High-quality open-access database for digital lensless holographic microscopy and its application on the improvement of deep-learning-based autofocusing models

Carlos Buitrago, Heberley Tobón, Alejandra Gómez, Samuel Zapata Valencia, Maria Lopera Acosta, Carlos Trujillo, and Jorge Garcia-Sucerquia

DOI: 10.1364/AO.507412 Received 03 Oct 2023; Accepted 22 Nov 2023; Posted 22 Nov 2023  View: PDF

Abstract: Among modern optical microscopy techniques, Digital Lensless Holographic Microscopy (DLHM) is one of the simplest label-free coherent imaging approaches. However, the hardware simplicity provided by the lensless configuration is often offset by the demanding computational postprocessing required to match the retrieved sample information to the user’s expectations. A promising avenue to simplify this stage is the integration of artificial intelligence and machine learning (ML) solutions into the DLHM workflow. The biggest challenge to do so is the preparation of an extensive and high-quality experimental dataset of curated DLHM recordings to train the ML models. In this work, a diverse, open-access, high-quality dataset of DLHM recordings is presented as support for future research, contributing to the data needs of the applied research community. The database is comprised of 11,760 experimental DLHM holograms of bio and non-bio samples with diversity on the main recording parameters of the DLHM architecture. The database is divided into two datasets of 10 independent imaged samples. The first group, named multi-wavelength dataset, includes 8,160 holograms and was recorded using laser diodes emitting at 654 nm, 510 nm, and 405 nm; the second group, named single-wavelength dataset, is composed of 3,600 recordings and was acquired using a 633 nm He-Ne laser. All the experimental parameters related to the dataset acquisition, preparation, and calibration are described in this manuscript. The advantages of this large dataset are validated by re-training an existing autofocusing model for DLHM and as the training set for a simpler architecture that achieves comparable performance, proving its feasibility for improving existing ML-based models and the development of new ones.

Nanoporous anti-reflection coating for high temperature applications in the infrared

Lorna Alvarez, Luke J. Currano, Christine Zgrabik, Dajie Zhang, Robert Weiblen, Timothy Montalbano, Noah Talisa, Michael J Purcell, Cavin T Mooer, Michael Thomas, David Young, and Jacob Khurgin

DOI: 10.1364/AO.506714 Received 02 Oct 2023; Accepted 21 Nov 2023; Posted 22 Nov 2023  View: PDF

Abstract: Anti-reflection (AR) coatings are essential to the performance of optical systems, without them surface reflections increase significantly at steep angles and become detrimental to functionality. AR coatings apply to a wide range of applications from solar cells, laser optics to optical windows. Many times, operational conditions include high temperatures and steep angles of incidence (AOIs). Implementation of AR coatings is extremely challenging to operate in both of these conditions. Nanoporous coatings made from high-temperature–tolerant materials offer a solution to this problem. Careful selection of materials is needed to prevent delamination when exposed to high temperatures, and an optimal optical design is needed to lower surface reflections at both normal incidence and steep AOIs. This article presents nanoporous silicon dioxide and hafnium dioxide coatings deposited on sapphire substrate using oblique angle deposition by electron-beam evaporation, a highly accurate deposition technique for thin films. Developed coatings were tested in a controlled temperature environment and demonstrated thermal stability at temperatures up to 800° C. Additional testing at room temperature demonstrated reduction of power reflections near optimal for AOIs up to 70° for a design wavelength of 1550 nm. These findings are promising to help extend the operation of technology at extreme temperatures and steep angles.

High diffraction efficiency varied-line-space concave gratings for Lyman ultraviolet explorer

Tao Ren, Angran Li, Shuhu Huan, Keqiang Qiu, Zheng Lou, Li Ji, ShuangYing Li, and Yilin Hong

DOI: 10.1364/AO.506040 Received 20 Sep 2023; Accepted 21 Nov 2023; Posted 22 Nov 2023  View: PDF

Abstract: In this paper we present the design and fabrication of the reflection varied-line-space concave grating (VLSCG) for the project of CAFE (the Census of warm-hot intergalactic medium, Accretion, and Feedback Explorer), which aims to detect and map the warm-hot circumgalactic medium (CGM) via OVI emission at 103.2 nm, using two Rowland-Circle-like spectrograph channels. High diffraction efficiency at LUV is supposed for VLSCG and an aperture ratio as small as F/3.6 is desired for a compact design. The gratings are fabricated by holographic lithography and ion-beam etching techniques. We introduce an additional lens into the normal holographic exposing system to generate the varied-line-space grating patterns. Grooves with triangle profiles are obtained to increase the diffraction efficiency by oblique ion beam bombardment during etching process. Finally, several VLSCGs with a central line density of 3300 lines/mm have been fabricated successfully. The measured results show that the groove efficiency reaches 51% at 106.4 nm and 31% at 127.4 nm. We imitated the working optical path of the spectrometer and used the -1 order of the VLSCG to measure the image near the exit slit. The results showed that the image of point source has a vertical extent of 0.68 mm, and the aberrations have been corrected.

BRDF modeling and optimization of target surface based on gradient descent algorithm

pengfei yang, Lu Bai, Zifei Zhang, and Yanhui Li

DOI: 10.1364/AO.506672 Received 22 Sep 2023; Accepted 21 Nov 2023; Posted 22 Nov 2023  View: PDF

Abstract: Addressing the current challenges in modeling and optimizing the Bidirectional Reflectance Distribution Function (BRDF) for the target surface, an improved six-parameter semi-empirical model is proposed based on an existing five-parameter semi-empirical model. In comparison with the original five-parameter model, the new model considers reciprocity, and the results demonstrate that as the incident angle increases, the fitting accuracy of the six parameters gradually surpasses that of the five parameters. Additionally, this paper employs a machine learning optimization algorithm, namely the gradient descent method, for optimizing the BRDF. The algorithm was comprehensively compared with other optimization methods, revealing that for the same dataset, the gradient descent method exhibited the smallest fitting errors. Subsequently, utilizing this algorithm for fitting experimental data resulted in errors consistently within 3%, confirming the reliability and accuracy of this optimization algorithm.

Nematic D-shaped Liquid Crystal Voltage Sensor with Enhanced Performance for Power Monitoring and Fault Detection in Extreme Environments

Md Walid Hassan and Md Aslam Mollah

DOI: 10.1364/AO.503706 Received 21 Aug 2023; Accepted 21 Nov 2023; Posted 21 Nov 2023  View: PDF

Abstract: A novel nematic liquid crystal voltage sensor with enhanced performance is proposed in this paper. The sensor is designed D shaped using a single NLC filled core without the presence of air holes, which has made the sensor fabrication very much simpler than previous sensors. The sensor also consists of a circular slot that provides a vast space for the surface plasmon resonance (SPR) phenomenon with a minimum amount of gold. The performance of the proposed sensor is carried out using finite element method (FEM) based simulation. Following this, the sensor has obtained a maximum wavelength sensitivity of 10 nm/V for a wide range of 190V to 250V with 5V increments. The sensor also has a linearity of 0.9926 and a figure of merit (FOM) of 0.2V-1. It has the resolution of 0.01V. The proposed sensor is a promising technology with a wide range of extreme and sophisticated applications. The sensor’s simple structure, high sensitivity, resolution, linearity, and FOM make it perfectly suitable for a variety of sensing applications, including power monitoring, fault detection, medical diagnosis, voltage lines, electronics etc.

Application of Powell algorithm for estimating optical properties of semitransparent medium based on time-domain information

Zhonghao Chang, Shuangcheng Sun, Linyang Wei, and Guangjun Wang

DOI: 10.1364/AO.504903 Received 11 Sep 2023; Accepted 20 Nov 2023; Posted 21 Nov 2023  View: PDF

Abstract: Accurate estimation of the optical properties of a semitransparent medium is crucial in various engineering applications. This study introduces the Powell algorithm to estimate the optical properties of a 2D semitransparent slab. The time-domain radiative transfer equation is solved using the discrete ordinate method. The radiative intensity on the medium's surface serves as the measurement signal for the inverse analysis. The results demonstrate that the Powell algorithm accurately estimates the absorption coefficient, scattering coefficient, and scattering asymmetry factor. For simultaneous reconstruction of these three parameters, it is recommended to use 8 signal detectors on both the left and right sides of the medium. Even when the standard measurement error is increased to 15%, the relative errors for these three parameters remain low, at 1.87%, 1.379%, and 0.194%.

Lens-less single-fiber ghost imaging

Toshitaka Wakayama, Yudai Higuchi, Rikuto Kondo, Yasuhiro Mizutani, and Takeshi Higashiguchi

DOI: 10.1364/AO.507550 Received 06 Oct 2023; Accepted 20 Nov 2023; Posted 21 Nov 2023  View: PDF

Abstract: We demonstrate lens-less single-fiber ghost imaging, which allows illumination and collection using a single optical fiber without a transmission-type system. Speckle patterns with relative coincidence degrees of 0.14 were formed by image reconstruction using improved differential ghost imaging. Employing fiber with a diameter of 105 µm, we achieved a spatial resolution of 0.05 mm in an observing area of 9 mm2 at a working distance of 10 mm. Compared with a conventional neuroendoscope at a power density of 94 mW/cm2, our imaging could be realized by extremely weak illumination at a laser power density of 0.10 mW/cm2. Using our lens-less single-fiber ghost imaging, utilizing 30,000 speckle patterns and implementing a diffuser, we attained an average coincidence degree of 0.45.

