18 Oct 2021  View: PDF

Abstract: Emerging applications in the mid-infrared (MIR) stimulate the growth and development of novel optical light sources. Soliton self-frequency shift (SSFS) in soft glass fiber currently show great potential as an efficient approach towards the generation of broadly tunable femtosecond pulses in the MIR. In this work, we demonstrate highly efficient tunable soliton source based on SSFS in chalcogenide glass. We show a simple and fully fiberized system to generate these continuously tunable Raman solitons over a broad spectral range of 2.047-2.667 µm, and consumes no more than 87 pJ per pulse. The generated pulses are as short as 62 fs with a maximum power conversion efficiency of 43%. This result is realized thanks to an 8 cm long As2S3 microstructure optical fiber tapered into a microwire. Thanks to their broad transparency, their high nonlinearity and their adjustable chromatic dispersion, chalcogenide microwires are most promising components for the development of compact, low power consumption and highly efficient MIR optical sources.

Incoherent two-color pulse compounds

Oliver Melchert and Ayhan Demircan

DOI: 10.1364/OL.440567 Received 17 Aug 2021; Accepted 16 Oct 2021; Posted 18 Oct 2021  View: PDF

Abstract: We study the dynamical evolution of two-frequency pulse compounds, i.e.intriguing bound-states of light, kept together due to their incoherentinteraction.A special class of solutions describing such compounds is found to bedescribable in terms of a simplified model.They entail generalized dispersion Kerr solitons and yield their correspondingmetasolitons.We use these solutions to study when the interaction of their constituentpulses is independent of their phase. These results are relevant to understand the complex collision dynamics ofquasi group-velocity matched solitons across a vast frequency gap.

Remote Michelson interferometric phase sensor based on dual-core fiber transmission and linear phase demodulation

Weilin Xie, Yuanshuo Bai, Songhan Liu, Haijun Zhou, Yinxia Meng, Ling Zhang, Wei Wei, and Yi Dong

DOI: 10.1364/OL.436047 Received 08 Jul 2021; Accepted 16 Oct 2021; Posted 18 Oct 2021  View: PDF

Abstract: We present a remote Michelson interferometric phase sensor based on dual-core fiber transmission and linear phase demodulation. The former allows for synchronous transmission of both sensing signal and reference lights, enabling efficient suppression for the environmental disturbances along the transmission link and for the incoherent phase noise between the two lights. The latter is conducted by two optical phase-locked loops, one of which that consists of a fiber stretcher is used to eliminate the residual phase noises, thus stabilizing the operation point while the other that relies on a phase modulator is used to track the remote phase changes, thus achieving a highly linearized phase demodulation. A remote phase sensing over a 20 km fiber link with less than 3% nonlinear phase error over 3π range has been readily realized, corresponding to more than 10 times extension in linear phase demodulation range. The proposed system shows great potential in the field of remote phase sensing for a variety of physical quantities.

Mitigation of the supermode noise in a harmonically mode-locked ring fiber laser using optical injection

Valeria Ribenek, Dmitry Stoliarov, Dmitry Korobko, and Andrei Fotiadi

DOI: 10.1364/OL.441630 Received 27 Aug 2021; Accepted 16 Oct 2021; Posted 21 Oct 2021  View: PDF

Abstract: We report on a new technique enabling mitigation of the supermode noise (and timing jitter) in a soliton harmonically mode-locked (HML) fiber laser built on the nonlinear polarization evolution (NPE). An optical injection of an external continuous wave (CW) into the HML laser cavity results in an increase of the supermode noise suppression level (SSL) by two-three orders of magnitude for harmonics between 25-th and 135-th. The operation mechanism involves phase-locking between the injected light and soliton pulses and exhibits strong resonant dependence on the CW laser wavelength. Our findings offer important insights into the HML laser dynamics associated with an interaction between solitons and CW background in the laser cavity.

Cylindrical vector beam sorter with spin-dependent spiral transformation

Ziang Lin, Zhiqiang Xie, Yanliang He, Wang Xinrou, Haisheng Wu, Suilin Wang, Zhiwei Guan, Junmin Liu, Huapeng Ye, Ying Li, Dianyuan Fan, and Shuqing Chen

DOI: 10.1364/OL.440828 Received 23 Aug 2021; Accepted 16 Oct 2021; Posted 19 Oct 2021  View: PDF

Abstract: Here, we propose and experimentally demonstrate a cylindrical vector beam (CVB) sorter based on a spin-dependent spiral transformation. By exploiting the spin-orbital interaction of the geometric phase, a pair of conjugated spiral transformations are applied to modulate the two orthogonal circularly polarized components of the CVB, which are converted into the same linear phase gradient from opposite azimuthal phase gradients. Since the orthogonal spin components of CVBs with different polarization orders carry different phase gradients, under the convergence of a convex lens, the coaxially transmitted CVBs can be sorted with spatially separated positions, and the increased phase gradient provided by the spiral transformation yields the high resolution. We show that five CVB modes from -2 to +2 are successfully sorted with a separation efficiency of 3.65. As a proof-of-concept, we demonstrate a two-channel CVB multiplexing communication with a bit-error-rate approaching 10-6. In addition to providing an avenue for CVB demultiplexing, our results show potential applications in mode filtering and mode routing in all-optical interconnections.

Spatial resolution enhancement of coherent Doppler wind lidar using differential correlation pair technique

yunpeng zhang, Yunbin Wu, and Haiyun Xia

DOI: 10.1364/OL.442121 Received 03 Sep 2021; Accepted 15 Oct 2021; Posted 15 Oct 2021  View: PDF

Abstract: A high-spatial-resolution coherent Doppler wind lidar (CDWL) incorporating differential correlation pair (DCP) technique is proposed and demonstrated. By employing pulse pair with appropriate window functions, the spatial resolution can be enhanced, as the common parts of the correlation pair can be eliminated in the differential data processing. The performance of the new method is validated in the comparison experiment with CDWLs adopting conventional schemes. Under a given peak power, the DCP technique provides higher wind velocity accuracy compared with a conventional pulsed CDWL, where the laser spectral broadening caused by short pulses can be avoided and the carrier-to-noise ratio is improved. At laser peak power of 250 W, with spatial and temporal resolution of 3.3 m and 1 s, continuous radial wind profiling over 700 m is realized with maximum error of 0.1 m/s.

Geometric Optimization Method for Polarization State Generator of Mueller Matrix Microscope

Qianhao Zhao, Tongyu Huang, Zheng Hu, Tongjun Bu, ShuGang Liu, Ran Liao, and Hui Ma

DOI: 10.1364/OL.441492 Received 25 Aug 2021; Accepted 15 Oct 2021; Posted 18 Oct 2021  View: PDF

Abstract: We propose a geometric optimization method combined with the Coulombic energy indicator that can uniformly distribute N polarization states on the Poincaré sphere. Based on this method, we investigate the optimal frames of rotating polarizer and rotating quarter waveplate (RPRQ) based polarization state generator (PSG) at different number of modulations. We use the PSG on dual DoFP polarimeters based Mueller matrix microscope (DoFPs-MMM) to measure standard samples and pathological sections for testing the performance of optimized RPRQ. The experimental results show this method can effectively restrain noise and improve measurement precision.

