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Investigation of multi-pass Yb-doped fiber amplifiers

Julijanas Zeludevicius, Giedrius Dubosas, and Kestutis Regelskis

DOI: 10.1364/AO.443529 Received 17 Sep 2021; Accepted 24 Oct 2021; Posted 25 Oct 2021  View: PDF

Abstract: We investigated both numerically and experimentally single-pass, 2-pass and 4-pass Yb-doped core-pumped fiber amplifier configurations and compared their performance with respect to achieved pump-to-signal conversion efficiency. Despite numerical calculations predicted highest efficiency in 4-pass setup, experimentally highest conversion efficiency of 77 % was achieved in 2-pass setup and only 49 % in 4-pass setup. Reduced performance of experimental 4-pass fiber amplifier configuration was caused by excess coupling losses, which complicate practical implementation of such configuration using of-the-shelf components.

Analytical model for transmission dips in selfassembled two-dimensional colloidal crystals

Sameh Abdellatif, Khaled Kirah, and Frank Marlow

DOI: 10.1364/AO.442750 Received 13 Sep 2021; Accepted 23 Oct 2021; Posted 25 Oct 2021  View: PDF

Abstract: Self-assembled two-dimensional colloidal crystals (CCs) are utilized in various optical devices, lasers,biosensors, and light harvesting applications. Optical design tuning capabilities, in terms of sphere refractive index anddiameter size, can influence the optical characteristics for the close-packed single-layer or multilayer structures. Oftentransmission dips in two-dimensional colloidal crystals (2D CCs) are observed which cannot be explained by Braggdiffraction as for 3D photonic crystals. In this work, an analytical attempt to accurately model the transmission dipsobserved in the 2D CCs optical spectra is presented, aiming to explain the origin of these dips. The formation of a broaddip was also studied experimentally as well. A less than 1% mismatching error was found between experiment and theoryfor the two blaze peak positions as well as for the transmission intensity ratio. Finally, the 2D CCs were integrated inmesostructured solar cells as light trapping structures.

Electromagnetically induced transparency basedmetal dielectric metamaterial and its terahertzsensing application

REENA DALAL, Yogita Kalra, and Ajeet Kumar

DOI: 10.1364/AO.442948 Received 10 Sep 2021; Accepted 22 Oct 2021; Posted 25 Oct 2021  View: PDF

Abstract: In this paper, electromagnetically induced transparency has been reported in the metal-dielectric structure whichprovides the platform for high quality factor Fano resonance in the Terahertz region. Electric dipole in the metal ringprovides bright mode while electric and magnetic dipoles formed in the dielectric offer bright and dark modes,respectively. Two resonance dips have been obtained with high quality factor 89.5 and leads to high figure ofmerit of sensor equal to 6 and 4 for first and second resonance dips, respectively which is useful for the design anddevelopment of the metamaterial based sensing devices and bio-sensors.

Out-of-plane displacement measurement foroptical fiber image transmission elementusing optical flow

Ran Zhao, Peng Jiao, Bo Liu, Sun Ping, Yuxin Tang, and Jinsheng Jia

DOI: 10.1364/AO.443184 Received 13 Sep 2021; Accepted 22 Oct 2021; Posted 25 Oct 2021  View: PDF

Abstract: In order to realize the industrial in-situ deformation detection of optical fiber imagetransmission element, a novel method for out-of-plane displacement detection based on thecombination of optical flow and image spherizing algorithm is presented. After capturing thetwo-dimensional deformation images, the out-of-plane displacement can be extracted fromthe in-plane displacement estimated by optical flow method, which means that the out-ofplane displacement information contained in two-dimensional image is fully utilized. In thispaper, the principle of Brox optical flow method is introduced and the accuracy of the methodis proved by comparing the test results with digital image correlation method. Experimentalresults show that the relative errors and the absolute errors of the measurement are less than2% and 0.1 μm. Because the optical flow algorithm introduces the time parameter and thesmoothness assumption, the new method is more appropriate for dynamic and micro scaleout-of-plane displacement measurement. Unlike conventional methods, it does not need phaseunwrapping or frequency domain conversion during detection and has many attractivecharacteristics such as nondestructive test, simple arrangement and low errors. The newmethod has been applied to the industrial in-situ monitoring of optical fiber imagetransmission element.

Thermally-responsive polymer dispersed liquid crystaldiffusers fabricated using laser speckle patternirradiation

Akifumi Ogiwara and Hiroshi Kakiuchida

DOI: 10.1364/AO.443216 Received 14 Sep 2021; Accepted 22 Oct 2021; Posted 25 Oct 2021  View: PDF

Abstract: This study examines the thermal response of polymer dispersed liquid crystal (PDLC) diffusers patterned using a twolens imaging system. Optical modulation is achieved by modifying PDLC transmittance using temperature-inducedchanges to liquid crystal (LC) orientation. PDLCs with controllable scattering properties are obtained by irradiating LCpolymer composites with laser speckle patterns. The variation of the scattering characteristics of the PDLCs withtemperature, average speckle size, and LC orientation order is analyzed to determine the most suitable parameters for adiffuser for smart window solar-ray control applications. The findings of these experiments demonstrate that usingspeckle patterns, a one-time laser exposure process can provide a simple method for fabrication of novel optical devices.

Fast-iterative automatic reconstruction method forquantitative phase image with reduced phaseperturbations in off-axis digital holographicmicroscopy

Raul Castañeda Quintero and Ana Doblas

DOI: 10.1364/AO.437640 Received 15 Jul 2021; Accepted 21 Oct 2021; Posted 22 Oct 2021  View: PDF

Abstract: This works presents a reconstruction algorithm to recover the complex object information for an off-axis digitalholographic microscope (DHM) operating in the telecentric regimen. We introduce an automatic and fast method tominimize a cost function that finds the best numerical conjugated reference beam to compensate the filtered objectinformation, eliminating any undesired phase perturbation due to the tilt between the reference and object waves. Thenovelties of the proposed approach are a precise estimation of the interference angle between the object and referencewaves, reconstructed phase images without phase perturbations, and reduced processing time. The method has beenvalidated using a manufactured phase target and biological samples.

Concept and Application of a Linearized RingMultipass Optics

Björn Waßmuth, Guido Fuchs, Henrik Zimmermann, and Thomas Giesen

DOI: 10.1364/AO.441368 Received 25 Aug 2021; Accepted 21 Oct 2021; Posted 22 Oct 2021  View: PDF

Abstract: Multipass optics (MPO) are used to increase the optical path length in a limitedvolume. Well-known concepts such as the White-type cell and the Herriott-type cell are usedin many applications. The optical path in these cells leaves a large portion of the cell volumeunused. In contrast, multipass optics with astigmatic mirrors have significantly more beam passesper volume, but because of their reduced symmetry along the beam axis they are much moresensitive to misalignment. Our goal is to develop a multipass optics that has both: the highbeam densities of astigmatic MPOs and the optical robustness of Herriott-type optics. We showthat linearized optical ring resonators which can be constructed by means of simple 2x2 optical퐴퐵퐶퐷-matrices lead to suitable solutions. The concept is demonstrated using a MPO with abase length of 150 mm and a total beam path length of 19.2 m, optimized for laser beams of10 µm wavelength. The volume of the cell is 295 cm3 with a four times higher beam densitycompared to a standard Herriott-type cell.

An evaluation of the role of the beam homogeneity onthe mechanical coupling of thelaser-ablation-generated impulse

Jacopo Terragni, Pietro Battocchio, Nicola Bazzanella, Michele Orlandi, William Burger, Roberto Battiston, and Antonio Miotello

DOI: 10.1364/AO.432991 Received 02 Jul 2021; Accepted 21 Oct 2021; Posted 22 Oct 2021  View: PDF

Abstract: The material emitted from the target surface during laser ablation generates a net thrust (propulsion) inthe opposite direction. The momentum generation efficiency of this laser-driven propulsion is given bythe mechanical coupling coefficient (Cm). In this work we considered nanosecond UV laser ablation ofthe aluminum 6061 alloy to study the Cm behaviour with different irradiating conditions. This is done bysystematically changing: fluence, uniform/nonuniform intensity, and incident angle of the laser beam. Inparticular we found that, when dealing with nonuniform laser intensity, characterizing Cm exclusively interms of fluence is not fully satisfactory because the energy distribution over the irradiated area plays akey-role in the way material is removed -interplay between vaporization and phase-explosion- and thrustis generated.

Dynamic shape measurement for objects withpatterns by Fourier fringe projection profilometrybased on variational decomposition and multi-scaleRetinex

Qi zhao, Chen Tang, Min Xu, and Zhenkun Lei

DOI: 10.1364/AO.438992 Received 28 Jul 2021; Accepted 20 Oct 2021; Posted 22 Oct 2021  View: PDF

Abstract: In practical measurement, we often need to measure the shape of objects with patterns or letters. As far as weknow, no paper has ever reported the shape measurement for objects with patterns or letters by Fourier fringeprojection profilometry (FPP). In this paper, we propose a method based on variational decomposition TV-HilbertL2 model and multi-scale Retinex (MSR) to measure the shape of objects with patterns and letters by Fourier FPP. Inthis method, we first use the TV-Hilbert-L2 model to obtain the fringe part, then perform MSR enhancement on thefringe part, and finally decompose the enhanced fringe part with TV-Hilbert-L2 again. We evaluate theperformance of this method via application to one computer-simulated noisy fringe projection pattern and twoexperimental fringe projection patterns with different types of patterns or letters, and comparison with theFourier transform (FT) method, the variational image decomposition TV-Hilbert-L2 model. Furthermore, we applythe proposed method to the dynamic three-dimensional shape measurement of hand posture with pattern. Theexperimental results show that our method can effectively measure the dynamic shape of objects with patterns orletters from single frame fringe projection pattern.

Photosynthetic spectral response curves of saffronleaves

XINYU JI, DAN GAO, WEIZHONG PEI, XUE ZHANG, FUSHENG LI, Qiuyi Han, and Shanduan Zhang

DOI: 10.1364/AO.433486 Received 07 Jun 2021; Accepted 20 Oct 2021; Posted 20 Oct 2021  View: PDF

Abstract: Saffron (Crocus sativus L.) is an herb with outstanding medicinal functions and commercial value. Light is animportant factor in plant growth, and the sensitivity of plant photosynthesis to light quality can be characterized byphotosynthetic spectral response curves. This study aims to measure the spectral response curves of saffron leaves, so asto provide theoretical guidance for supplemental lighting spectrum to increase saffron production. The measurementresults show the peaks of spectral response curves of saffron leaves are at 480 nm and 660 nm respectively, whichprovides a reference for the peak wavelengths of supplemental lighting spectrum. Full-spectrum white light with lowcolor temperature or red light mixed with a little blue light might be most beneficial for saffron biomass accumulation.