Laser Tissue Welding by Using Collagen Excitation at 1,720 nm Near-Infrared Optical Window III

Stefan Thomas, Vidyasagar Sriramoju, and Robert Alfano

DOI: 10.1364/AO.500113 Received 18 Jul 2023; Accepted 20 Nov 2023; Posted 21 Nov 2023  View: PDF

Abstract: Laser tissue welding (LTW) is a method of fusing incised tissues together. LTW has the potential to revolutionize plastic surgery and wound healing techniques by its ability to produce water-tight, scarless seals with minimal foreign body reaction. While using thermal mechanisms to achieve LTW, energy from the incident laser is absorbed by water in the tissue. As the water temperature increases, partial denaturing of the collagen triple helix briefly occurs, which is quickly followed by renaturation of collagen as the tissue cools, thus providing a watertight seal. This research study investigates the efficacy of direct collagen excitation at 1,720 nm to accomplish LTW. This wavelength falls within the near infrared (NIR) optical window III. The tensile strengths of pig skin that have been welded with NIR continuous-wave (CW) diode lasers at 1,455 nm, which promotes thermal mechanisms of tissue welding, and 1,720 nm wavelengths are compared. Near infrared lasers tuned to 1,455 nm and 1,720 nm were used to weld incised pieces of porcine skin together without extrinsic solders or dyes. The tensile force of the welded tissues was measured using a digital force gauge. The average tensile force of the welded pig skin using the 1,720 nm laser was approximately four times greater than that using the CW 1,455 nm laser, suggesting that LTW accomplished through direct collagen excitation in NIR Optical Window III provides greater tensile strengths.

Research on pulse energy exchange of vector solitons in fiber laser

Zhou Yong, Xian Wang, Hao Liu, Shihao Tang, Zhiyu Xu, Xiaohui Ma, WENTAN FANG, Xiaolin Chen, Song Huang, Wei Zhang, Li Li, and Weiqing Gao

DOI: 10.1364/AO.507508 Received 04 Oct 2023; Accepted 19 Nov 2023; Posted 20 Nov 2023  View: PDF

Abstract: The energy exchange between orthogonal polarization components is crucial for the building-up of vector solitons (VSs). Unlike previous observations of energy exchange in the frequency domain, our experiments analyzed pulses energy flow in the time domain. We provide evidence to demonstrate the influence of the four-wave mixing (FWM) and cross-phase modulation (XPM) effect on VSs building-up in passive mode-locked fiber lasers through a novel perspective of pulse energy exchange. The results indicate that the energy exchange of PRVS caused by FWM and XPM is stronger than that of PLVS. The liner energy exchange caused by the birefringence of fiber and PC influences the period of energy exchange. After stabilization, the intra-cavity energy evolution period is one roundtrip for PLVS while serval roundtrips for PRVS. In the future, we can achieve PLVS by adjusting the linear energy exchange through cavity birefringence, thereby meeting the industrial demand for stable and uniform pulse trains.

MCNN-DIC: A mechanical constraints-based digital image correlation by a neural network approach

Lu Wang, Yawen Deng, Xianzhi Gao, and Guangyan Liu

DOI: 10.1364/AO.498872 Received 03 Jul 2023; Accepted 18 Nov 2023; Posted 20 Nov 2023  View: PDF

Abstract: Abstract：Digital Image Correlation (DIC) is a widely used photomechanical method for measuring surface deformation of materials. Practical engineering applications of DIC often encounter challenges such as discontinuous deformation fields, noise interference and difficulties in measuring boundary deformations. To address these challenges, a new DIC method called MCNN-DIC is proposed in this study by incorporating mechanical constraints using neural network technology. The proposed method applied compatibility equation constraints to the measured deformation field through a semi-supervised learning approach, thus making it more physical. The effectiveness of the proposed MCNN-DIC method was demonstrated through simulated experiments and real deformation fields of nuclear graphite material. The results show that the MCNN-DIC method achieves higher accuracy in measuring non-uniform deformation fields than a traditional mechanical constraints-based DIC and can rapidly measure deformation fields without requiring extensive pre-training of the neural network.

Impact of Laser Phase Noise on the Ranging Accuracy of Cooperative MIMO FMCW Photonic Radar System

AMITESH KUMAR and Annapurna Kumari

DOI: 10.1364/AO.505755 Received 13 Sep 2023; Accepted 17 Nov 2023; Posted 20 Nov 2023  View: PDF

Abstract: In this article, a FMCW based cooperative 2×2 MIMO photonic radar system using heterodyne detection is presented. The proposed system consists of two separate sensor node that uses linear frequency modulated continuous wave signal which allows simultaneous monostatic and bistatic radar measurement, where the target range and angle of arrival information is extracted. The additional bistatic information enhances the target detection and estimation capabilities with improved accuracy. This accuracy in practical is affected by the laser phase noise which degrades the overall system performance. Here, the analytical laser phase noise model for MIMO system is derived and implemented to analyze its impact on the ranging accuracy of the proposed system. Under the impact of standard white Gaussian laser phase noise assumption, the monostatic and bistatic response of the detected signal is measured and compared using statistics of measurement error. Further, the signal-to-noise ratio and SSB laser phase noise of the monostatic and bistatic response are measured and compared at different target ranges. At last, the phase noise limited ranging accuracy of the system is evaluated and analyzed. The concept shown in this work paves the way for advanced photonic radar system applications such as modern radar systems, electronic warfare systems, metrology, and automotive vehicle radar with multiperspective coherent detection.

Performance of Modulating Retro-reflector-Assisted Ground- HAP-Satellite uplink laser communication system

Xuewen Jiang, Xingyue Guo, and Yi Wang

DOI: 10.1364/AO.507297 Received 29 Sep 2023; Accepted 17 Nov 2023; Posted 20 Nov 2023  View: PDF

Abstract: Satellite-ground laser communication has attracted wide attention due to its advantages of rich spectrum resources, fast communication speed, strong anti-interference ability, and high security. Therefore, this paper proposes to use modulating retro-reflector(MRR) and assemble it on the HAP to improve the performance of the ground-satellite uplink laser communication system.Since the influence of the hovering fluctuation of HAP on the system can not be ignored, this paper introduces the angle of arrival (AoA) jitter to represent the influence of the random jitter of the HAP in the air on the channel and considers the light intensity scintillation, beam wander, atmospheric attenuation, pointing error. The combined effect of the system is analyzed. At the same time, the influence of key factors such as beam width, zenith angle, HAP position distance, wind speed, and cloud visibility on the performance of the ground-HAP-satellite system under different MRR effective areas is simulated and analyzed, and compared with the ground-HAP-satellite system without MRR(G-H-S). The results show that the MRR-assisted ground-HAP-satellite system(G-HM-S) has better communication performance. The work of this paper provides a good theoretical basis for the engineering implementation of the MRR-assisted ground-HAP-satellite laser communication system.

Improving Distance Imaging Accuracy through Temporal Position Correction with Phase Difference Compensation

zengyan wu, Cao Changqing, Zhejun Feng, xiaona wu, chenxuan duan, and Hongyuan liu

DOI: 10.1364/AO.502508 Received 03 Aug 2023; Accepted 17 Nov 2023; Posted 20 Nov 2023  View: PDF

Abstract: This study introduces a time-domain-based phase compensation method to address decoherence effects in optical heterodyne detection, which is critical for remote sensing and distance imaging. Numerical simulations demonstrate a substantial reduction in localization bias (6.56 to 2.85) and an increased probability of bias values below 2 (21.6% to 70.5%). Experiments show significant improvement in whiteboard distance imaging accuracy at 10m from the detector, with 91.7% of data falling within 10-12m compared to a mere 2.3% accuracy before compensation. The method effectively enhances intensity image quality, mitigates decoherence phenomena, and improves detection accuracy and reliability without additional hardware

A Posture Planning Method for Complex Feature Measurement Using a Single-Line Laser Scanner

Feng ShengQuan, Li Qi, Huang JianXin, and Chen Xiang

DOI: 10.1364/AO.506258 Received 21 Sep 2023; Accepted 17 Nov 2023; Posted 20 Nov 2023  View: PDF

Abstract: A single-line laser scanner is commonly utilized for measuring complex surfaces and contours. However, achieving automatic implementation of this scanner poses challenges in terms of designing a measurement posture that considers measurement accuracy, path planning, and the positioning of auxiliary equipment. This ensures non-interference during the measurement process. In this study, we focus on the application of T-SCAN. First, we construct a measurement posture parameter model for T-SCAN and analyze the viewpoint position’s coverage of the measurement area. Second, we propose a measurement path planning method based on scanning posture to minimize overlapping areas. Lastly, we present a measurement station planning method based on scanning postures and analyze the transfer error of the measurement targets to establish a unified measurement field. Experimental results demonstrate that, after the posture adjustment process, the average distance deviation between the measurement data and the theoretical model is significantly reduced.