Multiple Solution Solving in Plasmon Sensing by Deep Learning: Determination of Layer Refractive Index and Thickness in One Experiment

Qian Du, Quan Zhang, and Guohua Liu

DOI: 10.1364/OL.444442 Received 28 Sep 2021; Accepted 13 Oct 2021; Posted 15 Oct 2021  View: PDF

Abstract: Plasmons have received intensive attention owing to their significant spectrum shift in environmental sensing. Experimentally, the same spectral shifts may be caused by different combinations of structural parameters of a plasmonic nanoparticle. This multi-parameter problem can hardly be solved only via single feature analysis, but could be solved using the full scattering spectrum containing all features of the parameters. In this study, a deep learning method for solving multi-parameter problem is proposed based on the layer refractive index (n) and layer thickness (d) sensing of different nanorods and nanospheres. The full scattering spectrum can be theoretically simulated, precisely predicted using a well-trained deep learning method, and experimentally obtained using a homemade dark-field microscope. Error analysis of the simulation and experimental results indicates that this method is a potential way to determine n and d and further solve multi-parameter in plasmon sensing.

Flexible terahertz opto-electronic frequency comb light source tunable over 3.5 THz

Dominik Theiner, Benedikt Limbacher, Michael Jaidl, Karl Unterrainer, and Juraj Darmo

DOI: 10.1364/OL.434974 Received 23 Jun 2021; Accepted 13 Oct 2021; Posted 15 Oct 2021  View: PDF

Abstract: Using a combination of optical fiber communications technology and opto-electronic frequency conversion, we demonstrate a terahertz frequency comb that is flexible in terms of its frequency range and the number and spacing of comb lines. The quality of comb lines is proven by observing the pressure-dependent collisional broadening of an ammonia molecular absorption line at 572.498 GHz (19.09648 cm−1).

Polarization-independent parametric time magnifier based on four-wave mixing

Sheng Wang, Xin Dong, Bowen Li, and Kenneth Kin-Yip Wong

DOI: 10.1364/OL.438351 Received 21 Jul 2021; Accepted 13 Oct 2021; Posted 18 Oct 2021  View: PDF

Abstract: Time magnifier based on space-time duality has demonstrated comprehensive applications owing to its promising temporal resolution. However, conventional parametric time magnifiers are inherently polarization-sensitive, whose output intensity depends not only on the intensity but also the polarization of signal under test (SUT). Therefore, they are mainly applied to SUT with fixed polarization. On the other hand, many complex optical signals exhibit the simultaneous intensity and polarization dynamics. In this letter, a polarization-independent (PI) time magnifier at a 597-fs temporal resolution is first demonstrated, which provides accurate intensity information even for polarization related signals. The PI time magnifier de-convolves intensity and polarization information. It, therefore, paves the way for in-depth analysis of various complex ultrafast phenomena involving simultaneous intensity and polarization dynamics such as rogue waves and vector solitons.

Amplification of femtosecond pulses based on χ³ nonlinear susceptibility in MgO

T Hammond, Chathurangani Jayalath Arachchige, and Jake Stephen

DOI: 10.1364/OL.437749 Received 16 Jul 2021; Accepted 13 Oct 2021; Posted 14 Oct 2021  View: PDF

Abstract: We experimentally demonstrate large, widely tuneable gain using Kerr instability amplification in MgO. By pumping the crystal near optical damage at 1.4 × 10¹³ W/cm² by a femtosecond Ti:Sapphire laser, we amplify visible and near infrared pulses by factors > 5000, or a gain g~17 /mm. We temporally characterize the pulses to show that they are 42 fs in duration, much shorter than the pump pulse. In the non-collinear setup, the angle between the pump and seed selects the amplified wavelength, where we find certain angles amplify both the visible and near infrared simultaneously. We find that near the maximum pumping intensities, higher order nonlinearities may play a role in the amplification process.

Er:KY3F10 laser at 2.80 µm

liza Basyrova, Pavel Loiko, Jean-Louis DOUALAN, Abdelmjid Benayad, Alain BRAUD, Christophe Labbe, and Patrice Camy

DOI: 10.1364/OL.439457 Received 27 Aug 2021; Accepted 13 Oct 2021; Posted 14 Oct 2021  View: PDF

Abstract: We report on the mid-infrared laser operation of a cubic 15 at.% Er3+:KY3F10 crystal. In the quasi-continuous-wave regime, the peak power reaches 255 mW at 2.80 µm (the 4I11/2 → 4I13/2 transition) with a slope efficiency of 10.9% and a laser threshold of 58 mW. Two pumping schemes (to the 4I11/2 and 4I9/2 states) are compared. The emission properties of Er3+ ions in KY3F10 are studied, indicating high stimulated-emission cross-section of 0.57×10-20 cm2 at 2.80 µm, a large gain bandwidth of 40 nm and a long 4I11/2 state lifetime of 4.64 ms.

Complete two-dimensional photonic bandgap in refractive-index ratio 2.1 photonic crystals due to high-order bands

Jin Hou, Yusen Zhou, David Citrin, Xuejun Qiu, Chunyong Yang, and Shaoping Chen

DOI: 10.1364/OL.440763 Received 20 Aug 2021; Accepted 13 Oct 2021; Posted 14 Oct 2021  View: PDF

Abstract: It is found in a suitably designed photonic crystal that certain high-order photonic bands are less affected by the refractive-index ratio than low-order photonic bands, enabling the realization of a robust and complete two-dimensional photonic bandgap in a moderate-refractive-index-ratio photonic crystal. A detailed theoretical investigation of low- and high-order bandgaps in a series of photonic crystals with different configurations is performed, which shows that high-order bands may favor substantial complete photonic bandgaps for systems with moderate refractive-index ratio. Furthermore, the importance of the geometry and structural parameters on achieving a high-order complete photonic bandgap is found. Specifically, a hexagonal-lattice photonic crystal of annular-hole-peripheral-connecting-rods is proposed, which can support a complete photonic bandgap with a refractive-index ratio as low as nhigh: nlow = 2.1, the lowest RIR to our knowledge reported to obtain two-dimensional complete photonic bandgap in a photonic crystal.

