Early Posting

Accepted papers to appear in an upcoming issue

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

Estimation of atmospheric optical turbulence strength in realistic airborne environments

Matthew Kalensky, Mark Spencer, Eric Jumper, and Stanislav Gordeyev

DOI: 10.1364/AO.459461 Received 25 Mar 2022; Accepted 28 Jun 2022; Posted 30 Jun 2022  View: PDF

Abstract: In this paper, atmospheric optical turbulence strength is estimated for realistic airborne environments using a modified phase-variance approach, as well as a modified slope-discrepancy approach. The realistic airborne environments are generated using wave-optics simulations of a plane wave propagating through increasing strengths of homogeneous atmospheric optical turbulence, both with and without aero-optical contamination (from in-flight wavefront sensor data) and additive-measurement noise. In comparison to the modified phase-variance approach, the results show that the modified slope-discrepancy approach more accurately estimates atmospheric optical turbulence strength over a wide-range of conditions. Such results are encouraging for realistic airborne environments because they can be scaled to different free-stream conditions as long as the boundary layer is considered canonical.

Broadband and high-efficiency ultrathin wavefront manipulation based on Pancharatnam-Berry phase principle

Tengfei Wang and yuanwei tong

DOI: 10.1364/AO.459545 Received 28 Mar 2022; Accepted 28 Jun 2022; Posted 30 Jun 2022  View: PDF

Abstract: Broadband and high-efficiency Metasurfaces are proposed based on the Pancharatnam-Berry (PB) principle in this paper. The PB phase principle is analyzed using the scattering matrix approach and proposed to achieve the abrupt phase with equal amplitude of the transmission coefficient. This Metasurface can control the propagation direction and the wavefront of the outgoing wave depend on the chriality of the incoming wave. A 1-D gradient metasurface and a 2-D parabolic Metasurface is designed to achieve anomalous refraction and focusing effect in the pass band. The circular polarized waves in the frequency range 8.2-10 GHz were refracted anomalously when a LHCP or RHCP were irradiated vertically on the 1-D Metasurface. For the 2-D parabolic metasurface, it not only converts planar circular polarized waves into spherical circular polarized waves, but also converges the parallel wave into a focus point. The focal distance of the 2-D parabolic metasurface decreases as the frequency of the incident wave decrease and the half maximum of full width shows the opposite trend.

Terahertz polarization sensing for protein concentration and crystallization process on reflective metasurface

Tianrui Zhang, Fei Fan, Jie -Rong Cheng, Xianghui Wang, and Shengjiang Chang

DOI: 10.1364/AO.463924 Received 17 May 2022; Accepted 28 Jun 2022; Posted 30 Jun 2022  View: PDF

Abstract: Terahertz (THz) has attracted much attentions in the field of biosensing due to its advantages of non-destructive, label-free, and high-sensitivity detection. Here, we have experimentally demonstrated a THz polarization sensing method based on reflective metasurface sensors for detecting concentrations of protein solution and their crystallization process. The protein with varying concentrations has been detected by the five different polarization parameters, which show different spectral responses and sensing sensitivity. The sensing accuracy can reach the order of ng/mm2. Furthermore, the crystallization process of the protein sample from the dissolved state to the crystalline has been dynamically measured by polarization sensing, of which the highest sensitivity can reach 0.67 °/%. Therefore, this new sensing platform can have broad development prospects in the trace matter detection of biological sample.

Investigation of dielectric fiber-3D Dirac semimetals supported tunable terahertz hybrid waveguide

Guangqing Wang, Yubo Liang, Jin Leng, Xiaoyong He, Fangting Lin, and Feng Liu

DOI: 10.1364/AO.458551 Received 15 Mar 2022; Accepted 28 Jun 2022; Posted 30 Jun 2022  View: PDF

Abstract: Based on the proposed hybrid elliptical dielectric fiber- polyethylene gap-3D Dirac semimetals (DSM) plasmonic waveguide structure, the tunable propagation characteristics have been systematically investigated in the terahertz region, taking into account the influences of the elliptical fiber shape, the modified dielectric fiber and 3D DSM Fermi levels. The results manifest that as the ratio of the elliptical semi-axis along the y direction ay and the x direction ax (ay/ax) increases, the hybrid mode confinement increases. The real part of effective mode index and propagation length both increase with the refractive index of the elliptical fiber increases. The propagation length and figure of merit of the hybrid modes reach 1.56×104 μm and 300, respectively. In addition, by changing the Fermi level of 3D DSM substrate, the propagation properties of hybrid modes can also be modulated in a wide range, e.g. the modulation depth of propagation length reaches about 73.2% if Fermi levels varies in the range of 0.03-0.15 eV. The propagation properties of the hybrid modes are enhanced significantly by utilizing the modified three elliptical fiber structures, the real part of effective index and propagation length of the modified structure are enhanced simultaneously. The results are very helpful to understand the tunable mechanism of 3D DSM devices and aid the design of novel plasmonic devices, e.g. lasers, modulation and resonator.

Nondestructive detection of anthocyanin content in fresh leaves of purple maize using hyperspectral data

Gu Xiaohe, Yang Xin, Gao Shichen, Zhang Chao, Sun Qian, Hu Xueqian, Wei Zhonghui, and Qu Xuzhou

DOI: 10.1364/AO.458787 Received 18 Mar 2022; Accepted 28 Jun 2022; Posted 30 Jun 2022  View: PDF

Abstract: Anthocyanins are widely used in food industry as an additive, improving antioxidant capacity and strengthening the human immune system. However, the rapid and nondestructive detection methods are lacking. This study aimed to develop a rapid and nondestructive method to detect anthocyanin content in fresh purple maize leaves using hyperspectral reflectance. Sensitivity bands were screened by analyzing the correlation between the spectrum and anthocyanin, chlorophyll, and moisture content in maize leaves with models constructed. Through the combination of the sensitivity bands of the three components, the interference of chlorophyll and moisture on the spectral detection of anthocyanin in fresh leaves was analyzed. The results showed that the anthocyanin sensitivity band was approximately 550 nm. The determination coefficient and RMSE (root mean squared error) of the optimal hyperspectral model were 0.772 and 4.049 mg/g, respectively. After excluding chlorophyll and moisture interference, the anthocyanin content detection accuracy was only improved by 1.8% compared to that of the original. These results indicate that hyperspectral technology can be used to nondestructively detect anthocyanin content in fresh purple maize leaves with good accuracy. Chlorophyll and moisture in the leaves did not significantly influence anthocyanin content.

Integrative design and processing of conformal optical window pair in cylindrical isolator

Yang Ou, Yifan Dai, Shanyong Chen, Xiaoqiang FAN, Hao Hu, Yupeng Xiong, bing xiong, Chunyang DU, and Chaoliang Guan

DOI: 10.1364/AO.459913 Received 31 Mar 2022; Accepted 28 Jun 2022; Posted 30 Jun 2022  View: PDF

Abstract: In order to perform the flow visualization of shock train structure by schlieren imaging method in the cylindrical isolator, a novel integrative design and processing scheme of an aluminum alloy pipe with acrylic conformal optical window pair was proposed. The optical ray tracing and wavefront correction methods were applied to design the inner cylindrical surfaces and outer aspherical cylindrical surfaces of the optical window pair for parallel light correction based on the conjoint analysis with the processing capability. Under the tolerance analysis and the optimization of the machining path, the integrative model was fabricated on a 3-axis CNC machine using two-axis turning and fast slide servo (FTS) machining. The wavefront aberration (PV) and wavefront aberration (RMS) of the optical window pair were corrected within 12.189 λ and 2.658 λ (λ=632.8 nm) in the observation area which met the requirements of high precision schlieren observation.

