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Magnetically tunable metasurface comprising InAs andInSb pixels for absorbing terahertz radiation

Govindam Sharma, Akhlesh Lakhtakia, Somak Bhattacharyya, and Pradip Jain

DOI: 10.1364/AO.405023 Received 10 Aug 2020; Accepted 25 Sep 2020; Posted 25 Sep 2020  View: PDF

Abstract: A magnetically tunable metasurface comprising meta-atoms with InSb-patched, InAs-patched, and unpatched pixels was simulated using commercial software to maximize the absorption of normally incident radiation in the terahertz-spectral regime, the patches decorating the illuminated face of a goldbacked polyimide substrate. Maximum absorptance of 0.99 and minimum absorptance of 0.95 can be obtained in 0.14–0. -THz-wide bands in the 2 - 4-THz spectral regime, with an average tuning rate of 0.3 THz /T and 0.24-THz dynamic range when the controlling magnetostatic field is aligned parallel to the incident electric field. The use of both InSb and InAs patches is much superior to the use of patches of only one of those materials. The design can be adapted for neighboring spectral regimes by exploiting the scale invariance of the Maxwell equations.

Roughness model of optical surface in ultrasonic assisted diamond turning

Yintian Xing, Yue Liu, Chao Yang, and Changxi Xue

DOI: 10.1364/AO.402613 Received 17 Jul 2020; Accepted 25 Sep 2020; Posted 25 Sep 2020  View: PDF

Abstract: In this paper, the theoretical model is established to predict the optical surface roughness of difficult-to-cut material in ultrasonic assisted diamond turning (UADT). The effects of kinematics, material elastic recovery and plastic side flow aiming at the characteristics of vibration cutting are considered. The convincing results predicted can be obtained when main machining parameters change, such as cutting speed, cutting depth, tool feed rate, tool frequency and amplitude. Furthermore, the model is demonstrated to be correct on basis of comparing the results calculated by theoretical model and simulated by finite element method (FEM). The arithmetic average value of the vertical coordinates of the workpiece surface nodes is regarded as the surface roughness in the FEM. The minimum mesh size of workpiece is set as 5 nm in order to gain relatively exact results and avoid exceeded element distortions. The accuracy of predictive model is verified by cutting the MB5 magnesium alloy with UADT. The maximum error for surface roughness Ra is merely 10.98% and average error is only about 3% after analyzing experiment and prediction results. The optimal surface roughness Ra of magnesium alloy reflector can be 3.388nm with UADT so that the optical application level is realized only by turning means without subsequent abrasive machining. Therefore, the predicting model is valuable for theory guidance and engineering application in the optical manufactory field of difficult-to-cut material with UADT.

Daytime radiative cooler using porous TiO2: recent advances and new approach

zahir mounir and Mourad BENLATTAR

DOI: 10.1364/AO.403373 Received 28 Jul 2020; Accepted 24 Sep 2020; Posted 24 Sep 2020  View: PDF

Abstract: Passive daytime radiative cooling remains a topic of intense interest that has gained great attention recently, mainly due to its important applications, such as electronic heat dissipation and solar cells and photo-thermal technologies. In order to achieve the daytime radiative cooling for thermal management applications, a new smart shield should be designed to have both a low absorptivity in the solar range 0,3-2,5 μm and high emissivity in the atmospheric transparency windows 8-13μm and 16-26μm. In this work, we propose and analyze a new, inexpensive, and highly scalable double-layer coating to reach an efficient passive daytime radiative cooling. Double-layer coatings consisting of a top TiO_2porous adsorbent layer and a bottom providing excellent transmittance in the solar spectrum is achieved properly by borosilicate-crown glass (BK7). We have shown that the proposed device preserves an average absorptivity/emissivity well below of 7% in the entire solar spectrum and almost95% in the atmospheric transparency window. Besides, our design allows for sub-ambient temperature drops of 45° and diurnal cooling power output of 133 W⁄m^2 even in the case of solar irradiance up to1000 W⁄m^2 . The cooling performance persists under high humidity and even considering the non-radiative heat exchange progress into account. The results revealed that the proposed design can be easily applied to large area and promotes a significant step of achieving large scale application in solar cells and related systems.

A Few-mode Fiber Embedded Long-PeriodFiber Grating for Simultaneous MeasurementRefractive Index and Temperature

shuo zhang, Xiang li, Huiwen Niu, Qi Yan, cuiting sun, Feng Peng, Yiwei Ma, kai zhang, Tao Geng, Wenlei Yang, Weimin Sun, and Libo Yuan

DOI: 10.1364/AO.401444 Received 30 Jun 2020; Accepted 23 Sep 2020; Posted 23 Sep 2020  View: PDF

Abstract: A few-mode fiber (FMF) embedded long-period fiber grating (FE-LPFG) isproposed as a sensor for simultaneous measurement of refractive index and temperature.Periodically embedding the FMFs induces the local refractive index modulation to achieve acompact sensor size and obtains a low insertion loss. The simulated results show that the tworesonance dips have opposite waveguide dispersion coefficient. Therefore, they showdifferent refractive index and temperature sensitivities in the experiment. At the same time,the spectral characteristics of double resonance dips provides a condition for simultaneousmeasurement of two parameters. By monitoring wavelength shift of the two dips, thesimultaneous measurement of refractive index and temperature is easily realized.

Adaptive digital filter for the processing ofatmospheric lidar signals measured by imaging lidartechniques

Liang Mei, Chenguang Yang, Zhenfeng Gong, Hui Li, and Zifan Liu

DOI: 10.1364/AO.405049 Received 12 Aug 2020; Accepted 23 Sep 2020; Posted 24 Sep 2020  View: PDF

Abstract: The lidar signal measured by the atmospheric imaging lidar technique is subject to the sunlight background noise,the dark current noise and the fixed pattern noise (FPN) of the image senor, etc., which presents quite differentcharacteristics comparing to the lidar signal measured by the pulsed lidar technique based on the time-of-flightprinciple. Enhancing the signal-to-noise ratio (SNR) of the measured lidar signal is of great importance forimproving the performance of imaging lidar techniques. By carefully investigating the signal and noisecharacteristics of the lidar signal measured by a Scheimpflug lidar (SLidar) based on the Scheimpflug imagingprinciple, we have demonstrated an adaptive digital filter based on the Savitzky–Golay (S-G) filter and the Fourieranalysis. The window length of the polynomial of the S-G filter is automatically optimized by iteratively examiningthe Fourier-domain frequency characteristics of the residual signal between the filtered lidar signal and the rawlidar signal. The performance of the adaptive digital filter has been carefully investigated for lidar signalsmeasured by a SLidar system under various atmospheric conditions. It has been found out that the optimal windowlength for near horizontal measurements is concentrated in the region of 90-150, while it varies mainly in theregion of 40-100 for slant measurements due to the frequent presence of the peak echoes from cloud, aerosollayers, etc. The promising result has demonstrated great potential of utilizing the proposed adaptive digital filterfor the lidar signal processing of imaging lidar techniques in future.

Practical Interrogation Scheme for Abrupt Taper Mach-Zehnder Interferometer Refractive Index Sensor

Xiaoyu Gong and Scott Yam

DOI: 10.1364/AO.400156 Received 15 Jun 2020; Accepted 23 Sep 2020; Posted 23 Sep 2020  View: PDF

Abstract: A practical interrogation scheme of refractive index (RI) sensing system based on the abrupt fiber taper Mach-Zehnder interferometer (MZI) sensor is designed, implemented and demonstrated by experiment. The broad band light source (BBS) and optical spectrum analyzer (OSA) in the conventional design are replaced by two single wavelength laser diodes modulated by a periodical square waveform and a simple photodetector (PD), which can significantly lower the cost and lead to easier integration. The photocurrent of the PD output signal is used as the indicator of the surrounding RI. Automatic data acquisition and processing is realized by LabVIEW programming. The experiment proves the feasibility of the new scheme and shows a high sensitivity ( 71 mV/RIU) and high stability.

Lossy mode resonance in an etched out optical fibertaper covered by ITO thin layer

Eugene Golant, Andrei Pashkovskii, and Konstantin Golant

DOI: 10.1364/AO.401495 Received 02 Jul 2020; Accepted 22 Sep 2020; Posted 23 Sep 2020  View: PDF

Abstract: The results of rigorous calculation of mode-fields in double adiabatic single-mode etched out optical fiber taperscoated with thin Indium Tin Oxide films are discussed in the context of their application as environment refractiveindex sensors. It is shown that only at two particular thicknesses of covering homogeneous section of the taper ITOfilm amounting to about 100 nm and 177 nm, the lossy mode resonance is observed in the wavelength range of1.50 – 1.55 µm. Moreover, the sensitivity of a sensor based on a 177 nm coating is significantly higher, and theresonance width is significantly lower than that of a sensor with a 100 nm coating. Optimal from the viewpoint offigure of merit values for the diameter of a homogeneous section of the etched fiber are defined.

Dynamic calibration for enhancing thestability of a channeled spectropolarimeter

Benjamin Chrysler, Kazuhiko Oka, Yukitoshi Otani, and Nathan Hagen

DOI: 10.1364/AO.409054 Received 02 Sep 2020; Accepted 22 Sep 2020; Posted 23 Sep 2020  View: PDF

Abstract: Channeled spectropolarimeters are optical instruments that measure the spectraldependence of the polarization of light without any mechanically moving parts. An importantfactor in achieving stable and accurate measurements is the calibration process, especially indynamic environments where temperature fluctuations or other factors affect the retardance ofthe components in the polarimeter. In previous research, a self-calibration algorithm wasdeveloped that accounts for these variations without any additional reference measurements.In this paper we identify an ambiguity in the self-calibration phase that limits the allowedtemperature changes to surprisingly small ranges. We show how to adaptively estimate andcorrect for the phase ambiguity using a polynomial curve-fitting algorithm, extending thetemperature range to virtually all practical scenarios. Lastly, we demonstrate the ability of themodified self-calibration algorithm to provide stable reconstruction of the Stokes vector for atemperature range of greater than 40oC using an experimental channeled spectropolarimeter.

High-efficiency full-surface defects detectionfor ICF capsule based on null interferometricmicroscope

Cong Wei, Jianxin Li, Jun Ma, Mingliang Duan, zong yi, Xinyu Miao, Rihong Zhu, Caojin Yuan, GAO DANGZHONG, and Zongwei Wang

DOI: 10.1364/AO.404174 Received 31 Jul 2020; Accepted 21 Sep 2020; Posted 23 Sep 2020  View: PDF

Abstract: Laser inertial confinement fusion (ICF) triggers nuclear fusion reaction via theevenly compress of a capsule containing deuterium tritium fuel with high-power laser.However, isolated defects on the surface of the capsules reduce the probability of ignition. Inthis paper, we present a full-surface defects detection method based on null interferometricmicroscope (NIM) to achieve high-precision, high-efficiency, and full-surface defectsdetection on ICF capsules. A dynamic phase-shifting module is applied to NIM to achievesingle-shot measurement in a single subaperture. With the capsule controlling system, thecapsule is rotated and scanned along a planned lattice to get all subapertures measured. Theeccentricity error can be measured from wavefront aberrations and compensated online toguarantee the measurement accuracy during the scanning process. After the scanning process,all of the surface defects are identified on the full-surface map. Theories and experimentalresults indicate that for the capsule with 875-μm diameter, the lateral resolution could reach0.7 μm, and the measurement time is less than 1 hour. The number of sampling points canreach about 50 million. To the best of our knowledge, our proposed system is the first toachieve full-surface defects detection of ICF capsules with such high efficiency and highresolution at the same time.

Performance analysis of phase retrieval using transport of intensity with digital holography

Haowen Zhou, Elena Stoykova, Mallik Mohd Raihan Hussain, and Partha Banerjee

DOI: 10.1364/AO.404390 Received 05 Aug 2020; Accepted 21 Sep 2020; Posted 23 Sep 2020  View: PDF

Abstract: The performance of direct and unwrapped phase retrieval, which combines digitalholography with the transport of intensity, is examined in detail in this paper. In thistechnique, digital holography is used to numerically reconstruct the intensities at differentplanes around the image plane, and phase retrieval is achieved by the transport of intensity.Digital holography with transport of intensity is examined for inline and off-axis geometries.The effect of twin images in the inline case is evaluated. Phase-shifting digital holographywith transport of intensity is introduced. The performance of digital holography withtransport of intensity is compared with traditional off-axis single and dual wavelengthtechniques, which employ standard phase unwrapping algorithms. Simulations andexperiments are performed to determine and compare the accuracy of phase retrieval througha mean-squared-error figure of merit, as well as the computational speed of the variousmethods.

High-speed scanning chromatic confocal sensorfor 3-D imaging with modeling-free learningcontrol

Shingo Ito, Mathias Poik, Ernst Csencsics, Johannes Schlarp, and Georg Schitter

DOI: 10.1364/AO.399760 Received 08 Jun 2020; Accepted 21 Sep 2020; Posted 21 Sep 2020  View: PDF

Abstract: This paper presents a scanning system that integrates a chromatic confocal displacement sensor for topography measurement of a surface. To take an advantage of its compactnessand reliability, an off-the-shelf chromatic confocal displacement sensor is integrated. Instead ofmoving the sensor, a galvanometer scanner reflects the optical point to increase the scan speed,and fast and accurate scanning motion is realized by learning without a model. The resultingimages are corrected based on a geometric model to compensate for image distortion.

Parametric study of laser-induced damagegrowth in fused silica optics with large beams at351 nm, Part 2 : Fractal analysis

Matthieu Veinhard, Odile Bonville, Roger Courchinoux, Romain Parreault, Jean-Yves Natoli, and Laurent Lamaignère

DOI: 10.1364/AO.400696 Received 24 Jun 2020; Accepted 21 Sep 2020; Posted 22 Sep 2020  View: PDF

Abstract: The impact of laser fluence and pulse duration on both the rate and the probability ofgrowth of laser-induced damage sites has been reported and analyzed statistically in a companionpaper. In this paper, we report and analyze the volume morphology of damage sites during thegrowth process in fused silica optical components, at 351 nm, under various laser fluence andpulse durations. Fractal analysis has been used to quantify the bulk damage morphology. Aclear link between the damage morphology and the laser pulse duration has been observed. Theresults from fractal analysis allows for of a better understanding of the results from the stochasticapproach developed in our companion paper. More specifically, fractal analysis shows how thelaser parameters such as the fluence and the pulse duration impact the phenomenology and thedynamics of the growth process.

Detection of single human hairs with aterahertz nonlinear quantum cascade laser

Atsushi Nakanishi, Hiroshi Satozono, and Kazuue Fujita

DOI: 10.1364/AO.402997 Received 16 Jul 2020; Accepted 21 Sep 2020; Posted 22 Sep 2020  View: PDF

Abstract: We report the demonstration of imaging of a single human hair with a terahertzquantum cascade laser (THz QCL) source based on intracavity difference-frequencygeneration. A single human hair whose diameter was about 100 μm was detected using theTHz QCL source operating at 240 K, of which the THz beam had a linear polarization. Theresults show that the THz image of a human hair clearly depends on the polarization directionof the THz beam. The THz QCL sources that are capable of room temperature operation willbe useful for detection of small foreign objects like human hairs.

