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Accepted papers to appear in an upcoming issue

Optica Publishing Group posts prepublication articles as soon as they are accepted and cleared for production. See the FAQ for additional information.

Quantifying the quality of optical vortices by evaluating their intensity distributions

Mateusz Szatkowski, Brandon Norton, Jan Masajada, and Rosario Porras Aguilar

DOI: 10.1364/AO.456106 Received 15 Feb 2022; Accepted 13 May 2022; Posted 16 May 2022  View: PDF

Abstract: Optical vortices are widely used in optics and photonics, impacting the measurements and conclusions derived from their use. Thus, it is crucial to evaluate optical vortices efficiently. This work aims to establish metrics for evaluating optical vortex quality to support the implementation procedure and, hence, provide a tool supporting research purposes andtechnological developments. We propose to assess vortex quality using the following intensity parameters: eccentricity, cross-sectional peak-to-valley, cross-sectional peak difference, and the doughnut ratio. This methodology provides a low-cost, robust, and quantitative approach to evaluating optical vortices for each specific optical technology.

Measurement of stress-induced birefringence ina thin-disk laser with full-Stokes polarimetry

Johanna Dominik, Michael Scharun, Benjamin Dannecker, Veronika Bühner, Klaus Ertel, Dominik Bauer, and Thomas Dekorsy

DOI: 10.1364/AO.457145 Received 25 Feb 2022; Accepted 13 May 2022; Posted 16 May 2022  View: PDF

Abstract: Stress-induced birefringence leads to losses in solid-state laser resonators and amplifiers withpolarized output beams. A model of stress-induced birefringence in thin disks is presented, aswell as measurements of stress-induced birefringence in a thin disk in a multi-kilowatt oscillator.A full-Stokes imaging polarimeter was developed to enable fast and accurate polarimetricmeasurements. Experimental and simulated results are in good agreement qualitatively andquantitatively and show that the polarization loss due to stress-induced birefringence is negligiblefor ytterbium-doped thin disks with a thickness around 100 µm, but becomes relevant in thickerdisks. It is concluded that stress-induced birefringence should be taken into consideration whendesigning a thin-disk laser system.

Wavelength selection of multispectral imaging for oilpalm fresh fruit ripeness classification

Minarni Shiddiq, Herman Herman, Dodi Arief, Edy Fitra, IKHSAN HUSEIN, and Sinta Ningsih

DOI: 10.1364/AO.450384 Received 23 Dec 2021; Accepted 13 May 2022; Posted 16 May 2022  View: PDF

Abstract: Multispectral imaging has been recently proposed for high-speed sorting and grading machine vision of fruits.It is a prospective method applied in yet traditional sorting and grading of oil palm fresh fruit bunches (FFB). Theripeness of oil palm FFBs determines the quality of crude palm oil (CPO). Implementation of multispectral imaging forthe task needs wavelength selection from hyperspectral datasets. This study aimed to obtain the optimum wavelengthsand used them for oil palm FFB classification based on three ripeness levels. We have selected eight optimumwavelengths using Principal Component Analysis (PCA) regression which represented the ripeness levels.

Engineering tunable terahertz radiation from anelectron bunch using graphene metasurfaces

Limei Qi, mingjing Wu, and Xiaojun Han

DOI: 10.1364/AO.456494 Received 22 Feb 2022; Accepted 12 May 2022; Posted 13 May 2022  View: PDF

Abstract: We propose an approach to generate tunable terahertz radiation from an electron bunch passing over the uniquegraphene metasurface. We not only control the frequency of the terahertz radiation, but also tune the amplitude and directionof the radiation by varying chemical potential of the graphene. Several new phenomena are observed. The radiation has thesame frequency with the resonant frequency of the graphene metasurface at the normal incidence. The radiation frequencymeets the linear relationship with the chemical potential. The radiation magnitude is inverse to the reflection magnitude andthe sum of them is close to be a constant. The strong Smith-Purcell radiation on the graphene metasurface is due to theinteraction between the electron bunch and periodic surface plasmon polaritons. The stronger of the surface plasmonpolariton is, the higher radiation magnitude is obtained. These results would provide a promising way for developing tunableradiation in the terahertz band.

Areal measurement of vibration modes ofhemispherical shell resonator bydeflectometry

Rui Zhu, Xiangchao Zhang, Shaoliang Li, Yunuo Chen, Wei Lang, and Lu Ye

DOI: 10.1364/AO.456744 Received 22 Feb 2022; Accepted 12 May 2022; Posted 13 May 2022  View: PDF

Abstract: The hemispherical shell resonator (HSR) is the core and sensitive part of ahemispherical resonator gyro. The geometrical accuracy and vibration properties of HSRdetermine the navigating performance of the system. Lack of the areal measurement methodsof the vibration modes limits the investigation of kinetic mechanism and the improvement ofnavigating performance. Consequently, an areal measurement method is developed based ondeflectometry. The blurry spots on the image plane reflected from the vibrating HSR areextracted, and the blurring trajectories are obtained by the Wiener deconvolution method. Thevibrating amplitude distribution of a standing wave mode is transformed into the swing angledistribution of the normal vectors. The parameters of vibration mode are fitted by theLevenberg-Marquardt method. This method can find widespread applications in the arealinspection of vibration modes.

Improvement of beam shaping on metasurface byeliminating the interaction between coding units

Pengwei Zhou, Bo Fang, Tianqi Zhao, and Xufeng Jing

DOI: 10.1364/AO.458232 Received 11 Mar 2022; Accepted 12 May 2022; Posted 13 May 2022  View: PDF

Abstract: The interaction between sub-wavelength elements must be considered when constructing the metasurface. Generally, onesignore the interaction between cell structures when designing metasurface optoelectronic devices, which results in a significant decrease inthe design performance and efficiency of the overall metasurface structure. In order to reduce or further eliminate the interaction betweencell structures, we propose a cell structure with borders to construct coded metasurface sequences. At the same time, we design a commonframeless cell structure to construct a traditional coding metasurface. By numerical simulation of the near field distribution and far fieldscattering characteristics of these two types of coded metasurface sequences, we find that the element structure with medium frame canattenuate the interaction between adjacent encoded particles. In the process of transmission on the encoding metasurface with frame,different encoded particles can express their transmission phase independently, and are not affected by adjacent structures, thus realizingthe low coupling coding metasurface.

Effect of Doppler shift on preamplifier DPSK receiversusing balanced detection for optical satellite networks

Liying Tan, XINLEI WANG, Sixun Cheng, Jing Ma, and Dongpeng Kang

DOI: 10.1364/AO.458555 Received 15 Mar 2022; Accepted 12 May 2022; Posted 13 May 2022  View: PDF

Abstract: Optical satellite networks using laser intersatellite links (ISLs) are recognized to be capableto satisfy the increasing broadband communication demands. However, the Doppler shift due torelative movement between satellites must be taken into consideration. A convenient method thatdoesn't require complicated mathematical calculation is adopted to derive the relationship between theresidual Doppler shift and the bit error rate (BER) for the balanced differential phase-shift keying(DPSK) detection system. It is shown that the DPSK receiver is sensitive to the residual Doppler shift,when the residual Doppler shift Δfres ∕ Rb is 0.05, 0.10, and 0.15, the BER has been reduced from a levelof 10−17 to a level of 10−14, 10−11, 10−6, respectively. The BER of a certain node pair under different linkassignment schemes is calculated in a typical multi-hop LEO constellation. The results show that thecommunication BER of the same node pair is not related to the step change of the Doppler, but relatedto the accumulation of the Doppler shift in the optical path. The results obtained here will be helpful inrouting selection, link assignment, topology design, and system compensation.

Tunable and switchable multi-wavelength randomdistributed feedback fiber laser based on SMF andcascaded Sagnac ring filter

HongGang Pan, Zhanzhi Liu, Ailing Zhang, Taotao Guo, and yaomei peng

DOI: 10.1364/AO.458792 Received 18 Mar 2022; Accepted 12 May 2022; Posted 13 May 2022  View: PDF

Abstract: A tunable and switchable multi-wavelength random distributed feedback fiber laser based on cascaded Sagnac loops isproposed and experimentally demonstrated. The random distribution feedback of the laser is provided by the Rayleighscattering generated by the single-mode fiber. The cascaded Sagnac loops act as a filter and a reflector in the half-opencavity laser. The single, dual, three and four wavelength channels can be realized by adjusting the angle of the polarizationcontroller at the pump power of 300mW. In the single, dual and three wavelength channels, the wavelength spacing canbe maintained and the laser wavelength position can be changed at the same time. The maximum wavelength tuningranges of single, dual, and three-wavelength outputs are about 4.5 nm, 2.6 nm and 1nm respectively. The proposed multiwavelength random fiber laser has the advantages of simple structure and low threshold, and has good applicationprospects in remote sensing and imaging systems.

Methodology of Designing Compact Schlieren SystemsUsing Off-Axis Parabolic Mirrors

Lingzhi Zheng, Adam Susa, and Ronald Hanson

DOI: 10.1364/AO.457151 Received 03 Mar 2022; Accepted 11 May 2022; Posted 12 May 2022  View: PDF

Abstract: Schlieren imaging is widely adopted in applications where fluid dynamics features are of interest. However, traditional Z-type schlieren systems utilizing on-axis mirrors generally require large system footprintsdue to the need to use high f-number mirrors. In this context, off-axis parabolic (OAP) mirrors providean attractive alternative for permitting the use of smaller f-number optics, but well-documented methodologies for designing schlieren systems with OAP mirrors are lacking. The present work outlines aray-tracing-based workflow applied to the design of a modified Z-type schlieren system utilizing OAPmirrors. The ray-tracing analysis evaluates the defocus and distortion introduced by schlieren optics. Theresults are used along with system size and illumination efficiency considerations to inform the selectionof optimal optical components capable of producing high-quality schlieren images while minimizing thesystem footprint. As a step-by-step demonstration of the design methodology, an example schlieren systemdesign is presented. The example schlieren design achieved an image resolution of 1.1 lp/mm at 50%contrast with a 60% reduction in system length compared to traditional Z-type systems with f/8 mirrors;distortion characterizations of the designed schlieren system showed good agreement with ray-tracingpredictions and the distortion can be corrected through image post-processing. The current work providesa systematic approach for the design of compact schlieren systems with OAP mirrors and demonstrates theutility of this underutilized option.