Using deep learning to automatically generate design starting points for free-form imaging optical systems

Chengxiang Fan, Bo Yang, Yunpeng Liu, Qianyang Zhao, Bowen Qian, and Shishuang Chen

DOI: 10.1364/AO.460977 Received 11 Apr 2022; Accepted 28 Jun 2022; Posted 30 Jun 2022  View: PDF

Abstract: In this paper, we propose a method to automatically generate design starting points for freeform three-mirror imaging systems with different folding configurations using deep neural networks. For a given range of system parameters, a large number of data sets are automatically generated using the double seed curve extended algorithm and coded optimization. Deep neural networks are then trained using a supervised learning approach and can be used to generate good design starting points directly. The feasibility of the method was verified by designing a free-form three-mirror system with three different folding configurations。This method can significantly reduce the design time and effort for free-form imaging systems, and can be extended to complex optical systems with more optical surfaces.

Improved YOLOX detection algorithm for contraband in X-ray images

Yinsheng Zhang, Wenxiao Xu, shanshan yang, yongjie Xu, and xinyuan Yu

DOI: 10.1364/AO.461627 Received 21 Apr 2022; Accepted 28 Jun 2022; Posted 30 Jun 2022  View: PDF

Abstract: Abstract: In order to ensure the safety of people's lives and property, it is very important to carry out contraband inspection on items before entering public places. Yet mainstream inspection methods are mostly based on human X-ray image observation of objects, the presence of contraband judgment, contraband is often hidden behind other normal items, in the high-intensity work environment, the inspectors occurred leak and fault detection probability increase greatly, and the results of the artificial inspection great subjectivity, consciousness change and change with the inspectors. To this end, a detection framework based on computer vision technology is proposed, which is trained and improved on the basis of the current state-of-the-art YOLOX target detection network, and adopts strategies such as feature fusion, adding double attention mechanism, and classifying regression loss. Compared with the benchmark YOLOX-S model, the proposed method achieves a higher average accuracy, with an improvement of 5.0% on the public safety SIXray dataset, it opens up a way for large-scale automatic detection of contraband in public places.

An automatic detection method for BGA defects based on X-ray imaging

Kai Xiao, Qin Li, and Yang Chen

DOI: 10.1364/AO.462074 Received 25 Apr 2022; Accepted 28 Jun 2022; Posted 30 Jun 2022  View: PDF

Abstract: Ball Grid Array (BGA) packaging technology is a high-density surface mount packaging technology, with the advantages of small size, good heat dissipation and electrical properties, which is widely applied in the production of large-scale integrated circuits. With the rapid development of integrated circuit integration, devices assembled using BGA technology generally have greater complexity. However, BGA defects can seriously affect device performance and bring challenges to product quality inspection. More importantly, during the detection process of BGA defects, the high complexity of the device brings unprecedented challenges to personnel operations, and the associative detection methods also need to be improved. Aiming at this problem, this paper proposes an automatic detection method for BGA defects based on X-ray imaging. Firstly, the X-ray imaging technology is utilized to achieve non-destructive detection of the BGA area inside the device and generate image data. On this basis, a set of algorithms including threshold separation, detection filling, closing operation are designed to complete automatic detection of BGA defects. Furthermore, the method is validated with actual BGA imaging data and obtains satisfactory results. At the same time, this paper uses an automatic robust detection method to conduct comparative experiments with our method. The comparative experimental results indicate that the algorithm in this paper has matchable accuracy and automatic detection performance. In particularly, our method shows good performance in precisely finding BGA edge defects and subtle defects.

Study on defect detectability based on square wave lock-in thermography

Pengfei Zhu, Dan Wu, Yifang Wang, and Zhifei Miao

DOI: 10.1364/AO.460385 Received 08 Apr 2022; Accepted 28 Jun 2022; Posted 28 Jun 2022  View: PDF

Abstract: In this work, a lock-in thermography technique based on periodical square wave is used to detect stainless steel plates with defects. Combining neural network with lock-in thermography, a new image processing technique is proposed. The results are compared with the traditional image processing methods. A full-field defect reconstruction technology is proposed, which combines pulse phase thermography, threshold segmentation technology and lock-in thermography technology to reconstruct the full-field depth image. This method has fast processing speed and high detection accuracy. Finally, the effects of excitation frequency and duty cycle on thermal image quality, defect detection range and defect detection accuracy are investigated through extensive experiments to arrive at the optimal excitation frequency and duty cycle.

Single-shot blind deconvolution with coded aperture

Hideyuki Muneta, Ryoichi Horisaki, Yohei Nishizaki, Makoto Naruse, and Jun Tanida

DOI: 10.1364/AO.460763 Received 07 Apr 2022; Accepted 28 Jun 2022; Posted 28 Jun 2022  View: PDF

Abstract: In this paper, we present a method for single-shot blind deconvolution incorporating a coded aperture (CA). In this method, we utilize the CA, inserted on the pupil plane, as support constraints in blind deconvolution.Not only an object but also a point spread function of turbulence are estimated from a single captured image by a reconstruction algorithm with the CA support. The proposed method is demonstrated by a simulation and an experiment in which point sources are recovered under severe turbulence.

Meta-Ensemble Learning for OPM in FMF Systems

mohammad ali Amirabadi, S. Alireza Nezamalhosseini, and Mohammad Hossein Kahaei

DOI: 10.1364/AO.461473 Received 18 Apr 2022; Accepted 28 Jun 2022; Posted 28 Jun 2022  View: PDF

Abstract: Optical performance monitoring (OPM) is crucial for facilitating the management of future few-mode fiber (FMF)-based transmissions. OPM deploys fault detection and link diagnosis by measuring the physical layer states and providing feedback to the controller. Recently, machine learning (ML) collected a lot of attention for OPM, and various ML algorithms were developed where the selection of the proper one is a challenge. State-of-the-art ML algorithms are based on combining ML models which in turn suffer from being extremely costly and the dependency on different algorithms sometimes damages the performance. In this work, we propose meta-ensemble learning (MEL)-based approach that at each instance chooses the ML models to be employed from a subset. We compare the proposed MEL method with Naive ensemble learning (NEL) as a well-known method for combining ML models. Results indicate that MEL has great potential to simultaneously reduce the required training dataset size and improve the performance.

Nickel-vanadium layered double hydroxide for mid-infrared 2 μm Tm:YAG ceramic ultrafast laser

Enlin Cai, Xiangzheng Kong, Shuaiyi Zhang, Jianyi Xu, Yan Liu, and Guanghai Guo

DOI: 10.1364/AO.462620 Received 29 Apr 2022; Accepted 28 Jun 2022; Posted 28 Jun 2022  View: PDF

Abstract: In this study, nickel-vanadium layered double hydroxide (NiV-LDH) nanosheet was prepared as a saturable absorber (SA), by liquid phase exfoliation and drop coating method. The microstructure and optical transmission properties of the obtained NiV-LDH nanosheet were then systematically studied. An “X” type fold cavity was designed, to evaluate the ultrafast laser modulation performance of the NiV-LDH nanosheet, with Tm:YAG ceramic gain medium. A stable passively Q-switched mode-locked (QML) pulse, centered at 2011.6 nm has successfully been realized, with a repetition frequency of 145 MHz and a pulse duration of 320 ps. To the best our knowledge, this is the first time that the LDH has been used as a SA, in a mid-infrared range ultrafast laser.

