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Blue noise coding for coherent X-raydiffraction imaging system

zhen fang, Xu Ma, CARLOS RESTREPO, and Gonzalo Arce

DOI: 10.1364/AO.416226 Received 07 Dec 2020; Accepted 04 Mar 2021; Posted 05 Mar 2021  View: PDF

Abstract: Coded X-ray diffraction imaging (CXRDI) is an emerging computational imagingapproach that aims to solve the phase retrieval problem in X-ray crystallography based on theintensity measurements of encoded diffraction patterns. Boolean coding masks (BCM) withcomplementary structures have been used to modulate the diffraction pattern in CXRDI.However, the optimal spatial distribution of BCMs still remains an open problem to bestudied in depth. Based on the spectral initialization criterion, this paper provides a theoreticalproof for the premise that the optimal complementary BCMs should obey the blue noisedistribution in the sense of mathematical expectation. In addition, the benefits of the bluenoise coding strategy are assessed by a set of simulations, where better reconstruction qualityis observed compared to the random BCMs and other complementary BCMs.

Experimental demonstration of a chaoticcommunication system with switchable chaotic carrierwavelength based on two weak-resonant-cavityFabry-Perot laser diodes

Chun-Xia Hu, Guang-Qiong Xia, Dian-Zuo Yue, Zao-Fu Jiang, Bing Cui, Yanfei Zheng, Gong-Ru Lin, and Zheng-Mao Wu

DOI: 10.1364/AO.420239 Received 19 Jan 2021; Accepted 03 Mar 2021; Posted 04 Mar 2021  View: PDF

Abstract: Based on two weak-resonant-cavity Fabry-Perot laser diodes (WRC-FPLDs) with almost identical mode interval,we propose and demonstrate experimentally a chaotic communication system with switchable chaotic carrierwavelength. In such a scheme, a WRC-FPLD (named as T-WRC-FPLD) subject to filtered feedback provided by atunable optical filter (TOF) is taken as the transmitter, and the other WRC-FPLD (named as R-WRC-FPLD) underoptical injection from the T-WRC-FPLD is taken as the receiver. Through setting the central wavelength of theTOF and selecting the matched feedback strength, a desired mode in T-WRC-FPLD can be chosen to stimulateand output a chaotic signal, whose central wavelength is determined by the central wavelength of the TOF. Thus,the T-WRC-FPLD under filtered feedback can provide a chaotic carrier with a switchable central wavelength. Thechaotic signal output from the T-WRC-FPLD is sent to the R-WRC-FPLD, and drive the R-WRC-FPLD for generatinga chaotic signal. Under proper injection strength, high-quality chaotic synchronization between T-WRC-FPLDand R-WRC-FPLD can be realized, and therefore the chaotic communication with switchable carrier wavelengthcan be implemented in this system. We adopt three different chaotic signals provided by three different modesin T-WRC-FPLD as the chaotic carriers, respectively, and inspect the performances of the system. Theexperimental results show that, for the three cases, all the correlation coefficients between T-WRC-FPLD and RWRC-FPLD can be larger than 0.94 under optimized operating parameters. Via the high-quality chaoticsynchronization between T-WRC-FPLD and R-WRC-FPLD, a 5 Gb/s message encrypted in each chaotic carrier canbe successfully decrypted at the receiver for the three cases, and the bit error ratios (BERs) are all lower than3.8×10−3, which is the hard-decision forward error correction (HD-FEC) threshold.

Design of a high throughput telescope based on scanning off-axis Three-Mirror Anastigmat system

Huiru Ji, Zhu Zhengbo, Hao Tan, Yuefan Shan, wei tan, and Donglin Ma

DOI: 10.1364/AO.421998 Received 04 Feb 2021; Accepted 03 Mar 2021; Posted 04 Mar 2021  View: PDF

Abstract: High throughput optical system is defined to possess the features of both large field of view (FOV) and high resolution. However, it is full of challenge to design such a telescope with the two conflicting specifications at the same time. In this paper, we propose a method to design a high throughput telescope based on the classical off-axis Three-Mirror Anastigmat (TMA) configuration by introducing a scanning mechanism. We derive the optimum initial design for the TMA system with no primary aberrations through characteristic ray tracing. During the design process, a real exit pupil is necessitated to accommodate the scanning mirror. By gradually increasing the system’s FOV during the optimization procedure, we finally obtained a high throughput telescope design with an F-number of 6, a FOV of 60°×1.5°, and a long focal length of 876mm. In addition, the tolerance analysis is also conducted to demonstrate the instrumentation feasibility. We believe that this kind of large rectangle FOV telescope with high resolution has broad future applications in the optical remote sensing field.

Optimization of structured illumination seriesfor compressive X-ray tomosynthesis

Hao Xu, Xu Ma, Qile Zhao, CARLOS RESTREPO, and Gonzalo Arce

DOI: 10.1364/AO.417643 Received 28 Dec 2020; Accepted 02 Mar 2021; Posted 02 Mar 2021  View: PDF

Abstract: Compressive X-ray tomosynthesis (CXT) uses a set of encoded projectionmeasurements from different incident angles to reconstruct the object under inspection. Thispaper considers the variable motion of objects on a conveyor mechanism, and establishes animaging model based on the sensing geometry of dynamic CXT system. Then, a numericalalgorithm is proposed to optimize the structured illumination series to improve reconstructionaccuracy with reduced radiation dose. Compared to the state-of-the-art method, the proposedstrategy increases the degrees of optimization freedom by jointly optimizing the coding maskpatterns, locations of X-ray sources and exposure moments in CXT system, and thusobtaining better reconstruction performance. A genetic algorithm is applied to achieve theoptimization results. It shows that the proposed method outperforms the traditional CXTapproach by further improving reconstruction performance under comparable radiation dose.

Polyvinyl Chloride Gels based-Fresnel Zone PlateFabricated by Solvent Evaporation Under DC ElectricField

Cuifei Chen, Miao Xu, Peiwen Xu, and Hongbo Lu

DOI: 10.1364/AO.419431 Received 08 Jan 2021; Accepted 02 Mar 2021; Posted 02 Mar 2021  View: PDF

Abstract: Electroactive polymers-polyvinyl chloride (PVC) gels are useful for the fabrication of microlenses. In this work, a facilemethod was developed for the fabrication of PVC gels-based Fresnel zone plate (FZP). To this end, a concentric zonedelectrode with odd zones connected to both the anode and cathode was used to shape the PVC gels solution. Next, thesolvent of PVC gels solution was evaporated under 60V DC applied electric field at room temperature. During thisprocess, the electric charge injected from the cathode was carried by the plasticizer toward the anode to accumulate onits surface. After full evaporation of the solvent, the PVC gels formed undulated shapes with concentric zoned patternsdue to the electrostatic repulsions. The focal length of the as-obtained PVC gels based-FZP was estimated to 40 cm, andmaximum diffraction efficiency was ~79.4%. In sum, the proposed PVC gels based-FZP with the simple fabricationprocess, compact structure, high transmittance, good optical isotropy, and low power consumption looks verypromising for applications in long-distance optical communication, space navigation, and optical trapping.

Beam Steering in a Narrow-beam Phosphor Down-converted White Light Visible Light Communication Link using Transmitter Lens Decentering

Faheem Ahmad, Rabindra Biswas, and Varun Raghunathan

DOI: 10.1364/AO.416333 Received 01 Dec 2020; Accepted 02 Mar 2021; Posted 03 Mar 2021  View: PDF

Abstract: Indoor visible light communication (VLC) systems with narrow beams can achieve practical few meters long wireless optical links operating at low power levels and high data-rate for supporting point-to-point or multi-point communication. Such narrow beam VLC links can benefit from beam steering to support the mobility of user equipment and cater to multiple users. Simple implementations of beam steering with minimal change to the existing optical hardware are required to enable widespread adoption of beam-steering in VLC implementations. In this work, we study the performance of a simple transmitter lens decentering based beam steering in a narrow-beam, VLC link using phosphor down-converted blue laser transmitter. The beam steering angle and hence the receiver coverage are proportional to the lens decenter and the focal length of the transmitter and receiver lenses. Optical ray tracing is used to quantify the collection efficiency achievable, choose a suitable receiver lens, and understand the role of off-axis aberration in the system performance. In the experimental implementation, the transmitter lens decentering results in steering angle of 5.7º and corresponding receiver displacement of 30 cm per cm of lens decenter for a link length of 300 cm. We measure the VLC beam color content, and illuminance level, which are found to be acceptable for eye-safe indoor usage. We also quantify the data communication performance as a function of beam-steering for +/- 1.25 cm lens decenter using on-off modulated data. Bit-error rates below the forward error correction limit are obtained for receiver displacement or coverage diameter of 75 cm and 60 cm for 1.25 Gbps and 1.5 Gbps data rates respectively.

Reducing backscattering and the Kerr noise inresonant micro optic gyro using twoindependent lasers

Wenyao Liu, Jian Niu, ziwen pan, Yu Tao, Zhou Yanru, Enbo Xing, Jun Liu, and Jun Tang

DOI: 10.1364/AO.417006 Received 09 Dec 2020; Accepted 02 Mar 2021; Posted 03 Mar 2021  View: PDF

Abstract: In resonator micro optic gyroscope (R-MOG), backscattering noise and the Kerrnoise have been a key effect on optical gyro output, and they are difficult to completelysuppress. In this paper, A novel method are proposed for the R-MOG using two independentlasers, by locking the two optical signals at different resonance peaks, the differential outputof the two optical signals is achieved to suppress backscattering noise. At same time, a lightintensity feedback loop based on a light intensity modulator (IM) is added to the loop toensure the same optical power into the cavity. Experimental results show that the lightintensity fluctuation into the gyro system is reduced nearly two orders of magnitude and thebias stability is improved to 9.06 deg/h by using light intensity feedback loop with twoindependent lasers.