Laser Radius Measurement Method Based on Amplitude Jumpof Thermal Wave

Dongjin Hu, Xunpeng Qin, Zeqi Hu, Yan Zeng, and Feilong Ji

DOI: 10.1364/AO.460098 Received 04 Apr 2022; Accepted 11 May 2022; Posted 12 May 2022  View: PDF

Abstract: The laser radius should be calculated with high precision when analyzing the characteristics of Lamb wave excited bylaser in thin plate. However, the traditional methods are complex and rough to measure the radius of laser beam. The paper aimsto propose a novel convenient method for measuring the radius of laser ultrasonic beam. A laser interferometer (receiving laser)is used to receive ultrasonic signals excited by the laser to be measured (exciting laser) on the surface of a test block. Consideringthe characteristics represented by the thermal wave, positions where the receiving beam contacts and separates from the excitingbeam is determined, and the spot of radius is calculated by corresponding geometric relations. Experiments show that accuracyof the proposed method is in the order of magnitude of 0.01 mm, and the relative errors of experimental data are within 2%. Thispaper provides an alternative method for the measurement of laser beam radius, and has great significance for the application oflaser ultrasonic detection in thin plate and the calculation and analysis of dispersion curve.

A dissolved oxygen sensor based on fluorescence quenching method with optimal modulation frequency

Zhixuan Er, Ping Gong, Jian Zhou, Yiming Wang, Xiaokang Jiang, and Liang Xie

DOI: 10.1364/AO.457805 Received 09 Mar 2022; Accepted 11 May 2022; Posted 11 May 2022  View: PDF

Abstract: Measurement of dissolved oxygen (DO) in liquid samples is of vital importance inboth industrial and biomedical fields. In this paper, a DO sensor based on fluorescencequenching method has been built. The measurement principle is based on fluorescencelifetime detection, which is indicated by the phase difference between excitation light signaland fluorescence signal. The nonlinear effect of the fluorescent material has been taken intoconsideration to obtain a more accurate fitting model. The performance of the system varyingwith the modulation frequency of excitation light signals is also reported in this paper.Modulation frequency mainly affects the sensitivity and phase resolution ratio of the system.The system at the optimized modulation frequency has a good degree of fitting with 2 R valueof 0.9981 and a small relative error of 0.79%. The study presents that this kind of sensor withoptimal modulation frequency has a good performance, which can be used in many important fields.

Performance of resonant fiber-optic gyroscope basedon broadband source

shuang Liu, Lu Liu, Junyi Hu, Li hanzhao, Qingwen Liu, Huilian Ma, and Zuyuan He

DOI: 10.1364/AO.460419 Received 04 Apr 2022; Accepted 11 May 2022; Posted 11 May 2022  View: PDF

Abstract: A resonant fiber-optic gyroscope (RFOG) based on a broadband source can avoid the fundamental drawback ofcoherence detection process while possess the greater sensitivity afforded by the finesse of the fiber-optic ring resonator.In this paper, the basic operation principle is presented and demonstrated in detail, and various noise sources as wellas temperature effect encountered in this broadband source-driven RFOG are studied and analyzed. Then a combinedmodulation technique is proposed to suppress the residual backscattering noise. To further reduce the effect oftemperature transience, an asymmetric fiber ring resonator is designed. In the experiment, a bias stability of 0.01°/h issuccessfully demonstrated with a 100-meter-long fiber ring resonator of 8 cm diameter in a laboratory environmentwithout temperature control.

Phase response measurement of spatial lightmodulators based on a Shack Hartmann wavefrontsensor

Xing Zhou, Shuhai Jia, Hongqiang Yu, Zihan Lin, Huajian Zhang, and Yunlong Zhang

DOI: 10.1364/AO.458374 Received 14 Mar 2022; Accepted 10 May 2022; Posted 11 May 2022  View: PDF

Abstract: It is known that the phase response of spatial light modulators (SLMs) measured by double-beam interferometers issensitive to mechanical and environmental disturbances, this paper proposes a Shack-Hartmann wavefront sensor(SHWS) method to measure the phase response characteristics of the SLM. The results show that the phase modulationdepth measured by the proposed method is 1.7581λ, while 1.7993λ by the Twyman–Green interferometer method. Thedifference in the phase modulation depth between the two methods is only 0.0412λ, and its relative error rate is 2.29%.It proves that the phase modulation accuracy obtained by the SHWS with the lenslets of 73*73 used in this paper isequivalent to that of the Twyman–Green interferometer. Compared with the interferometer method, the SHWS method issimple, compact, robust, has good real-time performance, and is relatively vibration insensitive. In the future, the SHWSmethod will play a more important role in the detection of the SLM’s phase response.

Pulsed laser source digital holography efficiency measurements

Steven Owens, Mark Spencer, Douglas Thornton, and Glen Perram

DOI: 10.1364/AO.453344 Received 07 Jan 2022; Accepted 10 May 2022; Posted 12 May 2022  View: PDF

Abstract: In this paper, a 1064 nm pulsed laser source and a short-wave infrared (SWIR)camera are used to measure the total-system efficiency associated with a digital-holographysystem in the off-axis image plane recording geometry. At zero path-length difference betweenthe signal and reference pulses, the measured total-system efficiency (15.9%) is consistent withthat previously obtained with a 532 nm continuous-wave laser source and a visible camera[Appl. Opt. 58, G19-G30 (2019)]. In addition, as a function of temporal delay between thesignal and reference pulses, the total-system efficiency is accurately characterized by a newcomponent efficiency, which is formulated from the ambiguity function. Even with multi-modebehavior from the pulsed laser source and substantial dark-current noise from the SWIRcamera, system performance is accurately characterized by the resulting ambiguity efficiency.

Silicon Photonics 2×2 Trench Coupler Design andFoundry Fabrication

Hiva Shahoei, Ifeanyi Achu, Evan Stewart, Unaiza Tariq, William Oxford, Mitchell Thornton, and Duncan McFarlane

DOI: 10.1364/AO.453464 Received 12 Jan 2022; Accepted 10 May 2022; Posted 13 May 2022  View: PDF

Abstract: A 2×2 photonic coupler is realized at the intersection of two 480nm × 220nm Silicon on Insulator (SOI) ridgewaveguides. The designed 2×2 coupler is simulated in both HFSS and Lumerical and shows an equal split of an inputsignal into a transmitted and a reflected signal for a 45 degree, ~100 nm SiO2 filled trench. The principle of operationof the coupler is frustrated total internal reflection (FTIR). Thus, this behavior is reasonably flat across wavelengthwhich is confirmed by Lumerical simulations and by experiment in this paper. Also, it has been shown that this couplerhas a flat behavior across trench thickness for the chosen geometry and material system which makes it insensitive tofabrication variation and resolution. We are interested to make this coupler a part of silicon photonic foundry processdevelopment kits. Therefore, fabrication is done at the AIM Photonics foundry to study the performance in the contextof the foundry’s design rules and process flow of the foundry. Good agreement between theory and experiment isreported herein. A 2×2 trench coupler is, when operated in the single photon or quantum regime, an integratedphotonic realization of a Hadamard gate.

Viscosity of fluoride glass fibers for fused componentsfabrication

Edith Ducharme, Stephane Virally, Rodrigo Itzamnà Becerra-Deana, Caroline Boudoux, and Nicolas Godbout

DOI: 10.1364/AO.455528 Received 04 Feb 2022; Accepted 09 May 2022; Posted 10 May 2022  View: PDF

Abstract: Fluoride glasses show great promise for mid-IR fiberbased applications. Their brittleness and low glass transition temperature have thus far been obstacles towardsobtaining low-loss fused components. Here, we suggest a simple method to measure glass viscosity over arange of process temperatures of interest for fused coupler fabrication. We achieved tapers of inverse taper ratio (ITR) 0.12 in multimode fluoroindate fibers. Taperswith loss < 0.1 dB at ITR 0.3 and no visible defects werefabricated with high repeatability. This work paves theway towards low-loss fused optical coupler in fluorideglass fiber.

Photonics-assisted joint communication-radar systembased on a QPSK-sliced linearly frequency-modulatedsignal

Shi Wang, Liang Ding, and Yang Chen

DOI: 10.1364/AO.456287 Received 16 Feb 2022; Accepted 09 May 2022; Posted 10 May 2022  View: PDF

Abstract: A photonics-assisted joint communication-radar system is proposed, by introducing a quadrature phase-shift keying(QPSK)-sliced linearly frequency-modulated (LFM) signal. An LFM signal is carrier-suppressed single-sidebandmodulated onto the optical carrier in one dual-parallel Mach–Zehnder modulator (DPMZM) of a dual-polarization dualparallel Mach–Zehnder modulator (DPol-DPMZM). The other DPMZM is biased as an IQ modulator to implement QPSKmodulation on the optical carrier. The polarization orthogonal optical signals from the DPol-DPMZM are furthercombined and detected in a photodetector to generate the QPSK-sliced LFM signal. The QPSK-sliced LFM signal is used torealize efficient data transmission and high-performance radar functions including ranging and imaging. An experimentis performed. Radar range detection with an error of less than 4 cm, ISAR imaging with a resolution of 14.99 cm × 3.25cm, and communication with a data rate of 105.26 or 210.52 Mbit/s are successfully verified.

Detection model of plasma spectrum based onpolarization recognition rate induced by low energydensity laser

jiang xu, xiao wang, Mingyin Yao, and Liu Muhua

DOI: 10.1364/AO.460092 Received 31 Mar 2022; Accepted 09 May 2022; Posted 10 May 2022  View: PDF

Abstract: Laser-induced breakdown spectroscopy (LIBS) is a fast recognition method for heavy-metal detection. The recognition rateof the characteristic spectrum is related to the laser-induced energy. In order to analyze the polarization characteristics ofplasma and the improvement mechanism of recognition rate under the condition of low energy density, a detection model ofpolarization recognition rate was established by exploring the intensity formula of discrete spectral. At Brewster angle, theLIBS and discrete spectral data of Cd and Cu elements induced by five energies were measured in combination with thepolarization spectrometric path. According to the model of polarization recognition rate, the optimization effect ofpolarization separation approach on plasma spectrum of heavy-metal elements has been clarified, and the recognition rate ofdiscrete spectrum induced by low energy density has been improved. This study shows that the increase of laser energy ishelpful to the recognition of characteristic spectral lines. Under the same energy induction, the polarization recognition rateof heavy-metal elements is larger, and this characteristic is more obvious under low energy density. This model not onlyimproves the recognition rate of plasma spectrum, but also greatly reduces the requirement of laser energy and the damageof medium surface, which is a more effective nondestructive testing technology.