Quantitative analysis of energy-dispersive X-ray fluorescence spectroscopy based on machine learning and generative data enhancement technique

Wei Zhao, Xianyun Ai, and Hui Zhao

DOI: 10.1364/AO.506027 Received 18 Sep 2023; Accepted 16 Nov 2023; Posted 16 Nov 2023  View: PDF

Abstract: This paper proposes a data enhancement technique to generate expanded datasets for machine learning by developing an X-ray fluorescence spectra simulator based on the physical process. The simulator consists of several modules, including the excitation source, the interaction process, and the detection system. The spectra generated by the simulator are subject to dimension reduction through feature selection and feature extraction algorithms, and then serve as the input for the XGBoost (Extreme Gradient Boosting) model. Six elements of metal samples with various content ranges were selected as the research target. The results showed that for simulated data, the R2 value for elements with concentrations ranging from 0% to 100% is greater than 95%, and for elements with concentrations of <0.3%, the R2 value is greater than 85%. The experimental data were able to be predicted by the model trained by the simulated spectra. Therefore, this approach provides reliable results for practical application and can supply additional datasets to obtain reasonable prediction results for machine learning with inadequate reference materials.

An automatic marker-based alignment method for nano-resolution full-field transmission X-ray microscope

Chenpeng Zhou, Yan Wang, wang shanfeng, jin zhang, Fu Tianyu, Wanxia Huang, Kai Zhang, and Qingxi Yuan

DOI: 10.1364/AO.506046 Received 18 Sep 2023; Accepted 16 Nov 2023; Posted 16 Nov 2023  View: PDF

Abstract: Driven by the development of X-ray optics, the spatial resolution of full-field transmission X-ray microscope (TXM) has reached tens of nanometers and plays an important role in promoting the development of biomedicine and materials science. However, due to the thermal drift and the radial/axial motion error of the rotation stage, the TXM computed tomography (CT) data are often associated with random image jitter errors along the horizontal and vertical directions during CT measurement. A nano-resolution 3D structure information reconstruction is almost impossible without a prior appropriate alignment process. To solve this problem, a fully automatic gold particle marker-based alignment approach without human intervention was proposed in this study. It can automatically detect, isolate, and register gold particles for projection image alignment with high efficiency and accuracy, facilitating a high quality tomographic reconstruction. Simulated and experimental results confirmed the reliability and robustness of this method.

Athermal panoramic annular lens design with thermal anlysis method

Yiru Pan, Shaohua Gao, Yiqi Liao, Jian Bai, and Kaiwei Wang

DOI: 10.1364/AO.507191 Received 28 Sep 2023; Accepted 16 Nov 2023; Posted 16 Nov 2023  View: PDF

Abstract: An athermal 360°×(30°-100°) and F/3.5 panoramic annular lens (PAL) system is designed. Through the optical mechanical thermal analysis results based on finite element analysis (FEA) , it is expected that the system will have excellent performance under extreme temperature fluctuations. Simulation shows that the system is thermally insensitive in the temperature range from -40 °C to 60 °C, consistently providing great imaging capability with the modulation transfer function (MTF) value at 133 lp/mm greater than 0.5. The proposed design and analysis workflow contains an evaluation of thermal-optical performance with a higher accuracy, thus has significance in future athermal optical design. We expect the designed PAL system to have a broad application prospect in the field of outdoor applications, including automatic navigation of vehicles and all-weather surveillance systems etc.

IRS-Assisted Vehicular Visible Light Communication Systems: Channel Modeling and Performance Analysis

Arash Rabiepoor, S. Alireza Nezamalhosseini, and Lawrence Chen

DOI: 10.1364/AO.502663 Received 21 Aug 2023; Accepted 16 Nov 2023; Posted 17 Nov 2023  View: PDF

Abstract: Visible light communication (VLC) is a promising solution as an alternative for the fully-occupied radio frequency bands in the near future. The rear (tail) and front of vehicles have lamps that can be used for vehicular visible light communication (VVLC) systems. On the other hand, one of the main challenges of VLC systems is the line-of-sight (LoS) blockage issue. In thispaper, we propose the installation of intelligent reflecting surfaces (IRSs) (i.e., smart mirrors) on the back of vehicles to overcome the aforementioned issue in VVLC systems. We assume three different patterns of angular distribution for the radiation intensity, such as a commercially available LED with an asymmetrical pattern (Philip Luxeon Rebels), a symmetrical Lambertian pattern, and an asymmetrical Gaussian pattern. In the first section of this paper, we obtain the channel model for the IRS-Assisted VVLC systems, then the evaluation of the path loss results versus link distance under different conditions such as weather type (clear weather, rainy, moderate fog, and thick fog) and radiation patterns are investigated. Moreover, the impact of system parameters such as the aperture size of the photodetector (PD), side-to-side and front-to-front distances, number of IRS elements, and IRS area are studied. In the second part, we derive a closed-form expression for the maximumachievable link distance versus the probability of error for the IRS-Assisted VVLC systems. In addition, the impact of parameters in single-photon avalanche diode (SPAD), background noise, as well as the system parameters evaluated for the path loss is analyzed in this section.

Carrier-frequency estimation for digital holograms of phase objects

Nishant Goyal and Kedar Khare

DOI: 10.1364/AO.505663 Received 12 Sep 2023; Accepted 16 Nov 2023; Posted 17 Nov 2023  View: PDF

Abstract: Accurate estimation of carrier fringe frequency is essential for the demodulation of off-axis digital holograms. The carrier frequency location is often assumed to be the amplitude peak of the cross-term in the two-dimensional Fourier transform of a digital hologram. We point out that this definition of carrier frequency is generally invalid for holograms associated with phase objects. We examine the carrier-envelope representation for digital holograms from the viewpoint of Mandel's criterion [J. Opt. Soc. Am. 57, 613 (1967)]. An appropriate definition of carrier frequency is observed to be the centroid of the power spectrum associated with the cross-term. This definition is shown to apply uniformly to holograms associated with phase objects and leads to the smoothest (or least-fluctuating) envelope representation for the demodulated object wave. The proposed definition is illustrated with simulated as well as experimentally recorded off-axis holograms.

Liquid crystal wavefront correction based on improved machine learning for free space optical communication

HONGYANG GUO, wei tang, zihao wang, liangzhu yuan, yang li, Dong He, Qiang Wang, and Yongmei Huang

DOI: 10.1364/AO.505697 Received 13 Sep 2023; Accepted 15 Nov 2023; Posted 15 Nov 2023  View: PDF

Abstract: In order to suppress the impact of atmosphere turbulence on the space laser communication link, the wavefront correction technology of liquid crystal spatial light modulator (LCSLM) is studied. Combining with the control mode of LCSLM, we propose an improved deep learning approach that restores the input image features into wavefront and then controls LCSLM to compensate for the phase distortion. This method does not have Zernike coefficient truncation and does not require the calculation of coefficient matrices, thus improving the accuracy and efficiency of the algorithm. At the same time, as for its powerful phase fitting ability, LCSLM can be used as a turbulence simulator to construct datasets. During the training process of the neural networks, a calibration between LCSLM and deep learning is established. Finally, a spatial optical coupling experimental system is built. The results show that under different atmospheric conditions, the liquid crystal wavefront correction method has a significant improvement in terminal coupling efficiency, and has certain application prospects in the field of free space optical communication.

Detecting Vibrations in Digital Holographic Multiwavelength Measurements Using Deep Learning

Tobias Störk, Tobias Seyler, Markus Fratz, Alexander Bertz, Stefan Hensel, and Daniel Carl

DOI: 10.1364/AO.507303 Received 29 Sep 2023; Accepted 15 Nov 2023; Posted 15 Nov 2023  View: PDF

Abstract: Digital holographic multiwavelength sensor systems integrated in the production line on multi-axis systems such as robots or machine tools are exposed to unknown, complex vibrations that affect the measurement quality. To detect vibrations during the early steps of hologram reconstruction, we propose a deep learning approach using a deep neural network trained to predict the standard deviation of the hologram phase. The neural network achieves 96.0~\% accuracy when confronted with training like data while it achieves 97.3~\% accuracy when tested with data simulating a typical production environment. It performs similar or even better as comparable classical machine learning algorithms. A single prediction of the neural network takes 35~µs on the GPU.

Model-driven terahertz image reconstruction method for debonding defects in thermal barrier coatings

Binghua Cao, Hongxi Li, Mengbao Fan, Fengshan Sun, and Bo Ye

DOI: 10.1364/AO.504870 Received 08 Sep 2023; Accepted 15 Nov 2023; Posted 16 Nov 2023  View: PDF

Abstract: Terahertz imaging system is considered to be an effective method to study the thermal barrier coating defects in gas turbine engines. However, due to the influence of system hardware and terahertz wavelength, the imaging system has slow acquisition efficiency, low image resolution and serious edge blur, which cannot meet the demand for defect detection. To overcome the above defects, a model-driven terahertz image reconstruction method is proposed, which uses simulation data to build data sets, reduces the dependence on experimental data, and has a good reconstruction effect on experimental images. A fusion loss function based on edge intensity was designed to optimize the edge effect of reconstructed images. Compared with the Bicubic, SRCNN and VDSR methods, the proposed method can achieve better results in terms of visual and evaluation indexes for the reduced terahertz images. It is proved that this method can effectively restore the defect contour in the terahertz image, sharpen the edge of the image and improve the image quality. It has good application value in the industry.