Improving accuracy of distance measurements basedon beat frequency detection of a dual-frequencyoptoelectronic oscillator

Bo Yang, jiwen yu, Hao Chi, shuna yang, Jun Ou, and Yanrong Zhai

DOI: 10.1364/AO.442306 Received 02 Sep 2021; Accepted 20 Oct 2021; Posted 21 Oct 2021  View: PDF

Abstract: A high accuracy distance measurements (DMs) approach based on beat frequency detection of a dual-frequency optoelectronicoscillator (OEO) is proposed and experimentally demonstrated. The dual-frequency OEO is formed with a single electro-opticalmodulator and a common length of energy storage fiber, and the beat frequency of the two oscillation signals can be directly achievedafter a photodetector. Since the environmental disturbance has same influence on the lengths of the two loops corresponding to thetwo oscillation frequencies, the environmental disturbance errors can be greatly reduced by beat frequency detection. In theexperiment, we achieved a sensitivity of 49.9375 kHz/mm and a measurement error of ±15 µm. The frequency stability of beatfrequency signal was 8.57 times higher than the oscillation signal of measurement loop.

The performance analysis of photonic crystal fiberpolarization filter filled with different materials

junjun Wu, Chao Wang, lichun Hu, and chao Dou

DOI: 10.1364/AO.441355 Received 06 Sep 2021; Accepted 20 Oct 2021; Posted 21 Oct 2021  View: PDF

Abstract: A compact square photonic crystal fiber polarization filter with high performance is proposed. Two larger holes filledwith different metal and fluid is designed to break the strict structure symmetry and forming high birefringence. Foursmall pores are designed to provide a wider channel for the coupling between defect mode and core mode. Its filtering andcoupling characteristics are analyzed by full vector finite element method. In addition, the properties of PCF filled with differentmaterials are compared and discussed. Different metal materials have different dielectric constants, different opticaldamping, interband transitions, molecular structure and physicochemical properties, which make their transmissionmodes and coupling strength different.The results show that the performance of PCF filter filled with gold and liquid isthe best, which is very suitable for adjustable PCF polarization filter. The calculation results show that the extinction ratioof the designed filter can reach 5383 dB for a device length of 3mm, the unwanted fiber loss can reach 2073 dB/cm, andthe applicable bandwidth of 2000 nm covers almost the whole communication band.The proposed polarization filtershows large unwanted loss, high extinction ratio, wide bandwidth and coordination, which is a good candidate forexcellent optical fiber filter device.

Photonic-based analog and digital RF self-interferencecancellation with high spectral efficiency

Taixia Shi, Moxuan Han, and Yang Chen

DOI: 10.1364/AO.439709 Received 04 Aug 2021; Accepted 19 Oct 2021; Posted 20 Oct 2021  View: PDF

Abstract: A photonic-assisted analog and digital radio frequency (RF) self-interference cancellation (SIC) approach with highspectral efficiency is reported for base stations in the in-band full-duplex (IBFD) radio-over-fiber (ROF) systems on thebasis of our previous research. One dual-polarization quadrature phase-shift keying (DP-QPSK) modulator is used as thecanceller for one base station. The two dual-parallel Mach-Zehnder modulators (DP-MZMs) of the DP-QPSK modulatorare both biased as carrier-suppressed single-sideband (CS-SSB) modulators and driven by the received signal andreference signal, respectively, to achieve high spectral efficiency while implementing the SIC in the optical domain. Thebaseband optical signal after SIC is further transmitted to the central station, where the electrical signal is recovered,sampled, and processed to further suppress the residual self-interference in the digital domain by using the recursiveleast squared (RLS) algorithm. An experiment is performed. The proposed system is demonstrated by employing twoindependent channels. The analog cancellation depths of the 200, 500, and 800-Mbaud QPSK-modulated selfinterferences are around 24, 20, and 20 dB, respectively, while the total cancellation depths are around 29, 28, and 25 dB,respectively, when the analog cancellation and the RLS algorithm digital cancellation are applied. Meanwhile, the fiberdistribution has no significant influence on the SIC performance.

Dynamic light scattering from singlemacroscopic particles

Lisa Dossow, Raphael Kessler, Matthias Sperl, and Philip Born

DOI: 10.1364/AO.441093 Received 19 Aug 2021; Accepted 19 Oct 2021; Posted 20 Oct 2021  View: PDF

Abstract: Here we present a methodology to characterize the light intensity fluctuations thatarise from rotations of individual granular particles. We describe a setup for dynamic lightscattering measurements on individual macroscopic particles and isolate the contribution fromrotations of the individual particles to the obtained correlation functions. The results showthat rotation of granular particles result in a significant contribution to scattered light intensityfluctuations, a phenomenon not considered so far in dynamic light scattering measurements onfluidized granular media. The results presented here may thus form the basis for an extendedlight scattering methodology for granular media, and improve the selection of granular particlesaccording to their dynamic light scattering signal.

Alkali etched fiber Mach-Zehnder interferometer withcompact sensor head

Rende Ma, Xiaoping Li, Hongzhong Cao, Yue Pan, Zhong-Xiao Man, and Yunjie Xia

DOI: 10.1364/AO.441894 Received 02 Sep 2021; Accepted 19 Oct 2021; Posted 20 Oct 2021  View: PDF

Abstract: We demonstrate a scheme for fabricating compact fiber Mach-Zehnder interferometer (MZI). A section of Ge-doped fiber(GDF) is sandwiched between two single-mode fibers. The sandwich structure is side polished to make the core of GDFexposed to the surroundings. Alkali solution is utilized to etch the core of GDF. A compact fiber MZI is achieved when abouthalf of the core is etched. Compared with the traditional ones, our scheme for fabricating fiber MZI has the characteristicsof low cost, environmentally friendly, and regular transmission spectrum. This fiber MZI not only reduces theconsumption of the sample, but also brings forth a good potential for micro-scale detection of refractive index.

High precise two-dimensional beam steeringwith a 64-element optical fiber phased array

xiaobin wang, Chunbo Liu, yang cao, RuYuan Liu, Lin Zhang, xu zhao, fang lu, zhifang miao, Qiwei Li, and Xiang E Han

DOI: 10.1364/AO.434473 Received 20 Jul 2021; Accepted 18 Oct 2021; Posted 19 Oct 2021  View: PDF

Abstract: Fine target imaging in large-scale optical fiber phased array (OFPA) system requires simultaneously achieving high-quality beams output and high-precise beam steering. However, this is difficult in practice due to the dephasing issue that occurs from the power amplifier as well as the optical fibers jitter. We demonstrate a two-dimensional 64-element OFPA system, using a stochastic parallel gradient descent(SPGD) algorithm to synchronize the phases to realize high-quality beams output and utilizing an APD array detector to get accurate voltages for beam steering precisely. Experimental results show that the peak side lobe ratio of the main beam is .7dB and the speed of the beam steering between two discretionary angles is 128kHz.

Axial resolution enhancement for planar Airy beam light-sheet microscopy via complementary beam subtraction method

Chao Liu, Xianghua Yu, Chen Bai, Xing Li, Yuan Zhou, Shaohui Yan, Junwei Min, Dan Dan, Runze Li, Shuangyu Gu, and Baoli Yao

DOI: 10.1364/AO.441070 Received 19 Aug 2021; Accepted 18 Oct 2021; Posted 19 Oct 2021  View: PDF

Abstract: Airy beam light-sheet illumination can extend the field of view (FOV) of light-sheet fluorescence microscopy due to the unique propagation properties of non-diffraction and self-acceleration. However, the side lobes create undesirable out-of-focus background, leading to poor axial resolution and low image contrast. Here, we propose an Airy complementary beam subtraction (ACBS) method to improve the axial resolution while keeping the extended FOV. By scanning the optimized designed complementary beam that has two-main-lobes (TML), the generated complementary light-sheet has almost identical intensity distribution to that of the planar Airy light-sheet except for the central lobe. Subtraction of the two images acquired by double exposure respectively using the planar Airy light-sheet and the planar TML light-sheet can effectively suppress the influence of the out-of-focus background. The axial resolution improves from ~4 μm to 1.2 μm. The imaging performance was demonstrated by imaging specimens of aspergillus conidiophores and GFP labeled mouse brain section. The results show that the ACBS method enables the Airy beam light-sheet fluorescence microscopy to obtain better imaging quality.

Quantum-illumination-inspired active single-pixelimaging with structured illumination

Tiantian Zhang, Zhiyuan Ye, Hai-Bo Wang, and Jun Xiong

DOI: 10.1364/AO.438642 Received 22 Jul 2021; Accepted 18 Oct 2021; Posted 20 Oct 2021  View: PDF

Abstract: Quantum-illumination-inspired single-pixel imaging (QII-SPI) or computational ghost imaging protocolis proposed to improve image quality in the presence of strong background and stray light. According tothe reversibility of the optical path, a digital micro-mirror device acts as a structured light modulator anda spatial light filter simultaneously, which can effectively eliminate 50% of stray light. Accompanied by a6dB gain of detection signal-to-noise ratio under an equivalent loss condition, our scheme only requiresa simple and minor modification on the placement of the single-pixel detector based on the original SPIsystem. Since QII-SPI will obtain almost the same reconstruction results as the passive SPI technologyin principle, one can therefore adjust the placement position of the detector, without exchanging therelative position of the detector and the light source, to realize the flexible conversion of the SPI systemfrom active to passive. Also, this work initially discusses the influence of relative coherence time onHadamard-based single-pixel imaging driven by a thermal source. This work brings new insights intothe optical path design of the SPI technology, paving the way for the practical application of active SPI instray-light environments.

A new type of autocollimator based on normal tracing method and Risley prisms

Chuanqian Peng, HENGXIANG GONG, ZHONGHUA GAO, GANG WANG, XIAO LIANG, YUMEI HE, Xiaohao Dong, and Jie Wang

DOI: 10.1364/AO.438457 Received 26 Jul 2021; Accepted 18 Oct 2021; Posted 18 Oct 2021  View: PDF

Abstract: In the present study, we proposed a new type of autocollimator for high accuracy angular measurement within a large angle range. The new system is composed of traditional autocollimator and Risley prisms, and employs the normal tracing method to measure the angle. By rotating the Risley prisms, the outgoing beam of the autocollimator can be deflected close to the normal direction of the reflecting mirror, and then reflected back to the system by the mirror along the near normal direction to realize normal tracing. Based on the angle measured by autocollimator and the rotation angles of Risley prisms, we can calculate the tilt angle of the mirror. Since the beam returns to the system close to the original path, the angle error caused by aberration, optical component processing defects, non-uniform refractive index and so on can be ignored. Due to the normal tracing measurement method, theoretically, the angle error is not affected by the working distance. ZEMAX non sequence simulation shows that the angle error caused by aberration in the new system can be significantly reduced.