Numerical Analysis on Viewing Angle Enhancement of Digital Hologram by Attaching a Pixelated-Random Phase Mask

Woo-Young Choi, Chang-Joo Lee, Bum-Su Kim, Kwan-Jung Oh, Keehoon Hong, Hyon-Gon Choo, Jisun Park, and Seung-Yeol Lee

DOI: 10.1364/AO.403534 Received 24 Jul 2020; Accepted 21 Sep 2020; Posted 22 Sep 2020  View: PDF

Abstract: In digital hologram, the maximum viewing angle of a computer-generated hologram (CGH) is limited by pixel pitch due to diffraction grating equation. Since reducing pixel size of display panel is challenging and costly, we propose a method to expand the viewing angle of a digital hologram by attaching an aligned pixelated random phase mask onto the CGH pattern based on analysis of simulation result. By introducing a phase averaging process to the widely used iterative Fourier transform algorithm, an optimized CGH pattern can be obtained in conjunction with a pixelated random phase mask. Based on scalar diffraction theory, viewing angle enhancement characteristics were verified by comparing the perspective views of a two-plane hologram using a virtual eye model. In addition, we performed full electromagnetic simulations that included effects due to potential fabrication errors such as misalignment, thickness variation, and internal reflections and diffractions between the CGH and random mask patterns.

Parametric study of laser-induced damagegrowth in fused silica optics with large beams at351 nm, Part 1: Stochastic approach

Matthieu Veinhard, Odile Bonville, Stephane Bouillet, Roger Courchinoux, Romain Parreault, Jean-Yves Natoli, and Laurent Lamaignère

DOI: 10.1364/AO.400691 Received 24 Jun 2020; Accepted 21 Sep 2020; Posted 22 Sep 2020  View: PDF

Abstract: Both rate and probability of growth of laser-induced damage sites in fused silicadepend on several parameters. We investigate, in this two part paper, the impact of the laserparameters on damage growth. In part 1, we present statistical measurements of damage growthat different energy densities, pulse durations and initial damage sizes. In part 2, we use fractalanalysis to quantify the evolution of the damage morphology as a function of the laser energydensity and pulse duration. Damage initiation is performed using phase masks. These phasemasks allow for the initiation of evenly-spaced damage sites that can then be exposed to the samelaser beam, and, therefore, the same pulse duration. Such configuration allowed for the study ofdamage growth on a large population of over 5000 damage sites. The results clearly indicate thatboth the probability and the rate at which a damage site will grow strongly depend on the laserpulse duration. These differences can be explained by hypotheses that we have developed froman observation of the bulk damage morphology. Such observations will be presented in details inthe second part of this article.

Fabrication of sealed sapphire microfluidicdevices using femtosecond lasermicromachining

Ahmed Elgohary, Erica Block, Jeffrey Squier, Mohammad Koneshloo, Rajib Shaha, Carl Frick, John Oakey, and Saman Aryana

DOI: 10.1364/AO.400184 Received 15 Jun 2020; Accepted 21 Sep 2020; Posted 22 Sep 2020  View: PDF

Abstract: Due to its hardness, strength, and transparency, sapphire is an attractive materialfor the construction of microfluidic devices intended for high pressure applications, but itsphysiochemical properties resist traditional microfabrication and bonding techniques. Here, afemtosecond pulsed laser was used to directly machine fluidic channels within sapphiresubstrates and to form bonds between machined and flat sapphire windows, resulting in thecreation of sealed microfluidic devices. Sapphire-sapphire bond strength was determined bydestructive mechanical testing and the integrity of the bond was verified by the capillaryfilling of the channel with oil and ethanol. This combination of optical micromachining andbonding establishes a fully integrated approach to the fabrication of sapphire-basedmicrofluidic systems.

Ultra-High Birefringent Low Loss Suspended Elliptical Core PhotonicCrystal Fiber for Terahertz Applications

Md. Anwar Sadath, Mohammad Islam, Md. Hossain, and Mohammad Faisal

DOI: 10.1364/AO.402530 Received 14 Jul 2020; Accepted 21 Sep 2020; Posted 22 Sep 2020  View: PDF

Abstract: This paper presents a low loss suspended core microstructured fiber with ultra-high birefringence for terahertzwave guidance. The finite element method (FEM) with a perfectly matched layer is applied to investigatedifferent important properties including effective material loss (EML), birefringence, dispersion, confinementloss, and percentage of power flow through the core. The suspended elliptical core in the design createsasymmetry and results in an unprecedented value of birefringence. The simulated results using FEM at 1 THzshow an extremely ultra-high birefringence (highest to the best of our knowledge) of 0.1116, nominal EML of0.04716 cm-1, negligible confinement loss of 2.65×10-7cm-1, higher power fraction in core air of 35%, and aneffective modal area of 1.24×105µm. The advancement in technology makes the fabrication possible. Theproposed fiber could be used satisfactorily in the terahertz regime for various polarization preservingapplications and coherent communication.

Enhanced terahertz modulation usingplasmonic perfect absorber based on blackphosphorus

Shuqi Wang, Shuangluan Li, Yuanguo Zhou, jianping huang, Qiang Ren, Jianming Zhuo, and Yijun Cai

DOI: 10.1364/AO.402639 Received 14 Jul 2020; Accepted 21 Sep 2020; Posted 22 Sep 2020  View: PDF

Abstract: In this paper, we design a plasmonic perfect absorber based on black phosphorus(BP) with enhanced terahertz modulation. By tuning the chemical potential (μc) of BP, themodulation depth can reach up to 95%. The influence of geometric size and bandgap of BPon reflection spectra is also investigated. Moreover, the effect of incident angle on thereflectance is discussed with different values of μc. Our results show that the plasmonicnanoslit mode contributes to the enhancement of the modulation effect. This simple periodicalstructure provides a potential route for designing tunable plasmonic BP-based modulator inthe THz range.

Study on the surface integrity control of laser cleaningof aluminum alloy surface paint layer

Teng Shan, Feng Yin, sijie wang, yulin qiao, and Pengfei Liu

DOI: 10.1364/AO.404030 Received 29 Jul 2020; Accepted 21 Sep 2020; Posted 22 Sep 2020  View: PDF

Abstract: Removal of paint layer of 2024 aluminum alloy was studied using nanosecond fiber pulsed laser with a maximumpower of 30 W and the influence of laser cleaning energy density on the surface integrity of substrate was explored.The cleaning energy density threshold of the paint layer is 17.69 J/ cm2 and the damage energy density threshold is24.77 J/cm2. The optimum cleanliness and surface integrity of laser cleaning were obtained when the energydensity was 21. J/cm2. Microhardness and Young's modulus of the surface after laser cleaning were improved by6% and 25%, respectively. The mechanical properties of the surface of the substrate after laser cleaning weresignificantly improved which is an advantage for this high-quality non-destructive cleaning technology of aircraftskin surface paint layer.

Threshold conditions of Electric field enhancementand energy confinement in low index core ofnanoscale waveguide

Suo-Ming Wang, Yan-Ying Zhu, Wanli Ma, Sijie Luo, Erkuang Zhu, and Shuqing Chen

DOI: 10.1364/AO.400081 Received 12 Jun 2020; Accepted 21 Sep 2020; Posted 24 Sep 2020  View: PDF

Abstract: Threshold conditions to realize electric field enhancement and energy confinement in the low refractive index coreof nanoscale waveguide are studied by solving the field function. When the incident light wave meets the relation ofspecial thresholds, we observe the enhanced electric field and a concentrated light energy be in the core. Theelectric field enhancement and the confined light power is highly dependent on the light wavelength. When thecore width is 30 nm, for a wavelength of 1.55 µm, we achieve a power confinement factor above 40% at the corewidth of 30 nm and the wavelength of 1.55 µm. As the basis for a growing number of potential applications, thethreshold conditions discovered in this work will find significant applications in many fields, such as opticalsensors and optical communication components.

Comparison of nonlinear filtering techniques forphotonic systems with blackbody radiation

Isaac Spotts, C Harrison Brodie, S Gadsden, Mohammad Al-Shabi, and Christopher Collier

DOI: 10.1364/AO.403484 Received 23 Jul 2020; Accepted 20 Sep 2020; Posted 21 Sep 2020  View: PDF

Abstract: This work explores a theoretical solution fornoise reduction in photonic systems using blackbodyradiators. Traditionally, signal noise can be reduced byincreasing the integration time during signal acquisition.However, increasing the integration time during signalacquisition will reduce the acquisition speed of thesignal. By developing and applying a filter using a modelbased on the theoretical equations for black bodyradiation, the noise of the signal can be reduced withoutincreasing integration time. In this work, three filters,extended Kalman filter (EKF), unscented Kalman filter(UKF), and extended sliding innovation filter (ESIF), arecompared for blackbody photonic system. The filters aretested on a simulated signal from five scenarios, eachsimulating different experimental conditions. Inparticular, the nonlinear filters, being UKF and ESIF,showed a significant reduction of noise from thesimulated signal in each scenario. The results show greatpromise for photonic systems using blackbody radiatorsthat require post-process for noise reduction.

The unambiguous measurement range and errortolerance in dual-wavelength interferometry

ting guo, Liu jun, Lei Li, Cheng Chen, and Yang Jun

DOI: 10.1364/AO.401876 Received 08 Jul 2020; Accepted 20 Sep 2020; Posted 21 Sep 2020  View: PDF

Abstract: In dual or multi wavelength interferometry, the traditional equivalent wavelength method is widely used for phaserecovery to enlarge the unambiguous measurement range (UMR). But in fact, this method ignores information ofsize and sign (positive or negative) of the single wavelength wrapped phases and the extension of UMR is notsufficient. For the reflective measurement, the largest UMR of the dual or multi wavelength interferometry is half ofthe least common multiple (LCM) of single wavelengths, called as the LCM effective wavelength, which is oftenseveral times to the equivalent wavelengths. But why do we often use the equivalent wavelength and seldom usethe larger UMR in practice? The existing research told us that the actual UMR is related to the measurement errorof single wavelength wrapped phases and half of the LCM effective wavelength is only the theoretical value. Buthow do errors affect the UMR? We think the quantitative analysis and description are lacking. In this paper, wecontinue to study this problem. We analyze it in a graphical method and give some quantitative descriptions. Thesimulation experiments are carried out and verify our analysis above.

Research on multi-parameter characteristicsof PCF sensor modified by GO composite films

Jiaxin Li, zhengrong tong, Weihua Zhang, and Jingwei Liu

DOI: 10.1364/AO.403064 Received 20 Jul 2020; Accepted 20 Sep 2020; Posted 21 Sep 2020  View: PDF

Abstract: We propose a photonic crystal fiber (PCF) sensor based on graphene oxide (GO)composite film modification to simultaneously measure the multi-parameter sensingcharacteristics of humidity, temperature and glucose concentration. The GO-polyvinyl alcohol(PVA) composite film is used to measure the humidity sensing characteristics of the sensorand the glucose oxidase (GOD) composite film is used to measure the sensing characteristicof glucose concentration, respectively. Experiment results show that the sensitivities oftemperature of the GO-PVA coating structure are 0.037 nm/℃, 0.047 nm/℃ and 0.031 nm/℃, the sensitivities of humidity are 0.059 nm/%RH, 0.121 nm/%RH and 0.047 nm/%RH andthe sensitivities of glucose concentration of the GO-GOD coating structure are 0.028nm/(g/L), 0.049 nm/(g/L) and 0.010 nm/(g/L) for three interference dips, respectively. Thestructure is simple to manufacture and can be used as a sensor for detecting multipleparameters. It can be widely used in biomedicine, environmental monitoring and other fields.

Ultra-compact silicon polarization splitterrotator using dual-etched and tapered coupler

changjian xie, Xihua Zou, Peixuan Li, Lianshan Yan, and Wei Pan

DOI: 10.1364/AO.404741 Received 07 Aug 2020; Accepted 20 Sep 2020; Posted 21 Sep 2020  View: PDF

Abstract: Polarization splitter-rotator (PSR) plays a significant role in telecom, datacom andquantum circuits to meet the significant requirements on polarization processing andmultiplexing. We design an ultra-compact PSR based on silicon asymmetrical directionalcoupler (ADC). One waveguide of the ADC is gradually etched in two levels along thecoupling region and the S-bend. A trapezoidal taper on the top of the dual-etched waveguideis firstly formed to satisfy the phase matching condition for the TM-TE mode conversion,such that the TM mode is cross coupled from the input waveguide without etching. Then theetching width gradually increases in the S-bend and the following, to achieve a single-modewaveguide for removing the residual TM mode further. In this way, high extinction ratio canbe achieved for the polarization mode splitting in an ultra-compact silicon ADC. For thedesigned PSR with a total length is about 24 μm, the simulation results reveal that theminimum extinction ratio is greater than 30 dB, 20 dB, or 15 dB within the bandwidth of 33nm, 100 nm, or 150 nm, respectively, while the maximum polarization conversion loss is lessthan 0.4 dB, 0.9 dB, or 1 dB.

Line-scan camera calibration: a robust linear approach

Ruben Usamentiaga, Daniel Garcia, and Francisco delaCalle

DOI: 10.1364/AO.404774 Received 07 Aug 2020; Accepted 19 Sep 2020; Posted 21 Sep 2020  View: PDF

Abstract: Camera calibration requires three steps: estimation of correspondences between world and image coordinates, computation of a linear solution, and non-linear optimization using the linear estimate as startingpoint. The resulting accuracy depends mostly on the first and final steps. However, the non-linear optimization method can only achieve an accurate result when given an initial estimate close to the globalsolution. Therefore, obtaining a good linear estimation is crucial for the performance of the camera calibration procedure. This work proposes a robust method to estimate a linear solution for the calibrationof line-scan cameras, resulting in the individual intrinsic and extrinsic parameters by only using a singleline-scan. The calculated parameters can then be used by non-linear optimization methods to finely adjust the estimation of all the line-scan camera parameters, including distortions. The proposed proceduredoes not impose restrictions on particular orientations, always generating a well-conditioned problemthan can be solved analytically with no optimization required. Extensive experiments are performed toverify the robustness and accuracy of the proposed method. The comparative results demonstrate that theproposed method provides excellent performance.

High consistency fiber-optic Fabry-Perot sensor basedon MEMS for simultaneous temperature and liquidrefractive index measurement

Shuang Wang, Wen Wu, Mei Sang, Junfeng Jiang, Kun Liu, Xue Wang, Xun Yu, and T. Liu

DOI: 10.1364/AO.401162 Received 24 Jun 2020; Accepted 19 Sep 2020; Posted 21 Sep 2020  View: PDF

Abstract: We proposed a hybrid Fabry-Perot fiber-optic sensor based on Microelectro-mechanical System technique formeasuring temperature and liquid refractive index simultaneously, and verify the consistency of four sensors inthe same batch. The sensor consists of a groove-arrays structured glass wafer and two silicon wafers, which areconnected by double-sided anodic bonding. The three parts form two independent Fabry-Perot cavities fortemperature and liquid refractive index sensing, respectively. We selected three sensors in the same batchrandomly and conducted temperature and refractive index experiments to establish the sensing equation. Theexperimental results demonstrate their high consistency with temperature sensitivities of 81.6 pm/°C, 81.8 pm/°C,81.4 pm/°C in the range of 10 °C to 80 °C, and refractive index sensitivities of 1040.11 nm/RIU, 1044.24 nm/RIU,1042.91 nm/RIU in the range of 1.333 to 1.374. The sensors have low cross-sensitivities which are less than 5.86 ×10-6 RIU/ °C, and high precisions of 0.047 °C, 2.14 × 10−6 RIU, respectively. To verify the validity of the sensingequation, we made another sensor in the same batch and got the maximum errors of 0.36 °C and 7.7 × 10-5 RIU,respectively.