Optical Wireless Communication PerformanceEnhancement using Hamming Coding and EfficientAdaptive Equalizer with a Deep Learning BasedQuality Assessment

Bidaa Abul hassan, Hannan Ghanem, Randa S. Hammad, Safie eldin Mohamed, Ahmed Sedik, Rania Eltaieb, Walid El-Shafai, ahmed rashed, Mohsen A. M. El-Bendary, Mohammed Salah F. Tabbour, Ghada M. El Banby, Ashraf A.M. Khalaf, Ahmed Farghal, HossamEl-din Ahmed, gamal A. Hussein, El-sayed M. El-Rabaie, Ibrahim M. Eldokany, Moawad I. Dessouky, Abdelnaser mohamed, Osama Zahran, Maha Elsabrouty, Hesham Fathi, Gerges Salama, Said El-Khamy, Hossam Shalaby, and Fathi Abd El-Samie

DOI: 10.1364/AO.418438 Received 11 Jan 2021; Accepted 02 Mar 2021; Posted 03 Mar 2021  View: PDF

Abstract: Optical Wireless Communication (OWC) technology is one of several alternative technologies for addressing the RFlimitations for applications in both indoor and outdoor architectures. Indoor optical wireless systems suffer from noise and InterSymbol Interference (ISI). These degradations are produced by the wireless channel multipath effect, which causes data ratelimitation, and hence overall system performance degradation. On the other hand, outdoor OWC suffers from several physicalimpairments that affect transmission quality. Channel coding can play a vital role in the performance enhancement of OWC systemsto ensure that data transmission is robust against channel impairments. In this paper, an efficient framework for OWC in developingAfrican countries is introduced. It is suitable for OWC in both indoor and outdoor environments. The outdoor scenario will be suitableto wild areas in Africa. A detailed study of the system stages is presented to guarantee the suitable modulation, coding, equalization,and quality assessment scenarios for the OWC process, especially for tasks such as image and video communication. Hamming andLow-Density Parity Check (LDPC) coding techniques are utilized with an asymmetrically clipped DC-Offset Optical OrthogonalFrequency Division Multiplexing (ADO-OFDM) scenario. The performance versus the complexity of both utilized techniques forchannel coding is studied, and both coding techniques are compared at different coding rates. Another task studied in this paper ishow to perform efficient adaptive channel estimation and hence equalization on the OWC systems to combat the effect of ISI. Theproposed schemes for this task are based on the adaptive Recursive Least Squares (RLS) and the adaptive Least Mean Square (LMS)algorithms with Activity Detection Guidance (ADG) and Tap Decoupling (TD) techniques at the receiver side. These adaptive channelestimators are compared with the adaptive estimators based on the standard LMS and RLS algorithms. Moreover, this paper presentsa new scenario for quality assessment of optical communication systems based on the regular transmission of images over the systemand quality evaluation of these images at the receiver based on a trained Convolutional Neural Network (CNN). The proposed OWCframework is very useful for developing countries in Africa due to its simplicity of implementation with high performance.

A method for exposure dose monitoring and control inscanning beam interference lithography

ying song, Yujuan Liu, Shan Jiang, Yang Zhu, Liu Zhang, and zhaowu liu

DOI: 10.1364/AO.420870 Received 27 Jan 2021; Accepted 02 Mar 2021; Posted 03 Mar 2021  View: PDF

Abstract: To improve grating manufacturing process controllability in scanning beam interference lithography (SBIL), anovel method for exposure dose monitoring and control is proposed. Several zones in a narrow monitoring regionare fabricated on a grating substrate by piecewise uniform scanning. Two monitoring modes are given based on thedifferent widths of the monitoring region. The monitoring curve of the latent image diffraction efficiency toscanning velocity is calculated by rigorous coupled wave analysis. The calculation results show that the exposuredose in SBIL can be monitored by the shape change of the monitoring curve, and an optimized scanning velocity canbe selected in the monitoring curve to control the exposure dose.

Wide dynamic laser ranging based on diodelaser and photon counting techniques

Yumei Tang, Runze Yang, JIAN QIU, and KEFU LIU

DOI: 10.1364/AO.418542 Received 30 Dec 2020; Accepted 01 Mar 2021; Posted 02 Mar 2021  View: PDF

Abstract: In this paper, ultrafast diode laser and photon counting techniques were united to solve thetradeoff between the dynamic range and high precision in the TOF laser ranging. Based onthe D-TOF laser ranging method, we developed a wide dynamic laser ranging system, thepeak power of the emission laser can reach 70W, the width of pulsed laser is less than 2ns,and the repetition rate can reach 500kHz. The experimental results show that the precision is7.5mm when echo signal is saturated,the maximum range can reach kilometer level,theaccuracy is within 30cm, and the repetition rate of measurement can reach 100kHz. Thesystem has the advantages of small size, low cost, fast detection speed and high accuracy,which can be used in the fields of dynamic target measurement and high-speed environmentperception.

Optimization method using Nodal Aberration Theoryfor coaxial imaging systems with radial basis functionsbased on surface slope

Shuai Zhang, Liuchang Xiao, Xing Zhao, Lipei Song, Yongji Liu, Lingjie Wang, Guangwei Shi, and Weiwei Liu

DOI: 10.1364/AO.418563 Received 30 Dec 2020; Accepted 01 Mar 2021; Posted 02 Mar 2021  View: PDF

Abstract: The radial basis functions based on the surface slope (RBF-Slope) freeform surfaces model has been demonstratedstronger fitting ability and better optical performance than the conventional RBF model. However, the large number ofbasis functions and optimization variables of RBF-Slope model may result in convergence problems during theoptimization for the optical system consisting of freeform surface characterized by RBF-Slope. To overcome thesedrawbacks, we use Zernike polynomials to link RBF-Slope model to aberration correction and propose a newoptimization method for coaxial imaging systems using RBF-Slope model based on Nodal aberration theory(NAT). Inthis paper, the aberrations generated by conic parameter and Zernike terms up to Z17/18 of Zernike freeform surface atnonstop in coaxial imaging system are analyzed, the gradient descent is implemented to obtain the optimal coefficientsof Zernike surface which is then fitted by RBF-Slope surface for further optimization. The method is applied to theoptimization of a secondary mirror using RBF-Slope model in two-mirror telescope and proved to have better resultsthan traditional commonly used direct optimization. This research offers an important reference for optimization usingNAT, and provides valuable insight into the optimization method for RBF-Slope freeform.

The investigation on phase stabilization technology of a near-infrared single-mode fiber Mach-Zehnder interferometer

Yun-Rui Bai, Haijuan Yu, Yong Zhang, Zhang Wang Miao, Chaojian He, Rui Dou, Yuting Zhang, and Xuechun Lin

DOI: 10.1364/AO.409588 Received 20 Nov 2020; Accepted 01 Mar 2021; Posted 02 Mar 2021  View: PDF

Abstract: We give an investigation on the phase noise in the single mode fiber Mach-Zehnder interferometer (MZI) using the light source of 1530 nm. According to theoretical analysis, the phase noise is mainly produced by the tiny fluctuation of external temperature while its amplitude is proportional to the optical path difference (OPD) between two arms of optical fibers in the interferometer. By minimizing the length difference between two fibers in MZI and specifying compensation signal applied to the piezoelectric transducer (PZT) modulator actively, the interferometry has worked stably at different phase point difference. When the interferometry working at the 90° phase difference, usually necessary in practical application, the RMS noise is minimized to 0.47°. For a given experiment conditions, the magnification ratio of noise amplitude at room temperature per unit OPD of MZI was measured to be 132.29° /m (0.73 rad/m). In this experiment, a good linear correlation between OPD and phase intensity was observed successfully.

Simulation of the influence of gain and SESAM oncharacteristics of mode-locked VECSELs

Yiwei Zhang, lingling Hua, zhuangbihui zhuang, Jinrong Tian, and Yangrong Song

DOI: 10.1364/AO.417615 Received 18 Dec 2020; Accepted 28 Feb 2021; Posted 01 Mar 2021  View: PDF

Abstract: Vertical external cavity surface-emitting lasers (VECSELs) have unique properties, such as excellent beam quality, highaverage output power and high repetition rate. Short pulses from VECSELs, especially picosecond and femtosecond, can be achievedby passively mode-locking mechanism with semiconductor saturable absorber mirror (SESAM). In this paper, we theoreticallysimulate the pulse evolution dynamics in VECSELs and investigate the influences of different parameters of gain and SESAM on pulsecharacteristics. To the best of our knowledge, this is the first time that the influences of fast recovery time and TPA coefficient on pulsecharacteristics have been studied.

Single-pixel imaging in the presence of specularreflections

mengchao ma, Qianzhen Sun, Xicheng Gao, Huaxia Deng, Guan Wang, Yilong Su, Qingtian Guan, and Xiang Zhong

DOI: 10.1364/AO.418925 Received 04 Jan 2021; Accepted 28 Feb 2021; Posted 01 Mar 2021  View: PDF

Abstract: Single-pixel imaging (SPI), which uses a photodetector to detect the reflected total light intensity of aset of structured illumination patterns modulated by a target scene, provides a method for visible waveband imaging, hyperspectral imaging, and terahertz imaging. However, it faces a challenge when thescene to be imaged has specular reflections. To deal with this problem, a multi-angle method withoutfeature matching is presented. With this method, the location of the detector does not affect the imagereconstruction, and the results of reconstruction at each location are matched at the pixel level automatically. In simulations, with the original image as a reference, the structural similarity index value of thepicture obtained by the proposed method is 10% higher than the picture obtained from a single angle.The signal-to-noise ratio value of the picture obtained by the proposed method is 4.424 which is higherthan 1.577 of the maximum value of the reconstruction result from a single angle. To evaluate the method,a metal key and an aircraft engine blade with specular reflections are taken as the target scene and arereconstructed from four different imaging perspectives, giving results that are matched at the pixel level.The final reconstructed image is obtained using the principal component analysis algorithm or the fourthorder partial differential equations and principal component analysis algorithm. Compared with the image obtained from a single angle, the correlation coefficient between the image obtained by the proposedmethod and the reference image is increased from the minimum value of 0.3139 to 0.7050, and the powerratio is increased from 4.52% to 73.63%. The proposed method has great potential specifically for improving the quality of SPI for scenes exhibiting specular reflections.