Computer-generated hologram manipulation and fastproduction with a focus on security application

Vladimir Cviljusac, Antun Lovro Brkic, Blaz Svilicic, and Marko Cacic

DOI: 10.1364/AO.439004 Received 16 Aug 2021; Accepted 18 Oct 2021; Posted 18 Oct 2021  View: PDF

Abstract: Motivated by the successful printing of the Computer-generated hologram (CGH) using the Computer-toFilm (CtF) graphic process, we present a further refined technique with increased resolution, applicablein security. The CtF process offers a low cost and fast production while persevering high resolution andit can make every hologram unique. In this paper, we present the improvement of the printing method,with several software modifications and the implementation of security features at different levels ofproduction.

Simultaneous Temperature and Refractive Indexsensor Based on a novel L-like MichelsonInterferometer

Yulan Zhang, Yongfeng Wu, Yang Han, Yaya Mao, Jing Wu, lilong zhao, Rong Tang, Jianxin Ren, and Bo Liu

DOI: 10.1364/AO.441963 Received 01 Sep 2021; Accepted 18 Oct 2021; Posted 18 Oct 2021  View: PDF

Abstract: We experimentally demonstrate temperature and refractive index (RI) sensing witha novel L-like Michelson interferometers (MI). Two MIs are designed respectively based onsingle-mode-fiber (SMF) and multimode fiber (MMF), which is fabricated via the oxyhydrogenflame and a rotatable platform. By comparative experiments on the size and bent degree of thesensor, the corresponding parameters of the sensor with excellent performance are determinedfor measuring experiments. The RI sensitivity of the SMF L-like MI reaches to -131.0 nm/RIUand the highest temperature sensitivity is 94.17 pm/°C. The highest RI of the MMF L-likeMI is 176.5 nm/RIU and the temperature sensitivity is 104.2 pm/°C. The L-like MI owns theadvantages of low cost, easy fabrication and is a promising temperature and RI sensor in widerange measurement.

Wave propagation visualisation through ducts using Schlieren technique for crack localisation with eSHM-system

Zoé Jardon, Patrick Guillaume, J.P.A.J Van Beeck, and Michaël Hinderdael

DOI: 10.1364/AO.438397 Received 28 Jul 2021; Accepted 18 Oct 2021; Posted 19 Oct 2021  View: PDF

Abstract: The effective Structural Health Monitoring (eSHM) system fully exploits the flexibility offered by the 3D printing process for the analysis of the structural integrity of additive manufactured parts by integrating a smart technology inside the component. The eSHM-system relies on the propagation of pressure waves through capillaries/small ducts embedded in 3D printed metallic components and allows the detection and localization of fatigue cracks. However, the nature and propagation of these waves seem to be determinant for the accuracy of the crack localization system. To achieve a better physical understanding of the propagating waves through the capillaries, Computational Fluid Dynamics simulations are performed and compared with experimental results, obtained by Schlieren flow visualisation and high-speed imaging techniques. The presence of propagating shock waves and contact discontinuities is observed in the simulations, as well as a complex reflection mechanism around the leak location. The Schlieren experiments exhibit the same wave shape behavior and complex reflection mechanism around the crack location for the contact discontinuity and the shock-tube analogy is confirmed.

Second-harmonic and sum-frequency generation based on birefringence phase matching of BaMgF4 crystal

Shuo Yan, Zhanwei Liu, Haigang Liu, Yiwei Sun, Yuanlin Zheng, and Xianfeng Chen

DOI: 10.1364/AO.438688 Received 20 Aug 2021; Accepted 17 Oct 2021; Posted 18 Oct 2021  View: PDF

Abstract: BaMgF4 is a ferroelectric nonlinear crystal with a very wide transparence window range from 125 nm to 13 μm of the wavelength. Therefore, it is a candidate material to generate ultraviolet or deep ultraviolet laser, which is very important in lithography, semiconductor manufacturing, and advanced instrument development. Here, the second-order birefringence phase matching processes of the BaMgF4 crystal were studied, including second-harmonic generation (SHG) and sum-frequency generation (SFG). In the experiments, we measured the phase matching angle, nonlinear frequency conversion efficiency, and angle bandwidth of SHG and SFG processes of BaMgF4 crystal, which are in well agreement with the theoretical calculations. This study may promote the research of nonlinear optical process of BaMgF4 crystal and also the further development of all solid-state vacuum ultraviolet lasers.

Freeform gradient index progressive addition lensraytrace performance evaluation

Nicholas Kochan, Greg Schmidt, and Duncan Moore

DOI: 10.1364/AO.441603 Received 27 Aug 2021; Accepted 16 Oct 2021; Posted 19 Oct 2021  View: PDF

Abstract: Raytrace evaluation capable of evaluating progressive addition lens (PAL) designs with freeform surfaceand gradient index (GRIN) contributions is presented. The method is validated on an analytically generated PAL start design and on optimized surface designs. Surface raytrace evaluations are compared withthe surface-geometric evaluation commonly presented for freeform surface PAL designs. The evaluationis also tested on analytically generated freeform GRIN PAL designs with spherical and plano surfaces.The raytrace method agrees with the analytic performance and surface-geometric performance near thecenter of the lens and deviates at the edge of the lens, due to ray obliquity with the surfaces and aggregatecontributions of surfaces and/or GRIN. These deviations are expected as the raytrace model accounts formore physical contributions to optical performance, including pupil diameter and eye position. This raytrace method enables the evaluation of lens performance contributions other than from polished surfaceson homogeneous materials, enabling further exploration of GRIN in PAL designs.

Design and fabrication of broadband terahertzmetamaterial absorber

Yan-Qing Qiu, Jinfeng Wang, Meiyu Xiao, and Tingting Lang

DOI: 10.1364/AO.440457 Received 16 Aug 2021; Accepted 15 Oct 2021; Posted 18 Oct 2021  View: PDF

Abstract: We proposed and experimentally demonstrated a broadband terahertz metamaterialabsorber based on a symmetrical L-shaped metallic resonator. The absorber structureproduces two absorption peaks at 0.491, and 0.73 THz, with the absorption rates of 98.6%,99.6%, respectively. Broadband absorption was obtained from 0.457 to 1 THz, achieving a >90% absorption bandwidth of 0.543 THz. By analyzing the distributions of the electric andmagnetic field at the two resonance frequencies, electric and magnetic dipole resonances wereproposed to explain the broadband absorption mechanism. Furthermore, various widths andlengths of the symmetrical L-shaped metallic resonator on the absorption characteristics wereinvestigated. Moreover, the broadband absorption characteristic can be maintained with anincident angle of up to 45° for transverse electric (TE) and 30° for transverse magnetic (TM)polarization. Finally, we experimentally observed a > 70% broadband absorptioncharacteristic from 0.42 to 1 THz. This proposed absorber has the potential for bolometricimaging, modulating and spectroscopy in the THz region.

Effects of phase flicker in digitally driven phase-onlyLCOS devices on holographic reconstructed images

Yuan Tong, Mike Pivnenko, and Daping Chu

DOI: 10.1364/AO.442644 Received 08 Sep 2021; Accepted 15 Oct 2021; Posted 18 Oct 2021  View: PDF

Abstract: Phase flicker can degrade the performance of holographic applications at both device level and application level. Onthe device side, the meaningful phase modulation resolution is proved to be limited by the overlapping betweenadjacent phase levels caused by flicker. Here the tolerance of the overlapping for different modulation levels isprovided. The frame rate of the device is also constrained by the phase flicker. The balance between low flicker andfast LC response for fast frame rate is quantitatively analysed. On the application side, the effects of real phase flickeron the performance of blazed gratings and image holograms are investigated using the temporal phase flickerprofiles measured from a phase-only LCOS device, and they are shown to be comparable to that introduced byquantization level and amplitude noise respectively.

Propagation of Pearcey Gaussian beams in a stronglynonlocal nonlinear medium

Huang Zhicong, Junjie He, Danlin Xu, Zheng Manhang, Chen Hechong, Wei Quanfeng, Dongmei Deng, and Long Xuewen

DOI: 10.1364/AO.443803 Received 20 Sep 2021; Accepted 15 Oct 2021; Posted 18 Oct 2021  View: PDF

Abstract: We introduce the propagation of Pearcey Gaussian (PG) beams in a strongly nonlocal nonlinear medium(SNNM) analytically. Our results show that PG beams propagating in the SNNM have two differentfocusing positions. The intensity peak appears at different focusing positions depending on the selectionof the nonlinear parameters. In addition, the effects of the nonlinear parameters and the scaling factor onthe trajectory, the position of the intensity focusing, the intensity evolution between focus locations andthe radiation force are studied.

Development of an ultra-precision metalmirror on additively manufactured Ti-6Al-4V

Tomas Thor, Anna Prochazkova, Frantisek Prochaska, Roman Dolecek, Michal Spina, Jan Vaclavik, Thomas Sedlmaier, and Marco Mulser

DOI: 10.1364/AO.436311 Received 16 Jul 2021; Accepted 14 Oct 2021; Posted 15 Oct 2021  View: PDF

Abstract: Metal mirrors for precise optical applications are commonly fabricated by coatingof a metal base substrate with a nickel-phosphorus alloy (NiP). The NiP layer is then processedby precision diamond turning and polishing to obtain a high quality mirror surface. In this work,Ti-6Al-4V samples that were made by additive manufacturing, also called 3D printing, wereused as a base for the development of metal mirrors. The additively manufactured samples wereelectroplated with NiP coating and machined using single point diamond turning (SPDT) toobtain a flat mirror with optical quality and low form error surface. The periodic structure ofthe SPDT toolmark was then removed by polishing post-processing. Polishing optimizationwas first performed on NiP-coated aluminum test samples to find an optimal polishing setup.Based on this optimization, post-processing of titanium samples was carried out by pitchpolishing in combination with 1 µm, 0.25 µm and 0.1 µm diamond slurries. Using thispolishing processing, a scratch-free surface was attained with surface micro-roughness below0.5 nm.