Security analysis on the interference-based optical image cryptosystem with a designed amplitude modulator

YI XIONG and Ravi Kumar

DOI: 10.1364/AO.458385 Received 14 Mar 2022; Accepted 28 Jun 2022; Posted 28 Jun 2022  View: PDF

Abstract: In this paper, the security strength of an interference-based optical image cryptosystem with a designed amplitude modulator (AM) is evaluated. Compared to previous improved interference-based cryptosystems in which iterative or post-processing algorithms are involved to remove the silhouette problem, there, a pre-designed AM is placed at the image plane to modulate the plaintext directly which eliminates this drawback. Moreover, the AM also serves an additional private key, which enlarges the key space and further enhances the security. However, we have noticed that one of the parameter used to design the AM has low key sensitivity, which is fault-tolerant and does not need precise reconstruction. Additionally, the AM is plaintext-independent which means it can be recovered separately by using a pair of known plaintext and the corresponding ciphertext. Based on these findings, we propose hybrid attacks, including two chosen-plaintext attacks (CPAs) and a ciphertext-only attack (COA) to break the cryptosystem. Specially, CPAs with an impulse function and an arbitrarily given plaintext, respectively, are proposed to retrieve two parameters in the designed AM. With the retrieved AM, information of the plaintext can be retrieved from the corresponding ciphertext by two kinds of COAs without any knowledge of private keys. To the best of our knowledge, it is the first time that the interference-based cryptosystem with a designed AM is cracked successfully, which may provide potential evidence for further security improvements. Numerical simulations are carried out to validate the feasibility of the proposed attack.

The impact of color on polarization-based 3D imaging and countermeasures

Cai YuDong, Fei Liu, Xiaopeng Shao, and Guo Cai

DOI: 10.1364/AO.462778 Received 02 May 2022; Accepted 28 Jun 2022; Posted 30 Jun 2022  View: PDF

Abstract: The diffuse polarization-based 3D imaging has flourished as their ability to obtain the 3D shape of objects without multiple detectors, active mode lighting, or complex mechanical structures, which are major drawbacks of other methods for 3D imaging in natural scenes. However, traditional polarization-based 3D imaging technology will introduce color distortion when reconstructing the surface of multi-colored targets. So we propose an innovative polarization-based 3D imaging model to recover the 3D geometry of multi-colored Lambertian objects. In particular, the chromaticity-based color removal theory is used to restore the intrinsic intensity which is only modulated by the target shape, and applying the recovered intrinsic intensity to address the orientation uncertainty of the target normals due to the azimuth ambiguity. Finally, integrating the corrected normals to reconstruct the high precision 3D shape. Experimental results demonstrate that the proposed model has the ability to reconstruct multi-colored Lambertian objects exhibiting non-uniform reflectance from single view under natural light conditions.

Stereo-phase rectification for metric profilometry with two calibrated cameras and one uncalibrated projector

Rigoberto Juarez-Salazar, Obed Rios-Orellana, and Victor Diaz-Ramirez

DOI: 10.1364/AO.461168 Received 14 Apr 2022; Accepted 28 Jun 2022; Posted 28 Jun 2022  View: PDF

Abstract: Fringe projection profilometry requires calibrating both cameras and projectors for metric measurements. Cameras are relatively simple to calibrate, but projectors require more sophisticated procedures. In this paper, a fringe projection profilometer with two calibrated cameras and one uncalibrated projector is developed for metric measurements. A new phase rectification method, which is crucial for stereo matching, is designed by minimizing the perspective distortion. Also, a simple method for point matching using the stereo rectified phase maps is proposed. The principles of metric profilometry using the proposed rectification method are introduced. The developed system is evaluated experimentally by the metric measurement of three-dimensional objects. The obtained results confirm a high accuracy of metric measurement and versatility in the design of fringe projection profilometers with uncalibrated projectors.

Design of All-Optical Insulator-Metal-Insulator and Metal-Insulator-Metal Feynman Logic Gates

Mahmoud Abbasi, Mojtaba Sadeghi, and Zahra Adelpour

DOI: 10.1364/AO.465712 Received 01 Jun 2022; Accepted 28 Jun 2022; Posted 28 Jun 2022  View: PDF

Abstract: In this paper, two Feynman logic gates based on insulator-metal-insulator (IMI) and metal-insulator-metal (MIM) waveguides are proposed. The gates are modeled using the finite-difference time-domain (FDTD) method. The IMI-based Feynman gate shows high extinction ratio (ER) values of 11.06 dB and 7.61 dB for the output bits P and Q, respectively. The ER values of the MIM-based Feynman gate are also 12.83 dB and 7.29 dB for the output bits P and Q, respectively. The footprint of the gates is less than 2 µm2. The proposed gates benefit from high ERs in the wavelength range of 1.5 μm-1.6 μm (band C), ultra-compact footprints, and CMOS-compatible structures which make them potential candidates for use in integrated photonic circuits.

Evaluation method for noise-induced phase error in fringe projection profilometry

Jianhua Wang

DOI: 10.1364/AO.463751 Received 12 May 2022; Accepted 27 Jun 2022; Posted 28 Jun 2022  View: PDF

Abstract: For noise-induced phase error and phase unwrapping reliability in fringe projection profilometry (FPP), a new evaluation method is proposed. By introducing a Gaussian noise model, the maximum variance of the noise-induced phase error and phase unwrapping reliability are obtained. According to the proposed evaluation method, the variance of the noise-induced phase error is proportional to the noise variance and inversely proportional to the fringe modulation. The phase unwrapping reliability is not only inversely proportional to the noise-induced phase error, but also inversely proportional to the fringe frequency ratio. We analyze and compare three efficient dual-frequency phase unwrapping algorithms. The results show that the wrapped phase accuracy and phase unwrapping reliability based on 4fH+4fL are the best, followed by 3fH+3fL and 3fH+2fL. However, 3fH+2fL has the highest measurement efficiency.

Improving Reliability Using Phase Distribution Aware LDPC Code for Holographic Data Storage

Qin Yu, Fei Wu, Meng Zhang, Yahui Zhao, and Changsheng Xie

DOI: 10.1364/AO.463930 Received 13 May 2022; Accepted 27 Jun 2022; Posted 28 Jun 2022  View: PDF

Abstract: The iterative Fourier transform (IFT) algorithm is an effective solution for phase retrieval in phase-type holographic data storage systems, but introduces a higher phase error rate. As a result, data reliability becomes a significant issue. In this paper, to improve reliability and decrease decoding latency, we propose a phase distribution aware low-density parity-check (LDPC) code (called PDAL) with outstanding error correcting capability. After experiencing IFT, we first investigate the phase distribution characteristics and find that the adjacent phase distribution is more likely to cross, resulting in higher phase shift percentages. Then, using phase distribution, PDAL optimizes LDPC codes with higher precision decoding information by dynamically applying the phase threshold based on the phase error rate. When the phase error rate is 0.04, the bit error rate, decoding iteration times, and decoding failure rate are all reduced by 51.5%, 26.9%, and 51.8% on average, respectively, compared with traditional LDPC code without exploiting phase distribution. PDAL, which is an efficient and practical error correction approach for phase-modulated holographic data storage, can improve data reliability by boosting error correction performance.