Solution-processed WS2 and its doping in PEDOT:PSS for tailoringhole injection in near ultraviolet organic light-emitting diodes

Honghang Wang, Dongliang Li, Yongfang Yuan, Feng Chi, Liming Liu, Lihui Wang, Zongliu Lu, Zhidong Lin, and Xiao-Wen Zhang

DOI: 10.1364/AO.419279 Received 13 Jan 2021; Accepted 28 Feb 2021; Posted 01 Mar 2021  View: PDF

Abstract: Solution-processed ethanol tungsten disulfide and its doping inpoly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS+WS2) fortailoring hole injection in near ultraviolet organic light-emitting diodes (NUV OLEDs)are investigated in details. Raman spectrum, scanning electron microscopy, atomicforce microscopy, X-ray/ultraviolet photoelectron spectroscopy and impedancespectroscopy measurements show that WS2 and its composites behave superior filmmorphology and exceptional electronic properties. Using2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole as emitter andPEDOT:PSS+WS2 as hole injection layer, the NUV OLED produces short-wavelengthemission peaking at 404 nm and full-width at half-maximum of 53 nm. The devicealso gives maximum radiance of 4.7 mW/cm2 and external quantum efficiency of2.1%, which are superior to the counterparts using WS2, UV-Ozone-treated WS2 andPEDOT:PSS. The hole injection ability is enhanced by using WS2, UV-Ozone-treatedWS2, PEDOT:PSS and PEDOT:PSS+WS2 in this order. The robust hole injectioncontributes to carrier balance and accounts for high device performance of thefabricated NUV OLEDs. Our results pave an alternative approach for advancingOLEDs and accelerating WS2 applications.

1.14 W mid-infrared supercontinuum generation in afluoroindate fiber pumped by a fast gain-switched andmode-locked thulium-doped fiber laser system

Jacek Swiderski, Pawel Grzes, and Maria Michalska

DOI: 10.1364/AO.422763 Received 16 Feb 2021; Accepted 27 Feb 2021; Posted 01 Mar 2021  View: PDF

Abstract: We report the generation of a spectrally flat supercontinuum in a fluoroindate (InF3) fiber with a maximum timeaveraged output power of 1.14 W and a spectrum extended to 4.75 μm. The pump source was a fast gain-switchedand semiconductor saturable absorber mirror mode-locked Tm-doped fiber laser followed by a cascade of two Tmdoped fiber amplifiers. The repetition rates of the gain-switched pulses and mode-locked sub-pulses were 50 kHzand 111 MHz, respectively, whereas the duration of sub-pulses recorded within a gain-switched pulse envelopewas ~110 ps. This home-built pump laser system provided an output average power of 2.45 W at a centralwavelength of 2 μm. To the best of our knowledge, this is the first report on supercontinuum generation in an InF3fiber pumped by a fast gain-switched and mode-locked laser system. This approach is promising for furtherefficient continuum generation in the mid-infrared region.

Development of an optical depth sensing technology with amechanical control lens and a diffuser

Chih-Hsiung Lin and Kun-Huang Chen

DOI: 10.1364/AO.415431 Received 03 Dec 2020; Accepted 27 Feb 2021; Posted 01 Mar 2021  View: PDF

Abstract: A novel depth sensing technology that enables a shallow depth of field wasdeveloped by adding a diffuser to the rear end of a mechanical control lens that cancapture 2D images. The sensor in the optical depth sensing system obtains thefunction curve between the motor step and the focus distance through calibration andimports the measured values into the database of the control program. The opticaldepth sensing system scans the visible range of an interval and the Laplacian equationcan be applied to confirm whether the interval was in focus by judging the sharpnessof the contour of the objects captured in the interval and define the outline of theobjects. Then, the depth information can be obtained by calculating the focus distancebased on the motor step during scanning. Finally, the focus images of individualobjects are used to calculate the contours of the image in the depth direction. Thefocus images of each object are combined to reconstruct a 2.5D model within thesensing range. The optical depth sensing system is not affected by sunlight and thematerial of the measured object. Furthermore, the system can be used to obtain colorimages by using a modified lens. The optical path is simple and does not requirecomplex calculations. Therefore, the proposed system is not easily affected by theenvironment and exhibits a high resolution and calculation speed.

Design of a hybrid mode and wavelength division(de)multiplexer based on contra-directional gratingassisted couplers on SOI platform

Manoranjan Minz and Ramesh Sonkar

DOI: 10.1364/AO.418942 Received 05 Jan 2021; Accepted 27 Feb 2021; Posted 01 Mar 2021  View: PDF

Abstract: This paper presents a hybrid mode and wavelength division (de)multiplexer (MDM-WDM), which isbased on the silicon-on-insulator (SOI) platform using the grating assisted contra-directional couplingprinciple. The proposed hybrid MDM-WDM is able to (de)multiplex six channels with two wavelengthsand three modes. The proposed device is designed by cascading three grating assisted coupler sections.Each coupler section is dedicated to reflecting a specific mode at two wavelengths. The operation of theproposed design is simulated by the 2.5D finite-difference time-domain technique. The analysis showsthat the proposed device structure exhibits an insertion loss of < 0.55 dB, return loss of > 12.81 dB, extinction ratio of > 15.49 dB, and crosstalk of < –12.74 dB. A fabrication tolerance study for the MDM-WDMdevice has also been performed by changing the inter waveguide distance between the respective multimode bus waveguide and the single-mode waveguide.

A needle-based deep-neural-network camera

Ruipeng Guo, Soren Nelson, and Rajesh Menon

DOI: 10.1364/AO.415059 Received 16 Nov 2020; Accepted 26 Feb 2021; Posted 26 Feb 2021  View: PDF

Abstract: We experimentally demonstrate a camera whose primary optic is a cannula(diameter=0.22mm and length=12.5mm) that acts a lightpipe transporting light intensity froman object plane (35cm away) to its opposite end. Deep neural networks (DNNs) are used toreconstruct color and grayscale images with field of view of 180 and angular resolution of~0.40. When trained on images with depth information, the DNN can create depth maps.Finally, we show DNN-based classification of the EMNIST dataset without and with imagereconstructions. The former could be useful for imaging with enhanced privacy.

Tunable Delay Line Based on High-Performance ThreeDimensional Switches Using Graphene Heaters

Qian Song, Kaixin Chen, and Zhefeng Hu

DOI: 10.1364/AO.419697 Received 21 Jan 2021; Accepted 24 Feb 2021; Posted 25 Feb 2021  View: PDF

Abstract: We propose a novel tunable delay line based on high performance three-dimensional (3-D) switches driven bygraphene heaters. The 3-D switches, which are constructed with vertical directional coupler (VDC), providecontinuous tuning functionality of the proposed delay line by tuning the coupling ratio of the VDC. The use of thegraphene electrode by placing it in direct contact with the upper waveguide core combined with the air slotsformed on both sides of the waveguides make the switch a quite low power consumption of 3.6 mW. Our designeddevice, which has a footprint of 20 mm × 6.8 mm, is capable of providing a continuous tunable delay from 0 to 150ps with minimum operating bandwidth of >3.34 GHz, from 0 to 100 ps with minimum operating bandwidth of >5GHz, and from 0 to 50 ps with minimum operating bandwidth of >10 GHz. The crosstalks for different delay casesare less than −21.6 dB over the whole C-band and less than −45.2 dB at 1550 nm.

Diffractive VO2 metagrating for strong multi-objectiveamplitude modulation of optical reflection

Sun-Je Kim

DOI: 10.1364/AO.417128 Received 09 Dec 2020; Accepted 24 Feb 2021; Posted 25 Feb 2021  View: PDF

Abstract: Ultracompact dynamic modulation of light wave in the free space has been one of the most desired goals in the fieldof optics and photonics. Recently, various strategies of combining optical metasurfaces and phase-change materialswith largely reconfigurable optical properties have been in spotlight for the challenging goal. However, an intuitive,step-by-step design rules of a dynamic metasurface to meet multi-objective, high performance modulation ofoptical beam has not been reported. In this paper, I propose theoretical and numerical study on the vanadiumdioxide based diffractive metagrating exhibiting strong modulation of reflectance meeting multiple on-demandperformance objectives, simultaneously. Leveraging the large dielectric-to-metal phase-change of vanadiumdioxide and a non-resonant diffractive metagrating, multi-objective modulation of reflection intensity is achievedwith large on-state reflectance, near-unity modulation depth, broad bandwidth, amplitude-only modulation, andfurther elimination of higher order diffractions.

Performance of InGaN green light-emitting diodes with on-chip photodetectors based on wire-bonding and flip-chip configurations

JIAHAO YIN, Liang Chen, YUMENG LUO, Qing Wang, HONGYU YU, and Kwai Hei Li

DOI: 10.1364/AO.419162 Received 07 Jan 2021; Accepted 24 Feb 2021; Posted 25 Feb 2021  View: PDF

Abstract: In this work, we report the performance study of InGaN-based green light-emitting diodes (LEDs) with on-chip photodetectors (PDs) based on wire-bonding and flip-chip configurations. Compared with the conventional wire bonding design, the LED-PD device incorporating a flip-chip design can offer superior optical and thermal performances, and under an LED current of 200 mA, its light output and detected photocurrent increase by 37.7 % and 14.7 %, respectively at an LED current of 200 mA. The different extents of enhancement in both light output and photocurrent are also studied by analyzing their optical, electrical, and thermal properties under varying LED currents. The results provide important guidance for the design of LED-PD integrated systems operating at different current densities.

Machine-learning informed macro-meteorological models for the near-maritime environment

Christopher Jellen, Miles Oakley, Charles Nelson, John Burkhardt, and Cody Brownellni

DOI: 10.1364/AO.416680 Received 09 Dec 2020; Accepted 24 Feb 2021; Posted 25 Feb 2021  View: PDF

Abstract: Macro-meteorological models predict optical turbulence as a function of weather data. Existing models often struggle to accurately predict the rapid fluctuations in C_n^2 in near-maritime environments. Nine months of C_n^2 field measurements were collected along an 890 m scintillometer link over the Severn River in Annapolis, Maryland. This time series was augmented with local meteorological measurements to capture bulk-atmospheric weather measurements. The prediction accuracy of existing macro-meteorological models was analyzed in a range of conditions. Next, machine-learning techniques were applied to train new macro-meteorological models using the measured C_n^2 and measured environmental parameters. Finally, the C_n^2 predictions generated by the existing macro-meteorological models and new machine-learning informed models were compared for four representative days from the data set. These new models, under most conditions, demonstrated a higher overall C_n^2 prediction accuracy, and were better able to track optical turbulence. Further tuning and machine learning architectural changes could further improve model performance.