Tunable single-mode laser on thin film lithium niobate

Xiangmin Liu, Yan Xiongshuo, Yi'an Liu, Hao Li, Yuping Chen, and Xianfeng Chen

DOI: 10.1364/OL.441167 Received 25 Aug 2021; Accepted 13 Oct 2021; Posted 14 Oct 2021  View: PDF

Abstract: Erbium-doped lithium niobate on insulator (LNOI) laser plays an important role in the complete photonic integrated circuits (PICs). Here, we demonstrate an integrated tunable whisper galley single mode laser (WGSML) by making use of a pair of coupled microdisk and microring on LNOI. A 974 nm single-mode pump light can have an excellent resonance in the designed microdisk, which is beneficial to the whisper gallery mode (WGM) laser generation. The WGSML at 1560.40 nm with a maximum 31.4 dB side mode suppression ratio (SMSR) has been achieved. By regulating the temperature, WGSMLs output power increased and the central wavelength can be changed from 1560.30 nm to 1560.40 nm. What's more, 1560.60 nm and 1565.00 nm WGSMLs have been achieved by changing the coupling gap width between microdisk and microring. We can also use the electro-optic effect of LNOI to obtain more accurate adjustable WGSMLs in further research.

Trans-Spectral Vector Beam Nonlinear Conversion via Parametric Four Wave Mixing in Alkali Vapor

Pan Churong, Yang chengdong, Huajie Hu, Jinwen Wang, Zhang Yingxin, Qin yan, wei dong, Chen Haixia, Hong Gao, and fuli li

DOI: 10.1364/OL.441573 Received 31 Aug 2021; Accepted 13 Oct 2021; Posted 14 Oct 2021  View: PDF

Abstract: Coherent frequency conversion of vector beams (VBs) without distorting their intensity profile and spatial polarization distribution is important for novel applications in quantum and classical regimes. Here, we experimentally and theoretically investigate VB transfer from near-infrared to blue light using a Sagnac interferometer, combining the parametric four-wave mixing process in atomic vapor. Under optimal phase-matching conditions, the vector probe beam is efficiently transferred with a large frequency band; the generated blue beam is highly similarity to the incident probe field. These results may provide a feasible solution for communication interfaces in classical and quantum science fields based on atomic ensembles.

Diode-side-pumped watt-level high-energy Q-switched mid-IR Er:YLF laser

Andrey Pushkin, Ivan Slovinsky, Alexey Shavelev, Ajnur Shakirov, and Fedor Potemkin

DOI: 10.1364/OL.442753 Received 07 Sep 2021; Accepted 13 Oct 2021; Posted 14 Oct 2021  View: PDF

Abstract: We report on a powerful mid-IR diode-side-pumped tunable Er:YLF laser electro-optically Q-switched with the help of a KTP crystal. At a 20-Hz repetition rate, the laser pulses with output energy of 82 mJ and 13-ns duration at the wavelength of 2.67 μm are obtained. At higher rep. rates (up to 50 Hz) one can extract up to 20 mJ from the laser cavity. The developed mid-IR laser source demonstrates high peak (up to 6.3 MW) and average (up to 1.7 W) power. Realized wavelength tuning provides access for MW-peak power level nanosecond laser pulses over the 2667-2851 nm wavelength region which are highly demanded for mid-IR laser systems development and light-matter interaction study in the view of extreme state creation in liquids and solids paving the way to novel microprocessing techniques.

Monolithic transceiver for lens-assisted beam-steering Lidar

Chao Li, Kan Wu, Xianyi Cao, Guangjin Zhang, Xinwan Li, and Jianping Chen

DOI: 10.1364/OL.438740 Received 26 Jul 2021; Accepted 12 Oct 2021; Posted 19 Oct 2021  View: PDF

Abstract: We demonstrate a monolithic transceiver based on CMOS-compatible silicon photonic platform for lens-assisted beam-steering (LABS) Lidar application. By implementing on-chip two-dimensional transceiver array and off-chip lens, beam emitting, steering and receiving are realized simultaneously on a single chip. The transceiver is designed with a structure of a U-shaped vertical Ge photodetector surrounding a grating for high-efficient light transmission and reception. The on-chip photodetector has a bandwidth of 87 MHz, a responsivity of 0.3 A/W, and a detection sensitivity of -20 dBm. For proof-of-concept demonstration, a time-of-flight Lidar system is achieved for target ranging with a detection distance of 5.2 m, a scanning angle of 2.86 °, and a scanning speed of 5.3 μs. This work demonstrates a feasible solution to integrated Lidar with beam emitting and receiving on one single chip based on LABS.

An ultra-narrow passband tunable filter based on high-Q silicon racetrack resonator

Jin Xu, Yujia Zhang, Xuhan Guo, Qingzhong Huang, Xinliang Zhang, and Yikai Su

DOI: 10.1364/OL.443723 Received 20 Sep 2021; Accepted 11 Oct 2021; Posted 18 Oct 2021  View: PDF

Abstract: An ultra-narrow narrow passband tunable optical filter employing a high-Q silicon racetrack resonator is proposed and experimentally demonstrated on SOI platform. The high-Q silicon racetrack resonator is realized by utilizing the multi-mode waveguide racetrack, and the Q factor is measured as high as 8.1×10⁵. The structure of the device is based on a thermally tunable MZI coupled racetrack. The tunability of bandwidth is realized by tuning the coupling coefficient between the racetrack resonator and the input or output ports. Finally, the bandwidth of the filter can be tuned from 1.92 pm to 11.00 pm (240 MHz to 1.375 GHz), and the free spectral range (FSR) of it is about 0.28 nm (35 GHz), with the footprint of 0.21 mm².

Deep-learning-based adaptive camera calibration for various defocusing degrees

jing zhang, Bin Luo, Qican Zhang, Yajun Wang, Xin Su, Jun liu, Lu li, Wei Wang, and Zhuolong Xiang

DOI: 10.1364/OL.443337 Received 15 Sep 2021; Accepted 11 Oct 2021; Posted 19 Oct 2021  View: PDF

Abstract: Camera calibration tends to suffer from the low-quality target image acquisition, which would yield inaccurate or inadequate extracted features, resulting in imprecise or even failed parameter estimation. To address this problem, this Letter proposes a novel deep-learning-based adaptive calibration method robust to defocus and noise, which could significantly enhance the image quality and effectively improve the calibration result. Our work provides a convenient multi-quality target dataset generation strategy and introduces a multi-scale deep learning framework that successfully recovers a sharp target image from a deteriorated one. Free from capturing additional patterns or using special calibration targets, the proposed method allows for a more reliable calibration based on the poor-quality acquired images. In this study, an initial training dataset can be easily established containing only 68 images captured by a smartphone. Based on the augmented dataset, the superior performance and flexible transferable ability of the proposed method have been validated on another camera in the calibration experiments.