Enhancing the Sensing Behavior of Reduced Graphene Maghemite Based Plasmonic Optical Fiber Sensor

Ali Alwahibb, Mohammed Saleh, and Mohammed jalal

DOI: 10.1364/AO.458564 Received 28 Mar 2022; Accepted 27 Jun 2022; Posted 28 Jun 2022  View: PDF

Abstract: In this research sensor work, the D-shaped optical fiber is a base for the maghemite-based graphene nanocomposite (rGO/Fe3O4) for Pb heavy metals sensing layer. The designed sensor was studied under the effects of rapped annealing, water circulation, and plasmonic tuning. Various annealing temperatures (100°C, 200°C, 300°C, and 400°C) were primarily investigated. The effect of RTA temperature on the bandgap energy is verified between 2.3 eV and 3.17 eV for 100oC and 400oC, respectively. The transmission, stability, and resonance dip of the plasmonic effect were compared for the best performance under the influence of RTA and furnace methods. The best performance of the proposed sensor was improved by introducing water circulating chamber into the initial design. The resonance shifts due to Pb ions concentration (5, 10, and 15 ppm) were studied for transmission and wavelength shifts. The Sensor-shift was enhanced by using a free space polarizer controller attached to the second design. The results give detection and sensing potential in the visible range at a possible remarkable response time. This article presents the optical characterizations of plasmonic sensor-based rGO/Fe3O4 for detecting Pb ions and enhancing the resonance shift.

Dynamic Input Shaping and Image Motion Compensation for a Dual-Mirror System

Alicia Dautt-Silva and Raymond de Callafon

DOI: 10.1364/AO.463634 Received 17 May 2022; Accepted 27 Jun 2022; Posted 28 Jun 2022  View: PDF

Abstract: This paper shows that the task of trajectory planning for a dual-mirror optical pointing system with mehcanical vibrations greatly benefits from carefully designed dynamic input signals, especially when high bandwidth throughput is required. The optical pointing system consists of two Fast Steering Mirrors (FSM) for which dynamically coupled input signals are designed, while adhering to mechanical and input signal constraints. The proposed procedure consists of three steps and starts from dual-input vibration data to formulate a dynamic model and then use the model to formulate an input shaped signal via a convex optimization. First, step-response data is used to estimate a dynamic model of each FSM via a realization algorithm. Second, via (inverse) kinematic analysis the planned trajectories for the dual-mirrors are determined. Third, a linear programming problem is used to compute the dynamic input signal for each of the FSMs, with one of the mirrors acting as an image motion compensation device,that guarantees tracking of a planned trajectory within a specified accuracy and the operating constraints of the FSMs. The end result is a dynamically coupled set of planned input signals that improve the overall tracking of the dual-mirror optical pointing system, as demonstrated on the FSMs exercise included in this paper.

Modulation of high-energy γ-rays by collision of ultra-high-energy electron with tightly focused circularly polarized laser pulse

Yifan Chang, Zirui Zeng, Chang Wang, Zhaonan Long, and Youwei Tian

DOI: 10.1364/AO.459665 Received 08 Apr 2022; Accepted 27 Jun 2022; Posted 27 Jun 2022  View: PDF

Abstract: Using ultra-high-energy (γ≥ 1000) electron to collide with laser pulses to generate high-energy γ-rays is an important way to treat cancer. In this paper, we investigate a method for modulating high-energy γ-rays with higher energy and more collimation using tightly focused circularly polarized laser pulses colliding with ultra-high-energy electron. Theoretical derivation and numerical simulation within the framework of classical electrodynamics show that higher electron initial energy, stronger laser intensity and longer pulse can generate higher γ-ray energy. The high-energy γ-rays generated by electron with higher initial energies are more collimated. The increase of the laser intensity and the increase of the pulse width will increase the angular range of the high-energy γ-rays. At the same time, the phenomenon of ``jumping point" in which the radiation energy varies with the laser intensity was found. Our findings have important implications for modulating better high-energy γ-ray sources.

Design of vortex metalens with high focusing efficiency using propagation phase

Shu Yuan Lv, YuChi Bai, WenFeng Luo, Fei Meng, and Rong Wang

DOI: 10.1364/AO.464090 Received 18 May 2022; Accepted 27 Jun 2022; Posted 27 Jun 2022  View: PDF

Abstract: In this paper, three vortex-focused beams are produced with linearly polarized (LP) light along the X-axis or Y-axis at the wavelength of 1550 nm. First, the polarization-independent vortex metalens of topological charge is 3 and focal length is 3000nm is designed by selecting the cylindrical-shaped elements. This design has a focusing efficiency of 83%. Second, vortex beams with different focal lengths and topological charges are achieved by combining various shapes of structures, respectively. Both designs have a focusing efficiency of greater than 92%. The designed metasurface is of great significance to optical communication and radar detection.

Construction of Classification Models for Pathogenic Bacteria Based on LIBS Combined with Different Machine Learning Algorithms

Haorui Sun, Canran Yang, Youyuan Chen, Yixiang Duan, Qingwen Fan, and Lin Qingyu

DOI: 10.1364/AO.463278 Received 09 May 2022; Accepted 26 Jun 2022; Posted 27 Jun 2022  View: PDF

Abstract: Bacteria seriously threaten human life and health, especially foodborne pathogens. Rapid discrimination techniques for foodborne pathogens are still urgently needed. At present, laser-induced breakdown spectroscopy (LIBS), combined with machine learning algorithms, is seen as fast recognition technology for pathogenic bacteria. However, there is still a blank on evaluating the differences between different bacterial classification models. In this work, five species of foodborne pathogens were analyzed via LIBS, then the preprocessing effect of five filtering methods was compared, to improve the accuracy. The preprocessed spectral data were further analyzed with a support vector machine (SVM), a backpropagation neural network (BP) and k-nearest neighbor (KNN). By comparing the capacity of the three algorithms to classify pathogenic bacteria, the most suitable one was selected. The signal-to-noise ratio and mean square error of the spectral data after applying a Savitzky-Golay filter reached 17.4540 and 0.0020. The SVM algorithm, BP algorithm, and KNN algorithm attained the highest classification accuracy for pathogenic bacteria, reaching 98%, 97% and 96%, respectively. The results indicate that with the support of a machine learning algorithm, LIBS technology demonstrates superior performance, and the combination of the two is expected to be a powerful tool for pathogen classification.

Terahertz Antireflection Coatings Employing Off-the-Shelf Adhesive Tapes

Akif Ahmed, Aime Braconnier, Josh Gibbs, and Jacob Burgess

DOI: 10.1364/AO.456254 Received 14 Feb 2022; Accepted 26 Jun 2022; Posted 27 Jun 2022  View: PDF

Abstract: We demonstrate the feasibility of making antireflection coatings (ARCs) for terahertz (THz) light using multilayered polymer films from commercial adhesive tapes. Efficient and low-cost ARCs in the THz range are not conveniently available. Our economical approach can mitigate many of the experimental challenges posed by Fresnel reflection. Harnessing a time-domain THz spectrometer, we demonstrate the performance of several types of multilayer coatings on a variety of substrates. By varying layer stacking and thicknesses, spectral performance can be tuned and optimized for specific applications. Good agreement is found between experimental measurement and analytic calculations evaluating the performance of these multilayer tape ARCs.