Design of free-form off-axis three-mirroroptical antenna with an integratedprimary/tertiary-mirror structure

Yuanqi Sun, Yuanhe Sun, XIAOYU CHEN, FANG Wang, Xin Yan, Xuenan Zhang, and Tonglei Cheng

DOI: 10.1364/AO.437587 Received 15 Jul 2021; Accepted 14 Oct 2021; Posted 19 Oct 2021  View: PDF

Abstract: In order to improve the performance and simplify the structure of the opticalantenna for space laser communication system, we design a free-form off-axis three-mirroroptical antenna with an integrated primary/tertiary-mirror structure. The adoption of theintegrated primary/tertiary-mirror structure improves the efficiency of light energy utilizationand reduces the complexity of optical processing and assembly. The introduction of free-formoptical elements and optical structure constraints help correct the off-axis aberration andrealize a large field of view. The obtained optical antenna has the magnification of 5 timesand the field of view of 2.4º×2.4º. The image quality obtained here reaches the diffractionlimited level. At the communication wavelength of 808 nm, the wavefront error is better than / 22 , and the system has a high energy concentration. The proposed optical antenna couldnot only improve the tracking accuracy of the optical antenna in space, but also greatly reducethe complexity of the laser communication system. It has reference significance andapplication value for free-space laser communication.

Optical properties of biochemical compositions ofmicroalgae within spectral range from 300 to 1700nm

Xingcan Li, Li Lin, Bowei Xie, MINGHUI WU, Lanxin Ma, and Jia-Yue Yang

DOI: 10.1364/AO.439477 Received 04 Aug 2021; Accepted 14 Oct 2021; Posted 18 Oct 2021  View: PDF

Abstract: The optical properties of biochemical compositions of microalgae are vital for the improvement of biosensor design,photobioreactor design, biofuel and biophotonics techniques. A combination method (DOPTM-EM) using both DOPTM(double optical pathlength transmission method) and EM (ellipsometry method) is used to acquire the opticalconstants of protein, lipid and carbohydrate of Haematococcus pluvialis, Nannochloropsis sp. and Spirulina in bothsolid and solution state within the visible and near-infrared spectral range. For different types of microalgae, therefractive index of protein and carbohydrate in solid state are similar to each other, but show an observed differencefrom lipid in solid state. The refractive index of protein and carbohydrate in solution state presents a visible distinctionin the researched spectral range. The absorption index of protein, lipid and carbohydrate in solid state for these threetypes of microalgae are close to each other in the spectral range 300-500 nm. However, an observed difference is shownin the spectral range 500-1700 nm. For ease of application, the refractive index of biochemical composition ofmicroalgae was fitted based on the Sellmeier equation. This work can provide a reference for obtaining the opticalproperties of biomaterial in high accuracy.

Etched Fiber Bragg Grating Probe using a Regular CNC Machine and a 3D Printer

Vicente Oliveira, Alexander Cascardo, Alexandre Santos, Andres Barbero, Fernando Peixoto, and Vinicius Silva

DOI: 10.1364/AO.439995 Received 11 Aug 2021; Accepted 14 Oct 2021; Posted 18 Oct 2021  View: PDF

Abstract: Etched Fiber Bragg Gratings (EFBG) have been widely employed for refractive index measurements which can be used to monitor sugar consumption during the fermentation of alcoholic beverages. EFBG are obtained by removing the cladding of a Fiber Bragg Gratings, which is traditionally performed by a chemical attack with hydrogen fluoride, an extremely hazardous corrosive substance that cause severe wounds and even death. To overcome such drawbacks, this technical note presents a simple, practical and low cost method for the diameter reduction of single mode optical fibers by mechanical polishing, employing a small scale computer numerical control (CNC) device and an ad hoc 3D-printed rod. The sensor probe obtained was tested using sucrose aqueous solutions which refractive indexes (RI) between 1.333 and 1.394, measured in an Abbe refractometer. The results show a linear shift of the Bragg wavelength in respect of RI with a correlation of 0.928.

Surface solution to correct a freeform wavefront

Rafael Gonzalez Acuña

DOI: 10.1364/AO.435758 Received 01 Jul 2021; Accepted 14 Oct 2021; Posted 18 Oct 2021  View: PDF

Abstract: In this manuscript, we present the equation to design a refractive surface such thatgiven an arbitrary wavefront the surface refracts it into a perfect spherical wave. The equationthat computes these refractive surfaces is exhaustively tested using ray tracing techniques and theperformance is as expected.

Optimized Fresnel phase hologram for ringingartifacts removal in lensless holographicprojection

Songzhi Tian, Lizhi Chen, and Hao Zhang

DOI: 10.1364/AO.441095 Received 19 Aug 2021; Accepted 13 Oct 2021; Posted 14 Oct 2021  View: PDF

Abstract: Ringing artifacts are the main noise sources in holographic projection whenquadratic phase is introduced to suppress speckle noise. In this study, the mechanisms of ringingartifacts on reconstructed images are theoretically analyzed, which illustrates the ringingartifacts are related to the bandwidth properties of the reconstructed wave field. Based on thefrequency analysis, a band-limited iterative algorithm is proposed to optimize the phasehologram in Fresnel domain. The proposed method can effectively suppress the ringing artifactsas well as the speckle noise of the Fresnel hologram by optimizing the phase distribution withbandwidth constraint. Numerical simulations and optical experiments have been performed tovalidate the proposed method for providing quality reconstructions in lensless holographicprojection.

Shaping of picosecond laser pulses with THz intensitymodulation in the infrared, visible, and ultravioletranges

Igor Kuzmin, Sergey Mironov, Mikhail Martyanov, Anatoly Poteomkin, and Efim Khazanov

DOI: 10.1364/AO.441421 Received 24 Aug 2021; Accepted 13 Oct 2021; Posted 14 Oct 2021  View: PDF

Abstract: The methods of shaping picosecond laser pulses withperiodic intensity modulation tunable in frequencyand depth are considered. The schemes for shapingmodulated pulses "in-line" and with one output portare proposed. A picosecond modulation of the timeenvelope for IR laser pulses using a polarization interferometer is demonstrated experimentally. Shaping ofmodulated laser pulses of the UV range is shown by numerical modeling. The possibility to control the modulation depth of the fourth harmonic under the combined impact of material dispersion and nonlinear conversion in a classical collinear scheme of the fourth harmonic generation without distortion of the 3D pulseshape is demonstrated.

Universal nanosecond range pulse contrastmeasurement for kJ-class petawatt laser

Fucai Ding, Xiaoping Ouyang, Xuejie Zhang, Youen Jiang, ke Hou, Xiuqing Jiang, Hua Tao, Bao-Qiang Zhu, Dean Liu, Jian Zhu, and Jianqiang Zhu

DOI: 10.1364/AO.440823 Received 26 Aug 2021; Accepted 13 Oct 2021; Posted 15 Oct 2021  View: PDF

Abstract: : A single-shot measuring apparatus with optical limiting for temporal pulse contrastof kJ-class petawatt lasers in the nanosecond range is proposed. A temporal liner filtercomposed of an electro-optical switch and a polarizer and a temporal nonlinear filter composedof cascaded SHG crystals and a dichromatic mirror are respectively used as optical limitingapparatus for contrast measurement of nanosecond and picosecond pulses to improve dynamicrange and temporal resolution. The apparatus has been applied to pulse contrast measurementsat the SG-II petawatt facility, achieving a high dynamic range of 1010 and a fast time resolutionof 107 ps in 350-ns range. This technique can also be universally applied to the limiting of themain pulse of varying pulse widths to diagnose the pre-pulses during generation andtransmission.

Simple phase retrieval method based on twointensity measurements on a single plane

Junhe Zhou and Haoqian Pu

DOI: 10.1364/AO.440645 Received 16 Aug 2021; Accepted 12 Oct 2021; Posted 14 Oct 2021  View: PDF

Abstract: In this work, a simple phase retrieval method is proposed by observing twointensity patterns on a single plane, which are generated with and without a lens. Rigoroustheoretical derivations show that the two fields constitute the Fourier transform pairs, and amodified Gerchberg-Saxton algorithm is proposed to recover the phase patterns from theFourier pairs. The proposed method does not require the intensity patterns to be measured ontwo different planes along the propagation distance, and this is quite beneficiary in a systemwith a phase tuning element like a spatial light modulator (SLM), which can form a virtuallens by creating a parabola-like phase distribution. Experiments are conducted to demonstratethe effectiveness of the proposed phase retrieval method.

Complex Study of Solitonic Ultrafast SelfSwitching in Slightly Asymmetric Dual-CoreFibers

MATTIA LONGOBUCCO, Ignas Astrauskas, Audrius Pugzlys, Nhat Dang, Dariusz Pysz, Frantisek Uherek, Andrius Baltuska, Ryszard Buczynski, and Ignác Bugár

DOI: 10.1364/AO.430631 Received 17 May 2021; Accepted 12 Oct 2021; Posted 18 Oct 2021  View: PDF

Abstract: We present a complex study of pulse energy-controlled solitonic self-switching offemtosecond pulses at the wavelengths of 1700 nm and 1560 nm in two highly nonlinear highindex contrast dual-core fibers having different levels of slight asymmetry. In the case of thefiber with higher dual-core asymmetry excited by 1700 nm pulses, the highest switchingcontrast of 20.8 dB at 40 mm fiber length was demonstrated. It was accompanied by multipleexchanges of the dominant core at the fiber output, which is a strong signature of soliton-basedswitching process. In the case of the fiber with lower dual-core asymmetry, excited by 1560nm pulses, the highest switching contrast of 21.4 dB at 35 mm fiber length was achieved witha broadband character of the switching in the spectral range of 1450-1650 nm. Bothdemonstrations represent progress in all-optical switching studies at these particularwavelengths thanks to the fiber length optimization and proper choice of the excited core.Finally, the comparison between their results reveals the requirement of higher level of dualcore symmetry for applicable C-band operation.

Automated Defect Detection and Classification forFiber Optic Coil Based on Wavelet Transform andSelf-adaptive GA-SVM

ruifeng Yang, xiaole Chen, and chenxia Guo

DOI: 10.1364/AO.437625 Received 15 Jul 2021; Accepted 12 Oct 2021; Posted 13 Oct 2021  View: PDF

Abstract: The quality monitoring of fiberoptic coil (FOC) in winding system is usuallydone manually. Aiming at the problem ofinefficient and low accuracy of manualdetection, this article is dedicated toresearching a defect detection frameworkbased on machine vision, which provides areliable method for automatic defect detectionof FOC. For this purpose, a defect detectionscheme that integrates wavelet transform andnon-local means filtering is proposed toaccurately locate the defect region. Then,based on the features constructed by waveletcoefficients, support vector machine (SVM)is used as the classifier. Additionally,self-adaptive genetic algorithm (GA) isproposed to optimize parameters of SVM toform the final classifier. Through experimentson the dataset obtained by our designedimaging system, the results show that ourmethod has good defect detectionperformance and high classification accuracy,which provides an optimal solution for theautomatic detection of FOC.