Design of an objective for hyper-numerical-aperture immersion lithography tool with a multi-step alternative grouping design method

Xu Yan, Yanqiu Li, lihui liu, and Ke Liu

DOI: 10.1364/AO.457833 Received 08 Mar 2022; Accepted 08 May 2022; Posted 09 May 2022  View: PDF

Abstract: An adequate initial structure plays an important role to achieve a high-quality hyper-numerical-aperture (NA) catadioptric objective for an immersion lithography tool. In this paper, a design method of multi-step alternative grouping design method is innovated to achieve a feasible aspherical initial structure of the hyper-NA objective. The objective is firstly divided into three groups, the object-side group(G1), middle group(G2) and image-side group(G3). And then each group is designed separately. To guarantee the effective connection of these separate groups, G1 and G2 or G2 and G3 are connected as G12 or G32 on object or image side. Through the real ray tracing in the forward path and the reverse path, G2 will be recalculated to a new aspherical structure. Then, a hybrid iterating process is adopted to get a better G2 to better connect the whole system. Finally, G1, new G2 and G3 are connected as a feasible aspherical initial structure of the hyper-NA catadioptric objective, which can be considered as a good starting point for further optimization. A high-performance catadioptric objective with hyper-NA 1.35 was designed by the proposed method.

Artificial neural networks assisted the designof dual-mode photonic crystal nanobeamcavity for simultaneous sensing of therefractive index and temperature

ZIxing Gou, Chao Wang, Zhe Han, Tongyu Nie, and Huiping Tian

DOI: 10.1364/AO.453818 Received 14 Jan 2022; Accepted 07 May 2022; Posted 09 May 2022  View: PDF

Abstract: In this paper, we put forward a dual-mode photonic crystal nanobeam cavity forsimultaneous sensing of the refractive index (RI) and temperature (T) designed with theassistance of artificial neural networks (ANNs). We choose the structure of quadraticallytapered elliptical holes with a slot to improve the sensitivities of the two modes. To reduce thetime-consuming of the design, the ANNs are trained to predict the band structure and to inversedesign the geometric structure. For the forward prediction and the inverse design neuralnetworks, low mean square errors of 5.1×10-4 and 1.4×10-2 are achieved, respectively. Througha specific design of band properties by the well-trained neural networks, a dual-mode nanobeamsensor with high quality factors of 9.34×104 and 1.55×105 and a small footprint of .8×0.7μm2 is designed. The RI and T sensitivities of the air mode are 405 nm/RIU and 40 pm/Krespectively, whereas those of the dielectric mode are 531 nm/RIU and 27 pm/K, respectively.The present work shows significance in further research on the design and applications for dualmode cavities.

Freeform reflector design for uniform illumination in a space-based remote sensor calibration system

Jiewen Tian, xin ye, and wei fang

DOI: 10.1364/AO.454095 Received 24 Jan 2022; Accepted 07 May 2022; Posted 09 May 2022  View: PDF

Abstract: A freeform reflector for illumination in a space-based remote sensor calibration system is designed to produce the desired patterns and improve efficiency. Owing to the excellent cosine properties and light source stability of the integral sphere, it is widely used as the calibration source in optical remote sensing instruments. However, the light source cannot obtain uniform illumination on a large scale. Therefore, this study proposes a design method for a freeform reflector for an integral sphere source, which generates an independent irradiance distribution and achieves large-scale illumination by arraying the irradiance distribution. Irradiance uniformity is more than 98% and an efficiency greater than 90% were obtained.

Ultra-low Doppler frequency extraction for self-mixing dual-frequency laser velocimetry

Jun Chen, Xinmeng Wang, Yubao Wu, Yitao Yang, Mingyue Qiu, Ming Wang, and Yuzhi Li

DOI: 10.1364/AO.455671 Received 08 Feb 2022; Accepted 06 May 2022; Posted 09 May 2022  View: PDF

Abstract: In the self-mixing dual-frequency laser Doppler velocimetry, the self-mixingDoppler frequency shift of the optical frequency difference is a linear function of the velocityof an external dynamic object; however, it is always ultralow for the signal processing.Therefore, an ultra-low frequency extraction method based on artificial neural networks (NNs)is presented because NNs can accurately create a fitting function for a Doppler signal andextend the signal to the DC value, increasing the signal length and sampling points withoutyielding unnecessary influences on the Doppler frequency. We precisely measured Dopplerfrequencies in the frequency domain with a low sampling rate, and calculated the velocitiesfor a target with longitudinal movements. Compared with the time-domain extraction,frequency-domain extraction can reflect the complete information of the original Dopplersignal. This feature potentially contributes to the signal processing of velocimetry in practicalengineering applications.

Polarimetric characterization of segmented mirrors

Adur Pastor Yabar, Andres Asensio Ramos, Rafael Manso Sainz, and Manuel Colladas Vera

DOI: 10.1364/AO.460201 Received 01 Apr 2022; Accepted 06 May 2022; Posted 09 May 2022  View: PDF

Abstract: We study the impact of the loss of axial symmetry around the optical axis on the polarimetric propertiesof a telescope with segmented primary mirror when each segment is present in a different aging stage.The different oxidation stage of each segment as they are substituted in time leads to non-negligiblecross-talk terms. This effect is wavelength dependent and it is mainly determined by the properties of thereflecting material. For an aluminum coating, the worst polarimetric behavior due to oxidation is found forthe blue part of the visible. Contrarily, dust —as modeled in this work— does not significantly changethe polarimetric behavior of the optical system . Depending on the telescope, there might be segmentsubstitution sequences that strongly attenuate this instrumental polarization.

Surface plasma wave induced second harmonic generation on metal-semiconductorinterface: Effect of self-focusing of laser

Harleen Dua, Niti Kant, and Vishal Thakur

DOI: 10.1364/AO.456389 Received 23 Mar 2022; Accepted 06 May 2022; Posted 10 May 2022  View: PDF

Abstract: The effect of the self-focusing of laser on second harmonic generation (SHG) induced by thesurface plasma waves (SPW) over the metal-semiconductor interface has been investigated. Thenonlinear interactions of laser with metal generate SPW. These high amplitude waves couldpenetrate through plasmonic entities of metal and can interact with electrons of semiconductorleading to SHG. The behavior of beam width parameter for different metals has been analyzed.One may notice that the beam width parameter shows periodic variation with the distance but theamplitude shows maxima in the regions of the low beam width parameter. The focusingaccompanied by the increase in SHG amplitude is found to be better for the Cu-InSb interface ascompared to the Ag-InSb and Al-InSb. Present work put forwards metal-semiconductor interfaceas a good choice for SHG.

A parallelism measurement method for nontransparent flat parts

Juntao Zhang, Bo Pan, Huan Liu, Xu Zhu, Kang Renke, and Jiang Guo

DOI: 10.1364/AO.457755 Received 08 Mar 2022; Accepted 05 May 2022; Posted 10 May 2022  View: PDF

Abstract: Non-transparent flat parts with weak rigidity are widely used in precision physics experiments, aerospace, and otherfields, in which parallelism is highly required. However, existing methods cannot meet requirements due to the limitationof measurement size and accuracy. This paper proposes a new method for measuring parallelism of non-transparent flatparts with high accuracy, and then built a submicron level parallelism measuring system. The 3D model of the whole partis reconstructed by thickness and flatness, which are measured respectively. Subsequently, parallelism is evaluated bythe principle of minimum directional zone. The method is verified by an experiment with a thin copper substrate sizedø200 mm × 2.48 mm on the parallelism measuring system. The experiment result shows that the part's parallelism is 7.41µm and the expanded uncertainty of parallelism measurement system is 0.34 µm, k = 2.

Effects of water salinity on the multi-angularpolarimetric properties of light reflected from smoothwater surfaces

Zhongqiu Sun, Di Wu, and Yunfeng Lv

DOI: 10.1364/AO.458737 Received 18 Mar 2022; Accepted 05 May 2022; Posted 06 May 2022  View: PDF

Abstract: Salinity is an important environmental factor regulating the aquatic system structure of lakes and other waterbodies. Changes in salinity, which can be caused by human activities, can adversely impact the life of water organisms.The refractive index, which can be directly related to water salinity, also controls the polarimetric properties of lightreflected from the water surface. In this study, polarimetric measurements of smooth water surfaces with differentsalinity content were performed at different viewing zenith angles in the wavelength range of 450-1000 nm in thespecular reflection directions. The results show that the light reflected from water surface (defined as reflectance factor)in one measurement direction can be replaced by the reflectance factor derived from polarimetric measurements, if thepolarizer absorptance is considered, the average relative difference is less than 3%. The degree of linear polarization(DOLP) was used to retrieve the refractive indices of water with different salinity based on the Fresnel reflectioncoefficient. The inverted refractive indices not only have high accuracy (uncertainty from 0.9% to 1.8%), but also have avery strong relationship with the water salinity content. Our study shows the possibility of estimating the variation inwater salinity using multi-angular polarimetric measurements.

Dynamic PSF-Based Jitter Compensation and QualityImprovement for Push-Broom Optical Images ConsideringTerrain Relief and the TDI Effect

Shijie Liu, LIn Feng, Xiaohua Tong, Han Zhang, Hong Lin, Huan Xie, Zhen Ye, and Shouzhu Zheng

DOI: 10.1364/AO.453163 Received 10 Jan 2022; Accepted 05 May 2022; Posted 06 May 2022  View: PDF

Abstract: Platform attitude jitter is inevitable during satellite flight. The jitter reduces thequality of push-broom optical images, resulting in geometric deformation and image blur. Thispaper proposes an approach of jitter effect compensation and quality improvement for pushbroom optical images based on dynamic point spread function (PSF) estimation and iterativeimage restoration considering the effect of terrain relief and time delay integration (TDI). First,the attitude jitter is precisely detected and estimated by considering the terrain relief, and thejitter information is then used to establish the dynamic PSF of each image line considering thechanging jitter values and TDI effect. Finally, based on the constructed PSF, the image isimproved by iterative restoration using the optimal-window Richardson-Lucy algorithm. Themethod is validated by both simulation and real data experiments. In the simulation experiment,we apply jitter with different amplitudes and frequencies to generate the degraded images andthen restore the image using the proposed restoration method. The results show that theproposed method can effectively restore images affected by jitter. In addition, real dataexperiments are carried out with multispectral remote sensing images from the ZY-3 satellite,and the results show that in addition to the improvement of the radiometric quality, thegeometric quality is also significantly improved in both the across-track and the along-trackdirections. The experimental results validated that the proposed method outperformed othermethods without considering the terrain and TDI effect.

DIC measurement for large-scale structures basedon adaptive warping image stitching

Long Sun, Chen Tang, Min Xu, and Zhenkun Lei

DOI: 10.1364/AO.455564 Received 10 Feb 2022; Accepted 05 May 2022; Posted 06 May 2022  View: PDF

Abstract: As a representative method of optical non-interference measurement, digital image correlation (DIC) technology isa non-contact optical mechanics method that can measure the displacement and deformation of the whole field.However, when the measurement range of the field is too large, existing DIC method cannot measure the full fieldstrain accurately, which limits the application of DIC measurement in the case of large-size and wide-field view. Toaddress this issue, a DIC measurement method for large-scale structures based on adaptive warping imagestitching is proposed in this paper. Firstly, multiple adjacent high-resolution images are collected at differentlocations of large-scale structures. Secondly, the collected images are stitched by applying the adaptive warpingimage stitching algorithm to obtain a panoramic image. Finally, the DIC algorithm is applied to solve the wholedeformation field. In the experiments, we first verify the feasibility of the proposed method for image matching andfusion through the numerical simulation of rigid body translation experiment. Then, the accuracy and robustnessof the proposed method in practical application is verified by rigid body translation and three-point bendingexperiment. The experimental results demonstrate that the measurement range of DIC is improved significantlywith the adaptive warping image stitching algorithm.