Phase-only color rainbow holographic near-eye display

Xin Yang, Shuming Jiao, Qiang Song, Guobin ma, and Weiwei Cai

DOI: 10.1364/OL.431769 Received 18 May 2021; Accepted 11 Oct 2021; Posted 12 Oct 2021  View: PDF

Abstract: Color rainbow holography can realize color holographic 3D display without speckle noise under white light illumination. However, traditional color rainbow holograms used for high-resolution static color 3D display or near-eye color display are amplitude holograms, resulting in low diffraction efficiency. In this letter, a phase-only color rainbow holographic near-eye display is demonstrated. The calculation of phase-only color rainbow hologram is realized by combining band limited diffraction and bi-directional error diffusion algorithm with high frequency blazed gratings coded to control the longitudinal dispersion. The phase-only color rainbow holographic near-eye display of both 2D color image and 3D scene are implemented by optical experiments, which has the potential to be used in head mounted 3D Augmented Reality displays without vergence accommodation conflict.

Generation of long homogeneous plasma channels with high power long-wave IR pulsed Bessel beams

Paris Panagiotopoulos, Miroslav Kolesik, Sergei Tochitsky, and Jerome Moloney

DOI: 10.1364/OL.438633 Received 22 Jul 2021; Accepted 11 Oct 2021; Posted 12 Oct 2021  View: PDF

Abstract: Long wave multi-Joule ultrashort laser pulses are predicted to confine highly uniform electromagenitic energy and field intensities while sustaining high density uniform plasmas within nonlinear Bessel zones under extreme driving conditions in contrast to near-IR sources. This open up novel applications in laser wakefield generation, radiofrequency/microwave guiding and lightning control.

Neuromorphology in-sensor computing architecture based on optical Fourier transform

Hao Hao, Kang Yan, Zhongjie Xu, and Tian Jiang

DOI: 10.1364/OL.440057 Received 09 Aug 2021; Accepted 11 Oct 2021; Posted 12 Oct 2021  View: PDF

Abstract: We propose an object recognition architecture relying on neural network algorithm in optical sensors. Precisely, by applying the high-speed and low-power Fourier transform operation in optical domain, we can transfer the high-cost part of the traditional convolutional neural network (CNN) algorithm to the sensor side to achieve faster computing speed. An optical neuron unit (ONU) consisting of transition metal sulfide (TMD) material is fabricated, for a vivid validation of this architecture. Using the embedded gate pair structure inside our ONU, TMD materials can be electrically doped at different levels, forming an in-plane PN junction, which allows for effective manipulation of light response to imitate biological nerve synapses. The results demonstrated that our ONU could reach the ability of optic neurons, providing experimental support for the future in-sensor computing architecture.

Anisotropic nanostructure generated by a spatial-temporal manipulated picosecond pulse for multi-dimensional optical data storage

Zhi Yan, Peiyao Li, Jichao Gao, Yuan Wang, Lei Wang, Martynas Beresna, and Jingyu Zhang

DOI: 10.1364/OL.443370 Received 23 Sep 2021; Accepted 11 Oct 2021; Posted 12 Oct 2021  View: PDF

Abstract: Anisotropic nanostructures can be generated in fused silica glass by manipulating the spatiotemporal properties of a picosecond pulse. This phenomenon is attributed to the laser-induced interband self-trapped excitons. The anisotropic structures exhibit birefringence, and thus can be employed for multi-dimensional optical data storage applications. The generation of data voxels by such short laser irradiation enables on-the-fly high-speed data recording.

Ultra-compact in-core-parallel-written FBG and Mach-Zehnder interferometer for simultaneous measurement of strain and temperature

Ke Tian, Mingyuan Zhang, Zhiyuan Zhao, Ruo Wang, Dejun Liu, Xin Wang, Elfed Lewis, Gerald Farrell, and Pengfei Wang

DOI: 10.1364/OL.440118 Received 10 Aug 2021; Accepted 10 Oct 2021; Posted 11 Oct 2021  View: PDF

Abstract: An ultra-compact in-core-parallel-written fiber Bragg grating (FBG) and Mach-Zehnder interferometer (MZI) for simultaneous measurement of strain and temperature is described. The FBG and MZI are written spatially parallel in the same section of fiber core using a femtosecond laser, forming an ultra-compact device, which is different from the previously developed axial cascade of different structures. Due to the weak coupling between the FBG and the MZI, their individual extinction ratios are traded off by optimizing their writing position and separation, and extinction ratios of 5.9 dB for the FBG and 10 dB for the MZI are achieved. Experimental results show that the FBG and MZI have different sensitivities for strain and temperature, allowing this device to measure strain and temperature simultaneously. In addition, since both the FBG and MZI are written in the fiber core, this ultra-compact device is proven to be impervious to ambient humidity, making it a promising candidate for accurate industrial strain and temperature measurements.

Realizing normal group-velocity dispersion in free space via angular dispersion

Ayman Abouraddy and Layton Hall

DOI: 10.1364/OL.438978 Received 29 Jul 2021; Accepted 10 Oct 2021; Posted 11 Oct 2021  View: PDF

Abstract: It has long been thought that normal group-velocity dispersion (GVD) cannot be produced in free space via angular dispersion. Indeed, all experiments carried out to date using conventional diffractive or dispersive components such as gratings or prisms have produced only anomalous GVD. We show here theoretically and experimentally the conditions that must be fulfilled by the angular dispersion introduced into a pulsed field to yield normal GVD. We utilize a pulsed-beam shaper that can introduce arbitrary angular-dispersion profiles to produce both normal and anomalous GVD, which are realized on the same footing for the first time.

Wake dynamics of air filaments generated by high energy picosecond laser pulses at 1 kHz repetition rate

Adam Higginson, Yong Wang, Han Chi, Andrew Goffin, Ilia Larkin, Howard Milchberg, and Jorge Rocca

DOI: 10.1364/OL.439232 Received 03 Aug 2021; Accepted 10 Oct 2021; Posted 11 Oct 2021  View: PDF

Abstract: We investigated the filamentation in air of 7 picosecond laser pulses of up to 200 mJ energy from a 1.03 μm-wavelength Yb:YAG laser at repetition rates up to f = 1 kHz. Interferograms of the wake generated show that while pulses in a train of repetition rate f = 0.1 kHz encounter a nearly unperturbed environment, at f = 1 kHz a channel with an axial air density depression of ~20% is generated and maintained at all times by the cumulative effect of preceding laser pulses. Measurements at f = 1 kHz show that the energy deposited decreases proportional to the air channel density depletion, becoming more pronounced as the repetition rate and pulse energy increase. Numerical simulations indicate that contrary to filaments generated by pulses of shorter duration, electron avalanche is the dominant energy loss mechanism during filamentation. The results are of interest for the atmospheric propagation of joule-level picosecond pulses from Yb:YAG lasers, of which average powers now surpass 1 kW, and for channeling other directed energy beams.