An elemental image array generation algorithm with accurate depth information for integral imaging

Yan Mao, WEIFENG WANG, xiaoyu jiang, Teng Zhang, HAIYANG YU, PEI LI, xinlei liu, and SHUAI LE

DOI: 10.1364/AO.441189 Received 23 Aug 2021; Accepted 12 Oct 2021; Posted 14 Oct 2021  View: PDF

Abstract: In integral imaging, reproducing the depth information of three-dimensional (3D) objects accurately is one of the goals of scientific researchers. Based on the existing research, this paper proposes a new elemental image array (EIA) generation algorithm, which does not need to know the depth information of the spatial scene. By dividing the distance between the display lens array (LA) and the synthetic LA equally, and comparing the variances of the pixels corresponding to the partial of the display LA at different positions, it can accurately obtain the depth information of the 3D objects, and then the value of the synthetic pixel is obtained. Thus, a new EIA with accurate depth information is generated. Finally, the proposed algorithm has been verified in experiments of both two-dimensional (2D) objects and 3D objects.

A new approach for designing thick cemented doubletlenses based on the caustic surface

Maria del Carmen Lopez, Maximino Avendaño Alejo, Gabriel Castillo-Santiago, Luis Castañeda, and Edwin Román-Hernández

DOI: 10.1364/AO.441641 Received 27 Aug 2021; Accepted 12 Oct 2021; Posted 14 Oct 2021  View: PDF

Abstract: A new method for designing thick-lens achromatic doublet based on the concept of caustic surface, inorder to correct both third and fifth-order spherical aberration is presented. We have considered twodifferent wavelengths to be brought in coincidence at the Back Focal Length instead of Effective FocalLength as it is usually done, for calculating the radii of curvature assuming predefined values for axialthicknesses and their indices of refraction for both lenses. Alternatively, we have applied Taylor’s seriesaround the optical axis and demanding to vanish the approximate caustic surface, we obtain the values forthe conic constants, which reduce at third and fifth-order spherical aberration. Two designs for cementeddoublets will be provided assuming that the lenses are cemented. Finally, we propose a method to qualitatively test doublet lenses by using null screens, considering to place the detection plane at arbitrarypositions.

Some lens structural performance displays

Jose Sasian

DOI: 10.1364/AO.442880 Received 13 Sep 2021; Accepted 12 Oct 2021; Posted 14 Oct 2021  View: PDF

Abstract: Some useful displays that provide information about the performance of lens systems are presented and discussed. They are useful for comparing lenses, for identifying problematic lens elements, and for lens desensitizing and optimizing. An imaging simulation of a square wave is also presented to complement MTF plots.

A frequency quintupled laser at 308 nm foratomic physics applications

Maya Bueki, David Röser, and Simon Stellmer

DOI: 10.1364/AO.438793 Received 31 Aug 2021; Accepted 12 Oct 2021; Posted 14 Oct 2021  View: PDF

Abstract: We report on the development of a compact continuous-wave frequency-quintupled laser at 308 nm which is based on a fiber laser operating in the telecom band. Three consecutive frequency conversion stages in nonlinear crystals are employed to achieve a 2ν+3ν = 5ν frequency mixing. The performance of the laser system is demonstrated by linear absorption spectroscopy of a narrow intercombination line in zinc.

An efficient dynamic control method of lightpolarization using single phase-only liquid-crystal-onsilicon spatial light modulators (LCOS SLMs) for opticaldata storage

Daping Chu, jintao hong, and Jin Li

DOI: 10.1364/AO.443205 Received 13 Sep 2021; Accepted 12 Oct 2021; Posted 14 Oct 2021  View: PDF

Abstract: The technology of five-dimensional (5D) optical data storage in transparent materials paves a promising way tounlimited lifetime data storage for future cloud. Phase-only liquid-crystal-on-silicon spatial light modulator (LCOSSLM) has already exhibited its potential for this application in tailoring ultrafast laser writing beams for 5D opticaldata storage. A phase-only LCOS SLM can generate arbitrary data patterns by using diffractive holographic imagingfor the data writing light beam generation. However, the polarization control of the output holographic image isstill achieved by using an external polarization modulator, which leads to complications, bulkiness, and largedelays in current methods. In this paper, we presented an efficient phase and polarization modulation methodthrough a compact system based on a single phase-only LCOS SLM to simultaneously control both the holographicimage and its polarization state. The proposed method utilizes two-polarization-components coding in conjunctionwith a polarization components rotation technique in a compact system. Using this polarization rotation technique,two light components can be independently coded by separately using two holograms on two halves of LCOS SLM.We experimentally construct a proof-of-concept prototype of the compact system, and the effectiveness of thesystem has been experimentally verified.

M × N electrically controlled optofluidic matrixswitch

Jing Wan, MINGRUI GUO, WENZHI YUAN, XU ZHU, YIJING CHEN, and SHUO Wei

DOI: 10.1364/AO.440876 Received 23 Aug 2021; Accepted 12 Oct 2021; Posted 14 Oct 2021  View: PDF

Abstract: A completely non-blocking M × N electrically controlled optofluidic matrix switchis proposed. It uses a 1 × 3 optical switch with the V-shaped microchannel as the switchingunit. Its light paths and output ports are selected by a micro-actuator matrix and a controlcircuit. There are few reports of optofluidic matrix switches. Here the given electrostaticmicro-actuator and the basic switch structure provide an effective feasible manner for thematrix switch due to the simple and compact structure as well as the operation style. Theimpacts of microchannels and intersecting waveguides on the switch performance arediscussed, and multiple optimization schemes are proposed to reduce the insertion lossefficiently and significantly. The researching results indicate that the M × N matrix switch hasthe advantages of good matrix controllability, simple structure, wide waveband (400 – 1700nm), negligible polarization dependent loss, small insertion loss and low crosstalk. For 1550nm wavelength, the insertion loss of a 2 × 6 matrix switch is about 0.17 - 0.55 dB and themaximum crosstalk is less than -26.8 dB. In addition, the performance parameters of a 4 × 8matrix switch are given and compared with other reported matrix switches. The proposed M ×N matrix switch solves the problem on large insertion loss of general optical matrix switchesand can be expanded to a large-scale matrix switch. Moreover, the design of multiple outputports has more flexible applications in systems with multiple branch optical paths andnetwork-nodes.

Photonic generation of dual-chirp waveform with anoptoelectronic oscillator based on stimulated Brillouinscattering

Weinan Zhou, di wang, cong du, Yujiao Ding, and Wei Dong

DOI: 10.1364/AO.440925 Received 19 Aug 2021; Accepted 12 Oct 2021; Posted 14 Oct 2021  View: PDF

Abstract: A photonic method to generate dual-chirp microwave waveform (DCMW) is proposed and demonstratedby utilizing a stimulated Brillouin scattering (SBS) based optoelectronic oscillator (OEO) and a frequencyscanning laser source (FSLS). There are no radio frequency (RF) sources and Mach-Zehnder modulatorsin the proposed structure, which makes the system simple and stable. In the experiment, an alternativescheme is utilized to replace the FSLS and the DCMW signal with a central frequency of 4.69GHz is generated, the bandwidth of the generated DCMW signal up to 7GHz and the chirp rate up to ±5.3GHz/µs.The autocorrelation and the ambiguity function of the generated signal are also investigated, which showgood performance of pulse compression and reduction of range-Doppler coupling.

Highly Coherent and Multi-Octave Mid-InfraredSupercontinuum Generations in aReverse-Strip AlGaAs Waveguide with theThree Zero-Dispersion Wavelengths

Jinhui Yuan, Yujun Cheng, Jintao Lai, Chao Mei, Xian Zhou, Qiang Wu, Bin Liu, Binbin Yan, Kuiru Wang, Chongxiu Yu, and Xinzhu Sang

DOI: 10.1364/AO.440682 Received 17 Aug 2021; Accepted 11 Oct 2021; Posted 13 Oct 2021  View: PDF

Abstract: In this paper, a reverse-strip AlGaAs waveguide with the three zero-dispersionwavelengths (ZDWs) is designed. The corresponding three ZDWs are located at 3.74, 6.56,and 8.89 μm, respectively. The nonlinearity coefficient of the proposed reverse-strip AlGaAswaveguide is calculated as 2.09 W-1m-1 at wavelength 4.9 μm. The effects of the pump pulseparameters, waveguide length, and noise coefficient on the nonlinear dynamics of thesupercontinuum (SC) generation are investigated. When the hyperbolic secant pump pulsewith wavelength of 4.9 μm, peak power of 900 W, and duration of 100 fs is launched into theproposed waveguide and propagated after a 3 mm length, the highly coherent andmulti-octave mid-infrared (MIR) SC spanning from 2.2 to 14.5 μm (more than 2.7 octaves, at-40 dB level) is generated. Finally, a possible fabrication process of the reverse-strip AlGaAswaveguide is introduced. Our research results have important applications in the MIRphotonics, MIR spectroscopy, optical precision measurement, etc.

Fast on-line Rubidium DPAL atomic concentrationmeasurement by 420nm probe laser

Huizi Zhao, Hongyan Wang, Hao Tang, Liang Li, Zining Yang, Weiqiang Yang, Kai Han, and Xiaojun Xu

DOI: 10.1364/AO.440435 Received 16 Aug 2021; Accepted 11 Oct 2021; Posted 14 Oct 2021  View: PDF

Abstract: The alkali atom concentration plays an important role in the performance of the diode pumped alkali vapor lasers(DPALs). At the Rubidium DPAL operational region, the alkali concentration is as high as 1013-1014cm-3, which is “opticallythick”, or opaque, for the 780nm or 795nm doublet D-lines when the traditional scanning absorption spectrum method isused for concentration measurement. To solve this problem, we proposed to use a probe laser of 420nm, whichcorresponds to the 52S1/2 to 62P3/2 transition and has lower absorption cross-section compared with the D-line doublet.Due to the moderate absorption strength at a fixed 420nm probe wavelength, we realized fast and on-line measurementof the Rb concentration in a real-running DPAL. By combined with the quasi-two level model, we further gave thepopulation distribution in the lower three energy levels. The fast and on-line diagnostic method could be well applied inDPAL’s concentration measurement, and reveal the dynamics of the laser performance.