Analysis of an inverse weak-value tiltmeter in the kilohertz regime

Benjamin Baldwin, Garrett Josemans, and John Gray

DOI: 10.1364/AO.455631 Received 07 Feb 2022; Accepted 05 May 2022; Posted 06 May 2022  View: PDF

Abstract: We present a follow-on experiment to the recent study from The University of Rochester [Opt. Lett. 42, 2479 (2017)], which reported a new architecture for an inverse weak-value tiltmeter. We recreate the Rochester tiltmeter and specifically investigate mirror oscillations in the low kilohertz frequency regime, which is relevant to certain potential applications, such as Coriolis vibratory gyroscopes. We find that the inverse weak-value amplification effect persists in this regime, although our measured noise floors are higher than those obtained in the Rochester experiment – approximately 2 prad/√Hz for mirror oscillation frequencies between 1 and 25 kHz.

Enhanced photonic spin Hall effect of reflectedlight from a doubly linear gradient-refractive-indexmaterial

Jie Cheng, Jie Xiang, gaojun wang, Hao Xu, Peng Dong, Bin Li, Fengfeng Chi, and Shengli Liu

DOI: 10.1364/AO.457692 Received 07 Mar 2022; Accepted 05 May 2022; Posted 09 May 2022  View: PDF

Abstract: The photonic spin Hall effect (SHE), manifesting itself as spin-dependent splitting of light,holds potential applications in nano-photonic devices and precision metrology. However, the photonicSHE is generally weak, therefore its enhancement is of great significance. In this paper, we propose asimple method for enhancing the photonic SHE of reflected light by taking advantage of thegradient-refractive-index (GRIN) material. The transverse shifts for normal (homogeneous) layer andlinear GRIN structure with three different types (singly increasing, singly decreasing and doubly linearones) are theoretically investigated. We found that the doubly linear GRIN materials exhibit theprominent photonic SHE of reflected light, which is mainly due to the Fabry-Perót resonance. Byoptimizing the thickness and the lower (higher) refractive index of doubly linear GRIN layer, thetransverse shift for a horizontally polarized incident beam can nearly reach its upper limitation (i.e. halfof the beam waist). These findings provide us a potential method to enhance the photonic SHE, andtherefore establish a strong foundation for developing spin-based photonic devices in the future.

A hybrid ray-tracing/Fourier optics method to analyze multilayer diffractive optical elements

Victor LABORDE, Jerome Loicq, Juriy HASTANIN, and Serge Habraken

DOI: 10.1364/AO.456055 Received 11 Feb 2022; Accepted 05 May 2022; Posted 09 May 2022  View: PDF

Abstract: The performance (paraxial phase delay) of conventional diffractive optical elements is generally analyzedusing the analytical scalar theory of diffraction, based on thin-element approximation (TEA). However,the high thickness of multilayer diffractive optical elements (MLDOEs) means that TEA yields inaccurateresults. To address this, we tested a method based on ray-tracing simulations in mid-wave and long-waveinfrared wavebands and for multiple F-numbers, together with the effect of MLDOE phase delay on acollimated on-axis beam with an angular spectrum method. Thus, we accurately generated optical figuresof merit (point spread function along the optical axis, Strehl ratio at the "best" focal plane, and chromaticfocal shift), and by using a finite-difference time-domain method as a reference solution, demonstrate it asa valuable tool to describe and quantify the longitudinal chromatic aberration of MLDOEs.

In-Flight Measurement of Atmospheric-Imposed Tilt: Experimental Results and Analysis

Matthew Kalensky, Eric Jumper, Matthew Kemnetz, and Stanislav Gordeyev

DOI: 10.1364/AO.460717 Received 06 Apr 2022; Accepted 05 May 2022; Posted 09 May 2022  View: PDF

Abstract: The work presented here experimentally measured the tilt imposed onto a laser beam by the atmosphere from Shack-Hartmann Wavefront Sensor measurements collected in-flight. Tip/tilt is imposed onto the laser beam by propagating through optical turbulent structures larger than or on the order of the size of the beam diameter. This tip/tilt causes a dynamic, net deflection of the beam in the far-field, referred to as jitter, which poses a serious problem for tracking in directed energy applications. The practical measurement of turbulence-induced tip/tilt at altitude is challenging since mechanical contamination in the form of vibrations also manifests as tip/tilt. In this paper, a procedure referred to as the stitching method was used to quantify the turbulence-induced component of tilt without the influence of mechanical corruption. It was found that the measured tilt aligned with what analytic solutions predict and that the turbulence environment through which the beam propagated had Kolmogorov-like characteristics.

Simulation of asymmetric hexagonal microcavity with high-ratio fluorescence and high-efficiency directional emission

Lu Ren, Zhihui Chen, guang feng, Xiaowei Wang, Yibiao Yang, fei sun, and Yichao Liu

DOI: 10.1364/AO.458265 Received 14 Mar 2022; Accepted 04 May 2022; Posted 05 May 2022  View: PDF

Abstract: Ratiometric fluorescent sensors are widely used in biological sensing andimmunoassay due to its high sensitivity detection of analytes. The high ratio value of fluorescence can increase the sensitivity of the fluorescence sensor; in addition, the directional emission can improve the efficiency of light collection and improve the effective use of radiation power. In previous studies, low fluorescence ratios and low directional emission efficiency restrict the application of ratio fluorescence sensors. Based on the aboveconstraints, this paper proposes an asymmetric hexagonal microcavity structure. By destroying the complete rotational symmetry of the hexagon structure, it achieves high fluorescence ratios and high efficiency directional emission in far field range in the near19 infrared wavelength range, which is of significance for the development of high sensitivity fluorescence sensors.

Novel broadband tunable coding metasurfaces basedon metal patch and graphene for beam control at theterahertz frequencies

Panpeng Ge, ying Zhang, Lihua Xiao, and Binggang Xiao

DOI: 10.1364/AO.459343 Received 25 Mar 2022; Accepted 04 May 2022; Posted 05 May 2022  View: PDF

Abstract: In this paper, we present a broadband tunable coding metasurfaces structure usingcruciate metal patch and circular graphene on multilayer substrate. By changing the Fermilevel of graphene, we can achieve obvious reflection phase variation to design multi-bitcoding metasurfaces. In the research of 1-bit coding metasurfaces, we combine theadvantages of graphene and cooper to realize the real-time adjustment of reflected waves infour broadband frequency bands. In this case, we can control the number of far-fieldreflected waves in the frequency range of 5.45THz ~ 6.45THz. Then, we create 2-bit and3-bit coding mode on the basis of 1-bit coding metasurfaces to obtain a single beam ofreflected waves. Finally, we use the convolution calculation to realize the real-timeadjustment of the single beam reflection direction from 0° to 360° in the azimuthal plane.The research of 2-bit and 3-bit coding mode also provide a way to control the number anddirection of reflect beam specifically in 1-bit coding mode. The present codingmetasurfaces structure provide inspirations for the design of functional devices infuture-oriented intelligent communication.

Experimental and Analytical Evaluation of an Active Barium Vapor Notch Filter Functioning at 355 nm

Madison Hetlage and chris limbach

DOI: 10.1364/AO.457971 Received 24 Mar 2022; Accepted 04 May 2022; Posted 05 May 2022  View: PDF

Abstract: This work presents an experimental and theoretical study of the 6s2 1S0 → 6s6p 3P01(791 nm) and 5s5d 3D2 → 6s5f 3F02 (355 nm) transitions within a low vapor pressure barium vapor in the absence of a buffer gas. To the authors’ knowledge, this is the first measurement of the latter absorption feature. The study is motivated by the development of an optically controlled atomic vapor notch filter functioning at the third harmonic of the commonly used Nd:YAG laser at 355 nm. The low pressure environment within the vapor source has enabled a deeper understanding of barium vapor collisional and velocity changing properties with a simple pump-probe spectroscopy measurement. The results demonstrate depletion of the groundstate and subsequent population of the 5s5d 3D2 level. Most notably, the 5s5d 3D2 → 6s5f 3F02transition displays a non-thermal, cusped absorption curve. An analysis of this lineshape in the context of existing analytical collisional kernels is presented. Additionally, a six-level kinetic model of the low-lying energy levels, incorporating spatial diffusion and collisional and radiativetransitions, is introduced and compared to the measured level populations for the barium groundstate and excited 5s5d 3 D2 state.

Terahertz Metamaterials Biosensor for Diagnosis ofHepatocellular Carcinoma at Early Stage

DONGXIA LI, Lizhen zeng, Yuanli wang, Hong-Wen Tang, Wen Xing Lee, Zhencheng Chen, Longhui Zhang, Yingchang Zou, Duan Xie, and Fangrong HU

DOI: 10.1364/AO.459489 Received 30 Mar 2022; Accepted 04 May 2022; Posted 06 May 2022  View: PDF

Abstract: We propose a method for diagnosis of cirrhosis and hepatocellular carcinoma (HCC) by using a terahertz (THz)metamaterials (MMs) biosensor. The biosensor has a resonance frequency at about 0.801 THz, and can measure theconcentration of alpha-fetoprotein (AFP) in serum. The sensitivity of the sensor is 124 GHz/RIU (Refractive Index Unit)and the quality-factor (Q) is 6.913, respectively. When the surface of biosensor is covered with healthy serum (AFP ≤ 7ng/mL), the maximum resonance frequency shift is 50 GHz. However, when it is covered with serum from patients withcirrhosis and early HCC (AFP > 7 ng/mL), the resonance frequency shift is more than 59 GHz. Positive correlation existsbetween the frequency shift of the biosensor and serum levels of the AFP in the HCC patients. This study provides a methodfor quick diagnosis and prediction of the cirrhosis and the HCC.