Laser-driven photoelectron induced pulsed X-ray source for energy dispersive X-ray spectroscopy and radiography applications

Shakir Ullah, Zia Rehman, Usman Khurshid, Hamza Qayyum, Ashiq Dogar, and Abdul Qayyum

DOI: 10.1364/AO.419264 Received 08 Jan 2021; Accepted 23 Feb 2021; Posted 24 Feb 2021  View: PDF

Abstract: We present details of a nanosecond pulsed X-ray source developed for the Energy Dispersive X-ray spectroscopy (EDX) and low dose fast radiography. The source is based on the illumination of metal photocathode, placed in a Pierce type electron gun geometry, with a low energy Nd:YAG laser, delivering 266 nm wavelength pulses of 5 ns time duration and energy in the range of 15-100 mJ. The emitted nanosecond pulsed photoelectrons are accelerated up to 30 keV energy and impinge on the cathode. The influence of laser energy and electron accelerating potential on the X-ray flux is investigated. The X-ray flux found to follow power law as I_x∝(E_l )^α, where El is the laser energy and α depends of the accelerating potentials. The preliminary experiments confirm that this source can be used for rapid and efficient EDX analysis as compared to the commercial secondary electron microscope. Our radiography results indicate that the use of alkali-based photocathodes can facilitate to capture a radiograph in a single laser pulse of nanosecond time duration.

A QEPAS Sensor in a Butterfly Packageand its application

Tobias Milde, Morten Hoppe, Herve Tatenguem Fankem, Hanna Rohling, Sebastian Schmidtmann, Martin Honsberg, Wolfgang Schade, and Joachim Sacher

DOI: 10.1364/AO.415236 Received 04 Dec 2020; Accepted 23 Feb 2021; Posted 23 Feb 2021  View: PDF

Abstract: Molecular gases are highly relevant in healthcare, production control, safety, and environmental monitoring. Theyoften appear in small concentrations. The measurement of trace gases has increasingly become a key technique inthose domains. Quartz-enhanced photoacoustic spectroscopy (QEPAS) is a suitable method that can provide therequired low detection limits in such applications at comparatively low cost and small size. For mobileimplementation, the size of an entire sensor unit matters. In this paper, we present a QEPAS sensor that fits into astandard butterfly package, its characterization and its application on CH4 and CO2.

Fourier ptychographic reconstruction based onaugmented Lagrangian method and sparseapproximations for phase and magnitude

Xin Chen, Haobo cheng, yongfu wen, and Yunpeng Feng

DOI: 10.1364/AO.416477 Received 03 Dec 2020; Accepted 23 Feb 2021; Posted 23 Feb 2021  View: PDF

Abstract: Fourier ptychography microscopy (FPM) provides large field of view (FoV), high–resolution (HR) imaging by simultaneously recovering intensity and phase distributions.However, in real setups, the process of capturing large numbers of low–resolution (LR)images will inevitably suffer from imaging noise, which could seriously distort the resultsrecovered using the conventional Fourier ptychography approach. To suppress the effects ofimaging noise optimally, a novel iterative algorithm is proposed. This algorithm consists oftwo objective functions, one is based on the augmented Lagrangian function for the inversecomputation, and its solution is found by utilizing the alternating direction multiplier method(ADMM); the other is the separate sparse models built for amplitude and absolute phaseimage, and the filtering process is accomplished by exploiting the Block Matching 3D (BM3D)frames. In combination with the Nash equilibrium balancing theory, the proposed algorithm isrealized by alternately optimizing the two objective functions. The simulated andexperimental results demonstrate that the proposed algorithm is robust to noise, and is capableof reconstructing complete and well–contrast amplitude and phase images.

Automated segmentation for whole human eye OCTimage using RM Multi-stage Mask R-CNN

xuesong fu, jianlin wang, Zhixiong Hu, Yongqi Guo, and rutong wang

DOI: 10.1364/AO.415585 Received 20 Nov 2020; Accepted 23 Feb 2021; Posted 24 Feb 2021  View: PDF

Abstract: Accurate segmentation of structures in whole human eye optical coherence tomography (OCT) image can improvethe accuracy of disease diagnosis and ophthalmic biological parameter measurement. However, due to the complexfeatures of cornea, lens, and retina in the whole human eye OCT image and external interference, it is difficult toprecisely extract the object features, which restricts the segmentation accuracy of whole human eye OCT image. Inthis paper, an RM Multi-stage Mask R-CNN method for whole human eye OCT image segmentation is established.Based on Mask R-CNN, modulated deformable convolution is employed to produce irregular receptive field, andimprove the adaptability of the network to whole human eye OCT image’s object deformation. In the feature mapextraction, relation module (RM) is combined to learn the position relation feature map of the human eye structures,which is utilized to enhance feature maps. Multi-stage mask is constructed in the segmentation branch, and the erroris corrected through iterations of mask to improve the segmentation accuracy of the whole human eye OCT image.Combined with the above structures, the RM Multi-stage Mask R-CNN method is constructed for whole human eyeOCT image segmentation. The model is trained by whole human eye OCT images, and is applied to realize the highaccurate whole human eye OCT image segmentation. Comparison experiments with K-means, U-net, and FPNdeformable-mask R-CNN are performed to verify the segmentation accuracy and robustness of the proposed methodto complex deformation and external interference.

Multi-objective adaptive source optimization for full chip

Guanghui Liao, Yiyu Sun, Pengzhi Wei, Miao Yuan, Zhaoxuan Li, and Yanqiu Li

DOI: 10.1364/AO.417311 Received 11 Dec 2020; Accepted 23 Feb 2021; Posted 24 Feb 2021  View: PDF

Abstract: Source optimization (SO) is an extensively used resolution enhancement technique (RET) in optical lithography. To improve the computational efficiency, compressive sensing (CS) theory was applied to SO for clip-level applications in previous works. This paper proposes a multi-objective adaptive SO (adaptive-MOSO) with CS for full chip, the first time to our knowledge. The fast optimization of pixel illumination source pattern is achieved and the imaging fidelity of each clip is guaranteed simultaneously at full chip. In this paper, fast CS with contour sampling is applied to accelerate SO procedure by sampling all layout patterns. Novel cost function with adaptive weight distribution for every single clip is established to guarantee the lithography imaging fidelity for full chip. The simulation results prove the adaptive-MOSO method improve the efficiency of SO and the lithography performance for large-scale chip.

Nonparaxial Scalar Diffraction Theory Modifications for Improved Efficiency Estimation

Christi Madsen

DOI: 10.1364/AO.417696 Received 15 Dec 2020; Accepted 22 Feb 2021; Posted 24 Feb 2021  View: PDF

Abstract: Using the nonparaxial scalar diffraction equations to estimate the diffraction efficiency of multiple orders of a sinusoidal reflection grating, we compare the results to transverse electric (TE) and transverse magnetic (TM) vector wave simulations for a reflective surface on glass. Modifications are presented which enable the nonparaxial scalar solution to closely approximate simulation results across a 0 to 90-degree range of incidence angles when the diffracted power is predominantly carried in the first several orders, up to fourth order. The accuracy of simulations in this regime is particularly important for surface scattering where individual spatial frequencies typically have amplitudes much less than a hundred nanometers. A substantial reduction in the error between vector simulations and scalar estimation of diffraction efficiency is demonstrated with the following modifications: 1) an obliquity factor correction for the specular order based on a first-order power conservation criterion, 2) a “cut-off” factor modifying the obliquity factor as the diffracted angle approaches 90-degrees for each order, and 3) nonuniform unallocated power distribution near the low-order cut-offs. The second and third modifications are applied to higher diffraction orders for extending the range of applicable grating heights up to 200nm (peak-to-valley), periods down to twice the wavelength, and slopes up to 0.2 (root-mean-square).

Optical Characterization of Heat DependentCollagen Denaturation via Mueller MatrixPolarimetry in combination with PrincipalComponent Analysis

Deniz Derman, Ekin Opar, Onur Ferhanoglu, Ömer Polat, and Murat Kazanci

DOI: 10.1364/AO.418805 Received 30 Dec 2020; Accepted 22 Feb 2021; Posted 24 Feb 2021  View: PDF

Abstract: : Polarization properties of collagen in acidic and natural solutions were analyzed bya decoupling analytical technique based on Mueller Matrix Polarimetry (MMP). Theproposed method was employed to derive five effective optical parameters of collagensolutions: 1) Orientation angle of linear birefringence, 2) Phase retardance of linearbirefringence, 3) Orientation angle of linear dichroism, 4) Value of linear dichroism, and 5)Optical rotation angle of circular birefringence. The temperature dependence of opticalparameters was observed and denaturation (transition) temperature of collagen in twodifferent solvents were deduced as 33.38 ± 2. 78 °C, and 36.83 ± 2. 43 °C for acidic andnatural solutions, respectively based on a total of 100 measurements. Furthermore, usingscatter plots generated by the first two components of Principal Component Analysis, it waspossible to distinguish all denaturized vs. native and acid vs. dialysis samples based on scatterplots. In comparison to higher cost and complex alternatives, such as Circular DichroismSpectroscopy, our results show the promise of using combination of MMP and PCA as a lowcost method for identification and characterization of collagen denaturation.