Flat-top THz directional diagram of DC-biased filament

Irina Nikolaeva, Daniil Shipilo, Dmitrii Pushkarev, Georgy Rizaev, Daria Mokrousova, Andrew Koribut, Yakov Grudtsin, Nikolay Panov, Leonid Seleznev, Weiwei Liu, Andrey Ionin, and Olga Kosareva

DOI: 10.1364/OL.439901 Received 05 Aug 2021; Accepted 10 Oct 2021; Posted 11 Oct 2021  View: PDF

Abstract: In the experiment the laser pulse (744 nm, 0.5 mJ, 90 fs) focused into the air gap between the plane electrodes biased by 10-kV/cm field (DC-biased filament) produced terahertz (THz) radiation. At the selected frequencies ν = 0.3, 0.5, 1 THz a wide flat-top angular distribution was measured by a bolometer rotating in the plane of the electrodes. The simulations based on Unidirectional Pulse Propagation Equation with fine 0.01-THz resolution and 3-PHz frequency domain showed the transition of THz directional diagram from the flat-top at ν ≤ 1 THz to the conical one at ν > 8 THz due to the destructive interference of THz waves from the ionization front propagating with the superluminal velocity. Refraction on the plasma is not the major factor in conical ring formation.

Burst-mode 100 kHz N2 ps-CARS flame thermometry with concurrent nonresonant background referencing

Daniel Lauriola, Paul Hsu, Naibo Jiang, Mikhail Slipchenko, Terrence Meyer, and Sukesh Roy

DOI: 10.1364/OL.439996 Received 30 Aug 2021; Accepted 10 Oct 2021; Posted 11 Oct 2021  View: PDF

Abstract: A burst-mode nitrogen (N2) picosecond vibrational coherent anti-Stokes Raman scattering (ps-VCARS) system is presented for accurate flame thermometry at 100 kHz repetition rate. A frequency-tripled ps burst-mode laser is used to pump a custom optical parametric generator/amplifier (OPG/OPA) to produce 607 nm broadband Stokes pulses with 120 cm-1 bandwidth, along with a narrowband 532 nm pump/probe beam. A novel simultaneous shot-to-shot nonresonant background (NRB) measurement is implemented to account for Stokes spectral profile and beam overlap fluctuations. The 100-kHz ps-VCARS data are benchmarked in a near-adiabatic CH4/air Hencken calibration flame with an accuracy of 1.5% and precision of 4.7% up to peak flame temperatures. The use of N2 VCARS and simultaneous NRB measurements enables accurate high-speed thermometry for a wide range of fuels and combustion applications.

Phase correlation arc and universal decay of entangled orbital-angular-momentum qubit states in atmospheric turbulence

Donghui Yang, Zhengda Hu, ShuaiLing Wang, and Yun Zhu

DOI: 10.1364/OL.442577 Received 08 Sep 2021; Accepted 10 Oct 2021; Posted 14 Oct 2021  View: PDF

Abstract: In this study, we introduce the phase correlation arc of an orbital angular momentum (OAM) beam to investigate the evolution of the OAM entanglement. We reveal that the entanglement decay of all OAM states of Laguerre–Gaussian modes in atmospheric turbulence is universal via both numerical predictions and experimental data. A similar evolution law was also theoretically confirmed to exist in Bessel–Gaussian modes. Finally, by using phase correlation arc, the precise formula of the decay distance dependence on the OAM number is derived, and it exhibits excellent agreement with previous experimental conclusions.

End-to-end learned single lens design using fast differentiable ray tracing

Zongling Li, Hou Yu, Zhipeng Wang, Fanjiao Tan, Jin Liu, and Wei Zhang

DOI: 10.1364/OL.442870 Received 09 Sep 2021; Accepted 09 Oct 2021; Posted 11 Oct 2021  View: PDF

Abstract: We propose an end-to-end single lens design method, which can simultaneously optimize parameters of optical system and image processing algorithm in system-level. In the forward path, the sensor image is simulated by fast differentiable ray tracing, and then recovered by reconstruction network. In the backward path, the parameters of optical lens and recovery network are integratedly optimized with the regulation of recovered image quality. Simulation results show that our fast differentiable ray tracing model can reach high fidelity, and our end-to-end design has better imaging performance compared with conventional separated design of optical system and restoration algorithm. The designed single-lens imaging system possesses a high-quality large field-of-view with simple system architecture.

Active Hyperspectral Imager Using a Tunable Supercontinuum Light Source Based on a MEMS Fabry-Pérot Interferometer

Teemu Kääriäinen and Timo Dönsberg

DOI: 10.1364/OL.439551 Received 04 Aug 2021; Accepted 09 Oct 2021; Posted 19 Oct 2021  View: PDF

Abstract: We have developed an active hyperspectral imager, that is based on a tunable near-infrared supercontinuum light source. Non-dispersive wavelength selection of the supercontinuum laser source is achieved with a microelectromechanical Fabry-Pérot interferometer. The tunable light source enables the use of any monochromatic imaging sensor with a suitable spectral sensitivity for hyperspectral imaging. The imager is characterized and demonstrated in the laboratory for remote detection of ice.

3D Printed Metasurface for Generating Bessel Beam with arbitrary focusing direction

Meijun Qu, Wenyu Li, Ting Zeng, Jianxun Su, and Weili Song

DOI: 10.1364/OL.440977 Received 25 Aug 2021; Accepted 08 Oct 2021; Posted 11 Oct 2021  View: PDF

Abstract: In this letter, a metasurface combined with the emerging 3D printed technology is proposed. The proposed metasurface regards the simple cube as the unit cell, and the height of the cube is the only variable. A nearly linear transmission phase range covering 360° operating at 20 GHz is obtained when the height is regulated in [2.26mm, 11.20mm]. Therefore, the proposed unit cell can be adopted to any metasurface with various functions. Take the generation of non-diffractive Bessel beam as example, two metasurfaces composed of 30×30 units with different focusing directions are designed based on non-diffractive theory and generalized law of refraction. Two prototypes are 3D printed and measured by the near-field scanning system. The measured results have validated our design with satisfactory focusing and beam deflection performance. Additionally, 3D printed metasurface has lower cost and shorter processing cycle, and avoids metal loss. Therefore, 3D printed metasurface is an excellent candidate that can be applied in millimeter wave or even higher frequency bands.

Few-mode polymer optical waveguide amplifier for mode-division multiplexed transmission

cheng yu, Guijun Hu, Fei Wang, JIAYING LI, Dan Zhao, and Meiling Zhang

DOI: 10.1364/OL.441348 Received 24 Aug 2021; Accepted 08 Oct 2021; Posted 12 Oct 2021  View: PDF

Abstract: We propose a few-mode erbium-ytterbium co-doped polymer optical waveguide amplifier employing mode-selective photonic lanterns capable of multiplex and demultiplex. The scheme of reconfigurable pump configuration is adopted to balance the modal gain per mode. The square few-mode waveguide amplifier supporting LP01, LP11a and LP11b modes is designed and fabricated. The crosstalk effect and modal profiles are characterized. An average gain 10.4 dB per mode is obtained in a 1.5 cm waveguide at 1555 nm through pumping of LP01 mode with 320 mW and LP21b mode with 120 mW at 976 nm. The ultra-low differential modal gain is achieved to 0.4 dB. In addition, LP01, LP11a and LP11b modes amplified simultaneously are also demonstrated across the whole C-band with low noise figures. It is the first time to report experimental realization of a few-mode optical waveguide amplifier.