Femtosecond 3D photolithography through a digitalmicromirror device and a microlens array

Balaji Baskar, Aravind Jakkinapalli, and Sy Wen

DOI: 10.1364/AO.457847 Received 08 Mar 2022; Accepted 04 May 2022; Posted 09 May 2022  View: PDF

Abstract: Based on the microscale 3D point cloud projection with a digital micromirror device (DMD) and a microlens array(MLA) developed recently, we explored the capabilities of this specific type of 3D projection in 3D lithography withfemtosecond light in this study. Unlike 3D point cloud projection with UV continuous light demonstrated before, highaccuracy positioning between the DMD and the MLA is required to have rays simultaneously arrive at the designedvoxel positions to induce two-photon absorption with femtosecond light. Because of this additional requirement, anew positioning method through direct microscope inspection of the relative positions of the DMD and the MLA isdeveloped in this study. Because of the usage of a rectangular microlens array, around four rays can arrive at eachprojecting voxel at the same time. Thus, a new algorithm for determining the pixel map on the DMD to the 3D pointcloud projection with a femtosecond laser is also developed. It is observed that a very long exposure time is requiredto generate 3D patterns with the new 3D projection scheme because of the very limited number of rays used forprojecting each voxel with the new algorithm. It is also found that 3D structures with desired shapes should beprojected far away from the MLA (~15f to 30f, with f being the focal distance of the MLA) in the 3D lithography withthis femtosecond 3D point cloud projection. For patterns projected closer than 10f, shapes are distorted because ofunwanted voxels cured with the 3D projection technique using DMD and MLA.

PDMS-PUA bi-directional replication technology andits applications

Jian Jin, Zhong Wang, Xiaobao Cao, and Xudi Wang

DOI: 10.1364/AO.460724 Received 08 Apr 2022; Accepted 04 May 2022; Posted 06 May 2022  View: PDF

Abstract: Polydimethylsiloxane (PDMS) and Polyurethane acrylate (PUA) are excellent pattern transfer materials. In this study, thePDMS-PUA bi-directional replication technology was explored using the PDMS grating as a template, and relevanttechnical issues were discussed in detail. Special surface treatment and process optimization were applied to solve theproblems of demolding, PDMS polymerization inhibition, and substrate flatness. Further experiments show that thetechnology can be employed to replicate nanoscale structures and has the potential value of prolonging the longevity ofthe original template. Additionally, utilizing the advantage of the high elasticity of PDMS materials, two applications of thebi-directional replication technology are demonstrated. One is to increase the line-density of the grating by stretching andthe experimental results show that the line-density of the grating increased by 26.6%. The other one is to fabricate theconvex grating. Compared with the original planar PDMS grating, the resolution of the first-order diffraction spectrum ofthe convex grating at the focal point has been greatly improved. Since this technology requires simple equipment, andPDMS and PUA are reusable, it has the advantages of low cost, simplicity, and rapid fabrication. The two applicationexamples also indicate that the technology has good application value.

Solving optimal carrier frequencies of CGH null compensatorthrough a double-constrained searching method based oniterative ray-tracings

Zijian Liang, HONGYANG ZHAO, and Yongying Yang

DOI: 10.1364/AO.455315 Received 02 Feb 2022; Accepted 03 May 2022; Posted 03 May 2022  View: PDF

Abstract: Interferometry based on computer-generated hologram (CGH) null compensator is a general method for high-precisionmetrology of aspherics. In view that the most commonly used CGHs are of Ronchi-type with only two quantization steps, tilt anddefocus carrier frequencies are always required to be introduced to separate the disturbing diffraction orders (DDOs). Determiningthe amounts of carrier frequencies is a pivotal but difficult issue in the CGH design process. Previous studies can only drawqualitative conclusions or obtain some approximate results under specific conditions. This paper proposes a double-constrainedsearching method based on iterative ray-tracings (DCS-IRT), which can directly and accurately give the optimal combination oftilt and defocus carrier frequencies as long as the aspheric under test is a concave one and has an analytical expression. Theoptimal carrier frequencies solved by the proposed method will minimize the line-density of the CGH on the premise of separatingall disturbing diffraction orders, which will reduce the cost and difficulty of fabrication as much as possible. The proposed methodis almost error-free and holds a clear advantage over the previous methods in terms of versatility. Several typical design examplesare presented to verify the feasibility and versatility of the proposed method. Its accuracy is also verified through makingcomparisons of the ray-tracing results between another method and Zemax models based on these examples.

Direct experimental evidence for free-space fractionaloptical vortex transmutation

Fulin Cao and Changqing Xie

DOI: 10.1364/AO.458718 Received 17 Mar 2022; Accepted 03 May 2022; Posted 03 May 2022  View: PDF

Abstract: The emergence of vortex transmutation has opened new ways for vorticity modulation ofoptical vortex. Although several approaches have been proposed to realize vortex transmutation,fractional optical vortex (FOV) transmutation remains elusive owing to lack of effective generation anddetection methods. Here we report quantitative experimental evidence for free-space fractional opticalvortex transmutation rule. The key idea is to combine the advantages of a single optical element,termed as fractional spiral polygonal lenses (FSPLs), with deep learning approach. The desiredwavefront is simultaneously generated and manipulated at the focal plane of the FSPL, and thefractional output vorticity is measured by analyzing a single far-field diffraction pattern. Especially, adeep learning scheme using Bayesian optimization method is developed for output vorticityperformance prediction with data recovery rate up to 98.2%. The average error of recognized fractionalorbital angular momentum modes is as small as 0.02. We clearly observe the intriguing phenomenonthat the central vorticity of FOV is changed following modulo-n transmutation rule in free space. Ourresults have important implications for fundamental understanding of FOV systems in free space, andoffer a technological foundation for potential applications such as quantum information processing,particle manipulation and transportation.

All-fiber surface-enhanced Raman scatteringdetection system combining integratedmicrofluidic chip and micro-lensed fiber

Dongqiao Zheng, Wei Li, Benyang Zhao, zhao yang, and Li Xia

DOI: 10.1364/AO.457448 Received 03 Mar 2022; Accepted 03 May 2022; Posted 16 May 2022  View: PDF

Abstract: It is a challenge to perform simple and rapid detection of substances due to theircomplex structure. Biochemical molecules play a vital role in human health and environmentaltesting. Surface-enhanced Raman scattering (SERS) detection has the characteristics of strongspecificity and real-time performance. At present, most SERS systems are expensive and notportable. Here, we demonstrate a SERS detection system with all-fiber connection, combiningwith a microfluidic chip and micro-lensed. Compared with the conventional SERS system thatuses the spatial optical path, the devices in our system are connected by optical fibers, makingthe system more stable and operable. Besides, the microfluidic chips are introduced to furtherimprove the system integration and stability. Owing to the micro-lensed fiber probe, the detectedRaman signal intensity is increased by 2-3 times. We anticipate that the presented work will leadtowards a rapid and portability SERS system and corresponding detection system. It also lays thefoundation for real-time recognition in various complex environments in the design of futureoptical fiber system.

Analysis of RCF Crack Detection Phenomenon basedon Induction Thermography

XIAO LIANG, Jian Peng, Xiang Zhang, Jianqiang Guo, and Yu zhang

DOI: 10.1364/AO.455610 Received 04 Feb 2022; Accepted 03 May 2022; Posted 06 May 2022  View: PDF

Abstract: In this paper, eddy current pulsed thermography (ECPT) is used to detect rolling contact fatigue (RCF) cracks on therail. It is observed that some of the cracks disappear in the thermal image with the increase of heating time. Basedon the finite element method, with double-cracks as the basic unit, three different crack models are established, andthe mutual disturbance relationship between the double-cracks is discussed based on the eddy current distributionand thermal diffusion process. Simulation and experiment results show that different crack models are affected bythermal diffusion in different heating stages to different degrees, and the time of crack thermal image disappearanceis obtained. According to the above conclusions, the RCF cracks are extracted and classified based on the influenceof thermal diffusion. The possibility of rail condition assessment and maintenance based on disappearance time isexplained.

Wander and spread of perfect Laguerre-Gauss beamunder turbulent absorbent seawater

Hongbin Yang, yixin zhang, guoqing zhao, Lin Yu, and lifa hu

DOI: 10.1364/AO.457526 Received 02 Mar 2022; Accepted 03 May 2022; Posted 03 May 2022  View: PDF

Abstract: The wander and long-term spread of a beam caused by turbulence are two important factors affecting channeltargeting and information receiving in optics communication system. In this paper, the wander and long-termspread of perfect Laguerre-Gauss (PLG)/ circular perfect Laguerre-Gauss (CPLG) beam in a turbulentabsorbing seawater are studied. The analytical expression of the wander for CPLG beam in the weak turbulentfluctuation region and the analytical expression of the long-term spread for CPLG beam in weak to strongturbulent fluctuation region are derived by using the Rytov approximation and the generalized HuygensFresnel integral, respectively. Through numerical analysis, we find that the optimal beam diameter and selfconvergence effect of PLG beam exist under given communication link conditions, the long-term spread of PLGbeam is smaller than that of LG beam, but the wander evolution trend of PLG beam with increasing propagationdistance is opposite to that of LG beam. PLG and CPLG beams have stronger resistance disturbance ofturbulence than that of Laguerre-Gauss and circular Laguerre-Gauss beams, respectively.

Varifocal liquid lens driven by conical dielectricelastomer actuator

Yang Cheng, Zhikuo Li, CHUANXUN CHEN, jie CAO, CHUN BAO, and Qun Hao

DOI: 10.1364/AO.457886 Received 09 Mar 2022; Accepted 03 May 2022; Posted 04 May 2022  View: PDF

Abstract: A varifocal lens is an important part of optical systems with applications in biomedicine, photography, smartphone, andvirtual reality. In this paper, we propose and demonstrate a varifocal liquid lens driven by a conical dielectric elastomeractuator. When the conical dielectric elastomer is subjected to an actuation voltage, the conical dielectric elastomer worksas an out-plane actuator and makes the surface curvature of the liquid droplet increase, then the focal length of theproposed varifocal liquid lens changes. The overall dimensions of the proposed varifocal liquid lens are 9.4 mm indiameter and 12.5 mm in height. The focal length tuning range is 15.07 mm~9.50 mm when the actuation voltage increasesfrom 0 kV to 5.0 kV. The focal power variation of the proposed varifocal liquid lens is 35.5 D. The rise and fall times of theproposed varifocal liquid lens are 215 ms and 293 ms, respectively. The ability of the proposed liquid lens that can focuson objects at different distances without any moving parts is demonstrated. The compact varifocal liquid lens driven bythe conical dielectric elastomer actuator in the current study has the potential to be used in various compact imagingsystems in the future.

Modular-assembled laser system for long-baselineatom interferometer

Xu Rundong, Qi Wang, sitong yan, Zhuo Hou, chuan he, Yuhang Ji, Zhixin Li, Junjie Jiang, Biyan Qiao, Lin Zhou, Jin Wang, and Mingsheng Zhan

DOI: 10.1364/AO.458361 Received 15 Mar 2022; Accepted 03 May 2022; Posted 05 May 2022  View: PDF

Abstract: The Zhaoshan long-baseline Atom Interferometer Gravitation Antenna (ZAIGA) is a new type of large-scale atominterferometer facility under construction for the study of gravitation and related problems. To meet the differentrequirements of the laser system for the atom interferometer using various atoms (including 85Rb, 87Rb, 87Sr, and 88Sr), wedesign and implement a modular assembled laser system. By dividing the laser system into different basic units accordingto their functions and modularizing each unit, the laser system is made highly scalable while being compact and stable.Its intensity stability is better than 0.1% in 102 s and 0.5% in 104 s. We test the performance of the laser system with twoexperimental systems, i.e. an 85Rb-87Rb dual-species ultracold atom source and an 85Rb atom interferometer. The 85Rb87Rb dual-species magneto-optical trap and the 85Rb atom interference fringes are realized by using this laser system,indicating that its technical performance can meet the major experimental requirements.