Design and analysis of full-scale scanningsystem for curved glass based on motion and 3Dfeatures

Peng Wang, Yulin Fan, Guodong Chen, Wenzheng Chi, Zhenhua Wang, and Lining Sun

DOI: 10.1364/AO.403433 Received 11 Aug 2020; Accepted 18 Sep 2020; Posted 21 Sep 2020  View: PDF

Abstract: In recent years, mobile phones with glass curved screens have become more andmore widely used. The irregular shape of the curved screen and the light transmittancecharacteristic of the glass have brought great challenges to its automatic defect detection.Aiming at the defect detection of the glass cover of the curved screen, this paper designs afull-scale scanning system by combing motion and three-dimensional features. Firstly, ascanning system is constructed and a geometric error modeling method is proposed toimprove the accuracy of the scanning system; secondly, based on the point cloud of the 3Dglass cover obtained by the scanning system, a point cloud registration method is presentedby integrating the motion and 3D features; finally, the laser tracker is further used to calibratethe scanning system to analyze the mechanical error. Experimental results show that theintroduction of straightness error and perpendicularity error can effectively solve themismatch and fault problems of point cloud registration, and improve the accuracy of thescanning system. In addition, the registration method proposed in this paper can effectivelyreconstruct the complete point cloud of 3D glass cover for detection. The reconstructionaccuracy of the plane part can reach 0.031mm, and that of the curved part can reach0.091mm.

Size-dependent optical-electrical characteristic of blue GaN/InGaNmicro-light emitting diodes

Junhui Li and Wenya Tian

DOI: 10.1364/AO.405572 Received 18 Aug 2020; Accepted 18 Sep 2020; Posted 18 Sep 2020  View: PDF

Abstract: To figure out the impact of chip size reduction on optical efficiency in Micro-LEDarray panels, blue InGaN/GaN LED, which consists of 21×7 arrays (60 ppi display) with differentmesa sizes on sapphire substrates are designed and fabricated in this study. Changing the mesaarea of the chip is firstly proposed to investigate the luminous efficiency (cd/A) of the screen. Thecurrent efficiency with the peak wavelength of 450 nm reaches up to 14.29 cd/A for the biggestpixel 50×60 μm2 and to 12.25 cd/A for the 15×25 μm2 chip, delivering the high-level efficienciesto the current LED research field. The mechanisms of size-dependent efficiency variation trendsand efficiency droops of blue LED have been investigated experimentally, confirming that thecurrent efficiency is more efficient at high injection current density while exhibits poorerperformance at the low current density region for smaller chips. The peak efficiency willcorrespond to higher current density with the decrease of chip size according to the carrierrecombination ABC model. Moreover, the characteristic curve of spectrum and the change ofyellow light band under different incident light conditions (i.e., 355 nm and 375 nm) are analyzedby photoluminescence (PL).

Role of scattering by surface roughness in the photoacoustic detection of hidden micro-structures

Vanessa Verrina, Stephen Edward, Hao Zhang, Alessandro Antoncecchi, Stefan Witte, and Paul C.M. Planken

DOI: 10.1364/AO.397264 Received 08 May 2020; Accepted 18 Sep 2020; Posted 21 Sep 2020  View: PDF

Abstract: We present an experimental study in which we compare two different pump-probe setups to generate and detect high-frequency laser-induced ultrasound for the detection of gratings buried underneath optically opaque metal layers. One system is built around a high peak power, low repetition rate femtosecond laser (1 kHz) and the other around a low peak power, high repetition rate femtosecond laser (5.1 MHz). We find that the signal diffracted by the acoustic replica of the grating as a function of pump-probe time delay is very different for the two setups used. We attribute this difference to the presence of a constant background field due to optical scattering by interface roughness. In the high repetition rate setup the optical field diffracted by the acoustic replica is significantly weaker than the background optical field, with which it can destructively or constructively interfere. For the right phase difference between the optical fields, this can lead to a significant "amplification" of the weak field diffracted off the grating-shaped acoustic waves. For the low repetition rate system, the situation is reversed because the field diffracted off the acoustic wave-induced grating is significantly larger than the background optical field. Our measurements show that optical scattering by interface roughness must be taken into account to properly explain experiments on laser-induced ultrasound performed with high repetition rate lasers systems and can be used to enhance signal strength.

Closed-loop laser illumination algorithm using return photon counts

lanlan li, lei zhou, and ren ge

DOI: 10.1364/AO.401524 Received 29 Jun 2020; Accepted 18 Sep 2020; Posted 21 Sep 2020  View: PDF

Abstract: Boresight and jitter are two fundamental pointing errors of laser illumination systems. In this paper, a triangular-scanning algorithm is proposed to estimate the direction of the boresight via a three-step maximum boresight estimation and laser beam deflection procedure. On this basis, the closed-loop laser illumination (CLLI) for non-cooperative targets is realized, and the Cramer-Rao lower bounds (CRLB) performance in the lower limit of the pointing error is analyzed. Additionally, a Monte Carlo simulation system is built, and the performance of CLLI algorithm is analyzed. The simulation results demonstrate that the triangular-scanning algorithm has good performance and can accurately estimate the direction of boresight to achieve CLLI. Further study shows that the simulation results agree well with theoretical estimations and approximate the CRLB at the lower limit.

Aligning an Optical Cavity: with Reference toCavity Ring-Down Spectroscopy

Hamzeh Telfah, Anam Paul, and Jinjun Liu

DOI: 10.1364/AO.405189 Received 11 Aug 2020; Accepted 18 Sep 2020; Posted 23 Sep 2020  View: PDF

Abstract: A procedure for timely, accurate, and reproducible alignment of an opticalcavity is described.

Design of a chromatic confocal displacementsensor integrated with an optical laser head

Adrian Zakrzewski, Michal Cwikla, Piotr Koruba, Piotr Jurewicz, and Jacek Reiner

DOI: 10.1364/AO.403335 Received 23 Jul 2020; Accepted 17 Sep 2020; Posted 17 Sep 2020  View: PDF

Abstract: In this paper, the methodology for design of the system consisted of chromaticconfocal displacement sensor integrated with an optical laser head was presented.Furthermore, the results of experimental characterization of optical components of the laserhead were also included in the numerical analysis. The designed chromatic confocaldisplacement sensor was analyzed as an assembly of components widely available on theoptical market. Nevertheless, the main goal of the numerical optimization was to determinethe influence of individual components of the optical path of the system on its parameters, i.e.measuring range, FWHM of the characteristic spectral peak and intensity that reaches thedetector. The optimized solution was characterized in order to determine the calibrationcurve, as well as other important application parameters of the system. Moreover, taking intoconsideration the integration aspect the caustic of laser beam shaped by the laser head wasmeasured. Finally, the versatility of the system was presented and discussed.

Circuit Level Implementation of Photonic CrystalDevices

mohammad hasn Rezaei and mohammad hasan yavari

DOI: 10.1364/AO.395250 Received 17 Apr 2020; Accepted 17 Sep 2020; Posted 17 Sep 2020  View: PDF

Abstract: Different types of photonic crystal components have been modeled by approximate RLC circuits. The proposed lumpedcircuits exploit the analogy of photonic crystal elements and RLC circuits. They are either coupled to each other or insertedlike lumped circuits to imitate wave propagation within the photonic devices. Different examples such as side-coupledwaveguide-cavity systems, side-coupled cavity-cavity systems, and improved structures are investigated for evaluating thetheory. It is shown that the proposed circuits are exact enough to be substituted into the complicated calculations ofnumerical methods. In addition, the presented practical and straightforward procedure can be employed for flexible andefficient design. The results are verified using the finite-difference time domain (FDTD) numerical simulations and coupledmode theory (CMT) for various devices.

The appearance of circularly polarized light in an atmosphere-ocean system

James Adams

DOI: 10.1364/AO.401292 Received 26 Jun 2020; Accepted 17 Sep 2020; Posted 17 Sep 2020  View: PDF

Abstract: Using a Monte Carlo algorithm to simulate the scattering of sunlight in an atmosphere-ocean system, the degree of circular polarization of light in the ocean has been calculated at multiple depths and directions. In this system circularly polarized is produced by the reflection of light from beneath the surface at an incident angle greater than the critical angle. We present the results of these simulations for different solar angles, wavelengths of light, and models which include the presence of aerosols in the atmosphere and hydrosols in the water.

Monitoring and predicting Fusarium wilt disease in cucumbers based on quantitative analysis of kinetic imaging of chlorophyll fluorescence

Chunyan Zhou, Jiandong Mao, Hu Zhao, Zhimin Rao, and bai zhang

DOI: 10.1364/AO.399320 Received 03 Jun 2020; Accepted 17 Sep 2020; Posted 18 Sep 2020  View: PDF

Abstract: Cucumber (Cucumis sativus L.) is a widely cultivated and economically profitable crop. However, Fusarium wilt disease can seriously affect cucumber yields, as it is difficult to prevent and eliminate. Therefore, a reliable method is needed for the rapid and early detection of Fusarium infection in cucumbers, which could be provided via the kinetic imaging of chlorophyll fluorescence (ChlF). In this study, ChlF imaging and kinetic parameters were utilized with grey and radial basis function models to monitor cucumber Fusarium wilt disease. The results indicate that the disease can be detected and predicted using this imaging technique before symptoms become visible.

Complex wave-front sensing based on alternative structured phase modulation

Rujia Li and Liangcai Cao

DOI: 10.1364/AO.405630 Received 19 Aug 2020; Accepted 16 Sep 2020; Posted 16 Sep 2020  View: PDF

Abstract: Spatial light modulators (SLM), which generate varying phase modulation, is widely used in coherent diffraction imaging (CDI). Random patterns are uploaded on the SLM to modulate the measured wave-front. However, a random pattern is highly complex and requires a reliable SLM. In addition, the uncorrelated terms generated from the random modulations need to be sufficiently captured using an imaging sensor with a high signal-to-noise ratio(SNR) to avoid stagnation during iterations. We propose an alternative structured phase modulation (ASPM) method. The modulations are composed of orthogonally placed phase bars that introduce uncorrelated modulations. The ASPM modulation can act as the phase grating; in addition, the modulated intensities are concentrated, which can be captured with a high SNR. The complexity of the ASPM patterns is significantly reduced, which is helpful for utilizing the SLM to generate reliable phase modulation.

2.8 μm passively Q-switched Er:ZBLAN fiber laser withSb saturable absorber mirror

Jintao Wang, Jincheng Wei, Wen Liu, Peiguang Yan, Chunyu GUO, Linzhong Xia, and Shuangchen Ruan

DOI: 10.1364/AO.402227 Received 08 Jul 2020; Accepted 16 Sep 2020; Posted 16 Sep 2020  View: PDF

Abstract: A Q-switched Er:ZBLAN fiber laser operating at 2.8 μm was realized by employing Sb as the saturable material. TheSb material was deposited on a gold mirror by magnetron sputtering deposition (MSD) method to develop asaturable absorber mirror (SAM). By employing the Sb-SAM in an Er:ZBLAN fiber laser, the stable Q-switchingoperation was achieved at central wavelength of 2799.7 nm with the repetition rates ranging from 33.3 kHz to 58.8kHz and the pulse duration ranging from 5.7 μs to 1.7 μs. The Sb - SAM still works stably under the maximum pumppower of 5.6 W with an output power of 59 mW, corresponding to the pulse energy of 1.03 μJ. To our knowledge,this was the first demonstration of Sb-based saturable material in Er:ZBLAN fiber laser for mid-infrared Q-switchedpulse generation operating at 2.8 μm regime, indicating its potential applications in the mid-infrared waveband.

Digital holographic imaging and classification ofmicroplastics using deep transfer learning

Yanmin Zhu, Chok Hang Yeung, and Edmund Lam

DOI: 10.1364/AO.403366 Received 23 Jul 2020; Accepted 16 Sep 2020; Posted 16 Sep 2020  View: PDF

Abstract: We devise an inline digital holographic imaging system equipped with a lightweightdeep learning network, termed CompNet, and develop the transfer learning for classification andanalysis. It has a compression block consisting of a concatenated retified linear unit (CReLU)activation to reduce the channels, and a class-balanced cross-entropy loss for training. The methodis particularly suitable for small and imbalanced datasets, and we apply it to the detection andclassification of microplastics. Our results show good improvements both in feature extraction,generalization and classification accuracy, effectively overcoming the problem of overfitting.This method could be attractive for future in situ microplastic particle detection and classificationapplications.

Simulation and optimization of a broadband reflectivefar ultra-violet polarimeter

Maëlle Le Gal, Arturo Lopez Ariste, Coralie Neiner, and Martin Pertenais

DOI: 10.1364/AO.397984 Received 03 Jul 2020; Accepted 16 Sep 2020; Posted 17 Sep 2020  View: PDF

Abstract: Traditional transmissive polarimetric methods can be used for wavelengths above 1 nm where birefringent materials both transmit light and create a significant birefringence. Below 1 nm, no suitablesolution is known to measure the four Stokes parameters on a large wavelength range. Therefore, westudy here an innovative reflective (rather than transmissive) polarimeter working in the Far Ultra-Violet(FUV) range from 90 to 130 nm. We take advantage of the phase shift created by reflections as well asthe different reflectivities for p (orthogonal ⊥) and s (parallel k to the plane of incidence) polarizations todesign a FUV polarimeter. The simulation of both the analyzer and the modulator using Mueller matricescoupled to polarimetric efficiencies calculations allowed us to optimize reflective polarimeters for the firsttime. This opens up a new perspective for FUV polarimetry below 1 nm.

Quantitative analysis of analytical elements in Al-In-Sn-O thin filmbased on comprehensive calibration curves using picosecondlaser-induced breakdown spectroscopy

Shiming liu, qing gao, jiasen wu, yangmei xin, Junshan Xiu, zhao li, and Huiqiang Liu

DOI: 10.1364/AO.402542 Received 10 Jul 2020; Accepted 16 Sep 2020; Posted 17 Sep 2020  View: PDF

Abstract: Al-In-Sn-O (AITO) thin film refers to a novel wide-band gap transparent conductivematerial, which is formed by doping aluminum element into In-Sn-O (ITO)material. It is of promisingapplication in deep ultraviolet optoelectronic devices. Al/Al+In+Sn and Sn/Al+In+Sn are capable ofimpacting the optical and electrical properties of AITO thin film. Three groups of AITO thin filmsamples with different sputtering powers, sputtering pressures and different sputtering times wereprepared with magnetron sputtering. The concentration ratio of Al/Al+In+Sn and Sn/Al+In+Sn inAITO samples was quantitatively analyzed with LIBS technology. A single calibration curve wasdrawn based on the sputtering parameters of each group, and the comprehensive calibration curves oftwo concentration ratios under any sputtering parameters were plotted. The accuracy of thecomprehensive calibration curve was determined with samples prepared under random sputteringparameters, and the EDS analysis results were compared with the LIBS technical analysis results. Therelative error was less than 5%, so the LIBS technical analysis was demonstrated to be accurate. Bybuilding the comprehensive calibration curve, a novel method to conduct the rapid online analysis ofAITO thin films and the timely determination of the photoelectrical properties was presented, and thenew application of LIBS technology was developed in thin film semiconductor materials.