Broadband complementary vibrational spectroscopy with cascaded intra-pulse difference frequency generation

Takuro Ideguchi, Kazuki Hashimoto, Venkata Ramaiah Badarla, and Takayuki Imamura

DOI: 10.1364/OL.444003 Received 23 Sep 2021; Accepted 08 Oct 2021; Posted 14 Oct 2021  View: PDF

Abstract: One of the essential goals of molecular spectroscopy is to measure all fundamental molecular vibrations simultaneously. To this end, one needs to measure broadband infrared (IR) absorption and Raman scattering spectra, which provide complementary vibrational information. A recently demonstrated technique called complementary vibrational spectroscopy (CVS) enables simultaneous measurements of IR and Raman spectra with a single device based on a single laser source. However, the spectral coverage was limited to ~1000 cm-1, which partially covers the spectral regions of the fundamental vibrations. In this work, we demonstrate a simple method to expand the spectral bandwidth of the CVS with a cascaded intra-pulse difference-frequency generation (IDFG). Using the system, we measure broadband CVS spectra of organic liquids spanning over 2000 cm-1, more than double the previous study.

Stretchable optical diffraction grating from poly(acrylic acid)-polyethylene oxide stereocomplex

Jinghan He, Andre Kovach, Yunxiang Wang, Wei Wu, Andrea Armani, and William wang

DOI: 10.1364/OL.432699 Received 31 May 2021; Accepted 07 Oct 2021; Posted 15 Oct 2021  View: PDF

Abstract: Optical gratings are a key component in many spectroscopy, communications, and imaging systems. While initially static elements, advances in optical materials have enabled dynamically tunable gratings to be designed. One common tuning strategy is relying on mechanical deformation of the grating pitch to modify the diffraction pattern. To date, most mechanically adaptive optical gratings consist of a hybrid system where opaque elements, such as metal or semiconductor lines, are patterned on an elastomeric substrate. In the present work, we demonstrate an all-polymer tunable grating fabricated using a modified replica molding process. After forming thin films using a compression method from a poly(acrylic acid) (PAA)/polyethylene oxide (PEO) polymer stereocomplex, the optical transmittance and mechanical response are characterized. The films exhibit excellent optical transmittance at or above 80% from 500 nm to 1400 nm and stretchability over 800% strain. Subsequently, diffraction gratings are fabricated using imprinting of a nanofabricated mold and characterized at both 633 nm, and 1064 nm. Due to their high transmission and large elasticity, the stereocomplex diffraction gratings are able to reversibly modify the diffraction mode spacing at both wavelengths, and the observed tunability agrees well with the finite-difference time-domain (FDTD) theoretical modeling.

Imaging through scattering: the Fisher information and the generalized Abbe limit

Aleksei Zheltikov

DOI: 10.1364/OL.439132 Received 02 Aug 2021; Accepted 07 Oct 2021; Posted 08 Oct 2021  View: PDF

Abstract: Enhanced-resolution imaging in complex scattering media is revisited from a perspective of parameter estimation problem. A suitably defined Fisher information is shown to offer useful insights into the limiting precision of parameter estimation in a scattering environment and, hence, into the limiting spatial resolution that can be achieved in imaging-through-scattering settings. The Fisher information that defines this resolution limit via the Cramér–Rao lower bound is shown to scale with the number of adaptively controlled space–time modes of the probe field, suggesting a physically intuitive generalization of the Abbe limit to the spatial resolution attainable for complex scattering systems. In a conventional microscopy setting, this bound is shown to converge to the canonical Abbe limit.

The third- and fourth-order orbital angular momentum multiplexed amplification with ultra-low differential mode gain

She Gao, Weiping Liu, Ji Zhou, Bin Zhang, Wei Li, Cheng Du, Jiajing Tu, Zhaohui Li, and Tianjin Wen

DOI: 10.1364/OL.441111 Received 24 Aug 2021; Accepted 07 Oct 2021; Posted 08 Oct 2021  View: PDF

Abstract: In this Letter, a ring-core erbium-doped fiber (RC-EDF), with two-layer erbium-doped structure, supporting up to the fourth -order orbital angular momentum (OAM) mode is designed and fabricated for OAM mode multiplexed amplification. Using the RC-EDF, the third- and fourth-order OAM modes amplification with ultra-low differential mode gain (DMG) is demonstrated by observing both the modal intensity and phase distribution and measuring the modal gain under the fundamental mode core-pumping. The measured average gain of four modes (OAM±3,1 and OAM±4,1) multiplexed amplification is higher than 19dB cover the C-band and the DMG is less than 1dB. Additionally, the gain of two conjugate OAM modes are almost the same under different pump power no matter they are amplified simultaneously or separately.

High-peak-power structured beams by an Nd:YAG/Cr⁴+:YAG laser in a near-hemispherical cavity

Pi-Hui Tuan, Yu-Zhe Cheng, Wan-Chen Tsai, and Kuan-Ting Cheng

DOI: 10.1364/OL.442322 Received 02 Sep 2021; Accepted 07 Oct 2021; Posted 11 Oct 2021  View: PDF

Abstract: An Nd:YAG/Cr⁴+:YAG passively Q-switched (PQS) laser in a near-hemispherical cavity is exploited to generate the high-order structured pulsed fields. Under the tightly-focusing on-axis pumping, the radial-order Laguerre-Gaussian (LG) modes with controllable mode orders by the input pump power are realized to exhibit quite stable temporal behavior. The pulse repetition rate of the radial-LG modes can reach up to 78 kHz with an average output power of 0.57 W and a peak power beyond 300 W under a 5-W pump level. Furthermore, by introducing 1D off-axis pumping into the PQS laser, various structured pulsed fields with transverse morphologies as the high-order Ince-Gaussian (IG) modes are further created. With clean and well-defined beam structures, the IG pulsed fields can be nicely reconstructed by the resonant modes of the inhomogeneous Helmholtz equation for spherical cavities. More importantly, these high-order PQS IG modes reveal highly regular pulse trains with the maximum pulse repetition rate beyond 20 kHz and the overall peak power to be higher than 1.5 kW.

Optically formed rubbery waveguide inter-connects

Georgios Violakis, Athanasios Bogris, Stergios Pispas, George Fytas, Benoit LOPPINET, and Stavros Pissadakis

DOI: 10.1364/OL.435052 Received 23 Jun 2021; Accepted 06 Oct 2021; Posted 08 Oct 2021  View: PDF

Abstract: Self-written waveguides (SWWs) with unique elongation characteristics and low optical loss are formed in a monodispersed polyisoprene solution using a low-power laser photo-polymerization process, while their light transmission characteristics are exemplified in the flexible interconnection of two single mode optical fibers, operating in the visible/near infrared wavelengths. The SWWs formed exhibit rubbery properties, allowing extendabilities greater than 400%, without significant deterioration of the optical transmission properties. The rubber elasticity enables sustaining optical links at stressed lengths longer than 500μm while the propagation losses range from 1.0 dB/mm to 2.9dB/mm for the examined SWW lengths.