Compatibility of Al-doped ZnO ETL with various HTL and Absorber in Perovskite Solar Cells

Nur Syafiqah Nadiah Mohd Alias, FAIZ ARITH, FAIZ Muhammad Mustafa, Mohd Muzafar Ismail, Siti Amaniah Mohd Chachuli, and Ahmad Syahiman Mohd Shah

DOI: 10.1364/AO.455550 Received 04 Feb 2022; Accepted 02 May 2022; Posted 04 May 2022  View: PDF

Abstract: Perovskite Solar Cells (PSCs) have shown a significant improvement in cellperformance in photovoltaics technology. The commonly used light absorbing material ofhalide-based perovskite in PSCs have produced high efficiency cells with low cost and simplefabrication process. However, it contains harmful substance of Pb which affects theenvironment, and the cell still suffers from instability in the long run. Therefore, this workpresents a theoretical study of the Pb-free absorber layer of CH3NH3SnI3 and paired forcompatibility with various types of Hole Transport Layer (HTL). Several key parameters of theabsorbent layer and HTL have been optimized to produce the highest Power ConversionEfficiency (PCE) using 1D-SCAPS software under AM 1.5 illumination. It was found that thecombination of Cu2O and CH3NH3SnI3 used as HTL and absorbent layer, respectively hasresulted in great PCE as high as 27.72%. These findings prove that the use of inorganic HTLand Pb-free perovskite layer is promising for use in PSC.

High-efficiency InGaN blue LEDs with reducedpositive sheet polarization

Ravi Teja Velpula, Barsha Jain, Moulik Patel, FATEMEH MOHAMMADI SHAKIBA, Ngoc Quoc Toan, Hoang-Duy Nguyen, and Hieu Nguyen

DOI: 10.1364/AO.458463 Received 17 Mar 2022; Accepted 02 May 2022; Posted 03 May 2022  View: PDF

Abstract: The formation of positive sheet polarization charges at the interface of the last quantum barrier (QB) andconventional p-type electron-blocking layer (EBL) creates significant band-bending, leading to severeelectron leakage and poor hole injection in III-nitride light-emitting diodes. In this study, we report thatthe positive sheet polarization charges are mitigated by employing a lattice matched AlGaN last QB.Electron leakage is dramatically reduced due to the increased effective conduction band height at the lastQB and EBL. Furthermore, it favors the hole injection into the active region due to the reduced effectivevalance band height for EBL

Lens design for parallel cylindrical anamorphic attachments with finite objectdistance

Xiaobo Chen and Zhang Jinkai

DOI: 10.1364/AO.456177 Received 18 Feb 2022; Accepted 02 May 2022; Posted 03 May 2022  View: PDF

Abstract: A lens design method for parallel cylindrical anamorphic attachments with finiteobject distance is provided, which includes the paraxial lens design, the thin lens design, and the thick lens design. The paraxial lens data can be determined from geometric optics. The thinlens design adopts a total of 28 anamorphic aberrations. The anamorphic lens splitting method for each anamorphic lens module is also provided for anamorphic aberration balancing between anamorphic lens modules. With the proposed design method, an anamorphic attachment isdesigned and manufactured in this paper which proves the feasibility of the proposed design method.

Investigation and suppression of pre-pulses onnanosecond time scale in SULF-1PW laser

Peile Bai, Zongxin Zhang, XinLiang Wang, Fenxiang Wu, Jiabing Hu, Xiaojun Yang, Jiayi Qian, Jiayan Gui, Xiaoming Lu, Yanqi Liu, Yi Xu, Xiaoyan Liang, Yuxin Leng, and Ruxin Li

DOI: 10.1364/AO.456811 Received 22 Feb 2022; Accepted 01 May 2022; Posted 02 May 2022  View: PDF

Abstract: It is of crucial significance to investigate and suppress pre-pulses on nanosecond time scale, because the intense preplasma generated by them may have enough time to expand and thus cause fatal impact on laser-matter interactions. Inthis research, we analyze the potential origins of pre-pulses on nanosecond time scale in a typical Ti:Sapphire chirpedpulse amplification laser system. Based on the analysis, the initial status of these generated pre-pulses in SULF-1PW laseris measured and investigated. Then different measures, including fine control on the time synchronization and thereplacement for Ti:Sapphire, are adopted in SULF-1PW laser to suppress these pre-pulses with respective origins, whichcan promote the energy ratio between the main pulse and these pre-pulses by 2~3 orders of magnitude. This research notonly improves the temporal contrast of SULF-1PW laser on nanosecond time scale, but also provides beneficial guidancefor the design and construction of similar laser facilities.

A highly improved side mode suppressionratio and low phase noise optoelectronicoscillator

Yalan Wang, Chengji Lin, Xiang Li, Jin Zhang, Anle Wang, Depei Zhang, Shirui Du, and Xiaoniu Peng

DOI: 10.1364/AO.458038 Received 09 Mar 2022; Accepted 01 May 2022; Posted 05 May 2022  View: PDF

Abstract: By adopting self-injection locking (SIL) technology in an external injectionlocking (EIL) optoelectronic oscillator (OEO), a highly improved side mode suppression ratio(SMSR) and low phase noise microwave signal generator is designed. The external injectionlocking ranging is closely related to the frequency spacing ranging (FSR) of free-runningOEO, which is reverse to the oscillation loop length and limit the phase noise performance.Here an SIL technology is introduced to significantly increase the Q-factor of the OEOwithout degrading the SMSR by setting the longer loop without oscillation. Both thesimulation and experimental results are carried out to confirm the conclusion. And the bestSMSR up to 86 dB and the phase noise lower to -88.80 dBc/Hz@100 Hz and -122.83dBc/Hz@10 kHz, respectively, are demonstrated. Furthermore, the frequency overlappingAllan deviation (ADEV) of the proposed OEO scheme is also enhanced by 103 times whichbenefits from the external injection technology compared with free-running OEO. In addition,the SMRS and phase noise modification dependence on the fiber length, the RF sourcequality and external injection power, as well as the frequency tunability, are detaileddiscussed to reveal the compatibility combination mechanism of the EIL and the SIL.

LASER BEAM STEERING AUTOMATION WITHARDUINO-BASED CNC SHIELD FOR STANDOFFFEMTOSECOND FILAMENT INDUCED BREAKDOWNSPECTROSCOPIC STUDIES

lingamurthy Narlagiri, BYRAM CHANDU, Sampath Kumar Satani, and Venugopal Rao Soma

DOI: 10.1364/AO.453824 Received 16 Jan 2022; Accepted 30 Apr 2022; Posted 02 May 2022  View: PDF

Abstract: In this study, we report a novel instrumentation procedure in the automation of laserbeam steering for raster/spiral scanning of the samples used in standoff femtosecond laserinduced breakdown spectroscopy (LIBS) experiments. We have used a readily available andeasy-to-handle Arduino-based computerized numerical control (CNC) shield along with thefree software, universal G-code sender (UGS), for the automation. Standoff femtosecondfilamentation induced breakdown spectra (St-fs-FIBS) of metals, three compositions of Ag-Aualloy, and polyvinyl chloride (PVC), unplasticized polyvinyl chloride (UPVC), chlorinatedpolyvinyl chloride (CPVC) plastic samples were recorded using the developed automatedexperimental setup. The St-Fs-FIBS spectra were recorded at a standoff distance of ~5 metersutilizing a simple hand-held spectrometer. Furthermore, principal component analysis (PCA)technique was utilized for the successful classification of three compositions of Au-Ag alloyspectra using their St-Fs-FIBS spectral data.

Integrated refractive index sensor based onAlN-PSiO2 hybrid plasmonic microdiskresonator

Caixia Guo, Chenghao Wang, Tao Ma, Linqing Zhang, and fang wang

DOI: 10.1364/AO.458340 Received 14 Mar 2022; Accepted 30 Apr 2022; Posted 06 May 2022  View: PDF

Abstract: In this paper, a microdisk resonator (MDR) based on an AlN-PSiO2 hybridplasmonic waveguide (HPW) and its refractive index (RI) sensing characteristics areinvestigated. The plasmonic characteristics of the MDR based on the AlN-PSiO2 HPW(APHPW-MDR) in near-infrared wavelengths are studied by using the finite element method(FEM). Through the structure parameter optimizations, the propagation length (Lprop) of theAPHPW-MDR is ~165 µm, which is ~2.5 times as long as that of the MDR based on the AlNHPW (AHPW-MDR). The simulation results show that the quality factor (Q) and extinctionrate (ER) of the APHPW-MDR are ~3100 and ~30 dB, respectively. The RI sensing sensitivity(S) of the RI sensor based on the APHPW-MDR is ~276.6 nm/RIU. The RI sensor based on theAPHPW-MDR has wide application prospects in high-performance biochemical sensing, andit can also be used in integrated optical filters, modulators, switches, routers, delay circuits.

Experimental study on transit time broadeningin Rubidium 5S-5D Excitation

Vinay Shukla and Ayan Ray

DOI: 10.1364/AO.455563 Received 07 Feb 2022; Accepted 30 Apr 2022; Posted 05 May 2022  View: PDF

Abstract: In this experimental work we report our findings about a transition between states of even parity5S1/2→5D5/2 of Rb atoms under different conditions of transit time broadening. For this purpose the bluefluorescence emanating from associated decay 5D5/2→6P3/2→5S1/2 channel is monitored. Based on the relativemagnification of laser beam, which is used in excitation between levels of even parity, the system exhibits twophoton absorption (TPA) through blue fluorescence peaks. The TPA profile appears as a Lorentzian peak on aDoppler background. The change in laser beam diameter directly influence the relative contributions ofLorentzian and Doppler parts. A direct inference drawn from of this study may be utilised in optimizing theextraction of first derivative spectrum corresponding to 5S→5D transitions.