Designing an Ultrahigh Sensitive Surface PlasmonSensor using Nanofilm Coated D-type Photonic CrystalFiber

Xuanyi Liu, Melwin G, Aruna Gandhi M S, Hongyan Fu, P Ramesh Babu, Senthilnathan Krishnamoorthy, and Qian Li

DOI: 10.1364/AO.418191 Received 23 Dec 2020; Accepted 22 Feb 2021; Posted 23 Feb 2021  View: PDF

Abstract: In this paper, we investigate a novel D-type photoniccrystal fiber based refractive index microbiosensor thatworks on surface plasmon resonance. We study the performance of the proposed sensor using the finite element method. Further, we analyze the influence of various materials, namely, gold, silver and indium tin oxidethat are deposited on the D-type fiber surface. Of thesemetals, gold has been examined to be advantageous owing to its enhanced sensitivity and greater chemical stability. The proposed microbiosensor exhibits a highwavelength sensitivity of 19,000 nm/RIU in the sensingrange of 1.40-1.41, and its wavelength interrogation resolution is 5.26×10−6 RIU. Besides, this novel schemeprovides a high amplitude sensitivity of 569 RIU−1with the corresponding resolution of 2.63×10−6RIU.Further, the linearity of the proposed microbiosensor,R2, is 0.9745 in the sensing environment of refractiveindex ranging between 1.31 and 1.40. Finally, optimizing the metal coating thickness, diameter of the air-holeand the lattice-pitch of the proposed sensor have beenconducted systematically, in order to augment the sensing performance of the proposed sensor. We foreseethat the proposed microbiosensor would turn out tobe an appropriate candidate in sensing the analytes ofhigh refractive index.

A compact silicon-based TE-pass polarizer using asymmetrical triple-guide directional coupler includingSWG structures and hybrid plasmonic waveguides

Nan Yang and Jinbiao Xiao

DOI: 10.1364/AO.420026 Received 20 Jan 2021; Accepted 22 Feb 2021; Posted 23 Feb 2021  View: PDF

Abstract: A compact and broadband silicon-based transverse-electric (TE)-pass polarizer based on a symmetrical tripleguide directional coupler (TG-DC) is presented and analyzed in detail, assisted with subwavelength gratings (SWGs)and hybrid plasmonic waveguides (HPWs), where the SWGs are involved at the inner part of its central waveguideand two HPWs are introduced and symmetrically located at its outer sides. By carefully improving the structuralparameters, only the injected TE mode can be supported by SWGs and passes through the device directly due to thephase mismatching, while the input TM mode will be supported in the strip waveguides and coupled to theadjacent HPWs due to the phase matching and gradually attenuated via the metals. As a result, a compact TE passpolarizer is realized. Results show that the proposed polarizer has a total coupling length of 6 μm, with theextinction ratio (ER) of 31.2 dB, insertion loss (IL) of 0.1 dB and reflection loss (RL) of -26.2dB at 1.55 μm, and itsoperation bandwidth can be up to 270 nm (from 1.43 to 1.7 μm) for ER > 20 dB and IL < 1 dB. In addition,fabrication tolerances to the key structural parameters and field evolution through the proposed device are given.

Error Analysis and Optimization for Risley-prismimaging distortion correction

feng huang, He Ren, Ying Shen, and Pengfei Wang

DOI: 10.1364/AO.418007 Received 29 Dec 2020; Accepted 22 Feb 2021; Posted 23 Feb 2021  View: PDF

Abstract: The reverse ray tracing method has become a well-known technique to correct the dynamic imaging distortion causedby the Risley-prism imaging system due to its precision and computational efficiency. However, the reverse ray tracingmethod is sensitive to equipment error which seriously degrades the quality of distortion correction when using prismwith large wedge angle or camera with a large field of view. In this work, we optimize the distortion correction methodutilizing reverse ray tracing. In addition, we propose a distortion correction model with error parameters to investigatethe influence of prism orientation error, prism tilt error, prism parameters error, and model simplification errors onthe correction accuracy. The work on the optimized model clearly indicates the obvious image distortion introduced bydifferent kinds of errors, including model error and systematic error. Furthermore, we propose the error parameteridentification method to eliminate the negative results of error on the image correction. The simulations results showthat the boresight pointing error and distortion correction error is reduced to about 1% of the initial value after 10iterations, thus achieving high-precision imaging distortion correction and provides better data support for othersubsequent applications.

Experimental observations on cuttingperformance of laser assisted diamond turningof calcium fluoride with different crystalorientations

Hossein Shahinian, Charan Bodlapati, Di Kang, Jayesh Navare, Robert Turnbull, and Yuxiang Zhong

DOI: 10.1364/AO.415265 Received 01 Dec 2020; Accepted 22 Feb 2021; Posted 23 Feb 2021  View: PDF

Abstract: In this paper single point diamond turning (SPDT) of single crystal CaF2 usingµ-LAM process is studied. The paper focuses on how crystal orientation of the workpiece caninfluence tool wear and efficiency of the cutting process. Cutting experiments were performedon single crystal CaF2 with <100>, <111> and <211> crystal orientations. The tool failed atthe smallest cutting distance for <111> crystal orientation whereas the tool failure on <100>orientation occurred at the longest cutting length. Tool life for cutting the <211> and <111>crystal orientations was found to be 92% and 67% to that of tool life during cutting <100>orientation. To explain the results obtained, an anisotropic model based on schmid’s law wasused. It is shown that an inverse correlation between the resistance to plastic deformation andmaximum cutting distance without inducing any brittle fracture can be established.

Numerical comparison of pumping methods for highpower Er/Yb-codoped fiber lasers

Dan Wang, Qun Han, Qing Jia, Kun Ren, and T. Liu

DOI: 10.1364/AO.419783 Received 13 Jan 2021; Accepted 22 Feb 2021; Posted 24 Feb 2021  View: PDF

Abstract: The amplified spontaneous emission (ASE) in the Yb band and thermal damage are the main obstacles to thepower scaling of high-power Er/Yb-codoped fiber lasers (EYDFLs) and amplifiers. In this paper, three popular pumpingmethods, i.e., off-peak pumping, core pumping with ytterbium doped fiber laser, and cascade co-pumping, in conqueringthese obstacles are investigated and compared based on numerical simulation. The influence of the pump wavelength issystematically analyzed. Performances, including thermal effects, optimal fiber length, and maximum applicable pumppower and expectable output power are compared for EYDFLs with these pumping methods. The results show that, offpeak pumping and core-pumping can alleviate the Yb ASE problem and increase the achievable output power to someextent. We believe this improvement is fundamentally originated from the lower absorption coefficient that alleviatesthe bottlenecking effect. The Yb ASE problem is still inevitable for these two methods. Whereas the cascade co-pumpingmethod can completely extricate a EYDFL from the restrict of this problem and push the power to the thermal damagedlimitation.

Validity of the product rule and its impact on theaccuracy of a Mueller matrix polarimeter

Emanuel Chironi and C. Iemmi

DOI: 10.1364/AO.419983 Received 14 Jan 2021; Accepted 22 Feb 2021; Posted 24 Feb 2021  View: PDF

Abstract: In this work we analyze the validity of the product rule and its impact on the accuracy of a Mueller matrixpolarimeter. We demonstrate that the validity of the rule is essential to perform accurate measurements.The conditions for this rule to be valid in relation to different depolarization mechanisms are also discussed. The theoretical predictions are then experimentally verified by analyzing appropriately selectedsamples. Finally, we implement a real Mueller matrix polarimeter in order to show how its accuracydecreases when the product rule is not valid.

Dwell time algorithm in deterministicpolishing of free-form surface based oncontinuous tool influence function

Lei Zhang, Qizhi Zhao, and Cheng Fan

DOI: 10.1364/AO.418409 Received 28 Dec 2020; Accepted 22 Feb 2021; Posted 23 Feb 2021  View: PDF

Abstract: In deterministic polishing, solving the dwell time is one of the key factors. Usually,dwell time is solved by the tool influence function (TIF) and residual error. In previousresearch, single-point TIF (sTIF) is usually used in calculation, but it is not consistent withthe TIF in actual polishing process. In addition, when using the linear equation to solve thedwell time, a large TIF matrix results in a normal computer not having enough memory forcalculation. In order to solve the above problems, a continuous TIF (cTIF) that changes withpolishing path is proposed firstly by the discretization method to simulate the continuousmovement of the polishing tool in actual polishing, and then an optimized grouped LeastSquares QR-decomposition (LSQR) algorithm is proposed to solve the dwell time. In thispaper, an x-y polynomial free-form surface with different initial residual error (RMS= 30nmPV=120nm, RMS=70nm PV=280nm and RMS=100nm PV=400nm) were simulated by theproposed algorithm respectively. The final residual error was RMS= 1.8nm PV=13.3nm,RMS=2.6nm PV=10.1nm and RMS=2.8nm PV=17.4nm respectively. The convergence rateof RMS and PV basically reached 95%, and the validity of the algorithm is proved.

Error coupling analysis of laboratory calibrationmethod for star tracker

Shufan Zhao, Xingshu Wang, Wenfeng Tan, Dongkai Dai, and Shiqiao Qin

DOI: 10.1364/AO.416080 Received 17 Dec 2020; Accepted 21 Feb 2021; Posted 22 Feb 2021  View: PDF

Abstract: A star tracker should be well calibrated before it is equipped in order to achieve high accuracy. There exists, however,the coupling problem between the internal and external parameters for most commonly-used laboratory calibrationmethods, which affect the performance of the star tracker. In this paper we theoretically analyze the major aspects ofthe coupling mechanism based on the star tracker laboratory calibration model, which means the coupling between theprincipal point and the installation angle. The concept of equivalent principal point error, which illustrates theeffectiveness of the calibration even with poor decoupling accuracy between the principal point and the installationangle, is introduced in this paper. Simulation and bench experiments are conducted to verify the laboratory calibrationmethod and its coupling mechanism. The decoupling accuracy can be improved with more samples during calibration.In addition, the equivalent principal point error converges quickly and it hardly affects the attitude of the star tracker,which is verified by both theory and experiment. The comprehensive calibration accuracy can still reach a high leveleven with poor decoupling accuracy.

Combining Spectroscopic Techniques to Determine the OpticalConstants of Powdered Lactose

Michael Yokosuk, Thomas Tiwald, Danielle Saunders, Thomas Blake, and Tanya Myers

DOI: 10.1364/AO.414107 Received 10 Dec 2020; Accepted 20 Feb 2021; Posted 22 Feb 2021  View: PDF

Abstract: A method for deriving the optical constants (n/k) of organicpowdered materials using pressed pellets in the mid-infrared spectral range isintroduced that combines variable angle spectroscopic ellipsometry andtransmission spectroscopy. The approach is applied to anhydrous lactose, inwhich three different forms of pellets were pressed and measured: A purelactose pellet and a mixed lactose/KBr pellet with a large analyte percentagewere used for ellipsometric measurements, and a KBr transmission pellet withonly a small analyte percentage was used for transmission measurements. Thetransmittance data provide an initial set of oscillators and improve the spectralfitting of weak absorption features (k<0.01). Ellipsometric data for the pureand mixed pellets are then fit simultaneously to derive the final n/k values forlactose from 6,000-400 cm-1. An alternative method just using theellipsometric data from the mixed pellet and the transmittance data is alsopresented and shows good agreement with the multi-sample analysis,providing a simpler method for powders that do not press easily into purepellets. Finally, the derived optical constants were used to model thereflectance data, demonstrating a good match with the measured reflectancespectra if non-idealities are included.