A symmetrical indoor visible light communication system layout optimized by improved Grey Wolf algorithm

Zuo Yihang, BoJun Liu, and Kunming Shao

DOI: 10.1364/AO.458919 Received 28 Mar 2022; Accepted 26 Jun 2022; Posted 27 Jun 2022  View: PDF

Abstract: In this paper, we present a novel method for indoor visible light optimization based on improved Gray Wolf algorithm (IGWO) to reduce the mean square error(MSE) of the received power. The influence of conventional one single degree of freedom modification to change the power factor or position is analyzed for classical 4-lamp and 16-lamp layout. To improved the performance further, a two degree of freedom optimization scheme which will combine the position and power factor together for indoor visible light system with 4 to 16 lamps is explored. The results show that both the mean square error and fluctuation range of plane received power are greatly reduced with an significantly increased lighting coverage. Using the symmetry property of the square, a new layout can be obtained with the advantages of easy implementation, high lighting coverage and uniform receiving power.

Ceramic surface relief gratings imprinted on an optical fiber tip

Piotr Pala, Karolina Gemza, Piotr Kolodziej, Justyna Krzak, Andrzej Gawlik, Jacek Olszewski, Gabriela Statkiewicz-Barabach, Katarzyna Komorowska, and Tadeusz Martynkien

DOI: 10.1364/AO.459390 Received 24 Mar 2022; Accepted 26 Jun 2022; Posted 27 Jun 2022  View: PDF

Abstract: We report on the fabrication, experimental measurement, and numerical simulation of sol-gel diffraction grating structures deposited on the end-face of a single mode optical fiber using the imprint method. We demonstrate the high quality of the fabricated structures based on atomic force microscopy imaging and their operational characteristics, presenting measured and simulated far-field intensity distributions. Using a numerical model we simulated far-field diffraction patterns. We obtained strong agreement between the results of the simulations and experiments. We also investigated the tolerance of fabricated structures to high power lasers. Among the proposed structures, the most intriguing is the grism fabricated on a fiber end-face using sol-gel imprint technology for the first time.

A new method for improving the measurement accuracy of binocular stereo vision in a scattering environment

Yuanhang Liu, Jin Zhang, Lu Hong, Yangwei Fu, haojie xia, and Rui Zhang

DOI: 10.1364/AO.463391 Received 09 May 2022; Accepted 24 Jun 2022; Posted 27 Jun 2022  View: PDF

Abstract: In the scattering environment, binocular stereo vision measurement technology will produce large errors due to the change of refractive index of imaging light path and the decrease of target image contrast. For this problem, this paper proposes a new method for improving the measurement accuracy of binocular stereo vision in a scattering environment combined with polarization imaging theory. Firstly, the scattering images with different polarization angles are obtained and filtered by Gaussian low-pass filter to calculate the degree of polarization and angle of polarization. Then, the scattered light intensity is calculated by using the polarization information to obtain the images after removing the scattering. Secondly, feature extraction and matching are carried out for the images after scattering removal. Finally, the target is measured based on the binocular stereo vision measurement model. The experimental results show that when the scattering concentration is high enough, the conventional method can no longer be measured, but the method proposed in this paper can still obtain the target parameters at this time, and can also improve the measurement accuracy by at least 46.30%. In conclusion, the proposed method provides a valuable reference to improve the accuracy of binocular stereo vision measurement in a scattering environment by reducing the interference of scattering light.

Optimal Estimation Framework for Ocean Color Atmospheric Correction and Pixel-level Uncertainty Quantification

Amir Ibrahim, Bryan Franz, Andrew Sayer, Kirk Knobelspiesse, Minwei Zhang, Sean Bailey, Lachlan McKinna, Meng Gao, and Paul Jeremy Werdell

DOI: 10.1364/AO.461861 Received 20 Apr 2022; Accepted 24 Jun 2022; Posted 27 Jun 2022  View: PDF

Abstract: Ocean color remote sensing requires compensation for atmospheric scattering and absorption (aerosol, Rayleigh, and trace gases), referred to as atmospheric correction (AC). AC allows inference of parameters such as spectrally resolved remote sensing reflectance (Rrs(λ); sr-1) at the ocean surface from the top-of-atmosphere reflectance. Often, the uncertainty of this process is not fully explored. Bayesian inference techniques provide a simultaneous AC and uncertainty assessment via a full posterior distribution of the relevant variables, given the prior distribution of those variables and the radiative transfer (RT) likelihood function. Given uncertainties in the algorithm inputs, the Bayesian framework enables better constraints on the AC process by using the complete spectral information compared to traditional approaches that use only a subset of bands for AC. This paper investigates a Bayesian inference research method (Optimal Estimation, OE) for ocean color AC by simultaneously retrieving atmospheric and ocean properties using all visible and near-infrared spectral bands. The OE algorithm analytically approximates the posterior distribution of parameters based on normality assumptions and provides a potentially viable operational algorithm with a reduced computational expense. We developed a Neural Network (NN) RT forward model look-up-table-based emulator to increase algorithm efficiency further and thus speed up the likelihood computations. We then applied the OE algorithm to synthetic data and observations from the MODerate resolution Imaging Spectroradiometer (MODIS) on the NASA Aqua spacecraft. We compared the Rrs(λ) retrieval and its uncertainty estimates from the OE method with in-situ validation data from NASA’s SeaWiFS Bio-optical Archive and Storage System (SeaBASS) and Aerosol Robotic Network Ocean Color (AERONET-OC) datasets. The OE algorithm improved Rrs(λ) estimates relative to the NASA standard operational algorithm by improving all statistical metrics at 443, 555, and 667 nm. Unphysical negative Rrs(λ), which often appear in complex water conditions, was reduced by a factor of 3. The OE-derived pixel-level Rrs(λ) uncertainty estimates were also assessed relative to in-situ data and were shown to have skill.

Application of Ultraviolet-visible spectroscopy coupled with support vector regression for the quantitative detection of thiamethoxam in tea

Delong Meng, Ciyong Gu, Lin Li, Zhimin Zhao, Weichun Zhang, and ChaoLing Du

DOI: 10.1364/AO.463293 Received 06 May 2022; Accepted 24 Jun 2022; Posted 27 Jun 2022  View: PDF

Abstract: In this study, a new model combining UV-visible (UV-Vis) spectroscopy and support vector regression for the quantitative detection of thiamethoxam in tea is proposed. First, each original UV-Vis spectrum in the sample set is decomposed into some intrinsic mode functions (IMFs) and a residual via ensemble empirical mode decomposition (EEMD). Next, the decomposed IMFs are reconstructed into high frequency and low frequency matrices, and the residuals are combined into a trend matrix. Then, the support vector regression (SVR) is used to build regression sub-models between each matrix and the content of thiamethoxam in tea. Finally, the combination model is established by a weighted average of the sub-models. The prediction results are compared with SVR and SVR coupled with several preprocessing methods, and the results demonstrate the superiority of the proposed approach in the quantitative detection of thiamethoxam in tea.