Research on lunar laser ranging based on2 hundred hertz repetition frequency

Gao Tian-quan, ZHOU LIXIANG, ZHANG CAISHI, Hongchao Zhao, WU XIANLIN, and LI MING

DOI: 10.1364/AO.442263 Received 01 Sep 2021; Accepted 11 Oct 2021; Posted 14 Oct 2021  View: PDF

Abstract: High repetition rate lunar laser ranging (LLR) has great prospect and significance. We have successfully obtained the effective echo signals of the all five corner-cube reflectors (CCRs) on the lunar surface by using the 100Hz repetition rate. This method can effectively improve the detection ability, but it also has some defects, such as the main wave and echo signals overlap. In this paper, the frequency selection and signals overlap are theoretically analyzed. The results show that the repetition frequency can reach 7.5MHz when the target prediction accuracy is up to ±10m. In the experiment, using of high-repetition rate pulse laser can obtain CCRs detail information, such as the column number of CCRs, so it will prove that the effective echo signals of LLR are reflected by the CCRs. Finally, we propose to use the resolved data to calculate the precision of inner coincidence, the accuracy can reach the order of millimeter.

Dissolved gas sensing using an anti-resonant hollow core optical fiber

Jason Kapit and Anna Michel

DOI: 10.1364/AO.439787 Received 05 Aug 2021; Accepted 11 Oct 2021; Posted 14 Oct 2021  View: PDF

Abstract: Sensors that measure dissolved gases directly are needed for environmental, industrial, and biomedical applications. Here we present a hollow core fiber optic sensor capable of measuring dissolved methane gas in liquids using only nanoliters of sample gas. The sensor is based on an anti-resonant hollow core fiber combined with a permeable capillary membrane inlet which extracts gas from the liquid for analysis. Using a small capillary inlet for gas extraction is only possible due to the small amount of sample gas needed for analysis, and it presents new possibilities for dissolved gas analysis in a simple, robust, and compact sensor configuration. We demonstrate the sensing technique using wavelength modulation spectroscopy and measure methane dissolved in water with a 1σ lower detection limit of 0 ppm, a resolution of 45 ppb, and a response time of ~8 minutes.

Degree of Polymerization Mapping Raman FeatureExtraction of Oil-Paper Insulation Aging Based onQuadratic Mutual Information

Jian Fang, Weigen Chen, Dingkun Yang, Zhuang Yang, and Ruimin Song

DOI: 10.1364/AO.438781 Received 26 Jul 2021; Accepted 11 Oct 2021; Posted 14 Oct 2021  View: PDF

Abstract: To assess the aging status of oil-paper insulation, accelerate aging experiment has been executed. Ramanspectroscopy, a nondestructive detection method, with access to component identification and fault diagnosis, is used to analyzethe aging of oil-paper insulation. The degree of polymerization mapping Raman feature of oil-paper insulation aging wasextracted based on the concept of quadratic mutual information. By analyzing the relationship between the extracted featureand the degree of polymerization of samples at different aging degrees, the feasibility of reflecting the aging degree of oil-paperinsulation is determined. By load analysis, the chemical correlation between the extracted feature and oil-paper insulation agingis clarified. For 8 test samples, the prediction error of degree of polymerization based on Raman features is less than 50. Theresults show that the feature extracted in this paper is helpful to realize the Raman spectrum diagnosis of oil-paper insulationaging.

Learning scene and blur model for active chromaticDepth From Defocus

Benjamin Buat, Pauline Trouvé, Frederic Champagnat, and Guy Le Besnerais

DOI: 10.1364/AO.439139 Received 30 Jul 2021; Accepted 11 Oct 2021; Posted 14 Oct 2021  View: PDF

Abstract: In this paper we propose a new monocular depth estimation algorithm based on local estimation of defocus blur, approach referred to as Depth from Defocus (DFD). Using a limited set of calibration images, wedirectly learn image covariance which indeed encode both scene and blur (i.e. depth) information. Depthis then estimated from a single image patch using a maximum likelihood criterion defined using thelearned covariance. This method is applied here within a new active DFD method using a dense texturedprojection and a chromatic lens for image acquisition. The projector adds texture for low textured objects- which is usually a limitation of DFD - and the chromatic aberration increases the estimated depth rangewith respect to conventional DFD. We provide here quantitative evaluations of the depth estimation performance of our method on simulated and real data of fronto-parallel untextured scenes. The proposedmethod is then qualitatively experimentally evaluated on 3D-printed benchmark.

See-through aerial display using dihedralcorner reflector array and hologram mirrors

Isamu Nakao, Takumi Sakamoto, and Masahiro Yamaguchi

DOI: 10.1364/AO.440183 Received 16 Aug 2021; Accepted 10 Oct 2021; Posted 12 Oct 2021  View: PDF

Abstract: In this study, we intend a novel aerial display system that combines volumehologram mirrors with a dihedral corner reflector array (DCRA). The suggested aerial displayhas the see-through capability, which allows a new design of aerial displays by combining withother types of displays. Furthermore, the virtual image, which frequently disrupts theobservation of aerial images, can be suppressed by the Bragg condition of the volume hologram.The color dispersion in the holograms is efficiently compensated by employing the features ofthe DCRA. The findings of preliminary experiments are demonstrated using a DCRA deviceand full-color hologram mirrors.

Alternative constraints for improved multiplane hologram generation

Alejandro Velez, John Barrera Ramírez, and Roberto Torroba

DOI: 10.1364/AO.439708 Received 05 Aug 2021; Accepted 09 Oct 2021; Posted 12 Oct 2021  View: PDF

Abstract: In this work, we introduce a modified hologram plane constraint to improve the accuracy of the global Gerchberg-Saxton (GGS) algorithm used for multiplane phase-only hologram generation. This constraint consists of a modified phase factor that depends on the amplitude of the field in the hologram plane. We demonstrate that this constraint produces an increase in the mean correlation coefficient between the reconstructed planes from a multiplane hologram and the corresponding amplitude targets for each plane. Furthermore, this constraint can be applied together with a mixed constraint in the reconstruction planes, leading to a more uniform and controllable reproduction of a target intensity distribution. To confirm the validity of our proposal, we show numerical and experimental results for multiplane holograms with 6 discrete planes, using both high and low contrast targets. For the experimental results, we implement a holographic projection scheme based on a phase-only spatial light modulator.

3D image reconstruction of terahertz computedtomography at sparse angles by total variationminimization

Dayong Wang, Ran Ning, Gaochao Li, Jie Zhao, Yunxin Wang, and Lu Rong

DOI: 10.1364/AO.440847 Received 17 Aug 2021; Accepted 09 Oct 2021; Posted 12 Oct 2021  View: PDF

Abstract: Continuous-wave terahertz computed tomography (THz-CT) is an important three-dimensional (3D) imaging method forprobing the profile and inner properties of sample’s structure. we applied the total variational (TV) minimizationiterative algorithm to squeeze 75% data acquisition time of THz-CT without the loss of reconstruction fidelity. Theimaging system is built based on a 278.6 GHz avalanche diode source. A zero-order Bessel beam is generated by an axicon,for which the intensity profile remains essentially propagation invariant within the non-diffracting zone. Theeffectiveness of the proposed method is verified by using three optically opaque objects. The reconstruction results showthat the TV-minimization algorithm can effectively suppress noise, artefacts and shape distortion created in sparse anglereconstruction.

A method for tilt correction of calibration lines in highresolution spectra

Tanya Das and Ravinder Banyal

DOI: 10.1364/AO.434198 Received 21 Jun 2021; Accepted 09 Oct 2021; Posted 12 Oct 2021  View: PDF

Abstract: Technological advancement has led to improvement in the design capabilities of astronomical spectrographs, allowing for high precision spectroscopy, thereby expanding the realms of observational astronomy. High-resolution spectrographs use Echelle grating that operates in higher orders, giving more detailed spectra. Often, curvature and tilted lines are observed in the spectra, arising due to the designtrade-offs of the respective spectrographs. Removal of these artifacts can help avoid wrong flux calculation and line centroid position misinterpretation, which can aid in a better prediction of the wavelengthcalibration model. In this paper we present a post-processing technique that we developed to correct theobserved curvature and tilt in the spectra. We have demonstrated the correction technique on Fabry-Perotand Th-Ar calibration spectra obtained from Hanle Echelle Spectrograph (HESP), Magellan Inamori Kyocera Echelle (MIKE) spectrometer and X-shooter spectrograph.

Passively Mode-locked Thulium-doped FiberLaser Based on Saturable Absorption of CarbonNanofibers

FANG Wang, Dongfang Lan, Xuenan Zhang, and Tonglei Cheng

DOI: 10.1364/AO.442979 Received 13 Sep 2021; Accepted 09 Oct 2021; Posted 14 Oct 2021  View: PDF

Abstract: As a new type of carbon-based material, carbon nanofibers (CNFs) have attractedmuch attention due to their unique physical structure and optical properties. In this paper, weproposed the application of CNFs as the saturable absorber (SA) and established a passivelymode-locked thulium-doped fiber laser (TDFL) for verification. By mixing sodiumcarboxymethyl cellulose (CMC) solution with CNFs, CNF SA was prepared, the nonlinearityof which was tested as follows: the modulation depth was ~1.3% and the saturation intensitywas 18 MW/cm2. By inserting the CNF SA into the TDFL ring cavity, mode-locked laserpulses of a central wavelength of 1954.47 nm and a 3 dB bandwidth of 5.93 nm wereobtained. The spectral pulse width was 1.31 ps, the repetition frequency was 32.68 MHz, andthe signal-to-noise ratio (SNR) was calculated to be ~57 dB. To our knowledge, this is thefirst time that CNFs have been reported as SA for mode-locked lasers in the 2 µm wavelengthregion. Our work provides a new reference for using carbon-based materials in the realizationof ultrafast lasers, and the proposed CNFs are highly advantageous in the development ofultrahigh-speed optical modulators and next-generation high-performance nonlinear photonicdevices.

Defocused binary fringe phase error modelingand compensation using depth-discreteFourier series fitting

Jingcheng Hu, Shaohui Zhang, Yao Hu, and Qun Hao

DOI: 10.1364/AO.440408 Received 16 Aug 2021; Accepted 08 Oct 2021; Posted 14 Oct 2021  View: PDF

Abstract: The binary defocus fringe projection is an widely adopted way increasing the speed offringe projection profilemotry. However, the projected patterns may deviate from idealones at some depthes. In this paper, we proposed a theoretical model and a correspondingcompensation method to explain and calibrate the phase error of defocuse-projectedpatterns. We first low-pass filter the projected patterns at different depthes to obtaincorresponding ideal ones. Then, we calibrate the model coefficients based on the errorsbetween the original and ideal fringe patterns. The calibrated phase error model can beused to compensate the phase error at arbitrary depthes within the calibration volume.Experiments are conducted to verify the feasibility and performance of the proposedmethod.