A robust point light source calibration method for near field photometric stereo using feature points selection

Long Ma, Xu Liu, Yuzhe Liu, Xin Pei, and Shengwei Guo

DOI: 10.1364/AO.505234 Received 06 Sep 2023; Accepted 15 Nov 2023; Posted 16 Nov 2023  View: PDF

Abstract: In this paper, we present a robust method for non-isotropic point light source calibration through feature points selection. By analyzing the relationship between the observed surface and its image intensity under near field lighting, the feature points selection method is firstly developed to effectively address the noisy observations and improve the calibration robustness. Afterwards, to enhance the efficiency and the accuracy of calibration, a cost function of lp-norm is established based on the above relationship and an improved Newton method based iteration process is applied to calculate the light source parameters. The simulations demonstrate that the proposed method is capable of achieving robust calibration results with the estimation error less than 2.7mm and 0.8deg even the image intensities are corrupted by Gaussian white noise with standard deviation up to 0.4. The experimental validation is performed using a self-designed photometric stereo system, where the calibration of point light sources is conducted and measurements are taken on both a standard sphere and a compressor blade based on the obtained calibration results, which demonstrates the effectiveness of the new method.

An investigation on the acoustic characteristics of laser-induced plasmas from the forming dynamics perspective

Wendong Wu, Shu Chai, and Yuegui Zhou

DOI: 10.1364/AO.506394 Received 20 Sep 2023; Accepted 14 Nov 2023; Posted 15 Nov 2023  View: PDF

Abstract: The acoustic signal has demonstrated its capabilities in assisting laser-induced breakdown spectroscopy (LIBS) measurements. In this study, the acoustic characteristics of laser-induced plasmas (LIP) under different levels of energy deposition were analyzed, and its correlation with LIP forming dynamics was investigated. In the deposited energy space, two zones in the acoustic pressure and duration were observed, featuring a clear transition point in 100mJ. The analysis based on self-emission spectra and images suggested that, this transition is a result of the change in plasma forming dynamics. Above 100mJ, the plasma temperature and electron density were saturated; thus, any further increase in deposited energy only contributes to the plasma size. In this regime, the acoustic wave from the significantly elongated plasma no longer satisfied the ideal spherical assumption. The observation was also strengthened by the analysis in the frequency domain. Moreover, the correlation between acoustic and radiation signal was also changed significantly with plasma forming dynamics. This study offers a systematic analysis of LIP acoustic signals on the deposited energy space. The potential of using acoustic measurement to interpret the plasma forming dynamics was demonstrated, which could be beneficial for the successful implementations of acoustic-aided LIBS.

Structural stability design of optical mirror mount adjustment mechanism

Ziming Dong, Jianqiang Zhu, Zhigang Liu, Wei Fan, Ziming Sun, Donghui Zhang, Chao Zhang, and kaiqi zhang

DOI: 10.1364/AO.501644 Received 26 Jul 2023; Accepted 14 Nov 2023; Posted 15 Nov 2023  View: PDF

Abstract: The stability of beam pointing in a laser system depends on the consistency of the optical mirror mount. Typically, a locking mechanism is used to secure the adjustment mechanism after beam alignment, ensuring the mount's stability. However, this process can introduce errors, causing a drift in the optical path. To mitigate this issue, in this study, an interference fit adjustment screw was designed. This development enables the mechanism to self-lock after beam alignment, thereby preventing optical path drift and enhancing overall stability. Specifically, 14 long-term thermal shock stability tests, each lasting 2500 min, were conducted to validate the proposed design. The experimental results showed that the thermal drift of the interference fit adjustment screw was reduced by 47.16%, thermal shift was reduced by 79.59%, and the long-term stability improved by at least 48.67%.

Multichannel Laser Diode to Polymer Waveguide Array Coupling with a Double-Aspheric Lens

Katharina Kunze, Christian Goßler, Markus Reinhardt, Markus Arnold, Falk Schwenzer, Christian Helke, Danny Reuter, Daniel Keppeler, Tobias Moser, and Ulrich Schwarz

DOI: 10.1364/AO.505167 Received 13 Sep 2023; Accepted 14 Nov 2023; Posted 15 Nov 2023  View: PDF

Abstract: An optical system for multichannel coupling of laser arrays to polymer waveguide array probes with a single biconvex lens is developed. The developed cylindrical module with 13 mm and 20 mm in diameter and length, respectively, enables coupling of eight individual optical channels using an aspheric lens. Specific coupling with crosstalk below -13 dB for each channel and uniform coupling over all channels is achieved for a waveguide array with 100 μm lateral facet pitch at the incoupling site. The polymer waveguide technology allows for tapering the lateral waveguide pitch to 25 μm towards the tip of the flexible waveguide array. SU-8 and PMMA are used as waveguide core and cladding, respectively. The optical coupling module is designed as a prototype for preclinical evaluation of optical neural stimulators.

Efficiency enhancement and application of laser ultrasonic longitudinal wave based on glass constraint

Xing Guo, Youxing Chen, Xin Guo, jian wang, Yanqin Xun, and Ya Li

DOI: 10.1364/AO.503210 Received 14 Aug 2023; Accepted 14 Nov 2023; Posted 15 Nov 2023  View: PDF

Abstract: Due to the weak longitudinal signals generated by laser ultrasound in the thermoelastic mechanism, the characteristic echoes are weak when evaluating the interior of solids, thus limiting its application to internal defect detection. A glass confinement layer is introduced to enhance the longitudinal excitation effectiveness. Specially, a thermoelastic model of laser ultrasound with glass confinement is established to explain the mechanism of the enhancement of the longitudinal wave effectiveness, and the effect of glass width on the longitudinal wave generated by the base ultrasound is investigated. The effect of the glass confinement layer on the enhancement of the effectiveness of the internal defects detection is studied.. The simulation and experimental results show that the longitudinal waves with high signal-to-noise ratio induced from thermoelastic effect is similar to the ablation mechanism, which greatly improves the excitation efficiency of longitudinal waves. And thus the detection of deep defects and the accurate localization of depth information with an error of no more than 1.2% are realized.

Research on tunable extraordinary optical transmission spectrum properties of long wavelength infrared metamaterials

Peng Sun, Hongxing Cai, Yu Ren, Jianwei Zhou, Dongliang Li, Tingting Wang, Teng Li, and Guannan Qu

DOI: 10.1364/AO.505041 Received 06 Sep 2023; Accepted 14 Nov 2023; Posted 15 Nov 2023  View: PDF

Abstract: Metamaterial filters are an essential method to research the miniaturization of infrared spectral detectors. To realize 8~ 12 μm long-wave infrared tunable transmission spectral structure, an extraordinary optical transmission metamaterial model was designed based on the grating diffraction effect and surface plasmon polaritons resonance theory. The model consisted of an Al grating array in the upper layer and a Ge substrate in the lower layer. We numerically simulated the effects of different structural parameters on the transmission spectra, such as grating height (h), grating width (w), grating distance (d), grating constant (p), and grating length (S1), by utilizing the finite-difference time-domain method. Finally, we obtained the maximum transmittance is 81.52 % in the 8~12 μm band range, with the corresponding structural parameters set to h= 50 nm, w= 300 nm, d= 300 nm, and S1= 48 μm, respectively. After Lorentz fitting, a full width at half maximum of 0.94± 0.01 μm was achieved. In addition, the Ge substrate influence was taken into account for analyzing the model's extraordinary optical transmission performance. In particular, we firstly realized the continuous tuning performance at the transmission center wavelength (8~ 12 μm) of long-wave infrared within the substrate tuning thickness (D) range of 1.9~ 2.9 μm. The structure designed in this paper features tunability, broad spectral bandwidth, and miniaturization, which will provide a reference for the development of miniaturized long-wave infrared spectral filter devices.

Gain equalization for few-mode erbium-doped fiber amplifier supporting eight spatial modes

JIAO GAO, Fengping Yan, Guobin Ren, Hao guo, baoyuan wang, guangbo li, fuxi zhu, HAOYU TAN, and Ting Feng

DOI: 10.1364/AO.495215 Received 10 May 2023; Accepted 13 Nov 2023; Posted 13 Nov 2023  View: PDF

Abstract: A trench-assisted ring few-mode erbium-doped fiber amplifier (FM-EDFA) supporting eight spatial modes is designed and proposed in this work. The gain equalization for FM-EDFA is achieved by selecting the appropriate doping radius and concentration using a particle swarm optimization (PSO) algorithm when only the pump in the fundamental mode (LP01) is applied. When the signals in the eight spatial modes are simultaneously amplified, the average modal gain is about 20 dB, and the DMG is less than 0.3 dB for a signal at 1550 nm. Considering the gain competition of six-wavelength signals, the modal gain and DMG are more than 20 dB and 1 dB, respectively. In addition, the tolerance analysis for manufacturing with this design is also discussed. For a fluctuation in refractive index, the average modal gain is about 19.5 dB, and the DMG is 0.77 dB, indicating that the structure has good fabrication tolerance.