The optimal configurations for differentincident polarization states in linearpolarization calibration

XinKai Li, Lingping He, Xingjun Gao, and Bo Chen

DOI: 10.1364/AO.403647 Received 24 Jul 2020; Accepted 16 Sep 2020; Posted 17 Sep 2020  View: PDF

Abstract: The purpose of polarization calibration is to measure the response matrix of aninstrument and the deviation of noise to correct for subsequent flight measurements. Theprecision, however, is relative to the states of incident light. We investigate the influence ofpartially polarized light, in the presence of signal-independent additive noise or signaldependent Poisson shot noise. We obtain the estimation precision for different numbers ofthe polarization state generators and analyzers in linear Stokes measurements. To reduce theinfluence of incident light, we suggest that the numbers of the polarization state generatorsand analyzers should be greater than or equal to 4. In particular, for an instrument includingthree polarizers oriented at 0°, 60° and 120°, estimation precision is found to be dependenton the response matrix and incident polarization states.

Spectral-temporal hybrid modulation for channeledspectropolarimetry

Qiwei Li, Andrey Alenin, and J. Scott Tyo

DOI: 10.1364/AO.404623 Received 14 Aug 2020; Accepted 16 Sep 2020; Posted 17 Sep 2020  View: PDF

Abstract: Channeled spectropolarimeters are capable of estimating spectrally-resolved Stokes parameters from asingle modulated spectrum. However, channel crosstalk and subsequent spectral resolution loss reducethe reconstruction accuracy and limit the systems’ scope of application. In this paper, we propose aspectral-temporal modulation strategy with the aim of extending channel bandwidth and improving reconstruction accuracy by leveraging the hybrid carriers and allocating channels in 2-dimensional Fourierdomain that yield optimal performance. The scheme enables spectral bandwidth and temporal bandwidth to be traded off, and provides flexibility in selecting demodulation strategies based on the featuresof the input. We present an in-depth comparison of different systems’ performance in various input features under the presence of noise. Simulation results show that the hybrid-modulation strategy offers thebest comprehensive performance as compared to the conventional CSP and dual-scan technique.

Feature extraction algorithm of precession target based on image length and Doppler broadening

Li Bingchen, Di Mo, Ziqi Song, NING WANG, wang ran, Miao Lin, and Shiqiang Li

DOI: 10.1364/AO.401887 Received 20 Jul 2020; Accepted 15 Sep 2020; Posted 15 Sep 2020  View: PDF

Abstract: In space defense, utilizing the micromotion features to distinguish real targets from interfering targets and decoys is an effective means. Due to the imaging of the high-speed precession target by microwave radar is consisted of isolated scattering centers, there are many difficulties in using inverse synthetic aperture radar (ISAR) image for feature extraction. Whereas the inverse synthetic aperture ladar (ISAL) image is relatively continuous because of the short wavelength of laser, and the image sequence contains the Doppler information caused by the target precession, which can be used to inverse motion parameters. By establishing an observation model of the precession target and performing image processing on the obtained ISAL image, a new method is proposed for extracting micromotion features based on image length and Doppler broadening in the absence of prior information such as the radius and speed of the target motion. Finally, the effectiveness of the algorithm is verified by experimental results and the error is controlled within 2%.

Enhanced Fourier-transform method for high-density fringe analysis by iterative spectrum narrowing

Shigeru Nakayama, Hidemitsu Toba, Naoki Fujiwara, Takashi Gemma, and Mitsuo Takeda

DOI: 10.1364/AO.402415 Received 17 Jul 2020; Accepted 15 Sep 2020; Posted 16 Sep 2020  View: PDF

Abstract: We propose a model-based fringe analysis technique that enables the Fourier transform method to analyze dense fringe patterns with large phase variations. The conventional Fourier-transform method has a limited dynamic range of measurable phase because the Fourier spectra broadened by large phase variations cannot be separated by the spatial carrier frequency. Our model-based iterative technique effectively narrows the broad spectrum and reduces phase errors. Results of simulations and experiments are presented that demonstrate the validity of the proposed spectrum-narrowing technique for high-density fringe patterns.

Inline holographic microscopy through fiber imagingbundles

Michael Hughes

DOI: 10.1364/AO.403805 Received 28 Jul 2020; Accepted 15 Sep 2020; Posted 15 Sep 2020  View: PDF

Abstract: Fiber imaging bundles are widely used as thin, passive image conduits for miniaturised and endoscopicmicroscopy, particularly for confocal fluorescence imaging. Holographic microscopy through fiber bundles is more challenging; phase conjugation approaches are complex and require extensive calibration.This article describes how simple inline holographic microscopy can be performed through an imagingbundle using a partially coherent illumination source from a multimode fiber. The sample is imaged intransmission, with the intensity hologram sampled by the bundle and transmitted to a remote camera.The hologram can then be numerically refocused for volumetric imaging, achieving a resolution of approximately 6 µm over a depth range of 1 mm. The scheme does not require any complex prior calibrationand hence is insensitive to bending.

Simulation of Digital Holographic Recording andReconstruction Using Generalized Matrix Method

Brad Bazow, Thuc Phan, Thanh Nguyen, Christopher Raub, and George Nehmetallah

DOI: 10.1364/AO.404405 Received 03 Aug 2020; Accepted 15 Sep 2020; Posted 15 Sep 2020  View: PDF

Abstract: In recent years, research efforts in the field of digital holography have expanded significantly, due to the abilityto obtain high-resolution intensity and phase images. The information contained in these images have becomeof great interest to the machine learning community, with applications spanning a wide portfolio of researchareas including bioengineering. In this work, we seek to demonstrate a high fidelity simulation of holographicrecording. By accurately and numerically simulating the propagation of a coherent light source through a seriesof optical elements and the object itself, we accurately predict the optical interference of the object andreference wave at the recording plane including diffraction effects, aberrations, and speckle. We show that theoptical transformation that predicts the complex field at the recording plane can be generalized for arbitraryholographic recording configurations using a matrix method. In addition, we provide a detailed description ofdigital phase reconstruction and aberration compensation, for a variety of off-axis holographic configurations.Reconstruction errors are presented for the various holographic recording geometries and complex fieldobjects. While the primary objective of this work is not to evaluate phase reconstruction approaches, thereconstruction of simulated holograms provides validation of the generalized simulation method. The long-termgoal of this work is that the generalized holographic simulation motivates the use of phase reconstruction of thesimulated holograms to populate databases for training machine learning algorithms aimed at classifyingrelevant objects recorded through a variety of holographic setups.

Observation of vertical wind profiling with lidarbased on correction of sensitivity

Jiangfeng Shao, Dengxin Hua, and Li Wang

DOI: 10.1364/AO.400089 Received 11 Jun 2020; Accepted 14 Sep 2020; Posted 15 Sep 2020  View: PDF

Abstract: A high spectral resolution lidar (HSRL) for simultaneously detecting vertical wind, temperature, and thebackscattering ratio in the troposphere is developed. The atmospheric temperature and vertical wind aredetermined by the Rayleigh scattering spectrum width and Mie scattering spectrum Doppler shift, respectively.The influence of temperature and the backscattering ratio on vertical wind measurement accuracy is also analyzed.The temperature and backscattering ratio affect the wind measurement, which produces the vertical wind offset. Acorrection considering the effects of the method is conducted considering real-time and on-site temperatureprofiles and the backscattering ratio to correct wind measurement sensitivity. Measurements of HSRL taken underdifferent weather conditions (fine and haze days) are demonstrated. Good agreement between the HSRL and theradiosonde measurements was obtained considering lapse rates and temperature inversions. The maximumtemperature offsets were 1.3 and 4 K at a height of 1.5 km on fine and haze days, respectively. Then, real-time andon-site temperature profiles and backscattering ratios were applied to correct the real-time and on-site wind. Thecorrected wind profiles showed satisfactory agreement with the wind profiles acquired from the calibrated windlidar. The maximum detection offsets of the retrieved wind speed were reduced from 1 m/s to 0.55 m/s and from 1m/s to 0.21 m/s, respectively, which were decreases of 0.45 and 0.79 m/s in fine and haze days after correction ofsensitivity. It is evident that the corrected wind method can reduce the influence of temperature and thebackscattering ratio on the wind measurement and the offset of vertical wind. The reliability of the method is alsoproven.

Study of the intra-guide mode conversions of silicaon-silicon transfer waveguide and silicon-on-insulatortapered structure for highly efficient chip-fiber buttcoupling

Ting Yu, DeGui Sun, and Zhuo Chen

DOI: 10.1364/AO.402403 Received 10 Jul 2020; Accepted 14 Sep 2020; Posted 14 Sep 2020  View: PDF

Abstract: An advanced fiber-waveguide coupling scheme is crucial to the effective developments and applications of siliconon-insulator (SOI) waveguide photonic integrated (PIC) chips. In this paper, a regime of the mode conversionswithin guided-channels is investigated to realize a highly efficient fiber-chip coupling with a silica-on-silicon (SOS)transfer waveguide that can be realized with the local oxidation of silicon (LOCOS) technique. With this regime, theoptical-field overlap process of two optical guided modes between the fiber and the SOI waveguide through a gap issimplified to the two loss contributions: (i) the ultra-low coupling loss of the fiber-SOS transfer waveguide and (ii)the Fresnel loss of the SOS transfer waveguide and an SOI tapered structure. As a result, all the mode conversions inthis system are operated within the guided channels, so it is referred to be an intra-guide mode conversion.Further, the overall mode conversion efficiency of such a fiber-SOS-SOI system is modelled, in which the three keyprocesses: the fiber-SOS gap mode conversion, the SOS-SOI interface mode conversion and the SOI taper profilemode conversion are individually investigated and optimized. The simulation values and the experimental resultsare agreeable with each other for both the fiber-SOS waveguide coupling loss and the fiber-SOI waveguide couplingloss. Consequently, a fiber-chip butt coupling loss of 0.8-1.0dB/facet is demonstrated. Finally, the conditions forrealizing the effective LOCOS structure and process are discussed.

Passive UV imaging polarimeter

Samuel Pellicori, Elliot Burke, Carol Martinez, and Roberto Borda

DOI: 10.1364/AO.401221 Received 26 Jun 2020; Accepted 14 Sep 2020; Posted 14 Sep 2020  View: PDF

Abstract: The static polarimeter concept has a design and application flexibility potentially covering spectral ranges from <220nm to ~2500 nm. The original breadboard model of the passive UV polarimeter with sensitivity to 260 nm includedelliptical analysis for general application such as biomedical, industrial, and commercial technology. It was adapted tobe responsive to atmospheric and oceanic science and exo-atmospheric planetary missions to provide linearpolarization-resolved imagery in four spectral passbands between ~415nm and ~340nm in 5 x 10 fields of view.Simultaneous polarimetry is collected without electro-optical or mechanically moving or birefringent modulation ofretardance. The compact, lightweight, rugged architecture uses instead stable thin-film components with low systematicinstrumental polarization to provide high polarimetric accuracy. An internal polarization calibrator/ stability monitorsubsystem provides in-flight corrections for differential errors that might be induced by external environmentalstresses.

Evaluating edge-loss in the reflectance measurementof translucent materials

Lou Gevaux, Lionel Simonot, Raphael Clerc, Morgane Gerardin, and Mathieu Hébert

DOI: 10.1364/AO.403694 Received 12 Aug 2020; Accepted 14 Sep 2020; Posted 14 Sep 2020  View: PDF

Abstract: In many commercial instruments for measuring reflectance, the area illuminated on the measured objectis identical to the area from which light is collected. This configuration is suitable for strongly scattering materialssuch as paper, but issues arise with translucent materials, as a portion of the incident light spreads around theilluminated area by subsurface transport and escapes the detection system. This phenomenon, referred to as edgeloss, yields erroneous, underestimated reflectance measurements. In the case of colored and opalescent materials,the impact of edge-loss on the measured reflectance varies with wavelength, which is a significant issue forspectrophotometer and colorimeter users. In the present study, we investigate the edge-loss phenomenon with anemphasis on human skin measurement. In particular, we use a mathematical model to estimate the PSF oftranslucent materials, relying on the diffusion approximation of the radiative transfer theory, to predict edge-lossmeasurement error. We use this model to discuss the suitability of several commercial spectrophotometers foraccurately measuring translucent materials of various optical properties and show that not all devices are adaptedto all translucent materials.

Compact optical module to generate arbitrary vectorvortex beams

Yuan Zhou, xing li, YaNan Cai, Yanan Zhang, Shaohui Yan, Meiling Zhou, Manman Li, and Baoli Yao

DOI: 10.1364/AO.401184 Received 25 Jun 2020; Accepted 13 Sep 2020; Posted 14 Sep 2020  View: PDF

Abstract: We demonstrated a compact optical module that is capable of generating vector vortexbeams (VVB) efficiently. By this device, a linearly polarized input beam can be converted to a vectorbeam with arbitrary spatial polarization and phase distributions, accompanied by an energy utilizationup to 61%. Equally important, the area utilization of the spatial light modulator (SLM), a keycomponent in the device, is as high as 65.5%. With the designed vector-vortex-beam-generationmodule, several types of VVBs with different vortex topological charges and spatial polarizationdistributions were created experimentally. This device may find applications in optical tweezers, lasermachining, and so on.

Mass-specific light absorption coefficients of mineral particlesin aqueous suspension for the ultraviolet (UV) to near infraredradiation (NIR) spectral region (200 – 2500 nm)

Rüdiger Röttgers and Christian Utschig

DOI: 10.1364/AO.393289 Received 15 Jul 2020; Accepted 13 Sep 2020; Posted 16 Sep 2020  View: PDF

Abstract: Light absorption by in water suspended natural particles in the near infrared radiation (NIR; 780 – 3000 nm)region has received little attention. Minerogenic matter is thought to be one source for NIR light absorption inaquatic environments. Here, mass-specific particulate light absorption coefficients of several particulate singleminerals and mineral samples for the spectral range of 200 – 2500 nm are presented. The current methodologyallows very sensitive measurements of particle suspension with a detection limit of about 2x10-6 m2g-1 for the massspecific absorption coefficient. Except for one, all mineral materials examined possessed significant lightabsorption throughout the full spectral range considered. The spectra revealed absorption features of specificelements (like iron) and from water structures (H20, O-H bonds) in the mineral or crystal structure that have beenknown from reflectance measurements of minerals. The specific absorption in the NIR was as high as 0.02 m2g-1 forlaterite earths samples, but also below the detection limit for a steatite sample in a narrow spectral region (1600 –1800 nm). The specific absorption by mineral particles in the NIR was, hence, highly variable from strongabsorbing black minerals (magnetite) to low absorbing white clays. The information in the absorption coefficientspectrum can be used not only to identify specific mineral in natural particle assemblages but also to quantify theircontribution to total particulate absorption in the NIR.