High-purity orbital angular momentum vortex beam generator using an amplitude-and-phase metasurface

Sen Zheng, Hao Honggang, Yihao Tang, and Xuehong Ran

DOI: 10.1364/OL.441426 Received 25 Aug 2021; Accepted 04 Oct 2021; Posted 12 Oct 2021  View: PDF

Abstract: In this letter, we propose an approach to generate high-purity orbital angular momentum (OAM) vortex waves using an amplitude-and-phase metasurface (APM). By varying the square split ring opening and orientation angles, the cross-polarized reflection response of the proposed structure can yield full phase and amplitude coverage. Based on the traditional phase-only metasurface (POM), the Chebyshev synthesis method (CSM) is applied to array the metasurface amplitude distribution. Metasurfaces with modes l of 1, 2, 3, and 4 are designed. Compared with the POM, the APM can effectively improve the vortex beam quality and OAM mode purity. The measured results agree well with full-wave simulations. The presented method provides a new way to design high-purity OAM generators based on metasurfaces.

Low voltage surface-normal electroabsorption modulators

Stefano Grillanda, Ting-Chen Hu, David Neilson, and Mark Earnshaw

DOI: 10.1364/OL.441812 Received 30 Aug 2021; Accepted 04 Oct 2021; Posted 04 Oct 2021  View: PDF

Abstract: Surface-normal electroabsorption modulators (SNEAMs) are appealing for short-reach communication systems because of their outstanding properties, such as ultrawide bandwidth and polarization-insensitive response; however, due to their small active volumes, large voltage swings are typically required to obtain the best performance. Here, we propose and demonstrate a novel design that dramatically reduces the voltage needed by SNEAMs and significantly increases their extinction ratio. By shrinking the multiple quantum well stack of SNEAMs to the minimum, and by optimizing their reflectivity with dielectric coatings of suitable refractive index and thickness, we obtain modulators that require drive voltages of only 1-2 Vpp. We show that these novel devices largely outperform conventional SNEAMs.

High axial resolution single molecule localization under dense excitation with a multi-channel deep U-Net

Weihang Zhang, Zhihong Zhang, Liheng Bian, Haoqian Wang, Jinli Suo, and Qionghai Dai

DOI: 10.1364/OL.441536 Received 25 Aug 2021; Accepted 01 Oct 2021; Posted 04 Oct 2021  View: PDF

Abstract: Single-molecule localization microscopy (SMLM) can bypass the diffraction limit of optical microscopes and greatly improve the resolution in fluorescence microscopy. By introducing the PSF engineering technique, we can customize depth varying PSF to achieve a higher axial resolution. However, most existing three-dimensional single-molecule localization algorithms require the excited fluorescent molecules being sparse and captured at high signal-to-noise ratio, which results in a long acquisition time and precludes SMLM's further applications in many potential fields. To address this problem, we propose a novel three-dimensional single-molecular localization method based on a multi-channel neural network based on U-Net (MCU-Net). By leveraging deep network's great advantages in feature extraction, the proposed network can reliably discriminate dense fluorescent molecules with overlapped PSFs, and corrupted by sensor noise. Both simulated and real experiments demonstrate its superior performance in PSF engineered microscopes with short exposure and dense excitations, which holds great potential in fast 3D super-resolution microscopy.

P-doped 1300 nm InAs/GaAs quantum dot lasers directly grown on SOI substrate

Jing-Zhi Huang, Qi Wei, jiajian chen, Zihao Wang, Ting Wang, and Jianjun Zhang

DOI: 10.1364/OL.437471 Received 13 Jul 2021; Accepted 30 Sep 2021; Posted 08 Oct 2021  View: PDF

Abstract: The realization of monolithic integration of a stable III-V laser on standard silicon-on-insulator (SOI) substrate has been regarded as a challenging technology for silicon-based photonic integration circuits (PICs). Here, we successfully demonstrated the electrically pumped P-doped 1300 nm InAs/GaAs quantum dot (QD) laser epitaxially grown on {111}-faceted SOI hollow substrates. These III-V QD lasers epitaxially grown on SOI substrate generally exhibit strong thermal accumulation due to the oxide layer underneath. By applying double-side heat dissipation design, the maximum operation temperature of the SOI-based InAs/GaAs QD laser under continuous wave (CW) operation mode is ramped up to 35 °C from 20 °C. Moreover, the thermal profile simulation of three different structures has also been carried out to show the effectiveness of top heat sink design in order to improve laser performance. An integrated thermal shunt design is proposed to improve the heat dissipation without using the external top heat sink. The successful realization of room-temperature SOI-based InAs/GaAs QD laser paves a viable way for integration of light source in the PICs.

Transmission of 56Gbit/s PAM4 Signal with Low-resolution DAC and Pre-equalization Only over 80-km Fiber in C-band IM/DD Systems for Optical Interconnects

Mingzhu Yin, Dongdong Zou, Fan Li, Zhaohui Li, and Wei Wang

DOI: 10.1364/OL.441598 Received 26 Aug 2021; Accepted 30 Sep 2021; Posted 21 Oct 2021  View: PDF

Abstract: A low-cost intensity modulation and direct detection (IM/DD) scheme using a 4-bit digital-to-analog converter (DAC) for 56Gbit/s pre-equalized PAM-4 generation at the transmitter and without post-equalization at receiver over 80-km single-mode fiber (SMF) transmission in C-band is firstly experimentally demonstrated in this letter. Pre-equalization includes pre-emphasis for channel response, the proposed modified Gerchberg-Saxton (GS) algorithm with 5 iterations for chromatic dispersion compensation and noise shaping (NS) technique implemented by 5-taps finite impulse response (FIR) filter for quantization noise suppression. Enabled by the pre-equalized technique, bit-error-ratio (BER) of received signal without post-equalization can reach hard-decision forward error correction (HD-FEC) threshold with -2dBm receiver optical power (ROP). The experimental results indicate that our proposed scheme is a promising candidate for the standardization of 400GbE WDM-based long-haul distance data center interconnections (DCIs).