Self-Similar Pulse Compression in Tapered Pb-Silicate Photonic Crystal Fiber at 2 µm

Huizhen Hou, Tigang NIng, Changzheng Ma, Yong Wang, Dan Zhang, Wensheng Wang, Zhenyu Gu, Wei Jiang, and Li Pei

DOI: 10.1364/AO.503497 Received 16 Aug 2023; Accepted 13 Nov 2023; Posted 13 Nov 2023  View: PDF

Abstract: We report a 2 μm all-fiber nonlinear pulse compressor based on a tapered Pb-silicate photonic crystal fiber (PCF), which is capable of achieving large compression with low pedestal energy. A tapered Pb-silicate photonic crystal fiber with increased nonlinear coefficients is proposed for achieving self-similar pulse compression (SSPC) at 2 µm. The dynamic evolution of the fundamental order soliton is numerically analyzed based on the designed tapered fiber. After pulse compression in a tapered fiber with a length of 2.2 m, an initial 1.76 ps pulse can be compressed to 88 fs, increasing the peak power from 4.4 W to 86 W with a compression factor of 20 and a quality factor of 98%. The results reveal that exponential variation yields superior compression performance and provides a promising solution for generating high-power femtosecond pulses at 2 µm.

Blue micro-lasing from dye-doped hollow polymer optical fibre with Ag nanoparticles doped microring

Anugop B, Jessy Simon, and KAILASNATH MADANAN

DOI: 10.1364/AO.504341 Received 04 Sep 2023; Accepted 13 Nov 2023; Posted 13 Nov 2023  View: PDF

Abstract: We report the modified and improved whispering gallery mode laser emission from a hollow polymer optical fibre doped with 1,4-Bis(2-methyl styryl) benzene (Bis-MSB), a highquantum yield dye with a blue emission, with the introduction of a Ag nanoparticle-doped microring. In the presence of the Ag-doped microring, the laser emission was enhanced, and a reduction in the laser threshold from 40 µJ to 15 µJ was attained, along with a small redshift in the emission spectrum. The quality factor of the microring embedded hollow polymer optical fibre was found to be 1.3x103. Moreover, the introduction of Ag-doped microring inside the hollow polymer optical fibre helps enhance certain lasing modes. The observations open the way for the development of low-threshold polymer optical fibre lasers in the blue region.

Highly sensitive temperature and refractive index sensor based on no-core fiber cascaded with balloon-shaped bent single-mode fiber

Xiaojun Zhu, Mengqiang Song, Xing Liu, Wen Liu, Yongquan Pan, Juan Cao, Guoan Zhang, Yongjie Yang, Yuechun Shi, and Wuming Wu

DOI: 10.1364/AO.504976 Received 06 Sep 2023; Accepted 13 Nov 2023; Posted 13 Nov 2023  View: PDF

Abstract: A highly sensitive temperature and refractive index (RI) sensor based on no-core fiber (NCF) cascaded with a balloonshaped bent single-mode fiber (BSBSF) is proposed and demonstrated. The NCF can excite higher-order modes which will be concentrated and transmitted into the BSBSF due to the characteristic of self-imaging effect. The BSBSF has an excellent temperature performance due to the high thermo-optical coefficient (TOC) and thermal expansion coefficient (TEC) of the polymer coating. The NCF and BSBSF are both conducive to the excitation of higher-order modes, which induces the sensitivity of the sensor with an efficiency improvement. The experimental results show that the maximum temperature sensitivity is -3.19 nm/℃ in the range of 22 ℃-83 ℃, which is the highest temperature sensitivity in the cascaded BSBSF structure to our best knowledge. In addition, the maximum RI sensitivity is 2.16 nm/RIU when the RI changes from 1.3 4 to 1.3512. Compared with other cascaded BSBSF structures, this sensor has a higher temperature sensitivity and can be applicated in the prospects of food, biology, and environmental monitoring.

Radiometry and Contrast-to-Noise Ratio for Continuous Wave and Laser Range Gated Active Imaging Systems

Joshua Follansbee, Derek Burrell, Orges Furxhi, Christopher Renshaw, and Ronald Driggers

DOI: 10.1364/AO.505890 Received 20 Sep 2023; Accepted 13 Nov 2023; Posted 13 Nov 2023  View: PDF

Abstract: Both resolution and sensitivity must be considered in the design of an active imaging system. System sensitivity is characterized by the signal-to-noise \color{blue} or contrast-to-noise \color{black} ratio and is derived through radiometry. We present a tutorial for the radiometry associated with the contrast-to-noise ratio for active continuous-wave and laser range-gated imaging systems, giving a useful metric for determining reflective-band sensor performance against a target and background. A calculation of the full power and contrast-to-noise ratio terms is shown for \color{blue}an example case\color{black}, and all relevant radiometric signal terms are covered while describing the assumptions made. Coherent effects on signal-to-noise ratio are excluded from this analysis.

Intelligent Reflecting Surfaces Assisted Hybrid FSO/RF Communication with Diversity Combining: A Performance Analysis

Smriti Uniyal, Narendra Vishwakarma, Swaminathan R, and AS Madhukumar

DOI: 10.1364/AO.502196 Received 01 Aug 2023; Accepted 13 Nov 2023; Posted 14 Nov 2023  View: PDF

Abstract: Free space optics (FSO) and radio frequency (RF) communication systems exhibit complementary characteristics, with FSO being susceptible to fog, turbulence, and pointing errors, whereas RF being susceptible to rain and small scale fading. This inherent complementary characteristics between FSO and RF communication systems enable the hybrid configuration to effectively alleviate the impact of environmental impediments. This paper presents the performance analysis of intelligent reflecting surfaces (IRSs)-assisted hybrid FSO/RF system under atmospheric turbulence (AT), pointing errors (PE), small scale fading effects, and attenuation due to climatic conditions such as fog, rain, etc. To characterize the AT, Gamma-Gamma distribution is used and the multipath fading in the RF link is modeled using Nakagami-m distribution. At the receiver, signals from both the FSO and RF links are combined using either selection combining (SC) or maximal-ratio combining (MRC) technique. We derive the exact closed-form expressions for outage probability and average symbol error rate (SER) for both the diversity combining schemes. The proposed system is compared with the direct link (DL) FSO system, IRSs-assisted FSO system, and hybrid FSO/RF system with diversity combining schemes. Further, the performance comparison between the SC and MRC schemes is also reported. Finally, the accuracy of the analytical expressions is verified by utilizing Monte-Carlo simulations.

CNN neural network temporal feature storage structure fusion for visible channel equalisation algorithm

Xizheng Ke, Yang Zhang, and Qin huan

DOI: 10.1364/AO.502683 Received 07 Aug 2023; Accepted 13 Nov 2023; Posted 14 Nov 2023  View: PDF

Abstract: The visible optical communication channel has time-varying characteristics and is difficult to predict, this paper proposes a nonlinear composite channel equalisation algorithm based on the structure of Convolutional Neural Network (CNN) combined with the temporal characteristics of Long Short-Term Memory (LSTM). The visible channel equalisation coefficient vector is a time series that can reflect the channel noise characteristics and has memory characteristics, and the equaliser algorithm can accurately learn the complex channel characteristics and calculate the equalisation coefficients according to the channel characteristics to recover the original transmitted signal. The LSTM temporal feature memory structure can store the time series parameters reflecting the channel noise characteristics for a long time, and the CNN convolutional neural network reads the parameters and then directly compensates and corrects the received BER to reduce the BER. Simulation results show that the proposed algorithm can effectively eliminate the effect of visible channel fading characteristics, improve the system transmission BER performance, and accurately recover the original transmitted signals with faster convergence speed. In addition, compared with the traditional equalisation method, the method is able to achieve a better balance between performance and complexity, which demonstrates the good BER performance and efficiency of the proposed channel equalisation method.

Ocular aberrations measurement with and without an aperture stop using a Shack-Hartmann wavefront sensor

Yanrong Yang, Huang Linhai, Junlei Zhao, Naiting Gu, and YUN DAI

DOI: 10.1364/AO.505211 Received 07 Sep 2023; Accepted 13 Nov 2023; Posted 14 Nov 2023  View: PDF

Abstract: Pupil size is an important parameter since it governs the magnitude of ocular aberrations. The pupil size of human eye has significant individual differences and varies with light level and accommodation. In order to accurately measure ocular aberrations under different pupil sizes using a Shack-Hartmann wavefront sensor (SHWFS), two types of relationship matrices R(1) and R(2) were proposed, which corresponded to wavefront reconstruction with and without aperture stop, respectively. The numerical and experimental results indicated that matrix R(2) can significantly improve the accuracy of wavefront restoration when the incident beam size is inconsistent with the wavefront reconstruction aperture. Meanwhile, the impact of aperture stop on the reconstruction accuracy will become smaller and smaller as the ratio ρ of the outer area to the detection aperture decreases. This study not only can be used for accurately measuring ocular aberrations under different pupil sizes, but also for other variable aperture aberrations measurement in other applications.