Ultrashort all-fiber Fabry−Perot interferometerfabricated by CO2 laser

Qianqian Zhang, Zhiyuan Fan, Jianping Zhang, Fengbo Zhang, Qiang Zhang, and Yongmin Li

DOI: 10.1364/AO.402999 Received 16 Jul 2020; Accepted 12 Sep 2020; Posted 14 Sep 2020  View: PDF

Abstract: We proposed and demonstrated a method to fabricate ultrashort all-fiber Fabry−Perot interferometers by splicinga standard single mode fiber and another single mode fiber with concave surface, where the concave surface isconstructed by CO2 laser pulse. The geometric parameters of the concave surface could be controlled flexibly byadjusting the laser pulse and the relative position between the laser beam and the optical fiber. In our experiments,the minimum depth of the concave surfaces was 0.12 μm, which offers a means of fabricating an all-fiberFabry−Perot interferometer with submicron cavity length. Moreover, the ultralow-roughness concave surfacefabricated by CO2 laser pulse is beneficial to improve the fringe visibility of the interferometer. These advantagesmake it attractive for practical applications.

Spectral response of large-area luminescent solar concentrators

Yilin Li, Yongcao Zhang, Yujian Sun, and Tianhui Ren

DOI: 10.1364/AO.403354 Received 21 Jul 2020; Accepted 12 Sep 2020; Posted 14 Sep 2020  View: PDF

Abstract: Measuring the spectral response (SR) of large-area (> 100 cm2) luminescent solar concentrators (LSCs) has proven difficult because common laboratory photovoltaic (PV) instruments that offer monochromatic incidence measure devices with limited (typically < 50 cm2) sizes. This report addresses this issue through a method called regional measurements. In this method, large-area LSCs are configured to small surface and edge regions, which are sequentially illuminated and measured, respectively. The measured SRs of large-area LSCs are consistent with those from the conventional method and the Monte Carlo ray-tracing simulation (MCRTS). This method is also applied to analyze scattering effects in the LSCs, showing the relationships of the scattering-induced power gain and power loss to the surface root-mean-squared (RMS) roughness (Rq) of the devices. The results explain why the PV performance of the LSCs can be improved through proper surface scattering treatment.

Robust frequency stabilization and linewidthnarrowing of a laser with large intermittentfrequency jumps using an optical cavity andan atomic beam

Huidong Kim, Won-Kyu Lee, Chang Yong Park, Myoung-Sun Heo, and Dai-Hyuk Yu

DOI: 10.1364/AO.404817 Received 07 Aug 2020; Accepted 12 Sep 2020; Posted 14 Sep 2020  View: PDF

Abstract: An experimental method is developed for the robust frequency stabilization using ahigh-finesse cavity when the laser exhibits large intermittent frequency jumps. This isaccomplished by applying an additional slow feedback signal from Doppler-free fluorescencespectroscopy in an atomic beam with increased frequency locking range. As a result, a stableand narrow-linewidth 556 nm laser maintains the frequency lock status for more than a week,and contributes to more accurate evaluation of the Yb optical lattice clock. In addition, thereference optical cavity is supported at vibration-insensitive points without any vibrationisolation table, making the laser setup more simple and compact.

ALL-FIBER 1125 NM SPECTRALLY SELECTEDSUBNANOSECOND SOURCE

Irina Zhluktova, Serafima Filatova, Anton Trikshev, Vladimir Kamynin, and Vladimir Tsvetkov

DOI: 10.1364/AO.401668 Received 01 Jul 2020; Accepted 11 Sep 2020; Posted 14 Sep 2020  View: PDF

Abstract: Selection of radiation from the stimulated Raman scattered radiation was demonstrated whileusing a spectral filter, based on a high-reflection fiber Bragg grating and an optical circulator. As aresult, a stable pulsed signal was obtained at a wavelength of 1125 nm with a repetition rate of 1 MHz.The pulse duration and energy were varied from 120 to 173 ps and from 9 nJ to 15 nJ respectivelydepending on the operating regimes of the master oscillator and amplifier.

Diverging cyclic radial shearing interferometry forsingle shot wavefront sensing

Ki-Nam Joo, Dian Bian, Dae Hee Kim, Byunggi Kim, Liandong Yu, and Seung-Woo Kim

DOI: 10.1364/AO.402903 Received 15 Jul 2020; Accepted 11 Sep 2020; Posted 14 Sep 2020  View: PDF

Abstract: In this investigation, we describe a simple cyclic radial shearing interferometer for single shot wavefront sensing.Instead of using a telescope lens system used in typical radial shearing interferometry, a single lens is used to generatetwo diverging radial shearing beams. This simple modification leads to the advantages of conveniently adjusting theradial shearing ratio, compactness of the system and practical ease of the alignment. With the aid of a polarizationpixelated CMOS camera, the spatial phase shifting technique is used to extract the phase with a single image. Mostimportant feature is the fringe contrast enhancement by reducing the aberrations caused by the complicated opticalsystem even though an incoherent light is used. The experimental results show the fringe contrast enhancement is atleast 0.1 better than that of the conventional method, and the wavefronts are properly reconstructed with less than0.071λ root-mean-squared wavefront error regardless of the coherence of the light.

Influence of optical forces induced by paraxialvortex Gaussian beams on the formation of amicrorelief on carbazole-containingazopolymer films

Svetlana Khonina, Andrey Ustinov, Sergey Volotovsky, Nikolay Ivliev, and Vladimir Podlipnov

DOI: 10.1364/AO.398620 Received 26 May 2020; Accepted 11 Sep 2020; Posted 11 Sep 2020  View: PDF

Abstract: The article discusses photoinduced microrelief formation in a film of anazopolymer. A theoretical study of the effect of laser beam polarization on the balance ofoptical forces acting under the direct action of paraxial Gaussian beams on the irradiatedsubstance was made. We have shown that taking into account the gradient and scatteringcomponents of the force does not allow us to correctly describe the shape of themicroasperities obtained on a carbazole-containing azopolymer. An approximation function ispresented that describes the dependence of the microasperities shape on the non-gradientcomponent of the optical force of laser radiation in the absence and presence of a vortexphase. A comparative analysis of the approximation results and experimentally obtainedmicroreliefs was carried out.

Fabrication of micro-lens array for improving depthof-field of integral imaging 3D display

Peng Yan, xiongtu zhou, yongai zhang, and Tailiang Guo

DOI: 10.1364/AO.402704 Received 22 Jul 2020; Accepted 10 Sep 2020; Posted 11 Sep 2020  View: PDF

Abstract: In this paper, we present a new structure of double pinhole / micro-lens array (DP/MLA) with two centerdepth-planes, used for improving the depth-of-field (DOF) of integral imaging (II), which can fabricate bycombination of lithography and ink-jet printing. The results show that a black circular groove array prepared bylithography can be used for micro-lens’s location and reduce the stray light for II. By controlling the parameters ofink-jet printing system, DP/MLA with high precision, high alignment, and good focusing ability can be achieved.When the fabricated DP/MLA is applied in II system, the reconstructed image has a better three-dimensional(3D)image with higher DOF than that by traditional micro-lens array (MLA), and with higher quality than that byordinary double-layer MLA is obtained.

Evaluation of forward reflectance models and empirical algorithms for chlorophyll concentration of stratified waters

Zhongping Lee, Yongchao Wang, Xiaolong Yu, Shaoling Shang, and Kelly Luis

DOI: 10.1364/AO.400070 Received 10 Jun 2020; Accepted 10 Sep 2020; Posted 11 Sep 2020  View: PDF

Abstract: For waters with stratified chlorophyll concentration (Chl), numerical simulations were carried out to gain insight of the forward models of subsurface reflectance and empirical algorithms for Chl from ocean color. It is found that the Gordon and Clark (1980) forward model for reflectance using an equivalent homogeneous water with a weighted average Chl (Chl_avg) as the input works well, but the difference of reflectance between stratified and the equivalent homogeneous water can be more than 10%. Further, the attenuation of upward light is found better approximated as ~1.5 times of the diffuse attenuation coefficient of downwelling irradiance (Kd). On the other hand, although the forward model of reflectance developed in Zaneveld et al. (2005) using the equivalent homogeneous water with a weighted average of the ratio of the backscattering to absorption as the input works well, this model cannot be used to obtain equivalent Chl_avg to estimate reflectance. Further, for empirical Chl algorithms designed for “Case 1” waters, it is found that, for surface Chl in a range of ~0.06–22.0 mg/m3, the predictability of surface Chl is basically the same as that of Chl_avg from the blue-green band ratio or the band difference of reflectance. Because Chl_avg is wavelength and weighting-formula dependent, along that it is required to have profiles of Chl and the optical properties, these results emphasize that for empirical Chl algorithms, it is easier, less ambiguous, certainly more straightforward, simply to use surface Chl for algorithm development and then its evaluation, rather than to use Chl_avg regardless if the water is stratified or not.

Theoretical analysis of large negative dispersion photonic crystal fiber with small confinement loss

Md. Ekhlasur Rahaman, Md. Hossain, Himadri Mondal, Rekha Saha, and Ahmed Saif Muntaseer

DOI: 10.1364/AO.397420 Received 18 May 2020; Accepted 10 Sep 2020; Posted 11 Sep 2020  View: PDF

Abstract: A solid core circular and octagonal photonic crystal fibers (CPCF and OPCF) are proposed for analyzing different guiding properties such as dispersion, effective mode area, non-linearity and confinement loss from 0.8 µm to 2.6 µm wavelength. The proposed structures use three different types of background materials like SF10, BK7 and Silica, separately. Moreover, the fill fraction is varied by changing the diameter of the air-hole where the lattice pitch is unchanged. The proposed PCFs show a high negative dispersion with low confinement loss and small effective mode area. In the proposed design, finite element method (FEM) with a perfectly matched layer (PML) absorbing boundary condition is used. At 1.8 µm wavelength with 0.6 fill fraction,the maximum negative dispersion of -922.5 ps/(nm.km) is observed for CPCF when background material is SF10. In addition, at this particular wavelength, the confinement loss is observed very small. Moreover, -560.12 ps/(nm.km) dispersion is found for the similar condition at 1.55µm wavelength. On the other hand, using BK7 as background material, -706.77 ps/(nm.km)dispersion is found at 1.55 µm wavelength for CPCF. Results also imply that CPCF shows better performance than OPCF for wide wavelength range. Furthermore, at 1.55µm wavelength Silica-based glass exhibits maximum dispersion whereas increasing wavelength flint type glass shows the similar result. Analyzing different guiding properties of PCFs have significant impacton broadband dispersion compensation applications especially using SF10.

A Cost-effective Demonstration of Fiber-opticFrequency Transfer

Hao Zhang

DOI: 10.1364/AO.403634 Received 24 Jul 2020; Accepted 10 Sep 2020; Posted 11 Sep 2020  View: PDF

Abstract: In this paper, we demonstrate a cost-effective solution for fiber-optic frequency transfer. By employing thecommercially available small form pluggable (SFP) transceivers and other components for telecom, the scheme iscompatible with the existing telecom networks. An experiment testbed based on the common round-trip frequencytransfer is carried out to investigate the corresponding performance in detail. Compared with the conventionalanalog electro/optic interconversion scheme, the optical modulation and demodulation employing SFP transceiverhas an inferior performance in terms of additive phase noise. However, the scheme enables a much largeroperation range of receiving optical power for an optimal system performance, which indicates the certaincapability of adapting to different fiber transmission links. On the basis of the established testbed, fiber-opticfrequency transfer with a frequency of 1 GHz is demonstrated over a 50 km dispersion-compensated fiber link. Andthe additive phase noise of -84 dBc/Hz @1 Hz and -130 dBc/Hz @10 kHz is achieved. Simultaneously, the Allandeviation (ADEV) of 9.6×10-14 /s and 8.4×10-17 /104 s is reached, respectively. The scheme can provide a relativelyfeasible solution to implement fiber-optic frequency transfer over the existing telecom network infrastructure.

3D Reconstruction of Objects with Occlusion andSurface Reflection Using a Dual Monocular StructuredLight System

Kejing He, Congying Sui, Congyi Lyv, Zerui Wang, and Yunhui LIU

DOI: 10.1364/AO.402146 Received 08 Jul 2020; Accepted 10 Sep 2020; Posted 11 Sep 2020  View: PDF

Abstract: Three dimensional vision plays an important role in industrial vision, where occlusion and reflectionhas made it challenging to reconstruct the entire application scene. In this paper, we present a novel 3Dreconstruction framework to solve the occlusion and reflection reconstruction issues in complex scenes. Adual monocular structured light system is adopted to obtain the point cloud from different viewing anglesto fill the missing points in the complex scenes. To enhance the efficiency of point cloud fusion, we create adecision map that is able to avoid the reconstruction of repeating regions of left and right system. Besides,a compensation method based on the decision map is proposed for reducing the reconstruction error ofthe dual monocular system in fusion area. Gray-code and phase-shifting patterns are utilized to encodethe complex scenes, while the phase-jumping problem at the phase boundary is avoided by designingan unique compensation function. Various experiments including accuracy evaluation, comparison withtraditional fusion algorithm and the reconstruction of real complex scenes are conducted to validate themethod’s accuracy and the robustness to the shiny surface and occlusion reconstruction problem.

X-ray attenuation models to account for beamhardening in computed tomography

Qiheng Yang, Wilfred Fullagar, Glenn Myers, Shane Latham, Trond Varslot, Adrian Sheppard, and Andrew Kingston

DOI: 10.1364/AO.402304 Received 09 Jul 2020; Accepted 10 Sep 2020; Posted 11 Sep 2020  View: PDF

Abstract: We introduce a beam-hardening correction method for lab-based X-ray computedtomography (CT) by modifying existing iterative tomographic reconstruction algorithms. Ourmethod simplifies the standard Alvarez-Macovski X-ray attenuation model (Physics in Medicine &Biology, 5, 733 [1976]) and is compatible with conventional (i.e. single-spectrum) CT scans. Thesole modification involves a polychromatic projection operation which is equivalent to applying aweighting that more closely matches the attenuation of polychromatic X-rays. Practicality is apriority, so we only require information about the X-ray spectrum and some constants relatingto material properties. No other changes to the experimental setup or the iterative algorithmsare necessary. Using recontructions of simulations and several large experimental datasets, weshow that this method is able to remove or reduce cupping, streaking, and other artefacts fromX-ray beam hardening and improve the self-consistency of projected attenuation in CT. Whenthe assumptions made in the simplifications are valid, the reconstructed tomogram can even bequantitative.