On colors of stainless-steel surfaces polishedwith magnetic abrasives

Akash Tiwari Jag Prasad, Fang Xu, Akhlesh Lakhtakia, Hitomi Yamaguchi, and Satish Bukkapatnam

DOI: 10.1364/AO.419424 Received 13 Jan 2021; Accepted 20 Feb 2021; Posted 22 Feb 2021  View: PDF

Abstract: Multiple colors visible to the naked eye across the internal surfaces of SS 304tubes polished with a magnetic abrasive finishing (MAF) process were investigated at varyinglevels of magnification. The colors were found to result from microscopic colored features offour different hues — green, blue, red, and yellow — having irregular sizes, and shapes.Spectral analysis of their dispersion indicates that these colored features appear along the laymarks generated during MAF. Surface characterization employing Energy DispersiveSpectroscopy and Instrumented Indentation Testing indicates the presence of films of nonstochiometric chromium oxide and nickel oxide in the colored regions. Variations in themomentum and the density of the abrasive particles interacting with the surface during theMAF process, together with the chemical composition, and morphology of the surfaceprovide thermodynamic conditions favorable for a non-uniform growth of the oxides film.Local color then being dependent on the local composition and thickness of the oxides film.

A trinocular vision and spatial prior based method forground clearance measurement of transmission lines

Yaqin Zhou, Qingwu Li, Yi Wu, Yunpeng Ma, and Chunkuan Wang

DOI: 10.1364/AO.417533 Received 23 Dec 2020; Accepted 20 Feb 2021; Posted 22 Feb 2021  View: PDF

Abstract: It is an essential task to inspect ground clearance of transmission lines in time. However, the weak texture of transmission lines and the high complexity of the background make it difficult to balance efficiency and accuracy. In order to solve the problem, a trinocular vision and spatial prior based method isproposed, which is specifically designed for ground clearance measurement of transmission lines withUAV(Unmanned aerial vehicle). In the novel method, a perpendicular double-baseline trinocular visionmodule is applied to improve the accuracy of transmission line reconstruction. Then the spatial prior information of geometric models under different shooting attitudes is analyzed in detail. And it is adoptedto determine the ground crossing points and compute the ground clearance efficiently. Besides, an interactive software is developed and tested in the simulation environment of UAV inspection. Experimentalresults verified the feasibility of the novel measurement method. Finally, we discussed in detail how toapply the method effectively in practice and gave a set of recommended camera parameters.

Modelling of Optical Fiber Dispersion on QAMprotocol in Affordable RF-over-Fiber Channels

Erel Granot

DOI: 10.1364/AO.418138 Received 23 Dec 2020; Accepted 20 Feb 2021; Posted 22 Feb 2021  View: PDF

Abstract: We investigate the effect of optical chromatic dispersion on ordinary Double-SidedModulation (DSM) QAM-M protocol in RF over fiber (RFoF) channels (QPRFoF). QAM-Mprotocol is common in RFoF communications channels.An analytical expression was derived for: the channel's response, the standard deviation of theQAM's clusters, and for the estimations of the channel's BER.The main findings are: A) unlike Single-Sided Modulation (SSM) QAM or coherent QAM,ordinary DS QAM is governed by three length scales, and therefore it is much moresusceptible to dispersion. B) The QAM's clusters cannot be bounded, even for weakdispersion, and therefore, zero-error detection is unattainable without dispersioncompensation. C) A linear dispersion compensation filter was derived to mitigate thedispersion effects post detection.These results suggest that it is not essential to operate the channel in a SSM QAM mode. Asimpler and affordable option may be to operate the channel in an ordinary DSM QAMmethod and to apply a dispersion compensating filter in the electrical or even in the digitaldomain. This method is a low-cost solution to improve substantially the RFoF channelsperformances.

A megahertz-rate digital holography system

Mohammad Mansoor, James Trolinger, and Jacob George

DOI: 10.1364/AO.417304 Received 11 Dec 2020; Accepted 20 Feb 2021; Posted 25 Feb 2021  View: PDF

Abstract: Given the need for a simple and fieldable diagnostic that can enable the study ofultrahigh-speed phenomenon in three-dimensions, this work presents experiments andsimulations to optimize and demonstrate a digital in-line holography system with anacquisition rate of 1 million frames per second. The system is used to study the collision of asupersonic projectile and its bow shock with 1mm diameter water droplets. The hologramsacquired are 3-dimensionally reconstructed and de-twinned to analyze shock wavephenomena in these supersonic impact events and demonstrate the potential of ultrahighspeed digital holography as a robust technique to probe complex shock wave dynamics.

Machining characteristics of Ti6Al4V alloyturning assisted by laser heating andultrasonic atomization

Yizhong Hu, Jianbing Meng, Zhenwei Hu, Haian Zhou, Xiaosheng Luan, Bingqi Huang, and xiuting wei

DOI: 10.1364/AO.418023 Received 21 Dec 2020; Accepted 19 Feb 2021; Posted 22 Feb 2021  View: PDF

Abstract: Based on the heating and softening of laser as well as the lubrication and coolingeffect of ultrasonic atomized droplets, a new processing method of Ti6Al4V alloy turning wasproposed, which was assisted by laser heating and ultrasonic atomization. A series ofcomparison experiments were conducted by dry machining (DM), laser assisted machining(LAM), ultrasonic atomization assisted machining (UAAM), and laser heating and ultrasonicatomization assisted machining (LAM-UAAM). The results show that, while fully retainingthe advantages of laser assisted machining, LAM-UAAM gives full play to the advantages ofultrasonic atomization cooling, and overcomes the adverse effects of thermal effects on thequality of cutting tools and surface processing. While reducing tool wear, processing qualityhas been significantly improved.

Quantitative investigation of abrasive grit size dependency of subsurface damages for the metal-bounded abrasives on Zerodur® glass-ceramic

Özgür KARCI and Tugba B. Beldek

DOI: 10.1364/AO.419820 Received 14 Jan 2021; Accepted 19 Feb 2021; Posted 22 Feb 2021  View: PDF

Abstract: We present a quantitative evaluation of abrasive grit size dependency of subsurface damages on Zerodur glass-ceramic substrates induced during the precision grinding process. The depths of subsurface damages are evaluated for the metal-bonded grinding cup tools with four different grit sizes of D126, D64, D35, and D15 for the fixed nominal grinding process parameters. The micro-crack depth is driven by the mechanism between the abrasive grit and brittle glass, which generates median or radial cracks through the bulk material. The initiation and propagation of subsurface damages beneath the brittle glass-ceramic substrate of Zerodur are explored by the cross-sectional polishing method, which takes advantage of optical contact bonding method without any adhesive and imaged by scanning electron microscopy. A novel hybrid methodology, which comprises both destructive and non-destructive subsurface evaluation tools, is developed and employed for evaluating the micro-cracks quantitively. The procedure is based on a repetitive and consecutive chemical acid etching after the surface is ground, flat substrate polishing with a conventional polishing machine, a white light interferometer, and an image process. The four-grit sizes of D126, D64, D35, and D15 correspond to the rough through to the fine cut of the complete precision grinding process. The subsurface damages are evaluated and measured to be 71 μm, 35 μm, 25 μm, and 7 μm from rough to fine grit sizes. The results showed consistency with both the theoretical model and previous work.

Mid-infrared Polarization rotator based onSi3N4-CaF2 hybrid plasmonic waveguide withasymmetric metal claddings

yake chen, fang wang, Tao Ma, Heng Liu, Xu Wang, and Chan Jin

DOI: 10.1364/AO.418238 Received 22 Dec 2020; Accepted 18 Feb 2021; Posted 19 Feb 2021  View: PDF

Abstract: In this paper, a Si3N4-CaF2 hybrid plasmonic waveguide (HPW) with anasymmetric metal cladding is designed for the mid-infrared polarization rotator (PR). Themode characteristics and polarization rotation performances of the Si3N4-CaF2 HPW-basedPR are simulated by using the finite element method (FEM). Operating at the wavelength of3.5 μm, the polarization conversion efficiency (PCE) between two polarization modes (PM 1and PM 2) is larger than 99% at a Si3N4-CaF2 HPW length of 104 μm. The Si3N4-CaF2HPW-based PR maintains good polarization rotation performances within the fabricationtolerances from -10 to 10 nm. The polarization rotator based on the Si3N4-CaF2 HPW pavesthe way to achieve integrated waveplates, driving many important optical functions from freespace onto a chip.

Modeling, optimization, and validation of anextended depth-of-field optical coherencetomography probe based on mirror-tunnel

Chukwuemeka Okoro, Charles Cunningham, Aaron Baillargeon, Andreas Wartak, and Guillermo Tearney

DOI: 10.1364/AO.420591 Received 29 Jan 2021; Accepted 17 Feb 2021; Posted 19 Feb 2021  View: PDF

Abstract: The diagnostic capability of high-resolution optical coherence tomography (OCT)may be enhanced by using extended depth-of-field (EDOF) imaging that retains hightransverse resolution over long depths. A recently developed mirror-tunnel optical probedesign (single mode fiber-to-multimode fiber-to-lens structure) that generates coaxiallyfocused modes has been previously shown to enable EDOF for endoscopic OCT applications.Here, we present raytracing optical modeling of this optical configuration, which has thepotential to guide performance improvement through optimization. The Huygens wavepropagation of the field was traced through probe components with initial lengths. Theirradiance along the x-z plane was analyzed, yielding an average full-width-at-half-maximum(FWHM) of 9 µm over a 640 µm DOF (defined as the axial range over which the beam’stransverse FWHM is maintained). A custom merit function was then defined, based on thefocal region illumination intensity profile that yielded the maximum possible depth havingconstrained FWHM size. An orthogonal gradient descent optimization algorithm was thenapplied using this merit function, using the multimode fiber, spacer and lens’ lengths asvariables. Optimization resulted in a modeled mean 6 µm FWHM spot diameter over anEDOF of 1 mm. Following optimization, a probe was fabricated, and validated using acustom-built near-field scanning-pinhole beam profiler. The experimental results (6 µm meanFWHM over 800 µm EDOF) showed reasonable correspondence to the simulated predictions,demonstrating the potential utility of optical modeling and optimization for improving EDOFperformance in mirror-tunnel endoscopic OCT probes.