Temporal Compression of High-power IR Laser Pulses in a KDP Crystal

Sergey Mironov, Ivan Mukhin, Vladimir Lozhkarev, Anatoly Poteomkin, Mikhail Martyanov, Igor Kuzmin, and Efim Khazanov

DOI: 10.1364/AO.464579 Received 20 May 2022; Accepted 24 Jun 2022; Posted 27 Jun 2022  View: PDF

Abstract: It is shown that a KDP crystal can be used for temporal compression of powerful pulses of the near-IR range. A method of searching for laser beam and crystal parameters suitable for compression is proposed. Temporal compression of laser pulses at a central wavelength of 1034 nm from 266 fs to 94 fs during propagation along the optical axis in a 21-cm thick KDP crystal is demonstrated experimentally.

Assessment of a computed tomography imaging spectrometer using an optimized expectation-maximization algorithm..

Freddy Narea, Jorge Castro-Ramos, Juan Sánchez-Escobar, and Aaron Muñoz Morales

DOI: 10.1364/AO.460229 Received 01 Apr 2022; Accepted 23 Jun 2022; Posted 23 Jun 2022  View: PDF

Abstract: We designed and built a homemade Computed tomography imaging spectrometer (CTIS) with 250 × 250 pixels spatial resolution and 2 nm spectral resolution, based on a diffraction-limited optical design with thick aplanatic lenses. The optical design considers an experimental CTIS optical array coupled to a digital reflex camera, which generates an imagewith minimized wavefront error. We reconstructed the intensity spectra of a fluorescent source, the diffuse reflectance of a Colorchecker, and samples of Capsicum annuum, of three different colors, using the Expectation-Maximization sequential algorithm, optimized utilizing an array of indices to reduce the reconstruction time. The results obtained with a Colorchecker indicate a high positive correlation of 0.9745 with an average residual difference of 1.31% concerning the spectra obtained with a commercial integrating sphere spectrometer. The feasibility of the proposed CTIS system shows how to detect and evaluate the physiological changes resulting from the decomposition of the green fruit of the Capsicum annuum in a range from 500 to 650 nm.

An effective all-fiber pump recycler for kW fiber lasers

Weixuan Lin, Maxime Desjardins-Carrière, Victor Iezzi, André Vincelette, Marie-Hélène Bussières-Hersir, and Martin Rochette

DOI: 10.1364/AO.464425 Received 19 May 2022; Accepted 23 Jun 2022; Posted 23 Jun 2022  View: PDF

Abstract: We report an effective pump recycler for industrial kW fiber lasers. The pump recycler is a (6+1)x1 tapered fiber bundle, with signal ports of Ge-doped fibers (GDF) with core/clad diameters of 20/400 μm and pump fiber ports (PFP) with core/clad diameters of 135/155 μm. By splicing PFPs in pairs, 77.9% of the residual pump light reaching the pump recycler is sent back to the cladding of the GDF. The insertion of a pump recycler increases the power conversion efficiency (PCE) from 61.0% to 70.5% of a fiber laser using a 20 m long Yb-doped fiber (YDF), with a maximum output power of 2.78 kW. The laser with a 20 m long YDF and pump recycler compares well to a fiber laser using a 40 m long YDF without pump recycler. In both cases, the PCE is comparable but the laser with the 20 m long YDF and pump recycler benefits of reduced stimulated Raman scattering (SRS), thus enabling an 80% increase in Raman threshold. By giving access to short YDF length, the tapered fiber bundle represents an effective pump recycler since it enables reducing SRS while keeping a large PCE.

Blind and low-complexity modulation format identification based on signal envelope flatness for autonomous digital coherent receivers

xuedong jiang, Hao Ming, Lianshan Yan, lin Jiang, and Xingzhong Xiong

DOI: 10.1364/AO.457463 Received 08 Mar 2022; Accepted 22 Jun 2022; Posted 22 Jun 2022  View: PDF

Abstract: Modulation format identification (MFI) is a critical technology for autonomous digital coherent receivers in next generation elastic optical networks. In this paper, a novel and simple MFI scheme based on signal envelope flatness is proposed without requiring any training or other prior information. After amplitude normalization and partition, the incoming polarization division multiplexed (PDM) signals can be classified into QPSK, 8QAM, 16QAM and 32QAM signals according to envelope flatness R1, R2, and R3 of signals in different amplitude ranges. The feasibility of the proposed MFI scheme is firstly verified via numerical simulations with 28Gbaud PDM-QPSK/-8QAM/-16QAM/-32QAM signals. Only by using 4000 symbols, the proposed MFI scheme can achieve 100% correct identification rate for the four modulation formats over a wide optical signal-to-noise ratio (OSNR) range. Proof-of-concept experiments among 28GBaud PDM-QPSK/-8QAM/-16QAM, and 21.5 GBaud PDM-32QAM systems under back-to-back and long-haul fiber transmission links are implemented to further demonstrate the effectiveness of the proposed MFI scheme. Experimental results show that the proposed MFI scheme can obtain 100% correct identification rate when the OSNR value of each modulation format is higher than the threshold corresponding to 7% FEC and is resilient towards fiber nonlinearities. More importantly, compared with other two low-complexity MFI schemes, the proposed MFI scheme can significantly reduce computational complexity.

Reconfigurable Intelligent Surface-Assisted Free-Space Optical Communication System under the Influence of Signal Blockage for Smart City Applications

Prasad Naik Ramavath, K Prabu, and Goutham Simha G D

DOI: 10.1364/AO.459053 Received 04 Apr 2022; Accepted 21 Jun 2022; Posted 22 Jun 2022  View: PDF

Abstract: Atmospheric turbulence and pointing errors represent substantial hurdles to free-space optical communications (FSOs), impeding their practical design. The reconfigurable intelligent surface (RIS) is an emerging technology that enables reflective radio transmission conditions for next-generation 5G/6G wireless frameworks by intelligently adjusting the beam in the desired direction using low-cost inactive reflecting elements. In this paper, we proposed a RIS-assisted FSO system for mitigating the effects of atmospheric turbulence, pointing errors, and communication system signal blockage. The probability density function and cumulative distribution functions of an FSO system composed of $N$- RIS elements are evaluated in a free space environment that contains obstructions. We derived closed-form expressions for the proposed system's bit error rate (BER), outage probability, and channel capacity . The proposed system's performance is analyzed in terms of BER, outage probability, and channel capacity under various weather conditions, pointing errors, and signal blockage. The results are plotted as a function of number of RIS elements and average signal-to-noise ratio. The proposed system will be extremely beneficial in smart city applications since it will provide reliable connectivity in urban environments with high population density and high-rise buildings.

Fabrication of microgrooves in PMN-PT using femtosecond laser irradiation and acid etching

Tianlun Shen, Jinhai Si, Tao Chen, Yongyong Zhuang, and Xun Hou

DOI: 10.1364/AO.459556 Received 30 Mar 2022; Accepted 21 Jun 2022; Posted 22 Jun 2022  View: PDF

Abstract: A simple method of fabricating PMN-PT deep grooves with high aspect ratio using an 800 nm femtosecond laser with chemical selective etching is developed. The 567-μm-deep grooves with an aspect ratio of approximately 35 were obtained with no crack and thermal affected zone. The morphologies and chemical compositions of grooves were analyzed by scanning electron microscope with energy dispersive X-ray spectrometer. The formation mechanism of PMN-PT grooves is attributed to the chemical reactions of hydrochloric acid (HCl) and laser-induced structural changes (LISC). PMN-PT in LISC became amorphous or mixtures of metal oxide from crystal and all the compounds could react with concentrated HCl and form soluble matter, leaving no precipitation. Furthermore, influences of laser irradiation parameters on depths and aspect ratios of grooves are studied.