Digital holography for spatially resolved analysis of semiconductor optical response

Vira Besaga, Nils Gerhardt, and Martin Hofmann

DOI: 10.1364/AO.402488 Received 13 Jul 2020; Accepted 09 Sep 2020; Posted 10 Sep 2020  View: PDF

Abstract: We present spatially resolved measurements of the below-band-gap carrier-induced absorption and con-current phase change in a semiconductor with the help of transmission digital holography. The application is demonstrated for a bulk GaAs sample while the holograms are recorded with a conventional CMOS sensor. We show that the phase information enables spatially resolved monitoring of excess carrier distributions. Based on that, we discuss a phase-based approach for separation of carrier and heat related effects in the semiconductor optical response.

UV-vis spectroscopic studies of low-energy argon ion-bombarded ion-exchanged glasses

Zhimin Zhao, Wenjie Zhang, Zijuan Xie, Zhenlu Liu, and Xiaoyan Yu

DOI: 10.1364/AO.402058 Received 07 Jul 2020; Accepted 09 Sep 2020; Posted 10 Sep 2020  View: PDF

Abstract: Replacing small sodium ions in silicate glass with larger potassium ions from a molten KNO3 salt bath below Tg inducing a compressive stress in the subsurface region of glass leads to glass strengthening. The ion-exchanged glasses bombarded with 80 eV, 100 eV, 120 eV argon ions at room temperature are investigated. The optical and structural properties of the ion-exchanged glasses before and after bombardment were analyzed by means of UV-vis spectrometer, X-ray photoelectron spectroscopy and electron probe micro analysis, respectively. The optical absorption and transmittance spectra of ion-exchanged glasses appear obvious change in the UV-visible region after bombardment. Optical absorption band and transmittance properties of the ion-exchanged glasses at about 369-900 nm are less sensitive to the ion bombardment energy than that at about 200-280 nm. The changes in binding energy shift and peak area ratios of non-bridging oxygen (NBO) and bridging oxygen (BO) contributions to the O 1s lines were observed with increasing ion beam bombardment energy. Out-diffused of potassium cations as a consequence of bombardment, the peak of potassium cations concentration in the exchanged region decreases and moves into the interior of glasses in different degrees. The results show that variation of structure and optical properties of the ion-exchanged glasses are indicative of alterations of the silicate network structure induced by argon ion-bombardment, which provide importance information for application of the ion-exchanged glasses.

Numerical investigation of an ultra-broadband, wide-angle, and polarization-independent metasurface light absorber

Ke Zhang, Ruixiang Deng, Lixin Song, and Tao Zhang

DOI: 10.1364/AO.405135 Received 13 Aug 2020; Accepted 09 Sep 2020; Posted 10 Sep 2020  View: PDF

Abstract: In this paper, we propose and numerically investigate an ultra-broadband, wide-angle, and polarization-independent metasurface absorber based on periodic hexagon-latticed titanium (Ti) nanoring arrays over a continuous Ti film. The proposed absorber can achieve more than 90% absorptivity under normal incidence, ranging from 350 nm to 1453 nm, and the average absorption is up to 95.6%. Additionally, the absorptivity still remained beyond 70% when the incident angles vary from 0° to 60°. The simulations of electric field distributions indicate that the broadband absorption performance can be ascribed to the superposition of the localized surface plasmon resonance (LSPR) originated from the nanopillars and nanoholes, respectively. The proposed approach is simple and inexpensive, and the metal material is optional. Therefore, we believe that the proposed absorber will be a candidate for many potential applications, such as thermophotovoltaic cells, thermal emitters, and optoelectronic devices.

Designing Wideband Dielectric Polygonal DirectionalBeam Antenna Using Ray Inserting Method (RIM)

mohammad mahdi taskhiri and saeed fakhte

DOI: 10.1364/AO.397997 Received 21 May 2020; Accepted 08 Sep 2020; Posted 09 Sep 2020  View: PDF

Abstract: This paper presents the design of a wideband polygonal directional beam antenna based on the ray insertingmethod (RIM). The wideband characteristic of the directional beam antenna is achieved thanks to the use ofinhomogeneous dielectric material. Also, unlike most previous works, the present design can be implemented withthe isotropic and above unity refractive index materials, consequently simplifying its fabrication process. TheFinite Difference Time Domain (FDTD) scheme is used to evaluate the directional beam antenna.

Thermally stabilized operating mode of erbiumytterbium laser

Ivan Kosinskii, Yuri Rozhdestvensky, and Andrei Ivanov

DOI: 10.1364/AO.403389 Received 22 Jul 2020; Accepted 08 Sep 2020; Posted 09 Sep 2020  View: PDF

Abstract: We propose a theoretical method of pumping optimization for the Er-Yb laser based on the concept of a self-coolinglaser. The pumping optimization realizes the anti-Stokes fluorescence cooling and excitation transfer by the Ybions simultaneously. In this case, the Yb ions become the sources of cooling while the Er ions remain the heatingsources. With a certain ratio between the cooling and heating sources, the operating temperature of the lasermedium can be stabilized. We simulate the pumping process for the parameters of the Er, Yb: YAG system todemonstrate the possibility of getting a thermally stabilized operating mode of the laser for the ion ratios in therange of 40 to 60 Yb ions to one Er ion. The simulations show that the self-cooled laser medium can beimplemented for the laser intensities of kW/cm2 in the cavity.

Evaluation of systematic errors for the continuouswave NO2 differential absorption lidar employing amultimode laser diode

Liang Mei, Yuan Cheng, Zhen Zhang, Zheng Kong, Chenguang Yang, Zhenfeng Gong, and kun Liu

DOI: 10.1364/AO.403659 Received 29 Jul 2020; Accepted 08 Sep 2020; Posted 09 Sep 2020  View: PDF

Abstract: NO2-differential absorption lidar (NO2-DIAL) technique has been of great interest for atmospheric NO2profiling. Comprehensive studies on the measurement errors of the NO2-DIAL technique is vital for theaccurate retrieval of the NO2 concentration. This work investigates the systematic errors of the recentlydeveloped continuous-wave (CW) NO2-DIAL technique based on the Scheimpflug principle and a high-powerCW multimode laser diode. Systematic errors introduced by various factors, e.g., uncertainty of the NO2differential absorption cross-section, differential absorption due to other gases, spectral drifting of the λon andλoff wavelengths, wavelength-dependent extinction and backscattering effect, have been theoretically andexperimentally studied for the CW-DIAL technique. By performing real-time spectral monitoring on theemission spectrum of the laser diode, the effect of the spectral drifting on the NO2 differential absorptioncross-section is negligible. The temperature-dependent NO2 absorption cross-section in the region of 220 K –294 K can be interpolated by employing a linear fitting method based on high-precision absorption spectra at220 K, 240 K and 294 K. The relative error for the retrieval of the NO2 concentration is estimated to be lessthan 0.34% when employing the interpolated spectrum. The primary interference molecule is found to be theglyoxal (CHOCHO), which should be carefully evaluated according to its relative concentration in respect toNO2. The systematic error introduced by the backscattering effect is subjected to the spatial variation of theaerosol load, while the extinction-induced systematic error is primarily determined by the difference betweenthe aerosol extinction coefficients at λon and λoff wavelengths. A case study has been carried out todemonstrate the evaluations of the systematic errors for practical NO2 monitoring. The comprehensiveinvestigation on systematic errors in this work can be of great value for future NO2 monitoring using the DIALtechnique.

Confocal Laser Scanning Holographic Microscopy ofburied structures

Lena Schnitzler, Krisztian Neutsch, Falk Schellenberg, Martin Hofmann, and Nils Gerhardt

DOI: 10.1364/AO.403687 Received 30 Jul 2020; Accepted 08 Sep 2020; Posted 09 Sep 2020  View: PDF

Abstract: In this paper, we present a confocal laser scanning holographic microscope for the investigation of buriedstructures. The multimodal system combines high diffraction limited resolution and high signal-to-noiseratio with the ability of phase acquisition. The amplitude and phase imaging capabilities of the systemare shown on a test target. For the investigation of buried integrated semiconductor structures we expandour system with an optical beam induced current modality which provides additional structure-sensitivecontrast. We demonstrate the performance of the multimodal system by imaging the buried structures ofa microcontroller through the silicon backside of its housing in reflection geometry.

Full set of piezo-optic and elasto-optic coefficients of Ca3TaGa3Si2O14 crystals at room temperature

Bohdan Mytsyk, Yuriy Suhak, Natalya Demyanyshyn, Oleh Buryy, Nataliya Syvorotka, Dmytro Sugak, Sergii Ubizskii, and Holger Fritze

DOI: 10.1364/AO.398428 Received 25 May 2020; Accepted 07 Sep 2020; Posted 09 Sep 2020  View: PDF

Abstract: The photoelasticity of the Ca3TaGa3Si2O14 (CTGS) crystal was studied by an interferometric method based on a single-pass Mach-Zehnder interferometer. The maximum number of sample orientations for the piezo-optic experiments was applied to prove the accuracy in determination of piezo-optic coefficients. Basing on the matrices of piezo-optic coefficients and of elastic stiffness coefficients, all the coefficients pik of the elastic-optic matrix are calculated. For the highest pik coefficient, the acousto-optic efficiency is evaluated. The results obtained for CTGS are compared with the corresponding results for La3Ga5SiO14 (langasite) crystals. The highest acousto-optic figure-of-merit of CTGS М2 = 1.66•1015 s3/kg is two and three times higher, comparing to langasite, and strontium borate, respectively, which are often used for acousto-optic modulation of light in the UV spectral range.

LiFi Grid: A Novel Machine Learning Approach toUser Centric Design

Mohamadreza Pashazanoosi, S. Alireza Nezamalhosseini, and Jawad Salehi

DOI: 10.1364/AO.396804 Received 05 May 2020; Accepted 07 Sep 2020; Posted 09 Sep 2020  View: PDF

Abstract: A novel machine learning (ML) clustering algorithm, named LiFi Grid, is proposed to design amorphous cells ofLight-Fidelity (LiFi) access points (APs) in order to maximize theminimum SINR from the viewpoint of user-centric (UC) networkdesign. The algorithm consists of two phases. Explicitly, the firstphase consists of finding clusters of user densities based onMean-Shift (MS) clustering algorithm. In contrast to some otherclustering algorithms, such as K-means, MS does not need toknow the number of clusters in advance. Furthermore, CombinedTransmission (CT) scheme is assumed in each cell. In the secondphase, this paper proposes a novel clustering algorithm whichaddresses the problem of grouping APs based on the positionsof users–UC design–in optical wireless networks (OWNs). Hence,it addresses the dynamic resource allocation (DRA) problem inOWNs if APs are considered as network resources. Based on themaximization of minimum SINR metric, LiFi Grid demonstratesthe superior performance relative to conventional fixed-shapedcell-centric (CC) network designs. Additionally, full-compatibilityof LiFi Grid clustering algorithm with the standard IEEE802.15.7 is also shown.

Data-specific mask-guided imagereconstruction for diffuse optical tomography

Sohail Sabir, Sanghoon Cho, Duchang Heo, Kee Hyun Kim, Seungryong Cho, and Rizza Pua

DOI: 10.1364/AO.401132 Received 25 Jun 2020; Accepted 07 Sep 2020; Posted 09 Sep 2020  View: PDF

Abstract: Conventional approaches in diffuse optical tomography (DOT) imagereconstruction often address the ill-posedness of the inverse problem via regularization with aconstant penalty parameter, which uniformly smooths out the solution. In this study, wepresent a data-specific mask-guided scheme which incorporates a prior mask constraint intothe image reconstruction framework. The prior mask was created from the DOT data itself byexploiting the multi-measurement vector formulation. We accordingly propose two methodsto integrate the prior mask into the reconstruction process. First, as a soft prior by exploiting aspatially varying regularization. Second, as a hard prior by imposing a region-of-interestlimited reconstruction. Furthermore, the latter method iterates between discrete andcontinuous steps to update the mask and optical parameters, respectively. The proposedmethods showed enhanced optical contrast accuracy, improved spatial resolution, and reducednoise level in DOT reconstructed images compared with the conventional approaches such asthe modified Levenberg Marquardt approach and l1-regularization based sparse recoveryapproach.

Measurement of Supersonic Jet Screech withFocused Laser Differential Interferometry

Theron Price, Mark Gragston, John Schmisseur, and Phil Kreth

DOI: 10.1364/AO.402011 Received 03 Jul 2020; Accepted 07 Sep 2020; Posted 09 Sep 2020  View: PDF

Abstract: Focused laser differential interferometry (FLDI) is used to measure a well-characterized, 17kHz screech tone emitted from an underexpanded Mach 1.5 jet. Measurements are made atnumerous spatial locations in and around the jet flow-field, where intrusive diagnostics wouldotherwise influence the flow-field. Results from FLDI measurements are shown to agree withmeasurements from microphones and analysis of high-speed schlieren. The agreement is usedto demonstrate FLDI is a valid and accurate technique for measuring screech tones in jetflow-fields, and furthermore that FLDI can be used to measure jet screech at various spatiallocations around the jet, and notably inside of the jet, where microphones and other intrusivediagnostics cannot be used effectively.

Spatially modulated snapshot imaging polarimeter usingtwo Savart polariscopes

jian bo, Wenhe Xing, Yunting Gu, Changxiang Yan, Xiaodong Wang, and Xueping Ju

DOI: 10.1364/AO.403868 Received 30 Jul 2020; Accepted 07 Sep 2020; Posted 09 Sep 2020  View: PDF

Abstract: This paper presents a spatially modulated snapshot imaging polarimeter using two Savart polariscopes(SMSIPTS). It not only can avoid the alignment angle error and additional phase errors of thehalf-wave plate (HWP), but also can avoid changing HWP frequently when we want the targetpolarization state at different wavelengths and can increase some channel bandwidth to improve imagequality, comparing with spatially modulated snapshot imaging polarimeter (SMSIP). The alignmentangle error and additional phase errors of SMSIP and the optical layout and principle of SMSIPTS arederived firstly. The full Stokes polarization images can be obtained by processing interferogram. Basedon SMSIPTS, we determine the filtering method by simulation. We proved the feasibility of SMSIPTSand the effect of SMSIPTS and SMSIP on reconstruction is compared by simulation. At last, weexperimentally verified the feasibility of the theory of SMSIPTS.

An Open Channel Based Dual-Core D-shaped PCFPlasmonic Biosensor

Maliha Momtaj, Jannatul Robaiat Mou, QM Kamrunnahar, and Md. Ariful Islam

DOI: 10.1364/AO.400765 Received 01 Jul 2020; Accepted 07 Sep 2020; Posted 09 Sep 2020  View: PDF

Abstract: A simple dual-core D-shaped plasmonic Refractive Index (RI) sensor with an open arch channel is introduced in this paper. A thin plasmonic gold layer isinserted on the slotted portion, which makes the sensor cost-effective. By introducing a ring in the flat surface of the D-shaped structure coupling effect is increased, which enhances the sensor performance. Thecommonly used Finite Element Method (FEM) is applied to characterize the sensor performance. Numerical investigation under the wavelength interrogationmethod shows that the maximum spectral sensitivity of16,000 nm/RIU and 17,000 nm/RIU along with the corresponding resolution of 6.25×10−6 RIU and 5.88×10−6RIU for x- and y-polarization respectively. In tandemwith that, maximum amplitude sensitivity governed bythe amplitude interrogation method is calculated about2,603.7000 RIU−1 and 3,432.1929 RIU−1for x and ypolarization respectively. The proposed sensor exhibitsa high Figure of Merit (FOM) of 320 RIU−1 and 283.33RIU−1for x and y-polarization respectively, in the RIdetection range of 1.33 to 1.44. Moreover, impact onthe sensitivity with the overall sensor behavior is analyzed by altering the geometrical parameter like pitch,air hole diameter and gold layer thickness. So, with aneye toward the sensor performance and economic viability, this sensor is assignable to bio-sensing applications.