A planar metamaterial sensor with grapheneelliptical rings in transmission mode

Motahhare Farrokhfar, Saughar Jarchi, and Asghar Keshtkar

DOI: 10.1364/AO.418041 Received 21 Dec 2020; Accepted 17 Feb 2021; Posted 19 Feb 2021  View: PDF

Abstract: In this paper, a periodic planar metamaterial sensor in terahertz band based on surface plasmon polariton resonances is proposed and studied. The unit cell includes four half elliptical graphene rings located on a SiO2 substrate, an air gap, and another SiO2 layer. The air gap is embedded between the two layers of SiO2 to increase the sensitivity of the sensor. the refractive index sensitivity of the new sensor, by changing the refractive index sensing environment from 1.2 to 2, is 15.71 \sfrac{\mu m}{RIU} and the figure of merit is calculated as 4.3. Performance of the sensor is compared with previous works. The proposed sensor is suitable for biosensing applications.

Switchable terahertz metasurface with polarization conversion and filtering functions

Zhe Wen Li and Jiusheng Li

DOI: 10.1364/AO.419515 Received 11 Jan 2021; Accepted 17 Feb 2021; Posted 19 Feb 2021  View: PDF

Abstract: A switchable metasurface with dual functions of polarization conversion and filtering is proposed in this paper. The designed structure is composed of medium-metasurface-medium-metasurface-medium, and VO2 is embedded in the metal metasurface. When VO2 is insulated state, this structure can perform linear polarization conversion under terahertz wave normal incidence, and has good asymmetric transmission capabilities. At range from 2.01 THz to 2.86 THz, the polarization conversion rate (PCR) is higher than 93 %. When VO2 is metallic state, the structure becomes a broadband band-stop filter with a 3dB bandwidth of 1.73 THz. The polarization conversion and filtering functions of the proposed structure can be converted to each other by changing the external temperature. This switchable multifunction structure provides a new way for the design of terahertz devices.

Optical OFDM Based on Fractional Fourier Transformfor an Indoor VLC System

Mahdi Nasiri, Gholamreza Baghersalimi, and Zabih Ghassemlooy

DOI: 10.1364/AO.416565 Received 08 Dec 2020; Accepted 17 Feb 2021; Posted 19 Feb 2021  View: PDF

Abstract: The fractional Fourier transform (FRFT), which is a family of linear transformations generalizing the classical Fouriertransform, has been used in the fields of filter design, signal processing, phase retrieval, and pattern recognition due toits unique properties. The FRFT of a signal can be interpreted as a decomposition of the signal in terms of chirps. In thispaper, for the first time we introduce an optical FRFT (OFRFT)-based orthogonal frequency division multiplexing(OFDM) visible light communications (VLC) system and compare numerical results with a direct current-biased optical(DCO)-OFDM system. Firstly, the optimal fractional order is calculated to improve the performance of the proposedsystem by minimizing the bit error rate (BER). The numerical results show that, OFRFT-OFDM with the optimalfractional order offers a significantly improved BER performance compared with DCO-OFDM under the samecomputational complexity and spectral efficiency. In addition, the peak to average power ratio, which is an issue in lightemitting diode-based VLC systems, is reduced by < 1 dB using OFRFT-OFDM for the same BER compared with DCOOFDM.

CAOS Spectral Imager Design and Advanced HighDynamic Range FDMA-TDMA CAOS Mode

Nabeel Riza and Mohsin Ali Mazhar

DOI: 10.1364/AO.417472 Received 14 Dec 2020; Accepted 17 Feb 2021; Posted 19 Feb 2021  View: PDF

Abstract: In the first part of the paper, a CAOS line camera is introduced for spectral imaging of one dimensional (1-D) or linetargets. The proposed spectral camera uses both a diffraction grating as well as a cylindrical lens optics system toprovide line imaging along the line pixels direction of the image axis and Fourier transforming operations in theorthogonal direction to provide line pixels optical spectrum analysis. The imager incorporates the Digital Micromirror Device (DMD)-based Coded Access Optical Sensor (CAOS) structure. The design includes a line-by-line scanoption to enable two dimensional (2-D) spectral imaging. For the first time, demonstrated is line style spectralimaging using a 2850 K color temperature white light target illumination source along with visible band colorbandpass filters and a moving mechanical pinhole to simulate a line target with individual pixels along 1-D thathave unique spectral content. A ~412 nm to ~732 nm input target spectrum is measured using a 38 × 52 CAOSpixels spatial sampling grid providing a test image line of 38 pixels with each pixel providing a designed spectralresolution of ~6.2 nm. The spectral image is generated using the robust Code Division Multiple Access (CDMA)mode of the camera. The second part of the paper demonstrates for the first time the High Dynamic Range (HDR)operation of the Frequency Division Multiple Access (FDMA)-Time Division Multiple Access (TDMA) mode of theCAOS camera. The FDMA-TDMA mode also feature HDR recovery like the Frequency Modulation (FM)-TDMA mode,although at a much faster imaging rate and a higher Signal-to-Noise Ratio (SNR) as more than one CAOS pixel isextracted at a time. Experiments successfully demonstrate 66 dB HDR target recovery using both FDMA-TDMA andFM-TDMA modes, with the FDMA-TDMA mode operating at an encoding speed 8 times faster than the FM-TDMAmode given 8 FM channels are used for the FDMA-TDMA mode. The CAOS spectral imager can be used to imageboth full spectrum stationary line targets as well as spectrally map 2-D targets using line scanning methods. Thedemonstrated FDMA-TDMA CAOS mode is suited for improved speed and SNR linear HDR imaging.

Spectral speckle-correlation imaging

Kunihiko Ehira, Ryoichi Horisaki, Yohei Nishizaki, Makoto Naruse, and Jun Tanida

DOI: 10.1364/AO.418361 Received 23 Dec 2020; Accepted 16 Feb 2021; Posted 17 Feb 2021  View: PDF

Abstract: We present a method for single-shot spectrally resolved imaging through scattering media by using thespectral memory effect of speckles. In our method, a single speckle pattern from a multi-colored objectis captured through scattering media with a monochrome image sensor. The color object is recoveredby correlation of the captured speckle and a three-dimensional phase retrieval process. The proposedmethod was experimentally demonstrated by using point sources with different emission spectra locatedbetween diffusers. This study paves the way for non-invasive and low-cost spectral imaging throughscattering media.

Transmission characteristics of electromagnetic wavesin 2D tunable plasma photonic crystals

Yichao Liang, Zhen Liu, Liguang Lin, Jun Peng, Rubing Liu, and Qi Lin

DOI: 10.1364/AO.419164 Received 07 Jan 2021; Accepted 16 Feb 2021; Posted 17 Feb 2021  View: PDF

Abstract: The transmission characteristics of electromagnetic waves in a 2D plasma photonic crystal (PPC) in the frequencyrange of 1-12 GHz are investigated by experiments and numerical simulations. For PPC composed of dischargeplasma tubes, the transmission characteristics of electromagnetic waves in PPC can be adjusted by changing thedischarge current. The transmission characteristics of PPC in different array arrangements and working modes areobtained experimentally; the finite difference time domain (FDTD) method is used to obtain the band structure ofPPC in different array arrangements and working modes. The results show that: the simulation results of thelayered model are close to the experimental results; whether the discharge plasma tube is working or not has animportant influence on the transmission characteristics of PPC; Bragg scattering and the localized surfaceplasmons (LSP) are the main reasons for the generation of the band gap. The increase in electron density causesthe band gap to move to higher frequencies and increases the number of the band gap. The research results can beused as a reference for the evaluation of electromagnetic properties of 2D plasma photonic crystals and the designof new devices such as plasma antennas.

An Efficient Implementation of Optical ScanningHolography in Cancelable Biometrics

Fathi Abd El-Samie, rana melegy, Mohamed Safan, Mohamed Abdelhamed, Ashraf Khalaf, Ghada ELBANBY, Osama Zahran, El-Sayed El-Rabaie, Abd Naser Mohamed, Ibrahim El-Dokany, HOSSAM EL DIN HUSSEIN, Saied El-Khamy, Noha Ramadan, Randa Soliman, and walid El-Shafai

DOI: 10.1364/AO.415523 Received 27 Nov 2020; Accepted 15 Feb 2021; Posted 16 Feb 2021  View: PDF

Abstract: This paper presents a new trend in biometric security systems that is the cancelable multi-biometrics. In general, traditional biometricsystems, depend on a single biometric for identification. These traditional systems are subject to different types of attacks. In addition, biometricsignature can be lost. In case of intrusion, biometric signatures can be stolen forever. To reduce the risk of losing biometric signatures, the trendof cancelable biometrics has evolved by using either deformed or encrypted versions of the biometrics for verification. In this paper, severalbiometric traits for the same person are treated to obtain a single cancelable template. First of all, Optical Scanning Holography (OSH) is appliedin the acquisition of each biometric. The resulting outputs are compressed simultaneously to generate a unified template based on the energycompaction property of the Discrete Cosine Transform (DCT). Hence, OSH is used in the proposed approach as a tool to generate deformedversions of human biometrics in order to get the unified biometric template through DCT compression. With this approach, we guarantee thepossibility of using multiple biometrics of the same user to increase security and the security of the new biometric template through theutilization of the OSH. Simulation results prove the robustness of the proposed cancelable multi-biometric system in noisy environments.