A Deep Learning Approach to Predict Optical Attenuation in Additively Manufactured Planar Waveguides

Keno Pflieger, Andreas Evertz, and Ludger Overmeyer

DOI: 10.1364/AO.501079 Received 20 Jul 2023; Accepted 12 Nov 2023; Posted 13 Nov 2023  View: PDF

Abstract: The booming demand for efficient, scalable optical networks has intensified the exploration of innovative strategies that seamlessly connect large-scale fiber networks with miniaturized photonic components. Within this context, our research introduces a novel neural network, specifically a \ac{CNN}, as a trailblazing method for approximating the nonlinear attenuation function of centimeter-scale multimode waveguides. Informed by a raytracing model that simulated many flexographically printed waveguide configurations, we cultivated a comprehensive dataset that laid the groundwork for rigorous \ac{CNN} training. This model demonstrates remarkable adeptness in estimating optical losses due to waveguide curvature, achieving an attenuation standard deviation of 1.5 dB for test data over an attenuation range of 50~dB. Notably, the \ac{CNN} model's evaluation speed, at 517 microseconds per waveguide, starkly contrasts the used ray tracing model that demands 5 to 10 min for a similar task. This substantial increase in computational efficiency accentuates the model's paramount significance, especially in scenarios mandating swift waveguide assessments, such as optical network optimization. In a subsequent study, we test the trained model on actual measurements of fabricated waveguides and its optical model. All approaches show excellent agreement in assessing the waveguide's attenuation within measurement accuracy. Our endeavors elucidate the transformative potential of machine learning in revolutionizing optical network design.

Axial shift mapping: a self-referencing test for measuring axial figure of near-cylindrical surfaces

Hayden Wisniewski, Ralf Heilmann, Mark Schattenburg, and Brandon Chalifoux

DOI: 10.1364/AO.504270 Received 28 Aug 2023; Accepted 12 Nov 2023; Posted 13 Nov 2023  View: PDF

Abstract: Lateral shearing self-referencing interferometry methods shift the surface under test between measurements to separate its topography from that of the reference surface. However, rigid body errors occur during shifting, creating an ambiguity in the quadratic term of the extracted surfaces. We present axial shift mapping, a lateral shearing self-referencing interferometry method for cylinders, in which the quadratic ambiguity is resolved by measuring the rigid body errors using known artifact mirrors residing in the interferometer's field of view. First, one-dimensional lines of a flat mirror are measured with $2.8\ \mathrm{nm}$ RMS difference compared to a three-flat test. Then, axial shift mapping is extended to cylindrical surfaces using a computer generated hologram. We find that axial shift mapping results in full surface extraction of cylindrical optics, along the axial direction, with a repeatability of $4.4\ \mathrm{nm}$ RMS. We also find that the reference surface extracted through axial shift mapping is within $4.5\ \mathrm{nm}$ RMS of the transmitted wavefront error of the computer generated hologram substrate, which was expected to be the largest contribution of reference wavefront error.

Signatures of resonantly driven laser-wakefield excitation by a pulse train generated by an optical delay mask

Andrea Marasciulli, Luca Labate, Paolo Tomassini, and Leonida Antonio Gizzi

DOI: 10.1364/AO.506107 Received 15 Sep 2023; Accepted 12 Nov 2023; Posted 13 Nov 2023  View: PDF

Abstract: Electron plasma waves can be efficiently excited by a resonant train of ultrashort pulses, spatially separated by a plasma wavelength. Generating a pulse train from a single amplified ultrashort pulse may be challenging when dealing with large beams. Here we discuss a pulse splitting technique using a simple delay mask that can be adapted to large diameter PW beams. We show via detailed numerical simulations that unique signatures of electrons accelerated by resonantly excited wakefield can be obtained from realistic focused double-pulse trains obtained from a single region delay mask.

A highly discriminative and adaptive feature extraction method based on NMF-MFCC for event recognition of Φ-OTDR

Yi Huang, Jingyi Dai, Wei Shen, Xiaofeng Chen, CHENGYONG HU, Deng Chuanlu, Lin Chen, Xiaobei Zhang, Wei Jin, Jianming TANG, and Tingyun Wang

DOI: 10.1364/AO.506307 Received 19 Sep 2023; Accepted 11 Nov 2023; Posted 13 Nov 2023  View: PDF

Abstract: To enhance the capability of phase-sensitive optical time-domain reflectometers (Φ-OTDR) in recognizing disturbance events, an improved adaptive feature extraction method based on NMF-MFCC is proposed, which replaces the fixed filter bank utilized in the traditional method of extracting Mel-Frequency Cepstral Coefficient (MFCC) features by a spectral structure obtained from the Φ-OTDR signal spectrum using non-negative matrix factorization (NMF). Three typical events on fences are set as recognition targets in our experiments, and the results show that the NMF-MFCC features have higher distinguishability, with the corresponding recognition accuracy reaching 98.47%, which is 7% higher than that using the traditional MFCC features.

Material classification based on a SWIR discrete spectroscopy approach

Anju Manakkakudy, Andrea De Iacovo, Emanuele Maiorana, Federica Mitri, and Lorenzo Colace

DOI: 10.1364/AO.501582 Received 26 Jul 2023; Accepted 08 Nov 2023; Posted 08 Nov 2023  View: PDF

Abstract: A crucial yet difficult task for waste management is the identification of raw materials like plastic, glass, aluminum, paper. Most previous studies use the diffused reflection spectroscopy for classification purposes. Despite the benefits in terms of speed and simplicity offered by modern compact spectrometers, their cost, and the need for an external, widespectrum source of illumination create complications. To address this issue, the present paper proposes a discrete spectroscopy method that utilizes short-wave infrared (SWIR) reflectance to identify waste materials, exploiting a small set of selected wavelengths. This approach reduces the complexity of the classification data analysis and offers a more practical alternative to the conventional method. The proposed system comprises a single Germanium photodetector and 10 different light emitting diodes (LED). The LED wavelengths are selected to maximize the system sensitivity towards a set of seven different waste materials. Using a classification strategy relying on support vector machines, the proposed methodology reaches a classification accuracy up to 98%.

Direct generation of vector vortex modes from an end-pumped Pr3+:LiYF4 laser

TAKUYA MOROHASHI, Srinivasa Rao Allam, and Takashige Omatsu

DOI: 10.1364/AO.509263 Received 12 Oct 2023; Accepted 08 Nov 2023; Posted 08 Nov 2023  View: PDF

Abstract: We report the direct generation of vector vortex laser modes at 640 nm from a compact, diode end-pumped continuous-wave Pr3+:LiYF4 (YLF) laser which utilizes an intra-cavity lens and diaphragm. On-axis displacement of the intra-cavity lens combined with appropriate choice of intracavity diaphragm, enables selective generation of a desired radial and azimuthal vector laser mode. Such compact, vector vortex laser sources based on Pr3+:YLF in the visible wavelength region, are a significant enabling technology for a wide range of applications.

Broadband tunable terahertz metamaterial absorber having near-perfect absorbance modulation capability based on a patterned vanadium dioxide circular patch

Qian Zhao, Xuefeng Qin, Chongyang Xu, Haiquan Zhou, and Ben-Xin Wang

DOI: 10.1364/AO.499641 Received 03 Jul 2023; Accepted 08 Nov 2023; Posted 08 Nov 2023  View: PDF

Abstract: A new tunable broadband terahertz metamaterial absorber has been designed based on patterned vanadium dioxide(VO2). The absorber consists of three simple layers, the top VO2 pattern layer, the middle media layer and the bottom metal layer. Based on phase transition properties of VO2, the designed device has excellent absorption modulation capability, achieving the functional transition from broadband absorption to near-perfect reflection. When VO2 is in the metallic state, there are two absorption peaks observed at frequencies of 4.16 THz and 6.05 THz, exhibiting near-perfect absorption characteristics, the combination of these two absorption peaks gives rise to the broadband phenomenon, the absorption bandwidth, where the absorbance exceeds 90%, spans from 3.40 THz to 7.00 THz, with a corresponding relative absorption bandwidth of 69. %, impedance matching theory, near-field patterns, and surface current distributions are provided to analyze the causes of broadband absorption. Furthermore, the broadband absorption could be completely suppressed when VO2 presents the dielectric phase, its absorbance could be dynamically adjusted from 100% to less than 0.70%, thereby achieving near-perfect reflection. Owing to its symmetrical structure, it exhibits excellent performance in different polarization directions and at large incidence angles. Our proposed absorber may have a wide range of promising applications and can be applied in a variety of fields such as communications, imaging, sensing and security detection.

Ultra-sensitive refractive index sensor based on stainless steel metamaterial

guifang wu, Fengping Yan, Wei Wang, Lanju Liang, Xin Yan, Haiyun Yao, Meng Wang, and Hao guo

DOI: 10.1364/AO.501793 Received 27 Jul 2023; Accepted 08 Nov 2023; Posted 08 Nov 2023  View: PDF

Abstract: Terahertz metamaterial technology, as an efficient nondestructive testing method, has shown great development potential in biological detection. This paper presents a stainless steel terahertz metamaterial absorber that achieves a near-perfect absorption of incident metamaterial waves with a 99.99% absorption at 2.937 THz. We demonstrate the theoretical discussion about the absorber and the application in sensing. The effects of the metamaterial absorber's structural parameters on sensing performance are also analyzed. Simulation results show that the sensor can detect analytes with a refractive index between 1.0 and 1.8. Additionally, the performance of the sensor in detecting analytes in three states (solid, liquid, and gas) is analyzed in detail, and the sensitivity and FoM of the sensor for detecting methane are 22.727 THz/RIU and 568.175 RIU-1, respectively. In addition, the terahertz sensor has the advantage of wide incident angle insensitivity, maintaining good sensing performance within a wide manufacturing tolerance range of -10% to 10%. Compared to metal-dielectric-metal or dielectric-metal structures, the proposed sensor adopts stainless steel as the only manufacturing material, which has the advantages of simple structure, low manufacturing costs, and high sensitivity, and has potential application prospects in label-free highsensitivity biomedical sensing.