Triple-band black phosphorus-based absorption usingcritical coupling

yanli xu, hongxu li, xin zhang, Bai zhongchen, Zhengping Zhang, and Shuijie Qin

DOI: 10.1364/AO.405225 Received 14 Aug 2020; Accepted 07 Sep 2020; Posted 09 Sep 2020  View: PDF

Abstract: Black phosphorus (BP) is an important two-dimensional (2D) material and plays a key role in new photoelectric devices. In this work, a triple-band BP-based absorber is proposed, in which monolayer BP is coupled with the missing angle rectangular structure. Due to the critical coupling of the guided resonance, the BP absorber achieves triple-band absorption. The results show that the absorption spectra at 2901.76 nm, 3810.71 nm and 4676.97 nm under TM polarization reach a high absorption of 95.45%, 98.68% and 98.06%, respectively. In addition, the absorption peak and resonance wavelength can be flexibly adjusted by the electron doping of BP, the geometrical parameters of the structure and the refractive indexof the dielectric substrate. Because of the anisotropy properties of BP, the structure exhibits polarization dependent. Thus, the missing angle rectangular structure provides a potential for designing mid-infrared absorbers and has practical application significance in many photoelectric devices such as photodetectors, modulators and optical switches.

Novel Acoustic Resonance Laser Calorimetry forMeasurements of Low Optical Absorption

Aleksey Konyashkin, Aleksey Molkov, and Oleg Ryabushkin

DOI: 10.1364/AO.403164 Received 21 Jul 2020; Accepted 06 Sep 2020; Posted 09 Sep 2020  View: PDF

Abstract: A novel method of acoustic resonance laser calorimetry (ARLC) for the determination of low optical absorptioncoefficients of crystals and glasses is introduced. It is based on measurements of the kinetics of the equivalenttemperature of the sample irradiated by the pulse laser radiation. The equivalent temperature of the sample isdirectly obtained by measuring the frequency changes of its temperature-calibrated acoustic resonances excited bythe laser radiation with a corresponding pulse repetition rate. In contrast to the conventional resonantphotoacoustic spectroscopy, where the resonance is used for the enhancement of the registered signal, in ARLC thelaser-excited acoustic resonance of the sample acts as a high-sensitive temperature probe for measurements of itsheating kinetics. The ARLC approach was verified by measuring the optical absorption coefficient of the potassiumdihydrogen phosphate (KDP) crystal (α=4.2·10–2 cm–1) at 1064 nm wavelength.

Broadband/multiband absorption through surfaceplasmon engineering in graphene- wrapped nanospheres

shiva hayati raad and zahra atlasbaf

DOI: 10.1364/AO.400775 Received 22 Jun 2020; Accepted 06 Sep 2020; Posted 09 Sep 2020  View: PDF

Abstract: In this paper, a thin film constructed by a periodic assembly of graphene-wrapped particles with sphericalgeometry has been proposed as a polarization-insensitive reconfigurable perfect absorber. The performance of theproposed structure is based on the cooperative excitation of the quadrupole localized surface plasmons ongraphene shells. By sweeping the quality of graphene shells, it is recognized that the low-quality graphene materialis the best choice for the absorber design. Moreover, the effect of graphene chemical potential and periodicity ofthe particles on the absorptivity of the structure is investigated. The physical mechanism of performance isclarified by investigating the excited localized surface plasmon resonances. Besides, the angle independentbehavior up to around 60 degrees for both transverse electric (TE) and transverse magnetic (TM) waves is proved.Interestingly, by engineering the substrate height, our proposed absorber exhibits dynamic broadbandperformance due to the impedance matching and multiband absorption by enhancing the Fabry-Perot resonancesof a micron-sized substrate. The possibility of attaining a similar static broadband response by stacking multiplelayers is also proved. Our proposed sub-wavelength absorber can be suitable for novel optoelectronic devices dueto its simple geometry.

A measurement algorithm for real front and backcurved surfaces of contact lenses

Kentaro Saeki, Decai Huyan, Mio Sawada, Yijie Sun, Akira Nakamura, Masaki Kimura, Shin Kubota, Kenji Uno, Kazuhiko Ohnuma, and Tatsuo Shiina

DOI: 10.1364/AO.399190 Received 03 Jun 2020; Accepted 05 Sep 2020; Posted 09 Sep 2020  View: PDF

Abstract: : The optical measurement algorithm for the real front and back surfaces of contactlenses from its center to periphery accurately and simultaneously is proposed. It is analgorithm that makes light be incident vertically along the curved surfaces of contact lensesunder the condition that the difference of curvature radii between the front and back surfacesis small enough within N.A. of the optical probe. For this purpose, we adopted Time-DomainOptical Coherence Tomography (TD-OCT) with translation and rotation mechanism. Theshape, thickness distribution and curvature radii of both surfaces were estimated with theOCT signals analysis and the circular approximation. The measured results were comparedwith the designed values and the measured data from a conventional shape measurementdevice. The curved shape of both surfaces and thickness were well matched with the designedvalues from lens center to periphery. In a curvature radius of front surface, there was aproportional bias with a limit of agreement (LoA) of -0.77 % to -2.09 % and a correlationcoefficient was 0.57. On the back surface, there was no systematic bias and minimaldetectable change (MDC) was 0.178 mm in a range of 95 % confidential interval (CI). Theproposed algorithm well visualized the real shape and optical characteristics of the contactlens with the enough accuracy to the design.

High-order DBR semiconductor lasers: Effect of gratingparameters on grating performance

siyu e, Yinli Zhou, Xing Zhang, Jian Zhang, youwen huang, Zeng gang, jinjiang cui, Liu Yun, Yongqiang Ning, and Lijun Wang

DOI: 10.1364/AO.402699 Received 14 Jul 2020; Accepted 05 Sep 2020; Posted 09 Sep 2020  View: PDF

Abstract: In this paper, a high-order Distributed Bragg Reflector (DBR) semiconductor laser operating at 1064 nm is demonstrated based onsimulation analysis. To get optimal Bragg grating characteristics, four parameters of Bragg grating were analyzed in detail. 49-order Bragg gratings were designed with a reflectivity of 6 % and a full width at half maximum (FWHM) of 3 nm, which can realizemode selection while lasing. The Bragg gratings were designed to maximize the use of light. Transmission of the rear laser facet istheoretically 0. This simulation result provides a simple and efficient DBR semiconductor laser scheme without cavity surface coating.

Analytical Approximation of theSecond-Harmonic Conversion Efficiency

John Daniel, Shan-Wen Tsai, and Boerge Hemmerling

DOI: 10.1364/AO.404993 Received 10 Aug 2020; Accepted 05 Sep 2020; Posted 09 Sep 2020  View: PDF

Abstract: The second-harmonic generation process of a focused laser beam inside a nonlinearcrystal is described by the Boyd-Kleinman theory. Calculating the actual conversion efficiencyand upconverted power requires the solution of a double integral that is analytically intractable.We provide an expression that predicts the exact gain coefficient within an error margin of lessthan 2% over several orders of magnitude of the confocal parameter and as a function of thewalk-off parameter. Our result allows for readily tuning the beam parameters to optimize theperformance of the upconversion process and improve optical system designs.

Concave aspheric test combining Dall with Offner nullcompensation using plane wave

Xin Wang, Qiang Liu, Hao Zhou, Zhiping He, and Shu Rong

DOI: 10.1364/AO.402451 Received 10 Jul 2020; Accepted 05 Sep 2020; Posted 10 Sep 2020  View: PDF

Abstract: A plane wave aspheric surfaces test is simpler and easier to assemble than a spherical wave test. An optical systemcombining Dall with Offner null compensation using plane wave is proposed for measuring large-aperture andlarge-relative-aperture aspheric concave surfaces. One back compensation lens and one front compensation lensare used in the system, which can realize high-precision null test. Based on the third-order aberration theory, thenormalized theoretical parameters are calculated by formula derivation, and then are optimized after zoom. Thedesign characteristic is compared with classical plane wave Offner null test. A prototype experiment of theproposed method is carried on a parabolic mirror with a diameter of 500mm and a relative aperture 1/1, and RMSsurface error λ/40 can be obtained.

Tolerancing and characterization of curved image sensor systems

Fabien ZUBER, Bertrand Chambion, Christophe Gaschet, Stéphane Caplet, Stéphane Nicolas, Simon Charrière, and David Henry

DOI: 10.1364/AO.400950 Received 25 Jun 2020; Accepted 05 Sep 2020; Posted 11 Sep 2020  View: PDF

Abstract: Curved image sensors, not having to correct the field curvature, are considered a relevant solution for improving the vast majority of optical systems. They offer the possibility of designing compact aberration-free optical systems. In this work, we explain the advantage of curved sensor system using the aberration theory. A complete procedure was developed to produce functional curved sensors and functional prototypes were carried out. This paper focuses on the tolerancing process of curved sensors and its inclusion in optical design. A compact objective prototype designed and produced demonstrates the advantage of curvature and the impact of tolerancing.

Adaptive Clip-Limit Based Bi-Histogram EqualizationAlgorithm for Infrared Image Enhancement

Abhisek Paul, TANDRA SUTRADHAR, paritosh bhattacharya, and Santi Maity

DOI: 10.1364/AO.395848 Received 23 Apr 2020; Accepted 04 Sep 2020; Posted 08 Sep 2020  View: PDF

Abstract: Infrared (IR) images are basically low-contrast in nature; hence, it is essential to enhance the contrast ofthe IR images to facilitate in real-life applications. Thiswork proposes a novel adaptive clip-limit oriented bihistogram equalization (HE) method for enhancing IRimages. Although HE methods are simple in implementation but often causes over-enhancement due tothe presence of long spikes. To reduce long spikes,this work suggests to apply a log-power operation onthe histogram, where log operation reduces the longspikes and power transformation regains the shape ofthe histogram. At first, histogram separation-point isgenerated applying the mean of the multi-peaks ofthe input histogram. After that, an alteration in theinput histogram is done using the log-power process.Subsequently, a clipping operation on the altered histogram followed by redistribution of the clipped portion is performed to restrict over-enhancement. Next,the modified histogram is sub-divided using histogramseparation-point. Finally, the modified sub-histogramsare equalized independently. Simulation results showthat the suggested method effectively improves thecontrast of IR images. Visual quality evaluationsand quantitative assessment demonstrate that the suggested method outperforms the state-of-the-art algorithms.

Predicting the refractive index of amorphous materials using the Bruggeman effective medium approximation

Carynelisa Haspel and Maor Sela

DOI: 10.1364/AO.402103 Received 06 Jul 2020; Accepted 04 Sep 2020; Posted 09 Sep 2020  View: PDF

Abstract: Previous studies have shown that the Lorentz-Lorenz relationship or molar refractivity/specific refractivity effective medium approximation (EMA) gives a reasonable prediction of the refractive index (RI) of amorphous water ice given the RI of crystalline water ice. In the current study, we show that the Bruggeman EMA, which is based on volume fraction rather than mass density, provides an even closer match to measurements of the RI of several amorphous materials given the RI of their crystalline phase. We show that the Bruggeman EMA provides a good match to measurements of the RI of amorphous ice as well. We conclude that with respect to amorphous materials, the volume fraction of the scattering centers is a more consistently representative quantity than the measured mass density of the amorphous material. Worded another way, assuming that the volume fraction of the scattering centers is a constant for a given amorphous material (with respect to a given range of wavelengths) seems to be a more robust assumption than assuming that the molar mass and molar refractivity or specific refractivity are preserved in going from the crystalline state to the amorphous state of the same material. Our results have implications for astrophysics applications, as well as for the optics of non-crystalline materials in general.

Manipulation of infrared dispersive wave in customized microstructured optical fibers for 1.7 and 2.0 µm light sources

Surajit Bose, Harshavardhan Reddy Pinninty, Jintao fan, Ayhan Demircan, Axel Ruehl, Uwe Morgner, Samudra Roy, Mrinmay Pal, Shyamal Bhadra, and Debashri Ghosh

DOI: 10.1364/AO.398966 Received 01 Jun 2020; Accepted 04 Sep 2020; Posted 09 Sep 2020  View: PDF

Abstract: We demonstrate the controllable generation of infrared dispersive waves (DW) from customized in-house silica microstructured optical fibers (MOFs) by manipulating the location of zero-dispersion wavelength (ZDW) through the structure of the fibers. The highly enriched shaping mechanism of arrested soliton in MOFs with two ZDWs provides a technique for efficient energy transfer into the targeted eye-safe wavelengths at 1.7 and 2.0 𝜇m by the virtue of DW formation.

Diffraction by perfect and imperfect amplitude grating

Antonin Miks and Petr Pokorny

DOI: 10.1364/AO.401606 Received 30 Jun 2020; Accepted 04 Sep 2020; Posted 09 Sep 2020  View: PDF

Abstract: The paper presents theoretical formulas for calculation of diffraction by a perfect infinite and finite amplitude grating with Fresnel and Fraunhofer approximation. Further, general formulas for diffraction byimperfect diffraction grating are derived if edges of the grating are described with general harmonic functions. Such a formalism provides enough power to accurately characterize imperfections of diffractiongrating, and it serves as a simple tool for solution of a diffraction problem.

Data augmentation of optical time series signals for small samples

Xuezhi Zhang, Zhaozhu Liu, Junfeng Jiang, Kun Liu, Fan Xiaojun, Boyue Yang, Min Peng, Guanlong Chen, and T. Liu

DOI: 10.1364/AO.404799 Received 07 Aug 2020; Accepted 03 Sep 2020; Posted 03 Sep 2020  View: PDF

Abstract: It is difficult to obtain a large amount of labeled data, which has become a bottleneck for the application of deep learning to analyze one-dimensional optical time series signals. In order to solve this problem, a deep convolutional generative adversarial network (DCGAN) model suitable for augmenting optical time series signals is proposed. Based on the acoustic emission (AE) data set obtained by optical sensor with small amount, the model can learn the corresponding data features and apply them to generate new data. The analysis results show that our model can generate stable and diverse AE fragments in epoch 500, and there is no model collapse. All the features between the generated data and the original data are not significantly different at the 0.05 level, which confirms that the method in this paper can generate the optical time series signals effectively.