DBDNet for denoising in ESPI wrapped phase patternswith high density and high speckle noise

Jianming Li, Chen Tang, Min Xu, zirui fan, and Zhenkun Lei

DOI: 10.1364/AO.442293 Received 01 Sep 2021; Accepted 08 Oct 2021; Posted 14 Oct 2021  View: PDF

Abstract: In this paper, we propose a dilated-blocks based deep convolution neural network, named DBDNet for denoising inESPI wrapped phase patterns with high density and high speckle noise. In our method, the proposed dilated-blockshave a specific sequence of dilation rate and a multilayer cascading, fusion structure, which can better improve theeffect of speckle noise reduction especially for phase patterns with high noise and high density. Furthermore, wehave built an abundant training dataset with varieties of densities and noise level to train our network, thus, thetrained model has a good generalization and can denoise ESPI wrapped phase in various circumstances. Thenetwork can get the denoised results directly and does not need any pre-process or post-process. We test ourmethod on one group of computer-simulated ESPI phase patterns and one group of experimentally obtained ESPIphase patterns. The test images have a high degree of speckle noise and different density. We compare our methodwith two representative methods in spatial domain, frequency domain, named OCPDE, WFLPF and a method basedon deep learning, named FFDNet. The denoising performance has been evaluated quantitatively and qualitatively.The results demonstrate that our method can reduce high speckle noise and restore the dense areas of ESPI phasepatterns, and get better results than the compared methods. We also apply our method to a series of phase patternsfrom a dynamic measurement and get successful results.

Simultaneous measurement of liquid-filmthickness and solute concentration ofaqueous solutions of two urea derivativesusing NIR absorption

Marc Lubnow, Thomas Dreier, Christof Schulz, and Torsten Endres

DOI: 10.1364/AO.440465 Received 13 Aug 2021; Accepted 08 Oct 2021; Posted 12 Oct 2021  View: PDF

Abstract: We present a method to simultaneously measure the film thickness and individualconcentrations of two urea derivates (urea CH4N2O and dimethylurea C3H8N2O) mixed in anaqueous solution at constant temperature using near-infrared (NIR) absorption at multiplespecific wavelengths. Fourier transform infrared (FTIR) spectra of aqueous mixtures of ureaand dimethylurea solutions were recorded in the 1250–2500 nm wavelength range in thin-layerquartz cuvettes at room temperature. The spectra reveal suitable detection wavelengths, i.e.,1450, 1933, 2200, and 2270 nm, for which both the absorption coefficient and its variation withthe species concentration are large enough to achieve satisfactory detection sensitivity andselectivity. For validation measurements, samples were prepared in thin-layer quartztransmission cells with known path lengths and mixture compositions in the range 100–1000 µm and 0–40 wt.%, respectively. Film thickness and mass fractions of both species weredetermined from measured absorbance ratios in the determined characteristic wavelengthbands.

Adaptive Color Correction and Detail Restoration for UnderwaterImage Enhancement

quan wang, Chengtao Cai, Weidong Zhang, Peitong Li, and Baiyu Xin

DOI: 10.1364/AO.433558 Received 08 Jun 2021; Accepted 08 Oct 2021; Posted 21 Oct 2021  View: PDF

Abstract: Underwater images have different color cast due to different attenuation conditions, such as bluish, greenish and yellowish.In addition, due to floating particles and special illumination, underwater images will have problems such as the lack of detail andunnecessary noise. To handle the above problems, this paper proposes a new three-step adaptive enhancement method. For the firststep, adaptive color correction. the three channels are adjusted based on the intermediate color channel, which is calculated byconsidering the positional relationship of the histogram distribution. For the second step, denoise and restore details. We firstlytransform the space to HSV, a detailed restoration method based on the edge-preserving decomposition is proposed to restore the lostdetail while removing the influence of some noise. For the third step, improve the global contrast. Still in the HSV space, a simplelinear stretch strategy is applied to the Saturation channel. Experiments on the standard underwater image enhancement benchmarkdataset has proved that our method yields more natural colors and more valuable detailed information than several state-of-the-artmethods. Besides, our method also improves the visibility of underwater images captured by low-light scenes and different hardwarecameras.

High efficiency broadband polarizationindependent binary fused silica gratings forlarge astronomical telescopes

Yanan Wang, Zhiwen Chen, Maoqi Cai, Keqiang Qiu, Zhengkun Liu, Qingfeng Zhu, and Yilin Hong

DOI: 10.1364/AO.439287 Received 02 Aug 2021; Accepted 07 Oct 2021; Posted 12 Oct 2021  View: PDF

Abstract: The binary fused silica gratings (BFSGs) with high diffraction efficiency arepresented for large astronomical ground-based telescopes. Calculations demonstrate that theBFSGs could obtain high diffraction efficiency in a wider wavelength range and AOI (angle ofincident) range compared with volume phase holographic gratings (VPHGs). Several gratingswith a size of 60 mm×60 mm have been fabricated by holographic lithography and reactive ionbeam etching (RIBE) technology. The measured peak diffraction efficiency reaches 94% andresults show that there are 130 nm wavelength bandwidth and 12° AOI bandwidth, in whichdiffraction efficiency is higher than 70%. The stray light causes the diffraction efficiency todecrease by about 0.48%. All measurements have indicated good consistency with thesimulation results.

Plasmonic Octamer Objects: Reversal of Near-field Optical Binding Force without theAid of Background

Rafsan Jani, Saikat Das, Fatematuz Zahura, Haniful Islam, Golam Dastegir Al-Quaderi, and Mahdy Mahdy

DOI: 10.1364/AO.435982 Received 15 Jul 2021; Accepted 07 Oct 2021; Posted 12 Oct 2021  View: PDF

Abstract: In recent years, near-field optical binding force has gained a lot of interest in the field of opticalmanipulation. The reversal of near field binding force, a new kind of optical manipulation, has so farbeen investigated mostly between dimers and in a very few cases among tetramers by utilizing thehelp of suitable substrates or backgrounds. Until now, no known way to control the near-field opticalbinding force among octamer configuration has been found. In this article, we have proposed aplasmonic (silver) octamer configuration where we have demonstrated the control and reversal(attraction and repulsion) of near-field optical binding force of octamers by illuminating the systemwith a TM polarized Bessel beam. The control of the binding force and its reversal has been explainedbased on the polarization and gradient forces created by the Bessel beam. As the aid of thebackground or substrate is not required, our proposed simplified approach has the potential ofopening up of novel ways for the manipulation of multiple particles. Our investigation also implicitlysuggests: for the future research on controlling the reversal of near-filed optical binding force ofmultiple-particles, Bessel beams can be the appropriate choice instead of the plane waves.

Fiber Fabry-Pérot Astrophotonic CorrelationSpectroscopy for Remote Gas Identificationand Radial Velocity Measurements

Ross Cheriton, Adam Densmore, Suresh Sivanandam, Ernst de Mooij, Pavel Cheben, Dan-Xia Xu, Jens Schmid, and Siegfried Janz

DOI: 10.1364/AO.430540 Received 12 Jul 2021; Accepted 07 Oct 2021; Posted 14 Oct 2021  View: PDF

Abstract: We present a novel remote gas detection and identification technique based oncorrelation spectroscopy with a piezoelectric tunable fiber-optic Fabry-Pérot filter. We showthat the spectral correlation amplitude between the filter transmission window and gasabsorption features is related to the gas absorption optical depth, and that different gases canbe distinguished from one another using their correlation signal phase. Using a previouslycaptured telluric-corrected high-resolution near-infrared spectrum of Venus, we show the radialvelocity of Venus can be extracted from the phase of higher order harmonic lock-in signals.This correlation spectroscopy technique has applications in the detection and radial velocitydetermination of weak spectral features in astronomy and remote sensing. We experimentallydemonstrate remote CO2 detection system using a lock-in amplifier, fiber-optic Fabry-Pérotfilter, and single channel avalanche photodiode.

Partial-Field Illumination Ophthalmoscope: improvingthe contrast of a camera-based retinal imager

Léa Krafft, Elena Gofas-Salas, Yann Lai-Tim, Michel paques, Laurent Mugnier, Olivier Thouvenin, Pedro Mece, and Serge Meimon

DOI: 10.1364/AO.428048 Received 20 Apr 2021; Accepted 06 Oct 2021; Posted 14 Oct 2021  View: PDF

Abstract: Effective and accurate in-vivo diagnosis of retinal pathologies requires high performance imaging devices, combining a large field of view and the ability to discriminate the ballistic signal from the diffusebackground in order to provide a highly contrasted image of the retinal structures. Here, we have implemented the Partial-Field Illumination Ophthalmoscope, a patterned illumination modality, integrated toa high pixel rate adaptive optics full-field microscope. This non-invasive technique enables us to mitigatethe low signal-to-noise ratio, intrinsic of full-field ophthalmoscopes, by partially illuminating the retinawith complementary patterns to reconstruct a wide field image. This new modality provides an imagecontrast spanning from the full-field to the confocal contrast, depending on the pattern size. As a result,it offers various trade-offs in terms of contrast and acquisition speed, guiding the users towards the mostefficient system for a particular clinical application.

Calibration of intrinsic cameraparameters with a conic and its asymptotes

Xiang Liu, Yue Zhao, and Xiaomei Kou

DOI: 10.1364/AO.434156 Received 15 Jun 2021; Accepted 06 Oct 2021; Posted 08 Oct 2021  View: PDF

Abstract: This study proposes a method for camera calibration using the properties of conicasymptotes, which are combined with the projective invariance, images of the asymptotes,and image of the absolute points to obtain the vanishing line. Constraints for the image of theabsolute conic are obtained from a set of orthogonal vanishing points, the homography, andimages of the circular points. The proposed calibration algorithm is compared with severalother research methods and its effectiveness and robustness are verified within a certain errorrange from the results of simulated and real experiments for different eccentricities withoutloss of generality.

Research on the Abnormal Distortion and Controlof Gaussian Beam in Circular Aperture Diffraction

qizhen wang and Xia P

DOI: 10.1364/AO.438086 Received 16 Jul 2021; Accepted 06 Oct 2021; Posted 08 Oct 2021  View: PDF

Abstract: On the basic theory of wave diffraction, the Zernike polynomials have been builtby using the aberration function method to study the abnormal distortion and control of laserbeams with Gaussian distribution in circular aperture diffraction. In the process of research, itis found that under the different deformation degrees of Zernike polynomial in Z7 and Z8, thediffracted beam will produce comets optical aberrations, further causing the diffraction beam todeviate from the Gaussian profile and produce anomalous distortion. By optimizing the imagingposition and adding phases compensator, the imaging errors such as aberration and distortionon the structure can be effectively controlled and the beam transmission quality can be improved.