Selective Band Amplification in the Ultra Broad BandSuperimposed Quantum Dots ReflectiveSemiconductor Optical amplifiers

Farshad Serat Nahaei, Ali Rostami, and Samiye Matloub

DOI: 10.1364/AO.427496 Received 16 Feb 2022; Accepted 28 Apr 2022; Posted 03 May 2022  View: PDF

Abstract: In this paper, a novel approach has been proposed for having an ultra-broadband quantum dot reflective semiconductoroptical amplifier (QD-RSOA) using superimposed quantum dots with switching and band selection capability in the supportedband. Furthermore, about 1 µm optical bandwidth has been covered (O, E, S, C, and L bands), which is the desired region in mostoptical communication applications. Three optical windows have been selected for having an optimized amplification (1.55 µm,1.5 µm, 1.31 µm). Also, they can either be amplified simultaneously or one at a time, which guarantees independent modulation.This is a remarkable property in fast data transmission. Besides, this amplifier has been devised by solution processnanotechnology, which guarantees its synthesizing feasibility with low costs. Finally, by introducing this amplifier one step is takentoward the development of fast WDM-PONs.

Quantifying the Shape Effect of Plasmonic GoldNanoparticles on Photoacoustic Conversion Efficiency

Guanpin Ren, Limin Xu, Huan Zhan, Shuang Liu, Wei Jiang, and Ru Li

DOI: 10.1364/AO.457426 Received 07 Mar 2022; Accepted 28 Apr 2022; Posted 05 May 2022  View: PDF

Abstract: Gold nanoparticles with strong localized plasmonic effects have found wide applications in photoacoustic imaging, whichare ascribed to their unique microscopic mechanism of converting photons to ultrasound. In this report, we quantitativelymodeled the time-resolved temperature field, thermal expansion, and pressure distribution based on the finite elementanalysis method, two-dimensional gold nanoparticles spanning from triangle, square, pentagon, hexagon to circle havebeen systematically studied. Results show that the shape of gold nanoparticles has a nontrivial effect on photoacousticconversion efficiency, square-shaped gold structure exhibits the best performance. Our findings could shed light on theshape design of high-performance photoacoustic agents in the future.

Passivation of Black Phosphorus Nanoflakes:Embedded in Silica Glass Matrix AffordsAmbient Saturable Absorption StabilityEnhancement

Xueting Ma, Jialiang Liu, Chan Zheng, Li Huang, Wei Li, Shuguang Cai, and Xueqing Xiao

DOI: 10.1364/AO.458653 Received 16 Mar 2022; Accepted 28 Apr 2022; Posted 05 May 2022  View: PDF

Abstract: Black phosphorus (BP) is a graphene analogue with ultra-fast broad-band nonlinearoptical properties that make it a promising nanomaterial for saturable absorption. However, BPnanoflakes chemically degrade in ambient conditions. We developed air- and photo-stable BPnanoflakes via incorporation in inorganic-organic hybrid matrices. This realized passivationand materialization through a sol-gel method that produced high-quality, transparent bulkmaterials. Saturable absorption parameters of the passivated BP were maintained after fivemonths in ambient storage and after 8000 300-µJ nanosecond laser shots. The nonlinearabsorption coefficient was still 62% after twelve months in open air, which was higher thanthat for non-passivated BP after three days. The stability was attributed to dense silica-gelglasses that enveloped the BP, essentially eliminating oxygen and water penetration. Thesimplicity of this approach may stimulate potential applications for environmentally sensitivehigh-performance solid-state devices.

Scalable all-fibre coherent beam combination using digital control

Samuel Legge, Christian Freier, Lyle Roberts, Paul Wigley, John Close, and Kyle Hardman

DOI: 10.1364/AO.456360 Received 21 Feb 2022; Accepted 27 Apr 2022; Posted 29 Apr 2022  View: PDF

Abstract: This article describes a filled-aperture coherent beam combining (CBC) system based on Locking of Optical Coherence via Single-detector Electronic-frequency Tagging (LOCSET). The sensing and control architecture is implemented using a field-programmable gate array (FPGA) and high-bandwidth electro-optic phase modulators (EOMs). The all-fibre optical configuration consists of a narrow linewidth 1560 nm seed laser separated into three channels, each containing 7 W Erbium-doped fibre amplifiers. The system was demonstrated experimentally, achieving a total stabilized output power of 20 W, a combination efficiency greater than 95%, and an output RMS phase stability of 휆/493. As this architecture employs an entirely digital sensing and control scheme based on LOCSET, it presents a highly scalable and cost-effective solution for CBC that is wavelength agnostic and can support an arbitrarily large number of channels.

Single-mode fiber curvature sensor based onSPR

yong wei, Chunbiao Liu, Chunlan Liu, Lingling Li, Lei Hu, Yonghui Zhang, Xiaoling Zhao, Tianci Jiang, Rui Wang, and Chen Shi

DOI: 10.1364/AO.456788 Received 21 Feb 2022; Accepted 27 Apr 2022; Posted 04 May 2022  View: PDF

Abstract: Fiber surface plasmon resonance (SPR) sensor is often widely used in high-sensitivity refractive index measurement,and there is less research on curvature measurement. In this paper, a new single-mode fiber curvature sensor basedon SPR was designed and fabricated. By employing the bending, the transmitted light in the fiber core will leak intothe cladding. A 50 nm gold film was coated outside the cladding, the evanescent field of cladding after bendingcontacted the gold film to occur SPR. When the curvature changes, the coupled cladding mode and intensity aredifferent, that is, the SPR incident angle and evanescent field intensity are different, so as to realize the dualparameters of SPR resonance wavelength and depth of resonance valley change with curvature. By experiments, theinfluence of different cut-off wavelengths of single-mode fiber on the performance of the sensor was studied. Thetesting results indicate that, with the decrease of the cut-off wavelength of single-mode fiber, the valley depthsensitivity of the sensor increases and the half height width (FWHM) decreases. When the cut-off wavelength of thesingle-mode fiber is 630 nm, the valley depth sensitivity of the sensor is 0.0088 a.u/m-1, the wavelength sensitivity is0.26 nm/m-1, and the average FWHM is only 21 nm. The proposed single-mode fiber curvature sensor based on SPRhas a narrow FWHM and an opening threshold. It can also realize no opening threshold by introducing corelessfiber, which provides a new solution for the diversified detection of fiber SPR sensors.

HIGH-SPEED, LARGE DYNAMIC RANGE SPECTRALDOMAIN INTERROGATION OF FIBER-OPTIC FABRY PEROT INTERFEROMETRIC SENSORS

Kit Pan Wong, Hyun-Tae Kim, Keshav Rajasekaran, Amirhossein Yazdkhasti, Bala Manoghar, An Wang, Samuel Lee, Kenneth Kiger, James Duncan, and Miao Yu

DOI: 10.1364/AO.458196 Received 11 Mar 2022; Accepted 27 Apr 2022; Posted 29 Apr 2022  View: PDF

Abstract: We report high-speed, large dynamic range spectral domain interrogation of fiber optic Fabry-Perot (FP) interferometric sensors. An optical interrogation system employing apiezoelectric FP tunable filter and an array of fiber-Bragg-gratings for wavelength referencingis developed to acquire the reflection spectrum of FP sensors at a high interrogation speed witha wide wavelength range. A 98 nm wavelength interrogation range was obtained at theresonance frequency of ~110 kHz of the FP tunable filter. At this frequency, the resolution ofthe FP cavity length measurement was 1.8 nm. To examine the performance of the proposedhigh-speed spectral domain interrogation scheme, two diaphragm-based fiber-tip FP sensors (apressure sensor and acoustic sensor) were interrogated. The pressure measurement results showthat the high-speed spectral domain interrogation method has the advantages of being robust tolight intensity fluctuations and having a much larger dynamic range compared with theconventional intensity-based interrogation method. Moreover, owing to its capability ofmeasuring the absolute FP cavity length, the proposed interrogation system mitigates thesensitivity drift that the intensity-based interrogation often suffers from. The acousticmeasurement results demonstrate that the high-speed spectral domain interrogation method iscapable of high-frequency acoustic measurements of up to 20 kHz. This work will benefit manyapplications that require high-speed interrogation of fiber-optic FP interferometric sensors.

One Metal Ions-regulated AgTNPs Etching Sensor Array for Visual Discrimination of Multiple Organic Acids

Miao He, Jiawei Li, Dong Zhao, Yi Ma, Jing Zhang, Cailin Qiao, Zhihua Li, qun Huo, and Changjun Hou

DOI: 10.1364/AO.456278 Received 16 Feb 2022; Accepted 26 Apr 2022; Posted 27 Apr 2022  View: PDF

Abstract: The detection and discrimination of organic acids (OAs) is of great importance in the early diagnosis of specific diseases. In this study, we established an effective visual sensor array for the identification of OAs. This is the first time metal ions were used to regulate the etching of silver triangular nanoprisms (AgTNPs) in OAs discrimination sensor array. The sensor array was based on the oxidation etching of AgTNPs by three metal ions (Mn2+, Pb2+, and Cr3+) and accelerated etching of AgTNPs by OAs. The introduction of metal ions alone led to a slight wavelength shift of the AgTNPs colloid solution, signifying the incompletely etching of the AgTNPs. Nevertheless, when both metal ions and OAs were introduced simultaneously to the solution, a significant blue shift of the localized surface plasmon resonance (LSPR) peak was detected, and a color change of the AgTNPs was observed, which were the consequences of morphological transitions of the AgTNPs. The addition of different OAs accelerated AgTNPs etching in varying degrees, generating diverse colorimetric response patterns (i.e., RGB variations) as “fingerprints” associated with each specific organic acid. Pattern recognition algorithms and neural network simulation were employed to further data analysis, indicating the outstanding discrimination capability of the provided array for eight OAs at 33 μM level. Moreover, excellent results of selective experiments as well as real samples tests demonstrate that our proposed method possessed great potential for practical applications.