Single speckle image analysis for monitoring the hardening kinetics of glass ionomer cements

Aicha EL CHEIKH, Fabrice Pellen, Bernard Le Jeune, Guy Le Brun, and Marie Abboud

DOI: 10.1364/AO.403027 Received 23 Jul 2020; Accepted 02 Sep 2020; Posted 03 Sep 2020  View: PDF

Abstract: In this paper, we monitor the setting reaction of commercial glass ionomer cements using laser speckle technique and adopting a spatial approach in the analysis of recorded speckle images. Experimental results showed that spatial contrast and speckle grain size increased as two studied cements underwent their setting reactions. After combining two geometrical configurations to measure the intensities of backscattered and transmitted light, we concluded that the increase in speckle grain size was caused by an increase in size of the scattering centers, since cement components aggregate and hence transition from a Rayleigh to a Mie scattering regime. Finally, two main phases were distinguished in the hardening process, as reported in the literature, however, the technique we propose has the advantage of easily identifying these two phases. The analysis of a single speckle image offers multiple advantages over the temporal analysis of a series of speckle images, in particular due to the low number of images recorded and a far shorter image processing time.

Strong Enhanced Efficiency of NaturalAlginate for Polymer Solar Cells throughModifying ZnO Cathode Buffer Layer

Xiaolin Zhang, Wenfei Shen, Fanchen Bu, Yao Wang, Xiaoshuang Yu, Wenna Zhang, Jiuxing Wang, Laurence Belfiore, and jianguo tang

DOI: 10.1364/AO.398545 Received 03 Jun 2020; Accepted 02 Sep 2020; Posted 14 Sep 2020  View: PDF

Abstract: Sodium alginate, as a natural marine biopolymer, possesses many merits such assuper-easy accessibility from ocean, low cost, nontoxicity and no synthesis for practicalapplication. For the chemical structure, SA has enough lone electron pairs of oxygen atoms inthe backbone and short branched chains, which is expected to passivate oxygen vacancy onthe surface of ZnO cathode buffer layer to improve the photovoltaic performance. Herein, itwas applied to modify the surface trap of ZnO layer in fullerene and non-fullerene PSCs. Thedefects were successfully reduced and the trap-assisted recombination decreased. In PTB7-Th:PC71BM system, PCE was improved from 8.06% to 9.36%. In PM6:IT-4F system, PCEwas enhanced from 12.13% to 13.08%. The addition of SA did not destroy the stability of thedevice. Overall, this work demonstrates the potential for preparing the devices with long-timestability and industrial manufacture of PSCs by using biological materials in the future.

Estimation of maximum temperature and thermal crosstalkbetween two active elements in a PIC: development of a thermalequivalent circuit

Shusmitha Kyatam, Stanislav maslovsky, Hugo Neto, Luis Nero, and Joana Mendes

DOI: 10.1364/AO.397684 Received 26 May 2020; Accepted 01 Sep 2020; Posted 02 Sep 2020  View: PDF

Abstract: The operating temperature plays a key role in the performance and lifetime ofphotonic integrated circuits (PIC). Miniaturization and increasing heat dissipation promotethermal crosstalk effects and pose additional challenges to the PIC designer. The EuropeanPhotonics Industry Consortium recommends thermal modeling during design phase; however,a fully numerical optimization of a particular layout requires an unrealistically large numberof simulations. Here, we propose a compromise approach: a set of carefully chosensimulations are performed with a multi-physics software. The obtained results are used toderive a linearized equivalent thermal circuit which can be used to maximize the power levelsand to minimize the distance between the chosen components while guaranteeing the absenceof thermal crosstalk. For simplification, this model is derived considering a PIC with only twoactive components. Other parameters are varied, such as the material of the holder (silicon ordiamond) and the layer of epoxy that is used to attach the PIC to the holder. The obtainedcircuit is used to determine the maximum dissipated power or the minimum distance betweenthe components while keeping some predetermined specifications. The model can be extendedto contain more elements or to include transient analysis of the temperature distribution.

Optics mounting and alignment for the central opticalbench of the dual cavity enhancedlight-shining-through-a-wall experiment ALPS II

Li-Wei Wei, Kanioar Karan, and Benno Willke

DOI: 10.1364/AO.401346 Received 17 Jul 2020; Accepted 01 Sep 2020; Posted 02 Sep 2020  View: PDF

Abstract: ALPS II is a light-shining-through-a-wall experiment seeking axion-like particles. ALPS II will featuretwo 120 m long linear optical cavities that are separated by a wall and support the same photon mode.The central optical bench at the core of the experiment will be equipped with a light-tight shutter and twoplanar mirrors for the cavities. We show that the mounting concept for ALPS II provides sufficient angularstability and verify that a simple autocollimator assisted alignment procedure for crucial componentsof the ALPS II optical cavities can lead to the required overlap of the cavity eigenmodes. Furthermore,we show that mounted quadrant photodiodes added to the optical bench can have sufficient stability tomaintain this overlap even without clear line of sight between the two optical cavities.

Thermal lensing effects and nonlinear refractiveindices of fluoride crystals induced by highpower ultrafast lasers

Liam Andrus and Adela Ben-Yakar

DOI: 10.1364/AO.400242 Received 15 Jun 2020; Accepted 01 Sep 2020; Posted 02 Sep 2020  View: PDF

Abstract: Thermo-optical and nonlinear property characterization of refractive opticalcomponents is essential for endoscopic instrumentation that utilizes high power, highrepetition rate ultrafast lasers. For example, ytterbium-doped fiber lasers are well-suited forultrafast laser microsurgery applications, however, the thermo-optical responses of manycommon lens substrates are not well understood at 1035 nm wavelength. Using a z-scantechnique, we first measured the nonlinear refractive indices of CaF2, MgF2, and BaF2 at 1035nm and found values that match well with those from the literature at 1064 nm. To elucidateeffects of thermal lensing, we performed z-scans at multiple laser repetition rates and multipleaverage powers. The results showed negligible thermal effects up to an average power of 1Wand at 10 W material-specific thermal lensing significantly altered z-scan measurements.Using a 2D temperature model, we could determine the source of the observed thermallensing effects. Linear absorption was determined as the main source of heating in thesecrystals. On the other hand, inclusion of nonlinear absorption as an additional heat source inthe simulations showed that thermal lensing in borosilicate glass was strongly influenced bynonlinear absorption. This method can potentially provide a sensitive method to measuresmall nonlinear absorption coefficients of transparent optical materials. These results canguide design of miniaturized optical systems for ultrafast laser surgery and deep tissueimaging probes.

Few-layer GaSe nanosheets based broadbandsaturable absorber for passively Q-switched solidstate bulk lasers

bing nan shi, Guanbai He, Kejian Yang, Baitao Zhang, and Jingliang He

DOI: 10.1364/AO.404017 Received 31 Jul 2020; Accepted 01 Sep 2020; Posted 02 Sep 2020  View: PDF

Abstract: In this paper, few-layer two-dimensional (2D) GaSe nanosheets were fabricated and utilized as broadbandsaturable absorber (SA) for passively Q-switched (PQS) solid-state bulk lasers operating at 1.06 and 1.99 µm. For1.06 µm laser operation, the maximum average output power, the shortest pulse width, and the largest single pulseenergy were determined to be 438 mW, 285 ns, and 2.31 µJ, respectively. While for 1.99 µm PQS laser operation,they were 937 mW, 383 ns, and 9.56 µJ. Our results identified the great potential applications of few-layer 2D GaSenanosheets for practical optical modulators such as SAs for pulsed laser generation.

Soliton Mode-Locked Pulse Generation with Bulkstructured MXene Ti3AlC2 deposited onto D-shapedfiber

A.A.A. Jafry, A. H. A. Rosol, Nabilah Kasim, Ahmad Razif Muhammad, R RULANINGTYAS, M. Yasin, and Sulaiman Wadi Harun

DOI: 10.1364/AO.403122 Received 17 Jul 2020; Accepted 31 Aug 2020; Posted 01 Sep 2020  View: PDF

Abstract: For the first time, we proposed a bulk structured MXene, Ti3AlC2 deposited onto D-shaped fiber for soliton generation in an Erbium-doped fiberlaser (EDFL) cavity. Our saturable absorber (SA) device based on MAX phase was prepared by using stirring and ultrasonic vibration, which offerseasier sample preparation compared to its 2-dimensional counterparts. By means of the polishing wheel technique, we fabricated a D-shaped fiberwith a controlled polishing depth and incorporated the MAX phase Ti3AlC2 solution onto its polishing region. We obtained a mode-locked solitonpulse with the proposed MAX phase D-shaped (MAX-DS) SA in EDFL cavity. The pulse width, repetition rate, and central wavelength of the pulsetrain are 2.21 ps, 1.89 MHz, and 1557.63 nm, respectively. The polarization insensitive EDFL cavity initiated a soliton operation with superiorstability as the pump power tuned from 21 to 131 mW, the ML laser exhibits an average power of 15.3 mW, a peak power of 3.8 kW, and a pumpefficiency of 12.5%. The MAX-DS SA incorporated inside the EDFL reveals an efficient output performance, with a pulse energy of 8.14 nJ, thehighest ever reported among D-shaped fiber-based SA.

Multi-band metamaterial selective absorber for infraredstealth

jinglan zou, Jianfa Zhang, Yuwen He, Qilin Hong, Cong Quan, and Zhu Hong

DOI: 10.1364/AO.405015 Received 10 Aug 2020; Accepted 29 Aug 2020; Posted 01 Sep 2020  View: PDF

Abstract: Nanostructured selective absorbers have widespreadapplications ranging from artificial color to thermophotovoltaics and radiative cooling. In this paper, we propose a metamaterial selective absorber with a metalinsulator-metal structure for infrared stealth. It canrealize a multi-band absorption, one sharp peak isat 1.54 µm, which can be used to reduce the scattering signals in laser-guided missiles, and the other tworelatively broad absorption peaks are at 2.83 µm and6.11 µm, which can match the atmospheric absorptionband. It can reduce up to 90 % of the detected infrared signals while maintain a relatively high level ofthermal emission capability. The dependence of thespectral characteristics on the incident angle is studied.The infrared signatures of the structure could be suppressed across a wide temperature range.

Robust Polarimetry via Convex Optimization

Jacob Leamer, Wenlei Zhang, Ravi Saripalli, Ryan Glasser, and Denys Bondar

DOI: 10.1364/AO.400431 Received 16 Jun 2020; Accepted 28 Aug 2020; Posted 31 Aug 2020  View: PDF

Abstract: We present mathematical methods, based on convex optimization, for correcting non-physical coherency matrices measured in polarimetry. We also develop the method for recovering the coherency matrices corresponding to the smallest and largest values of the degree of polarization given the experimental data and a specified tolerance. We use experimental non-physical results obtained with the standard polarimetry scheme and a commercial polarimeter to illustrate these methods. Our techniques are applied in post-processing, which compliments other experimental methods for robust polarimetry.

Development of x-ray mirror foils using hotplastic deformation process

Nakaniwa Nozomi, Yuichiro Ezoe, Masaki Numazawa, Mai Takeo, Manabu Ishida, Kumi Ishikawa, Kohei Morishita, and Kazuo Nakajima

DOI: 10.1364/AO.401644 Received 03 Jul 2020; Accepted 27 Aug 2020; Posted 31 Aug 2020  View: PDF

Abstract: We present the development of a hot plastically deformed silicon reflector forlightweight and high angular resolution x-ray mirror. We deformed a silicon substrate usingconical dies with a curvature radius of 100 mm. The measured radii of the reflector areapproximately 100 µm larger than the design values. Owing to a gap between the die and thereflector toward the edge, it is probable that the substrate did not reach the yield point, and elasticspring back occurred. In addition, we evaluated the x-ray imaging capability of the plasticallydeformed silicon reflector for the first time. The estimated angular resolution is 1.76 arc minfrom the entire reflector, and 0.52 arc min in the best region. For the enhancement of theimaging capability, we may improve the shape of die and determine the best parameter set for thedeformation.

Spectrometric methods of grating constant for different holographic materials

Min Huang, Yonghui Xi, JIE PAN, and Xiu Li

DOI: 10.1364/AO.399250 Received 15 Jun 2020; Accepted 27 Aug 2020; Posted 28 Aug 2020  View: PDF

Abstract: In order to measure the grating constants of different holographic materials efficiently and conveniently, we use the spectrophotometers, and the macro angle-resolved spectrum system to measure the spectral information from three types of holographic materials, including the rainbow holographic materials with light pillars, the plain rainbow holographic materials and the matte holographic materials with light pillars. The information obtained from the instruments were used to investigate their grating constants by the grating equation, it was found that for the grating constant calculation, the 45/0 spectrophotometers applied in color measurement were applicable to the materials with rainbow effects, while for the matte holographic materials, the angle-resolved spectrum system was necessary. The results from the spectrometric methods were similar to those of the light optical microscope.

An Auto-Corrected Preconditioning RegularizationInversion Algorithm for Atmospheric TurbulenceProfile

Zhi Cheng, Lixin He, Xin Zhang, Chao Mu, and Ming Tan

DOI: 10.1364/AO.400202 Received 15 Jun 2020; Accepted 24 Aug 2020; Posted 25 Aug 2020  View: PDF

Abstract: Atmospheric turbulence profiles have great significance for adaptive optics, astronomical observations, laserpropagation in atmospheres and free space optical communications. The two-aperture differential scintillationmethod is a recent approach for analysing remote-sensing atmospheric turbulence profiles, which utilizes activebeacons to make it suitable for different measurement situations. The relationship between differentialscintillation and atmospheric turbulence profiles can be modeled using the Fredholm integral equation. To addressthis ill-posed integration problem, the discrete forward observation equation is first analyzed to obtain betterintegration intervals and measurement intervals needed for inversion. Then, an auto-corrected preconditioningconjugate gradient normal residual (PCGNR) algorithm is proposed to acquire atmospheric turbulence profiles.The algorithm contains a developed auto-correction strategy that incorporates incremental differences, adaptivethresholds and weighted averages to correct for artifacts and marginal errors that arise from the PCGNR method.Compared with other regularized methods, the proposed auto-corrected PCGNR method is more accurate androbust in the presence of noise.

3D Finger vein Biometric Authentication withPhotoacoustic Tomography

Ye Zhan, Aditya Rathore, Giovanni Milione, Yuehang Wang, Wenhan Zheng, Wenyao Xu, and Jun Xia

DOI: 10.1364/AO.400550 Received 17 Jun 2020; Accepted 24 Aug 2020; Posted 08 Sep 2020  View: PDF

Abstract: Biometric authentication is the recognition of human identity via uniqueanatomical features. Development of novel methods parallels widespread application byconsumer devices, law enforcement, and access control. In particular, methods based onfinger veins, as compared to face and fingerprints, obviate privacy concerns and degradationdue to wear, age, and obscuration. However, they are 2D and are fundamentally limited byconventional imaging and tissue-light scattering. In this work, for the first time, wedemonstrate a method of 3D finger vein biometric authentication based on photoacoustictomography. Using a compact photoacoustic tomography setup and a novel recognitionalgorithm, the advantages of 3D are demonstrated via biometric authentication of index fingervessels with false acceptance, false rejection, and equal error rates < 1. %, < 9.27%, and <0.13%, respectively, when comparing one finger, a false acceptance rate improvement > 10×when comparing multiple fingers, and < 0.7% when rotating fingers ±30.

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