Radiative Effects of Atmospheric Aerosols on theAverage Channel Capacity of Free-Space OpticalCommunication Systems

Sunilkumar K, Anand Sarma, S Satheesh, Krishnamoorthy K, and Ilavazhagan G

DOI: 10.1364/AO.438520 Received 22 Jul 2021; Accepted 05 Oct 2021; Posted 06 Oct 2021  View: PDF

Abstract: Free-Space Optical communication (FSO) systems employ unguided light beamspropagating through the atmosphere to carry a large volume of data. The reliability of such datatransfer can be hampered by various atmospheric effects. Based on an analytical model of aDifferential Phase-Shift Keying FSO system through exponentiated Weibull turbulence, weinvestigate the effectiveness of beamwidth optimization and improved beam alignment along withaperture averaging on the average channel capacity. Our results show significant signaldeterioration produced due to the aerosol-induced optical turbulence, which substantially shadowsthe performance gain achieved through beamwidth optimization. Strong aerosol-inducedatmospheric heating and the consequent enhanced optical scintillations result in reduction of thechannel capacity by as much as 50 % of its value when these effects are not considered ornegligible. FSO systems are more resilient to aerosol-induced optical turbulence when thenormalized beamwidth is less, and the average channel capacity can be significantly improved byimproved beam alignment. These variations are weakly dependent under poor transmitter –receiver alignment conditions. Furthermore, the receiver aperture has a strong control on the linkperformance. While FSO systems with higher magnitude of normalized beamwidth haveimproved performance under all aperture diameter conditions; for a given beam configuration,large aperture diameter ensures a significant improvement in the link performance due to reductionin effects of scintillations.

Blind modulation format identification based onimproved-PSO clustering in 2D Stokes plane

Ruqing Zhao, weibin sun, Hengying Xu, Chenglin Bai, Xue Tang, Zhiguo Wang, Lishan Yang, Lingguo Cao, Yanfeng Bi, Xinkuo Yu, Wenjing Fang, Baokun Li, Tanglei Zhou, and Peiyun Ge

DOI: 10.1364/AO.439749 Received 05 Aug 2021; Accepted 05 Oct 2021; Posted 06 Oct 2021  View: PDF

Abstract: Blind modulation format identification (MFI) is indispensable for correct signal demodulation and optical performance monitoring (OPM) in future elastic optical network (EON). The existing MFI schemes based on clustering algorithm in Stokes space have gained good performance, while only limited types of modulation formats (MFs) could be correctly identified and the complexities are relatively high. In this work, we have proposed an MFI scheme with a lower computational complexity, which combines an improved particle swarm optimization (I-PSO) clustering algorithm with two-dimensional (2D) Stokes plane. The main idea of I-PSO is to add a new field of view on each particle and limit each particle to only communicate with its neighbor particles, so as to realize the correct judgment of the number of multiple clusters (local extrema) on the density images of s2-s3 plane. The effectiveness has been verified by 28 GBaud polarization division multiplexing (PDM)-BPSK/PDM-QPSK/PDM-8QAM/PDM-16QAM/PDM-32QAM/PDM-64QAM simulation EON systems and 28 GBaud PDM-QPSK/PDM-8QAM/PDM-32QAM proof-of-concept transmission experiments. The results show that using this MFI scheme, the minimum optical signal to noise ratio (OSNR) values to achieve 100% MFI success rate are all equal to or lower than those of the corresponding 7% forward error correction (FEC) thresholds. At the same time, the MFI scheme also obtains good tolerances to residual chromatic dispersion (CD) and differential group delay (DGD). Besides that, the proposed scheme achieves 100% MFI success rate within a maximum launch power range of -2~+6 dBm. More importantly, its computational complexity can be denoted as O(N).

Reconfigurable and dual-polarization Bragggrating filter with phase change materials

Yanqun Wang, jieying zhang, Jin HuiMin, and Peipeng Xu

DOI: 10.1364/AO.439624 Received 10 Aug 2021; Accepted 05 Oct 2021; Posted 08 Oct 2021  View: PDF

Abstract: Fully reconfigurable optical filters are indispensable building blocks to realizereconfigurable photonic networks/systems. This paper proposes a reconfigurable and dualpolarization optical filter based on subwavelength grating (SWG) waveguide operating in theBragg reflection mechanism and combined with a low-loss phase-change materialGe2Sb2Se4Te1 (GSST). Numerical simulations indicate that, for TE (TM) polarization, thepresented Bragg grating filter offers up to 20 nm (17 nm) redshift with amplitude modulationof 6 dB (0.15 dB) at 1550 nm. Using the effective medium theory, we obtained the six-levelcrystallization performance of the optical filter. The proposed optical filter has potentialapplications in wavelength-division-multiplexing (WDM) systems, optical signal processing,and optical communications.

Analytical design of an advanced Littrow- Offnerspectrometer simultaneously off-axis in meridianand sagittal plane for ultraviolet(UV) imaging withhigh performance

Su Wu, Chan Huang, Lei Yu, Hui Xue, and Jing Lin

DOI: 10.1364/AO.441758 Received 27 Aug 2021; Accepted 05 Oct 2021; Posted 12 Oct 2021  View: PDF

Abstract: A novel configuration of Littrow- Offner spectrometer with off-axis in both meridian andsagittal plane is proposed. Through theoretical analysis, the relationship between the distance of eachelement and the aperture is obtained, and the theoretical basis of sagittal plane off-axis is given. Extraaberrations are corrected by a spherical lens with a convex grating. An example of design has beenpresented with F number as 4 working in 300-500 nm according to the theory. Its spectral resolutionachieves as 0.1 nm at central wavelength, and it’s volume decreases by 14.5% compared with traditionalOffner spectrometer.

Progress on the Starshot Laser Propulsion System

Simon Worden, Wesley Green, James Schalkwyk, Kevin Parkin, and Robert Fugate

DOI: 10.1364/AO.435858 Received 05 Jul 2021; Accepted 04 Oct 2021; Posted 06 Oct 2021  View: PDF

Abstract: The Breakthrough Starshot research program envisions a laser-propelled sail that will probe our neighboring stars within a human lifetime. Starshot spacecraft weigh no more than a few grams and are accelerated by photon momentum transfer with a beam generated by a kilometer-scale, ground-based 100 GW coherent phased array laser. We describe some of the major challenges for the laser and early exploratory efforts to overcome them. While challenges remain, we find no physical or economic restrictions that would preclude such a laser for the Starshot mission.

Measurement of phase refractive indexdirectly from phase distributionsdetected with a spectrally resolvedinterferometer

Kaining Zhang, Osami Sasaki, Samuel Choi, songjie luo, Takamasa Suzuki, and Jixiong Pu

DOI: 10.1364/AO.438267 Received 29 Jul 2021; Accepted 02 Oct 2021; Posted 05 Oct 2021  View: PDF

Abstract: Phase refractive index is measured directly from an unwrapped spectral phasedistribution whose 2π ambiguity is determined by fitting the spectral phase distributionwith functions based on Cauchy’s equation. A phase refractive index of a quartz glass with20 μm thickness is measured exactly from three spectral phase distributions detected intwo different configurations of a spectrally resolved interferometer. Since there is a highpossibility that the 2π ambiguity cannot be correctly determined when there is a largedifference between a function of real refractive index and Cauchy’s equation,characteristics of the fitting are examined.

Learned holographic light transport

Kaan Aksit, Hakan Urey, and Koray Kavakli

DOI: 10.1364/AO.439401 Received 05 Aug 2021; Accepted 23 Sep 2021; Posted 27 Sep 2021  View: PDF

Abstract: Computer-Generated Holography (CGH) algorithms often fall short in matching simulations with results from a physical holographic display. Our work addresses this mismatch by learning the holographic light transport in holographic displays. Using a camera and a holographic display, we capture the image reconstructions of optimized holograms that rely on ideal simulations to generate a dataset. Inspired by the ideal simulations, we learn a complex-valued convolution kernel that can propagate given holograms to captured photographs in our dataset. Our method can dramatically improve simulation accuracy and image quality in holographic displays while paving the way for physically informed learning approaches.

Remote angular velocity measurement by cascadedrotational Doppler effect

Zhenyu Guo, Jinglin Meng, Min An, Pengxiang Cheng, Junliang Jia, Zehong Chang, Xiaoli Wang, and Pei Zhang

DOI: 10.1364/AO.438997 Received 27 Jul 2021; Accepted 21 Sep 2021; Posted 12 Oct 2021  View: PDF

Abstract: Rotational Doppler effect has attracted extensive attention, which is caused by the angular momentumand energy exchange between rotating objects and waves. However, most of the previous works used asimple rotation frame, which only made use of a single-round angular momentum and energy exchange.In this article, we propose and demonstrate the frame containing spiral phase plate cascaded with rotatingtargets to make an amplification of traditional Doppler shift, and reduce the diffusion of orbital angularmomentum modes by half, which means the distance of practical application is doubled theoretically.To the end, an experiment is carried out to verify this frame. It shows a more practical, convenient andnon-destructive method to measure the rotational speed of a remote target.

This is a test submission

Dan McDonold and Keith Jackson

DOI: 10.1364/AO.443911 Received 20 Sep 2021; Accepted 20 Sep 2021; Posted 20 Sep 2021  View: PDF

Abstract: This is a test submission

LARAMOTIONS: A conceptual study on laser networks for near-term collision avoidance for space debris in the low Earth orbit

Stefan Scharring, Heiko Dreyer, Gerd Wagner, Jürgen Kästel, Paul Wagner, Ewan Schafer, Wolfgang Riede, Christoph Bamann, Urs Hugentobler, Pawel Lejba, Tomasz Suchodolski, Egon Döberl, Dietmar Weinzinger, Wolfgang Promper, Tim Flohrer, Srinivas Setty, Igor Zayer, Andrea Di Mira, and Emiliano Cordelli

DOI: 10.1364/AO.432160 Received 01 Jun 2021; Accepted 18 Sep 2021; Posted 21 Sep 2021  View: PDF

Abstract: A conceptual study has been carried out on laser station networks to enhance Space Situational Awareness and contribute to collision avoidance in the Low Earth Orbit by high precision laser tracking of debris objects and momentum transfer via photon pressure from ground-based high-power lasers. Depending on the network size, geographical distribution of stations, orbit parameters and remaining time to conjunction, multi-pass irradiation enhances the efficiency of photon momentum coupling by 1 – 2 orders of magnitude and has the potential to eventually yield a promisingly significant reduction of the collision rate in low Earth orbit.

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