Design of Unclad Single Mode Fiber-Optic Biosensor (USMFOBS) based on LSPR by using COMSOL Multiphysics (FEM)

Sara Tariq, Makram Fakhri, Evan T. Salim, Uda Hashim, and Forat H. Alsultany

DOI: 10.1364/AO.458175 Received 11 Mar 2022; Accepted 20 Jun 2022; Posted 27 Jun 2022  View: PDF

Abstract: In recent years, many simulation programs have been developed with various light propagation parameters in optical fiber, either for communication or sensing applications. In the present year, we proposed an unclad optical fiber biosensor based on localized surface plasmon (LSPR) phenomenon and operating at 650nm by using COMSOL Multiphysics 5.1 finite element method (FEM). Conventional single-mode fiber SM-28e has been used to develop this sensor. The central part (3cm) of the cladding was removed by using hydrofluoric acid (HF) and replaced by gold nanoparticles (AuNPs) of 50nm thickness which were coated on the core of the fiber. Air (empty fiber), water, blood plasma, liver tissue, colon tissue, and pentanol (C5H11OH) were used as the analytical layers with 3µm thickness to test this unclad single-mode fiber-optic biosensor (USMFOBS). We found that the largest sensitivity was obtained for blood plasma with the value of 10638.297 nm/RIU and a resolution of 9.4×〖10〗^(-6) RIU. Thus, we can use this sensor as a refractive index based-optical fiber biosensor.

Fast testing of non-independent imaging lens group based on liquid crystal spatial light modulator

Yufan Zhang, Yuanyuan Ren, Jiaan Chen, Zijian Hu, Jian Bai, Lei Zhao, and Kaiwei Wang

DOI: 10.1364/AO.460384 Received 06 Apr 2022; Accepted 19 Jun 2022; Posted 22 Jun 2022  View: PDF

Abstract: High requirements for the quality inspection of mobile phone cameras are put forward nowadays, as the design tolerance is getting critically tighter. In order to avoid unnecessary disposal of lens components when testing and assembling the complete cameras, an effective way is to test the quality of each single lens group in advance before the final assembly. However, as part of a whole camera, a single lens group cannot generate a sharp image independently, which needs to be cooperated with other elements in the testing system and be assembled precisely. In order to address this challenge, we propose a fast testing method based on spatial light modulators (SLMs). By taking advantage of the programmable feature of the SLM, the assembly misalignments caused by fixing the lens group to be tested into the testing system are dynamically scanned and compensated, in a fast speed. A design criterion of the phase map pattern to be loaded on SLM is also verified by simulation and applied on testing system. In this way, the proposed method significantly reduces the positioning requirement of the lens under test, and thus improves the efficiency. The passed yield of tested lens groups reaches 92.6%.

Detection of weak micro-scratches on aspherical lensesusing a Gabor neural network and transfer learning

HONG THAI NGUYEN, Tsao Yu Ming, and Hsiang-Chen Wang

DOI: 10.1364/AO.461380 Received 15 Apr 2022; Accepted 19 Jun 2022; Posted 22 Jun 2022  View: PDF

Abstract: Surface defect detection is a crucial step in ensuring the quality of lenses. One method to check for surface defects is to use an optical system integrated with an industrial camera to magnify and highlight the position of a defect on the surface of a lens. Therefore, automatic optical inspection systems are applied to detect micro-defects. In this study, we propose an automatic inspection platform based on a deep neural network for automatically imaging and examining the surface of a lens. High-resolution images of 2448 × 2048 pixels are acquired using a hybrid lighting system. A convolutional neural network integrated with a trainable Gabor filter is used as a machine vision algorithm to perform image classification and defect segmentation tasks. The platform’s results show that it outperforms traditional computer vision algorithms and other state-of-the-art neural networks, achieving a classification accuracy of 99% and a segmentation accuracy of 98%.

Projection Background-Oriented Schlieren

Joshua Weisberger and Brett Bathel

DOI: 10.1364/AO.458770 Received 17 Mar 2022; Accepted 19 Jun 2022; Posted 22 Jun 2022  View: PDF

Abstract: A background-oriented schlieren (BOS) system is developed with two benefits over traditional BOS systems. First, the dot pattern required for BOS is projected onto a retroreflective background instead of being painted/printed onto the material itself, allowing for on-the-fly updates to the size and distribution of the dot pattern. Second, a reference image is acquired for every flow image so that real-time BOS images can be displayed, and a flow-off reference image need not be acquired if the projected dot pattern is changed during a run for BOS signal optimization. The system can be made very compact, can be converted quickly to operate as a shadowgraph system, and can be integrated with polarization optics that reduce glare/reflections from wind tunnel windows.

Effect of wrinkles on extreme ultraviolet pellicle reflectivity and local critical dimension

Dong Gi Lee, Young Woong Kim, Seung Chan Moon, and Jinho Ahn

DOI: 10.1364/AO.461413 Received 15 Apr 2022; Accepted 18 Jun 2022; Posted 23 Jun 2022  View: PDF

Abstract: Extreme ultraviolet pellicles must have an extreme ultraviolet reflectance below 0.04 % to prevent the reduction of critical dimension. However, pellicle wrinkles cause localized critical dimension variation by locally amplifying the extreme ultraviolet reflectance. This study demonstrates that wrinkles can increase the pellicle’s extreme ultraviolet reflectance by approximately four times, and the critical dimension drop depends on the relative position of the reflected light from the wrinkle to the 0th- or 1st-order diffracted light. The critical dimension decreases by 6 nm. Thus, we have confirmed that, even if the pellicle reflectance requirements are satisfied, it is necessary to quantify and control the occurrence of wrinkles during the exposure process to suppress localized critical dimension variation due to wrinkles.

Relative humidity sensing using THz metasurfaces combined with polyvinyl alcohol

Jialin Gu, Jianyuan Qin, and Zhanghua Han

DOI: 10.1364/AO.463276 Received 20 May 2022; Accepted 17 Jun 2022; Posted 22 Jun 2022  View: PDF

Abstract: Relative humidity plays an important role in almost every industrial field. Thus the detection of relative humidity is of great significance in these fields. Terahertz waves are extremely sensitive to the changes in relative humidity because water absorbs strongly in this electromagnetic band. In this paper, a relative humidity sensor based on terahertz metasurfaces combined with polyvinyl alcohol is proposed. Being different from the conventional metasurfaces sensor, the polyvinyl alcohol layer is sandwiched between the metal structure and the substrate in our sensor. The improved design is able to enhance the interaction between the electric field and the water molecules absorbed in the polyvinyl alcohol layer, and the sensitivity of the humidity sensor can reach 0.34 GHz/%RH, which is more than twice that of the conventional metasurfaces sensor. These results show that the proposed sensor can be used for the detection of relative humidity with high reliability and high sensitivity. Our study opens a new avenue for relative humidity sensing using terahertz spectroscopy.