All-Silicon, Low-Crosstalk Terahertz Waveguide Crossing Based on Effective Medium

Harrison Lees, Weijie Gao, and Withawat Withayachumnankul

DOI: 10.1364/OL.436039 Received 07 Jul 2021; Accepted 28 Sep 2021; Posted 29 Sep 2021  View: PDF

Abstract: All-silicon effective-medium-clad waveguides are a promising candidate for an integrated terahertz platform with high efficiency and broad bandwidth. Waveguide crossings are essential circuit components, allowing for wave routing over shorter paths, to increase circuit density. However, the simple intersection of two orthogonal effective-medium-clad waveguides results in terahertz wave scattering leading to relatively high crosstalk. In this work, a low-loss, broadband crossing utilizing Maxwell-Garnet effective-medium theory and wavefront planarization techniques is proposed. This monolithic structure is fabricated on a single high-resistivity float-zone silicon wafer using a deep reactive ion etching (DRIE) process with a modest 4.4 mm diameter (4.03λ) structure footprint. Experimentally verified results show low insertion loss, less than 1 dB, and average crosstalk level of -39 dB for both E_{11}^x and E_{11}^y operating modes, over 220--330 GHz with a 40% fractional bandwidth. This waveguide crossing can be foreseen as a useful routing component for terahertz all-silicon integrated circuits. The proposed techniques are applicable to other dielectric waveguide platforms at infrared and optical frequencies.

Feedback-assisted transmission matrix measurement of a multimode fiber in a referenceless system

Zhengyang Wang, Daixuan Wu, Guoqiang Huang, Jiawei Luo, Ye Bolin, Zhaohui Li, and Yuecheng Shen

DOI: 10.1364/OL.437849 Received 14 Jul 2021; Accepted 28 Sep 2021; Posted 14 Oct 2021  View: PDF

Abstract: Recent development in wavefront shaping shows the promise to employ multimode fibers (MMFs) to deliver images in endoscopy and to realize mode-division multiplexing in optical communications. In these applications, retrieving the transmission matrix (TM) of the MMF is essentially important. Among existing non-holographic approaches, feedback-based wavefront shaping requires a large number of measurements while directly measuring the TM can be easily trapped into local optimums if the constraints are insufficient. To reduce the required number of measurements, we combine the concepts of these two approaches and develop a scheme termed feedback-assisted TM measurements. We show that under such a hybrid scheme, less than 3N intensity measurements are sufficiently enough to accurately retrieve one row of the TM that contains N unknown complex elements. As a proof of concept, we experimentally demonstrated retrieving the TM of an MMF using the proposed scheme with high fidelity, In particular, a single focus and dual foci through the MMF with enhancements larger than 75% of the theoretical values were reported.

Broadband terahertz rectification of ultrashort multiterawatt laser pulses near the beam breakup threshold

Maxim Nazarov, Pavel Shcheglov, Valery Teplyakov, Mikhail Chashchin, Alexander Mitrofanov, Dmitry Sidorov-Biryukov, Vladislav Panchenko, and Aleksei Zheltikov

DOI: 10.1364/OL.434759 Received 21 Jun 2021; Accepted 27 Sep 2021; Posted 11 Oct 2021  View: PDF

Abstract: We identify the physical factors that limit the terahertz (THz) yield of an optical rectification (OR) of ultrashort multiterawatt laser pulses in large-area quadratically nonlinear crystals. We show that the THz yield tends to slow its growth as a function of the laser driver energy, saturate, and eventually decrease as the laser beam picks up a spatiotemporal phase due to the intensity-dependent refraction of the OR crystal. We demonstrate that, with a careful management of the driver intensity aimed at keeping the nonlinear length larger than the coherence length, OR-based broadband THz generation in large-area OR crystals is energy-scalable, enabling an OR of multiterawatt laser pulses, yielding a few μJ/cm2 of THz output per unit crystal area. With a 27-fs, 10-TW, 800-nm Ti: sapphire laser output used as a driver for OR in large-area lithium niobate crystals, this approach is shown to provide a THz output with a pulse energy above 10 μJ and a bandwidth extending well beyond 6 THz, supporting single-cycle THz waveforms.

Improved temporal contrast of streak camera measurements with periodic shadowing

YUPAN BAO, Vassily Kornienko, David Lange, Wolfgang Kiefer, Tina Eschrich, Matthias Jaeger, Joakim Bood, Elias Kristensson, and Andreas Ehn

DOI: 10.1364/OL.438034 Received 13 Aug 2021; Accepted 25 Sep 2021; Posted 28 Sep 2021  View: PDF

Abstract: Periodic shadowing, a concept used in spectroscopy for stray-light reduction, has been implemented to improve the temporal contrast of streak-camera imaging. The capabilities of this technique are first proven by imaging elastically scattered picosecond laser pulses, and are further applied to fluorescence lifetime imaging, where more accurate descriptions of fluorescence decay curves were observed. This all-optical approach can be adapted to various streak camera imaging systems, resulting in a robust technique to minimize space-charge induced temporal dispersion in streak cameras while maintaining temporal coverage and spatial information.

Visible photon generation via four-wave mixing in near infrared near-zero-index thin films

Enrico Giuseppe Carnemolla, Wallace Jaffray, Matteo Clerici, Lucia Caspani, Daniele Faccio, Fabio Biancalana, Clayton DeVault, Vladimir Shalaev, Alexandra Boltasseva, and Marcello Ferrera

DOI: 10.1364/OL.433834 Received 15 Jun 2021; Accepted 13 Sep 2021; Posted 17 Sep 2021  View: PDF

Abstract: Optical nonlinearities can be strongly enhanced by operating in the so-called near-zero-index (NZI) regime where the real part of the refractive index of the system under investigation approaches zero. Here we experimentally demonstrate semi-degenerate four-wave mixing (FWM) in aluminum zinc oxides thin films generating radiation tunable in the visible spectral region where the material is highly transparent. To this end, we employed an intense pump (787 nm) and a seed tunable in the NIR window (1100-1500 nm) to generate a visible idler wave (~530-610 nm). Experiments show enhancement of the frequency conversion efficiency, with a maximum of ~2% and with a signal-to-pump detuning of ~360 nm. Effective idler wavelength tuning has also been demonstrated by operating on the temporal delay between pump and signal.

Towards a space-qualified Kerr-lens mode-locked laser

Feng Ye, TOBIAS LAMOUR, Hanna Ostapenko, Richard McCracken, Oliver Mandel, Dennis Weise, and Derryck Reid

DOI: 10.1364/OL.439965 Received 08 Aug 2021; Accepted 02 Sep 2021; Posted 08 Sep 2021  View: PDF

Abstract: We report a 1.5-GHz Kerr-lens-modelocked (KLM) Yb:Y2O3 ring laser, constructed by directly bonding the cavity components onto an aluminum baseplate. Stable unidirectional operation with an output power of 15 mW was obtained for pump-diode currents of 300–500 mA. After repetition-rate stabilization using active feedback to an intracavity piezoelectric transducer, a comparison with a conventionally-constructed identical laser showed a 50% reduction in phase noise. In free-running operation the bonded laser showed a 2.3× improvement in passive repetition-rate stability. The bonding procedure follows an already proven integration approach in space-borne instrumentation, providing a development pathway for the application of KLM lasers in aerospace applications.

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