Criterion to determine the wave vector range of heterodyne near field exposure time dependent spectrum

Yi Peng, Jian Qiu, Li Peng, Kaiqing Luo, Dongmei Liu, and peng Han

DOI: 10.1364/AO.452850 Received 24 Jan 2022; Accepted 26 Apr 2022; Posted 27 Apr 2022  View: PDF

Abstract: Recently, the image methods for measuring dynamics of nanoparticles have caughtgreat attention, however the low-speed frame rate of the normal image sensors limits theirapplication. Applying low-speed detector can extract fast dynamic information by the methodof exposure time dependent spectrum, whereas its accuracy is too sensitive to the wave vectorrange selection. In this paper, we present a criterion to determine the wave vector range for theimage method of heterodyne near-field light scattering, where the hologram of the particles isanalyzed to extract the dynamics of the particles. A normalized instrument factor ratio 퓇(푞) isdefined and the accuracy of the result can be guaranteed when the upper limit 푞푢 푝 is selectedat the maximum of 퓇(푞푢 푝) = 1 and the lower limit 푞푙표푤 is selected according to the optimalgoodness of fit 푅 2. The experimental results verify the effectiveness of the proposed criterion.Since most image methods for nanoparticle characterizing is related to the wave vector rangeselection, it is believed that the idea of this criterion can be extended generally

Numerical Analysis of Grating Embedded BidirectionalIntegrated Optical Coupler Pressure Sensor

Sanjeev Raghuwanshi, Yadvendra Singh, and Talabattula Srinivas

DOI: 10.1364/AO.456410 Received 21 Feb 2022; Accepted 24 Apr 2022; Posted 11 May 2022  View: PDF

Abstract: A numerical analysis of grating embedded bidirectional optical coupled waveguide structure is presentedfor the first time. Finite difference method (FDM) based scheme has been devised to extract the allowedeigen TE and TM modes of the structure. Sensing characteristics of grating employed between two highrefractive index couplers are then explored. The influence of strain on composite structure has beennumerically analyzed for better understanding of guiding phenomena. Numerical method based on3-point central finite difference scheme with proper boundary conditions at the point of discontinuity hasbeen developed. To be an accurate sensitivity analysis a large number of mesh points (N = 1000) havebeen used in FDM algorithm, while whole analysis being done on MATLAB software. To the best of theauthors knowledge, Bragg grating sensitivity of individual TE and TM modes have been estimated for thefirst time. It is found that higher order TE and TM modes shows the improved sensitivity performance.The physics behind the improved sensitivity of proposed structure is correlated with the existing cases.The proposed technique is based on effective refractive index theory, and hence easy to implement. ThisWork can be easily extended to obtain the temperature, humidity and vibration sensitivity of other novelstructure.

Robust Correction of Interferometer Phase Driftin Transmission Matrix Measurements

Ralf Mouthaan, Peter Christopher, George Gordon, Timothy Wilkinson, and Tijmen Euser

DOI: 10.1364/AO.454679 Received 25 Jan 2022; Accepted 22 Apr 2022; Posted 25 Apr 2022  View: PDF

Abstract: A complex-valued transmission matrix describing a scattering medium can beconstructed from a sequence of many interferometric measurements. A major challenge in suchexperiments is to correct for rapid phase drift of the optical system during the data acquisitionprocess, especially when the phase drifts significantly between consecutive measurements.Therefore, a new method is presented whereby the exact phase drift between two measurementsis characterised and corrected for using a single additional measurement. This approach removesthe need to continuously track the phase and significantly relaxes the phase stability requirementsof the interferometer, allowing transmission matrices to be constructed in the presence of fastand erratic phase drift.

Ultra-broadband high-efficiency circular polarizationconversion and terahertz wavefront manipulationbased on all-metallic reflective metasurface

Mucheng Wang, Yongzhi Cheng, and Ling Wu

DOI: 10.1364/AO.454099 Received 18 Jan 2022; Accepted 22 Apr 2022; Posted 27 Apr 2022  View: PDF

Abstract: In this paper, an all-metal metasurface (MS) is proposed and investigated theoretically,which can achieve the high-efficient reflective circular-polarization conversion and multi-functionalterahertz (THz) wavefront manipulation in an ultra-broadband frequency range. The proposed all-metalMS is consisted of the periodic array of the gold vertical-split-ring (VSR) structure adhered on the goldsubstrate. Numerical simulation results indicate that the proposed MS structure can convert the incidentcircular-polarization (CP) wave into its orthogonal component after reflection with conversioncoefficient over 95% from 0.8THz to 1.65THz (relative bandwidth of 68.3%). The full 2π phase shiftof the proposed MS in this frequency range can be obtained by changing the rotation angle of the VSRstructure along the wave propagation direction. As proofs of concept for the multi-functional wavefrontmanipulation, anomalous reflection, reflective planar focusing and vortex beam generation arenumerically demonstrated based on the Pancharatnam-Berry (PB) phase principle. Our work canprovide an effective method of enhancing the performance of reflective-type all-metal MS and showendless potential in wavefront manipulation and communication applications in THz and even opticalregion.

Design and Simulation of a Plasmonic Density Nano-Sensor for Polarizable Gases

Babak Moeinimaleki, Hassan Kaatuzian, and Abdolber MallahLivani

DOI: 10.1364/AO.457454 Received 03 Mar 2022; Accepted 22 Apr 2022; Posted 22 Apr 2022  View: PDF

Abstract: In this paper, an optical method of measuring the mass density of polarizable gases is proposed using a plasmonic refractive index nano-sensor. Plasmonic sensors are capable of detecting very small changes in therefracting index of arbitrary dielectric materials. However, attributing them to a specific application needs more elaboration of the material’s refractive index’s relation with the introduced application. In gaseous medium, the optical properties of molecules are related to their dipole moment polarizability. Hence, the theoretical index–density relation of Lorentz–Lorenz is applied in the proposed sensing mechanism to interpret changes in gas’ refractive index to changes in its density. The proposed plasmonic mass density sensor shows a sensitivity of 348.8 nm/ gr cm3 for methane gas in the visible light region. This sensor can be integrated with photonic circuits for gas sensing purposes.

GRINDING OF SILICON CARBIDE FOR OPTICALSURFACE FABRICATION, PART 1: SURFACE ANALYSIS

Prithiviraj Shanmugam, John Lambropoulos, and Matt Davies

DOI: 10.1364/AO.455863 Received 09 Feb 2022; Accepted 19 Apr 2022; Posted 20 Apr 2022  View: PDF

Abstract: This paper presents a study of the grinding of three different grades of silicon carbide (SiC) under the sameconditions. Surface topography is analyzed using coherent scanning interferometry and scanning electronmicroscopy. The study provides a baseline understanding of the process mechanics and targets effective selectionof process parameters for grinding silicon carbide optics with near optical level surface roughness, thus reducingthe need for post-polishing. Samples are raster and spiral ground on conventional precision machines with metaland copper-resin bonded wheels under rough, medium and finish grinding conditions. Material microstructureand grinding conditions affect attainable surface roughness. Local surface roughness of less than 3 nm RMS wasattained in both chemical vapor deposited (CVD) and chemical vapor composite (CVC) SiC. The tool footprint issuitable for sub-aperture machining of a large freeform optics possibly without the need for surface finish correctionby post-polishing. Subsurface damage is assessed in part 2 of this paper series.

Aircraft visibility in view from below in long waveinfrared band using infrared cross-section

Shripad Mahulikar, Pallavi Rastogi, Ashish Bhatt, and Shubendu Valodi

DOI: 10.1364/AO.459926 Received 31 Mar 2022; Accepted 18 Apr 2022; Posted 19 Apr 2022  View: PDF

Abstract: Aircraft low observables’ features are crucial in long wave infrared(LW-IR) band, due to imaging sensors used in IR Search and Trackand in latest generation of IR-guided missiles. Earthshineirradiance on aircraft bottom surface is an important source; hence,it is derived using data for atmospheric transmission. Emission dueto skin-friction heating (important at high bot) and earthshinereflection (important at low bot) are compared by a dimensionlessratio, for different bottom surface emissivities (bot). Infrared CrossSection of aircraft in direct view from below is obtained in LW-IRband, which shows that aircraft is seen also due to negative contrast.

Characterisation of liquid crystals in optically thin cells

Denitsa Bankova, Nicolas Brouckaert, Nina Podoliak, Benjamin Beddoes, Eleanor White, Oleksandr Buchnev, Malgosia Kaczmarek, and Giampaolo D'Alessandro

DOI: 10.1364/AO.456659 Received 01 Mar 2022; Accepted 16 Apr 2022; Posted 18 Apr 2022  View: PDF

Abstract: The current development of new liquid crystal devices often requires the use of thin cells and new, experimental materials. Characterising these devices and materials with optical methods can be challenging if (1) the total phase lag is small (``thin cells'') or (2) the liquid crystal optical and dielectric properties are only partially known. We show that even cells with a phase lag of ΔΦ≤π, such as E7 liquid crystal in a 1.5 μm cell, can be fully characterised using cross-polarised intensity measurements. The reliability of the optical method is also demonstrated for liquid crystals without precise values of the dielectric or refractive index coefficients.

All-fiber high-power erbium-doped laser systemgenerating optical pulses with a duration of 200 μs to5 ms for fractional photo-rejuvenation

Maksim Koptev, Andrey Morozov, Ksenia Shatilova, Sergey Muravyev, Alexander Zapryalov, Mikhail Likhachev, and Arkady Kim

DOI: 10.1364/AO.455761 Received 08 Feb 2022; Accepted 31 Mar 2022; Posted 13 May 2022  View: PDF

Abstract: An all-fiber high-power erbium-doped fiber laser (EDFL) source generating optical pulses from 200 μs to 5 ms with astable rectangular envelope for fractional photo-rejuvenation is proposed and experimentally demonstrated. A masteroscillator power amplifier (MOPA) configuration composed of a master oscillator, an acousto-optic modulator (AOM)and a one-stage amplifier is designed and employed in the EDFL to serve as an efficient laser system with excellentoutput performance. To avoid multistage amplifiers, the master oscillator generates 1.5 W and for one-stage poweramplifier a Yb-free Er-doped large-mode-area (LMA) active fiber is used, in which we have two benefits: first,modulation of both pump and seed pulses is used to achieve clear rectangular shaped pulses without amplifiedspontaneous emission (ASE) growth, and second, there are no power limitations in amplifier and undesirable 1 µm ASEcompared to Er/Yb systems. We have reached 28.6 W of peak power with 26% slope efficiency limited only by availablepump power, so the system can be easily scaled for achieving a higher peak power.

Ultra-Broadband Perfect Absorber Using Triple-layer Nanofilm in Long Wave Near-Infrared Regime

Kaili Kuang, qiao Wang, Xiaomin Yuan, Li Yu, Yuzhang Liang, Yang Zhang, and Wei Peng

DOI: 10.1364/AO.454217 Received 18 Jan 2022; Accepted 21 Mar 2022; Posted 30 Mar 2022  View: PDF

Abstract: Plasmonic absorbers have drawn extensive attentions because of their promising applications in solar cells, controllable thermal emission and infrared detection. Most proposed plasmonic absorbers are with precise designed surface-pattern, which require complex manufacturing process and high cost. Herein, we propose a simple plasmonic absorber composed of triple-layer Ti/SiO2/TiN nanosystem. The maximal absorption reaches 99.8% from 1554 nm to 1565 nm and an average absoption of 95.3% is achieved in the long wave near-infrared range (from 1100 nm to 2500 nm). The synergistic effect of the upper surface plasmon resonance and the Fabry-Perot resonance in the Ti/SiO2/TiN cause the high absorption. Besides, the effects of the incident angle, polarization state, structural materials and geometric parameters on the absorption performance are investigated in detail. The proposed near-infrared absorber has a potential application prospect in solar collectors, thermal emitters, and solar cells, owing to its high absorption, ultra-broadband bandwidth, insensitivity to incident angle and polarization state, low cost and simple preparation process.

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