Pedestal analysis of jitter in a 20.97GHzregeneratively mode-locked laser formed witha short polarization maintaining fiber-amplifier

Willie Wing Ng, Vladimir Stasyuk, and Virupaksha Reddy

DOI: 10.1364/AO.417306 Received 11 Dec 2020; Accepted 15 Feb 2021; Posted 17 Feb 2021  View: PDF

Abstract: We demonstrated a 20.97GHz regeneratively mode-locked Er fiber-laser that has ashort ring-length of 5.84 meter. The short active ring enabled large filter-rejection of adjacentsupermodes during extraction of its clock-signal for regenerative feedback. Very stablemode-locking was observed with the generation of ~5.5ps Gaussian pulses. Acharacterization of the noise pedestals in the photodetected first and second harmonics of themode-locked pulses indicated there was cross-correlated coupling between the timing- andamplitude-jitter. From an analysis of the noise pedestals in these harmonics, as well as thenoise spectrum at baseband, we estimated an amplitude-jitter of ~0.19%, and a timing-jitter of~60fs. Using a generalized harmonic analysis approach, we estimated a cross-correlation of0.11% between the timing- and amplitude-jitter.

Demonstration of a general scaling law for far-field propagation

Thomas Dolash, Matthew Cooper, Mark Spencer, and Sami Shakir

DOI: 10.1364/AO.419596 Received 15 Jan 2021; Accepted 14 Feb 2021; Posted 17 Feb 2021  View: PDF

Abstract: This paper conducts experiments that demonstrate the utility of a general scaling law (GSL) for far-field propagation. In practice, the GSL accurately predicts the diffraction-limited peak irradiance in a far-field plane, regardless of the beam shape in a near-field plane. Within the experimental setup, we use a reflective, phase-only spatial light modulator to generate various beam shapes from expanded and collimated laser-source illumination, including both flat-top and Gaussian beams with obscurations, in addition to phased arrays with these beam shapes. We then focus the resulting near-field source plane to a far-field target plane and measure the peak target irradiance to compare to the associated GSL prediction. Overall, the results show excellent agreement with less than 1% error for all test cases. Such experiments present a convenient and relatively inexpensive approach to demonstrating laser-system architectures (of varying complexity) that involve far-field propagation.

Measurement of the refractive index of lensesfrom the maximum phase difference of thetotal-internal reflection with a polarizationcamera

Ju-Yi Lee, Chia-Yu Chiang, Wei-Yi Sung, Tzu-Yang Weng, Jing-Heng Chen, and CHENG-CHIH HSU

DOI: 10.1364/AO.413732 Received 13 Nov 2020; Accepted 12 Feb 2021; Posted 17 Feb 2021  View: PDF

Abstract: The proposed total-internal reflection (TIR)-based technique can be used formeasuring the refractive index of lenses. The distribution of the phase difference between thes- and p- polarization states of the reflected light induced by TIR can be obtained by apolarization camera. The refractive index of the lens can be determined from the detectedmaximum phase difference, with the specific measurement equation. Only the maximumphase difference needs to be measured. Information about the incident angle, thickness of thelens and the matching liquid is not needed. The experimental results demonstrate that theresolution of the system can reach 4.8×10-4 RIU.

Boosting the deep learning wavefront sensor forreal-time applications

Esteban Vera, Felipe Guzmán, and Camilo Weinberger

DOI: 10.1364/AO.417574 Received 17 Dec 2020; Accepted 12 Feb 2021; Posted 17 Feb 2021  View: PDF

Abstract: The deep learning wavefront sensor (DLWFS) allows to directly estimate Zernike coefficients of aberratedwavefronts from intensity images. The drawback is related to the use of massive convolutional neuralnetworks (CNNs) that are lengthy to train or estimate. In this paper we explore several options to reduceboth training and estimation time. First, we develop a novel CNN that can be rapidly trained withoutcompromising accuracy. Second, we explore the effects given smaller input image sizes and differentamounts of Zernike modes to be estimated. Our simulation results demonstrate that the proposed WFNetusing images of either 8 × 8, 16 × 16 and 32 × 32 allow to dramatically reduce training time even boostingthe estimation accuracy of Zernike coefficients. From our experimental results we can corroborate that a16 × 16 DLWFS can be quickly trained and is able to estimate the first 12 Zernike coefficients–skippingpiston, tip and tilt–without sacrificing accuracy, achieving a significant prediction speedup aiming forlow-cost, real-time adaptive optics systems.

Direct measurement of the sound velocity inwater based on the acousto-optic signal

Chenxi Li, Bin Xue, and zihui yang

DOI: 10.1364/AO.417946 Received 18 Dec 2020; Accepted 12 Feb 2021; Posted 17 Feb 2021  View: PDF

Abstract: Based on the acousto-optic effect, we proposed a new method to directly measurewater sound velocity, which avoids the error like phase ambiguity brought by the piezoelectriceffect that is broadly adopted in current methods. In the experimental setup we designed, thelaser signal modulated by the propagating acoustic wave changes its phase suddenly when thewave crosses the two or more intercepting laser lines simultaneously. This new designcreatively realizes the possibility to capture time-information at the phase level in soundvelocity measurement, which is hardly realized in the piezoelectric-effect based methods.Utilizing the above principle and the derived mathematical calculation, the accuracy of soundvelocity with good traceability can be obtained. The experimental results show that therepeatability of the measurement results is less than 0.0159m/s, and the accuracy comparedwith the commercial SVP is better than 0.02 m/s.

Experimental design for collection andanalysis of laboratory passive infrared vaporspectra

Kevin Major, Jasbinder Sanghera, and Ken Ewing

DOI: 10.1364/AO.410353 Received 28 Sep 2020; Accepted 12 Feb 2021; Posted 01 Mar 2021  View: PDF

Abstract: Passive infrared (IR) spectroscopy is a powerful tool for the detection andclassification of chemical vapors in industrial, environmental, and defense applications. Forany application, it is critical that the collected passive IR spectra is supported via laboratoryand calculated spectral data. Here we demonstrate the importance of proper experimentalsetup to collect laboratory passive IR data which replicates that expected to be collected in thefield. We show how the type of IR detector element used affects the IR spectral results.Finally, we demonstrate calculated spectral results which show good agreement with thelaboratory collected data, and provide a method to model such vapor releases in externalenvironmental scenarios.

Velocity property of the optical chain generated by thetightly focused femtosecond radially polarization pulse

Dong Li, chen xin, Zhe Yang, Wei Zhang, and Jianlin Zhao

DOI: 10.1364/AO.418310 Received 24 Dec 2020; Accepted 07 Feb 2021; Posted 08 Feb 2021  View: PDF

Abstract: Based on the Richards-Wolf vector diffraction integration, we obtained the expressions of the intensityand velocity of femtosecond radially polarized pulses at the focus near a dielectric interface, and thepulses are modulated by the optical system consisting of the diffractive optical elements (DOE) and thehigh Numerical Aperture (NA) lens. The factors affected the intensity distribution and velocity evolutionof three-dimensional optical capture structural pulse (optical chain) are also analyzed. Such as the DOEstructural parameters (bandwidth, phase difference between the rings), the interception ratio of incidentbeam, the NA, the central wavelength of pulses, and the refractive index of exiting medium. The resultsshow that the velocity of the optical chain will increase with the increasing of the DOE bandwidth ordecreasing of the refractive index of the exiting medium, and the maximum of the optical chain velocitywill decrease versus the NA. Furthermore, the dependence of the optical chain velocity on its intensitydistribution is also revealed. And the superluminal and subluminal can also be found during the propagation of the optical chain. The velocity distribution difference between bright and dark areas alongz axis is more conducive to distinguish the trapping of the Rayleigh particles. These interesting resultshave great potential for improving the space-time resolution for detecting the particle positions duringthe high-speed optical trapping.

A study on optical scanning holography with apolarization directed flat lens

Chen-Ming Tsai, Hong-Yuan Sie, Ting-Chung Poon, and Jung-Ping Liu

DOI: 10.1364/AO.415387 Received 01 Dec 2020; Accepted 04 Feb 2021; Posted 05 Feb 2021  View: PDF

Abstract: Recently, optical scanning holographic system with a polarization directed flat lenswas proposed to realize coaxial scanning holography (CSH). The advantage of CSH is itssmall form factor and the stability. However, the diffraction efficiency of the polarizationdirected flat lens cannot be 100% and thus there is always zeroth order light in the scanningbeam. The imperfect diffraction property of the polarization directed flat lens results in anincomplete scanning Fresnel zone plate. Consequently, the reconstructed image is blurred andnoisy. In this paper, we compared different methods, including the back propagation, thephase correlation, and the inverse filtering, for the hologram reconstruction. It isdemonstrated that the inverse filtering is the only method which can retrieve the highfrequency component of the hologram. However, additional noise also arises with the use ofinverse filtering. Therefore, the imaging performance of CSH by using a polarization directedflat lens is inherently worse than that of conventional OSH.

Self-monitored and optically powered fiber optic device forlocalized hyperthermia and controlled cell death in vitro

Sondos Alqarni, William Willmore, Jacques Albert, and Christopher Smelser

DOI: 10.1364/AO.411576 Received 06 Oct 2020; Accepted 02 Feb 2021; Posted 03 Feb 2021  View: PDF

Abstract: Localized hyperthermia therapy involves heating a small volume of tissue in order to kill cancerous cells selectively andwith limited damage to healthy cells and surrounding tissue. However, these features are only achievable through realtime control of the tissue temperature and heated volume, both of which are difficult to obtain with current heatingsystems and techniques. This work introduces an optical fiber-based active heater which acts both as a miniature heatsource and as a thermometer. The heat-induced damage in the tissue is caused by the conductive heat transfer from thesurface of the device, while the heat is generated in an absorptive coating on the fiber by near infrared light redirectedfrom the fiber core to the surface by a tilted fiber Bragg grating inscribed in the fiber core. Simultaneous monitoring ofthe reflection spectrum of the grating provides a measure of the local temperature. Localized temperature increasesbetween 0 and 100oC in 10 mm-long/5 mm-diameter cylindrical volumes are obtained with continuous wave pumppower levels up to 1.8 W. Computational and experimental results further indicate that the temperature rise anddimensions of the heated volume can be maintained at a nearly stable level determined by the input optical power.

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