Realization of photoswitchable anapole metasurface based on phase change material Ge₂Sb₂Te₅

Xiaoyun Wang, Xiaojing Lu, and Zhengwei Xia

DOI: 10.1364/AO.503134 Received 11 Aug 2023; Accepted 08 Nov 2023; Posted 08 Nov 2023  View: PDF

Abstract: The electromagnetic anapole mode originates from the phase cancellation interference between the far-field radiation of oscillating electric dipole moment and toroidal dipole moment, which presents a radiation-free state of light while enhancing the near-field, and has potential applications in micro- and nano-photonics.The active control of the anapole is crucial for the design and realization of tunable photonic devices. In this paper, we realize dynamic tuning of anapole metasurface, and metasurface optical switching based on the phase change material Ge₂Sb$₂Te₅ (GST). By utilizing the destructive interference of electric dipole moment and ring dipole moment, we design the non-radiative anapole mode. At the same time, we introduced the phase change material GST to dynamically regulate the intensity and position of the far-field scattering, electric field and transmission spectra, and to realize the transition from anapole mode to electric dipole mode. At the same time, the modulation of transmission spectrum by metasurface after the addition of GST film is achieved.A relative transmission modulation of 640.62% is achieved.Our study provides ideas for realizing effective active modulation of active micro- and nanophotonic devices, and promotes active modulation of active micro- and nanophotonic devices in lasers, filters and potential applications in dynamic near-field imaging.

Optimization of cavity length and pulse characterization based on Germanene as a saturable absorber in an Er-doped fiber laser

Yanfeng Yu, Xueyao Liu, Tuo Li, Xiaofeng Zou, Jiashuai Ding, nannan xu, Xinxin Shang, Xing Wang, Pu Huang, Chen Cheng, Shuhao Si, Hua Lu, Huanian Zhang, and Degnwang Li

DOI: 10.1364/AO.504880 Received 06 Sep 2023; Accepted 08 Nov 2023; Posted 08 Nov 2023  View: PDF

Abstract: In this study, the germanene-nanosheets (NS) were synthesized by liquid-phase exfoliation, followed by experimental investigation into the nonlinear saturable absorption characteristics and morphological structure of germanene. The germanene-NS were employed as saturable absorbers, exhibiting saturation intensity and modulation depth values of 22.64 MW/cm² and 4.48%, respectively. This demonstrated the feasibility of utilizing germanene-NS passively mode-locked in an erbium-doped fiber laser (EDFL). By optimizing the cavity length, improvements in the output of EDFL characteristics were achieved, resulting in 883 fs pulses with a maximum average output power of 19.74 mW. The aforementioned experimental outcomes underscore the significant potential of germanene in the realms of ultrafast photonics and nonlinear optics.

A robust structured light 3D measurement method based on polarization-encoded projection patterns

Zhenmin Zhu, Yumeng Zhou, Wenquan Lu, Jing Zhang, lisheng zhou, and Haoran Liu

DOI: 10.1364/AO.502522 Received 03 Aug 2023; Accepted 08 Nov 2023; Posted 09 Nov 2023  View: PDF

Abstract: Fringe projection profilometry (FPP) is widely used in 3D vision measurement because of its high robustness and measurement accuracy. In the case of HDR objects, due to the problem of surface reflectivity, the obtained image will be overexposed. This will cause the sinusoidality of the fringes projected on the surface of the object in the acquired image to be interfered, resulting in a phase error in the calculated wrapped phase. Therefore, a polarization-encoded sinusoidal structured light is proposed to enhance the sinusoidality of the fringe. Because the phase information contained in the polarized sinusoidal structured light fringe is only related to the polarization state, but not to the light intensity. Polarization coding assisted structured light measurement strategy (PASM) is proposed. This method uses polarization coding assisted polarization phase-shifting fringes for phase unwrapping. This method does not need to rotate the polarizer, and only needs a single exposure to improve the fringe quality and obtain a more stable unwrapping phase. The experimental results show that the obtained polarization fringes have better sinusoidality, and the phase unwrapping can be more accurate. The reconstructed 3D point cloud also does not appear missing and has better accuracy. It is a reliable method for visual measurement of HDR objects.

Optical counting platform of shrimp larvae using masked k-means and side window filter

Kun Qian and Hong Duan

DOI: 10.1364/AO.502868 Received 09 Aug 2023; Accepted 08 Nov 2023; Posted 09 Nov 2023  View: PDF

Abstract: Accurate and efficient counting of shrimp larvae is crucial for monitoring reproduction patterns, assessing growth rates, and evaluating the performance of aquaculture. Traditional methods via density estimation are ineffective in the case of high density. In addition, the image contains bright spots utilizing the point light source or the line light source. Therefore, in this paper an automated shrimp counting platform based on optics and image processing is designed to complete the task of counting shrimp larvae. Firstly, an area light source ensures a uniformly illuminated environment, which helps to obtain shrimp images with high resolution. Then, a counting algorithm based on improved k-means and Side Window Filter (SWF) is designed to achieve an accurate number of shrimp in the lamp house. Specifically, the SWF technique is introduced to preserve the body contour of shrimp larvae, and eliminate noise, such as water impurities and eyes of shrimp larvae. Finally, shrimp larvae are divided into two groups, independent and interdependent, and counted separately. Experimental results show that the designed optical counting system is excellent in terms of visual effect and objective evaluation.

Multi-harmonic structured illumination based optical diffraction tomography (MHSI-ODT)

Ruihua Liu, Kai Wen, Jiaoyue Li, Ying Ma, Juanjuan Zheng, Sha An, Junwei Min, Zeev Zalevsky, Baoli Yao, and Peng Gao

DOI: 10.1364/AO.508138 Received 12 Oct 2023; Accepted 08 Nov 2023; Posted 09 Nov 2023  View: PDF

Abstract: Imaging speed and spatial resolution in optical diffraction tomography (ODT) are two key factors, while they are mutually exclusive in 3D refractive index imaging. This paper presents a multi-harmonic structured illumination based optical diffraction tomography (MHSI-ODT) to acquire 3D refractive index (RI) maps of transparent samples. MHSI-ODT utilizes a digital micromirror device (DMD) to generate structured illumination containing multiple harmonics. For each structured illumination orientation, four spherical spectral crowns are solved from five phase-shifted holograms, meaning that the acquisition of each spectral crown costs 1.25 raw images. Compared to conventional SI-ODT, which retrieves two spectral crowns from three phase-shifted raw images, MHSI-ODT enhances the imaging speed by 16.7% in 3D RI imaging. Meanwhile, MHSI-ODT exploits both the 1st order and the 2nd order harmonics, and therefore, it has a better intensity utilization of structured illumination. We demonstrated the performance of MHSI-ODT by rendering the 3D RI distributions of 5-μm polystyrene (PS) microspheres and biological samples.

Mean Wavelength-Based Fresnel Lens Solar Collector for Eliminating Hotpot Problem in Daylighting Systems

Vikas Kumar, Devendra Bisht, and harry garg

DOI: 10.1364/AO.503622 Received 17 Aug 2023; Accepted 07 Nov 2023; Posted 08 Nov 2023  View: PDF

Abstract: In this paper, the mean wavelength-based Fresnel lens was designed by merging the modified edge ray principle and idea of superposition. The bottom-to-top approach optimises the design of individual prism according to the predetermined plastic optical fibre (POF) bundle size. The simulated optical efficiency of the collector for the sun’s visible spectrum (380-740 nm) light is 82.93% with a uniformity ratio of 0.434. Based on the designed collector, the daylighting system can deliver 199.38 lumens via 10 m long POF bundle with an efficiency of .78%. The thermal analysis revealed that the maximum temperature on the focus plane was 49.7°C.

Design and Analysis of Photonic Crystal Nano-Cavity based Bio-Sensor

Akash Pradhan, KEERTHAN MADAGALA, Kalamcheti Naga Sravya, and chandra prakash

DOI: 10.1364/AO.503892 Received 23 Aug 2023; Accepted 07 Nov 2023; Posted 08 Nov 2023  View: PDF

Abstract: A new, design of photonic crystal nanocavity-based bio-sensor has been proposed to detect different blood components. A finite difference time domain (FDTD) numerical technique has been used to characterize the sensor by evaluating its frequency response. The shift in resonating wavelength of the proposed cavity is utilized to detect blood refractive index fluctuation due to the presence of various components. The obtained numerical findings show that the maximum sensitivity for a shift in resonating wavelength is reported as 760 nm/RIU for various blood components. Moreover, the fabrication of PhC is always prone to the fabrication induced disorders. Hence, the impact of fabrication imperfections on the sensor’s performance also has been included in the analysis.

A rapid calculation method for backscattered light of space gravitational-wave-detection telescope

Jinhang Zhou, Yi-ping Wang, Zhenning Luo, Zizheng Li, Hongchao Zhao, Lei Fan, and Yong YAN

DOI: 10.1364/AO.504524 Received 29 Aug 2023; Accepted 07 Nov 2023; Posted 08 Nov 2023  View: PDF

Abstract: Stray light is a key issue that must be considered in the optical design process of Tianqin telescope. In order to solve the problems of long simulation time and the inability of the simulation results to be fed back to guide the optical design for complex optical systems, and finally increase the speed of optical optimization, we propose a fast estimation method of stray light based on the mirror-to-mirror field of view with high accuracy. Compared with other models, the error between our model and the software simulation results is smaller, within one order of magnitude. Based on this method, we obtain the optical component target of the Tianqin telescope and propose an optimization method for the purpose of reducing stray light. By analyzing the optimized optical design, the stray light is indeed reduced, which verifies the correctness of the model and can realize the simulation and optimization design of the optical system taking into account the backscattered light.

VisAdapt: Catadioptric Adaptive Camera for scenes of variable density of visual information

Julien Ducrocq, Guillaume Caron, hervé midavaine, christian duriez, Jérémie Dequidt, and el mustapha mouaddib

DOI: 10.1364/AO.500663 Received 03 Aug 2023; Accepted 07 Nov 2023; Posted 08 Nov 2023  View: PDF

Abstract: This paper presents the design method of a multi-resolution camera, named Visadapt.It is made of a conventional compact camera with a sensor and a lens pointed to a new deformable mirror so that the mirror at flat state is parallel to the image plane.The main novelty of the latter mirror is the ability to control automatically strokes of several millimeters.This allows Visadapt to capture scenes with a spatially variable density of visual information. A grid of linear actuators, set underneath the mirror surface, deforms the mirror to reach the desired shape computed to capture several areas of different resolutions.Mechanical simulations allowed to iterate on Visadapt's design, to reduce the geometrical distortions in the images.Evaluations made on an actual prototype of Visadapt show that, by adapting the mirror shape, this camera can magnify a scene object up to 20\%, even off-centered in the field-of-view, while still perceiving the rest of the scene.

Performance Prediction of Circularly Polarized Graphene-Dielectric Resonator Based Antenna for THz Frequency Application using Machine Learning Algorithms

Kundan Kumar and Pradip Kumar Sadhu

DOI: 10.1364/AO.502463 Received 03 Aug 2023; Accepted 06 Nov 2023; Posted 07 Nov 2023  View: PDF

Abstract: In this article, a graphene-dielectric resonator based antenna is designed in THz frequency regime. Circular polarization is achieved by feeding the cylindrical shaped ceramic using perturbed square shaped aperture. Graphene loading over the alumina ceramic provides the frequency reconfigurable feature. In order to overcome the difficulty of simulating the THz antenna (i.e. very large simulation time), machine learning algorithms such as Artificial Neural Network (ANN) and Random Forest are used to effectively predict the performance of designed antenna. The proposed antenna works effectively in between 5.0-5.5 THz with a 3-dB axial ratio frequency range from 5.1-5.35 THz. There is good correlation found between predicted, measured and simulated reflection coefficient and axial ratio. Due to stable radiation properties and good diversity performance within the operating frequency band, the proposed antenna can be employable for different wireless application in THz frequency regime.

Broadband free space 2 × 4 90° optical hybrid for satellite laser communication

lingling xu, Jianfeng Sun, Qian Xu, Haisheng Cong, Weijie Ren, Zhang kun, Yuxin Jiang, Chaoyang Li, and hanrui pan

DOI: 10.1364/AO.505607 Received 12 Sep 2023; Accepted 03 Nov 2023; Posted 06 Nov 2023  View: PDF

Abstract: We design a broadband free space 2 × 4 90° optical hybrid over a spectral window of 1000 – 1200 nm and 1470 – 1650 nm, and verify the feasibility of the scheme experimentally. The hybrid consists of three broadband polarization beam splitters, an achromatic λ/4 wave plate, and three achromatic λ/2 wave plates. The fabricated hybrid exhibits a good quadrature phase response with an interchannel imbalance of 0.93~1.07 and a low phase deviation of less than 0.2° under the typical communication wavelengths of 1064 nm and C-band. The experimental results of the heterodyne method show that the proposed hybrid can effectively solve the wavelength incompatibility problem in satellite laser communication and realize interconnection at different wavelengths. The designed hybrid (without coupling) has a measured insertion loss of no more than 7.34 dB at 1064 nm and Cband. A high-speed transmission experiment with BPSK format has been conducted to verify the performance of the assembled device.

Calibration method for infrared division-of-focal-plane polarimeter considering polarizer reflection characteristics

Jianguo Yang, Hao Cui, Li Li, Weiqi Jin, and Zunyi Sun

DOI: 10.1364/AO.500712 Received 17 Jul 2023; Accepted 02 Nov 2023; Posted 02 Nov 2023  View: PDF

Abstract: Owing to the manufacturing defects of the micropolarizer array and the difference in the pixel response of the detector, the infrared division-of-focal-plane (DoFP) polarimeter has severe non-uniformity, which affects the imaging quality of the detector and the calculation of polarization information. This study proposes a calibration method for the infrared DoFP polarimeter considering polarizer reflection characteristics. The temperature-controlled adjustable infrared polarization radiation source (TAIPS) is calibrated by a division-of-time (DoT) polarimeter and is, in turn, used to calibrate the infrared DoFP polarimeter. Through laboratory blackbody and external scenes, the performance of the proposed method is compared with that of state-of-the-art techniques. The experimental results show that the extinction ratio is improved by 25% and the non-uniformity is reduced by 15% compared with other methods, which improves the imaging quality and the accuracy of polarization information.

Design and Implementation of Dual Aspheric Integrated Shaping Element for High-Power Fiber Laser

Yupeng Xiong, Shiyu Chen, Yanxing Ma, Yang Ou, Wenwen Lu, Yifan Dai, Shanyong Chen, and Cheng HUANG

DOI: 10.1364/AO.503237 Received 16 Aug 2023; Accepted 30 Oct 2023; Posted 17 Nov 2023  View: PDF

Abstract: A novel dual aspheric integrated beam shaper suitable for high-power laser situation was first designed and realized. The model for this new lens was derived theoretically and the performance is evaluated using detailed simulation. The ultrasonic vibration assisted cutting and the high precision grinding and polishing technology were used for the processing. The surface accuracy was less than 200nm measured with a profiler, and the roughness was smaller than 20nm with the help of the white light interferometer. And the shaping experiments were carried out, which verified the Gaussian beam has uniform intensity distribution with the uniformity of 85.13% in the near field and converges to a point in the far field which is exactly expected. It thus provides an actual selection for the high-power laser shaping.

Depth estimation from a single-shot fringe pattern based on DD-Inceptionv2-UNet

Linlin Wang, Wenke Xue, Chuanyun Wang, Qian Gao, Wenjie Liang, and yinwen zhang

DOI: 10.1364/AO.504023 Received 28 Aug 2023; Accepted 23 Oct 2023; Posted 24 Oct 2023  View: PDF

Abstract: The quick and accurate retrieval of an object's depth from a single-shot fringe pattern in Fringe Projection Profilometry has been a topic of ongoing research. In recent years, with the development of deep learning, the employment of deep learning technique to FPP for single-shot 3D measurement has become active. To improve the accuracy of depth estimation from a single-shot fringe pattern, we propose the Depthwise separable Dilation Inceptionv2UNet(DD-Inceptionv2-UNet) by adjusting the depth and width of the network model simultaneously. And we evaluate the model on both simulated and experimental datasets. The experimental results show that the error between the depth map predicted by the proposed method and the label is smaller, and the depth curve map is closer to the ground truth. And on the simulated dataset, the MAE of the proposed method decreased by 35.22%, compared to UNet. On the experimental dataset, the MAE of the proposed method decreased by 34.62%, compared to UNet. The proposed method is relatively outstanding in both quantitative and qualitative evaluations, effectively improving the accuracy of 3D measurement results from a single-shot fringe pattern.

High sensitivity optical fiber probe for simultaneous measurement of chloride ions and temperature

Xia Li, Yu Li, Chenxiao Wang, Wa Jin, Guangwei Fu, Xinghu Fu, and Weihong Bi

DOI: 10.1364/AO.500182 Received 10 Jul 2023; Accepted 02 Oct 2023; Posted 22 Nov 2023  View: PDF

Abstract: A fiber optic probe for the simultaneous measurement of chloride ions and temperature is presented. The Ag/alginate composite film is used as the reflective surface of the Fabry-Perot interferometer(FPI) and is also a sensitive film for the adsorption of chloride ions. The experimental results show that the Fabry-Perot (FP) response sensitivity is approximately 1.4689 nm/μM as the chloride ion concentration changes from 1µM to 9µM, but the fiber Bragg grating(FBG) is insensitive to chloride ions. When the temperature is changed from 35℃ to 80℃, the response sensitivities of the FP and the FBG are about 0.7nm/℃ and 0.01115nm/℃, respectively.