Polarization-sensitive tunable extraordinary terahertz transmission based on a hybrid metal-vanadium dioxide metasurface

Seyed Hadi Badri and Sanam Nahaie

DOI: 10.1364/AO.460547 Received 06 Apr 2022; Accepted 17 Jun 2022; Posted 22 Jun 2022  View: PDF

Abstract: A thermally tunable extraordinary terahertz transmission in a hybrid metal-vanadium dioxide (VO2) metasurface is numerically demonstrated. The metasurface consists of a metal sheet perforated by square loops while the loops are connected with strips of VO2. The frequency and amplitude of the transmission resonance are modulated by controlling the conductivity of the VO2. For y-polarized incident field, the resonance transmission peak redshifts from 0.88 to 0.81 THz upon insulator-to-metallic phase transition of VO2. For x-polarized incident field, the transmission resonance at 0.81 THz is observed in the insulator phase. However, in the metallic phase of VO2, the electromagnetic field is effectively reflected in the 0.5-1.1 THz range with a transmission level lower than 0.14. The proposed metasurface can be utilized as a terahertz modulator, reconfigurable filter, or switch.

High precision anamorphic lens calibration with 3D and 2D calibration targets

Zhang Jinkai and Xiaobo Chen

DOI: 10.1364/AO.456166 Received 14 Feb 2022; Accepted 15 Jun 2022; Posted 17 Jun 2022  View: PDF

Abstract: A high precision camera calibration method for anamorphic lenses is provided in this paper. For the adoption of cylindrical lenses, the distortion model for anamorphic lenses is much more complex than the distortion model of spherical lenses. Based on the aberration theory and numerical experiments, a polynomial type distortion model is provided for anamorphic lenses, with which high precision camera calibrations can be achieved using 3D calibration targets. Generally, 3D calibration targets have limited sizes which can’t cover the whole imaging field of anamorphic lenses. And the calibration results using 3D calibration targets tend to be unstable with the increased number of distortion coefficients in the anamorphic distortion model. Thus after the anamorphic lenses are calibrated using 3D calibration targets, they are re-calibrated using 2D calibration targets which can cover the field of view of the anamorphic lenses, and a more reliable and precise calibration results can be easily achieved. Four anamorphic lenses are calibrated with high precision at the end the paper, which proves the effectiveness of the proposed method.

Aberration analysis and compensate method of BP neural network and sparrow search algorithm in deep ultraviolet lithography

Shuang Zhang, Libin Zhang, Tianyang Gai, peng xu, and Yayi Wei

DOI: 10.1364/AO.462436 Received 02 May 2022; Accepted 15 Jun 2022; Posted 23 Jun 2022  View: PDF

Abstract: Mass production can be planned by utilizing the multiple patterning technology of 193nm immersion scanners at the 7nm technology node. In Deep Ultraviolet Lithography (DUVL), the imaging performance is significantly affected by distortions of projection optics. For 7nm immersion lithography layer patterns, distortions of the projection optics must be controlled very tightly. This paper proposes an optimization method to determine the distribution of Zernike aberration coefficients. Firstly, we build aberration prediction models using the backpropagation neural network (BPNN). Then, we propose an aberration optimization method based on Sparrow Search Algorithm (SSA), using the common indicators of the lithography process window, Depth of Focus (DOF), Mask Error Enhancement Factor (MEEF), and Image Log Slope (ILS) as the objective function. Some sets of optimized aberration distributions are obtained using the SSA optimization method. Finally, we compare the results of the SSA optimization algorithm with those obtained by rigorous computational simulations. The aberration combination distribution optimized by the SSA method is much more significant than the value under the zero aberration (ideal conditions), a non-optimal distribution in DUVL image simulation. Furthermore, the results indicate that the aberration optimization method has a high prediction accuracy.

Phase Image Correlation Spectroscopy for Detecting Microfluidic Dynamics

Peng Gao, Lan Yu, Yu Wang, Yang Wang, Kequn Zhuo, Ying Ma, min liu, Juanjuan Zheng, Jianlang Li, and Junhua Li

DOI: 10.1364/AO.458026 Received 08 Mar 2022; Accepted 12 Jun 2022; Posted 14 Jun 2022  View: PDF

Abstract: It is essential to quantify the physical properties and the dynamics of flowing particles in many fields, especially in microfluidic-related applications. In this paper, we propose phase image correlation spectroscopy (PICS) as a versatile tool to quantify the concentration, hydro-diameter, and flow velocity of unlabeled particles by correlating the pixels of the phase images taken on flowing particles in a microfluidic device. Compared with conventional image correlation spectroscopy (ICS), PICS is minimally-invasive, relatively simple, and more efficient since it utilizes the intrinsic phase of the particles to provide contrast instead of fluorescent labeling. We demonstrate the feasibility of PICS by measuring flowing PMMA microspheres and yeast in a microfluidic device. We can envisage that PICS will become an essential inspection tool in biomedicine and industry.

Temporal convolution network with dual attention mechanism for φ-OTDR event classification

Hui Dong, Manling TIAN, Xiaomin CAO, and Kuanglu YU

DOI: 10.1364/AO.458736 Received 17 Mar 2022; Accepted 09 Jun 2022; Posted 10 Jun 2022  View: PDF

Abstract: In this paper, we propose a hybrid model named channel attention based temporal convolutional network combined with spatial attention and bidirectional Long short-term Memory network (ATCN-SA-BiLSTM) for Ф-OTDR signal recognition. This hybrid model consists of 3 parts: ATCN which extracts temporal features and preserves causality of time-domain signal, SA mechanism which re-weights spatial sequence for better feature extraction, and BiLSTM which extracts spatial relationships considering the bidirectional propagation characteristics of disturbances in space-domain signals. Experimental results show that our method achieves better classification performance with an accuracy of 93.4% and zero NAR.

Terahertz Imaging using Optically-Controlled Fourier-BASIS Structured Illumination

Erich Grossman, Daniel Feldkhun, Kelvin Wagner, and Sean McComb

DOI: 10.1364/AO.455226 Received 15 Mar 2022; Accepted 01 Jun 2022; Posted 01 Jun 2022  View: PDF

Abstract: We demonstrate that a new type of structured-illumination imaging may be migrated10 from the optical to the terahertz domain. This Fourier-basis technique involves illuminating a11 target with rapidly moving sinusoidal fringes of controllable spatial frequency and orientation,12 while measuring the scattered radiation on a single fast detector. This initial proof-of-concept13 demonstration is purely one-dimensional since the fringe orientation is fixed, but the technique14 is readily extensible to two dimensions. The fringes are first generated in the near-infrared15 (808 nm) by passing a high power laser beam through an acousto-optic Bragg cell driven by a16 superposition of two RF signals slightly offset in frequency, blocking the undeflected beam, and17 refocusing the two diffracted beams onto a metal-backed semiconductor wafer. The laser can be18 amplitude-modulated to slow down the moving fringes to accommodate the semiconductor’s19 temporal response. The semiconductor acts as an optically addressed spatio-temporal modulator20 for a THz beam illuminating the same area. The periodic optical fringes effectively transform21 the semiconductor into a reflective THz diffraction grating with a programmable period. The22 diffracted THz radiation is then imaged onto the remote target plane, where the diffraction orders interfere pairwise to create traveling THz fringes. Scattered radiation from the target is collected24 by a simple receiver operating in "light bucket" mode, which produces an output signal consisting25 of a superposition of sinusoidal tones, one for each spatial Fourier component of the target. We26 present measurements of the THz fringe projector’s performance and compare with a model of27 the semiconductor modulator’s operation. Finally, we present Fourier-reconstructed images of28 pairs of point targets as an initial demonstration of THz F-BASIS imaging.

A 3D Noise Photon Transfer Curve

Bradley Preece and David Haefner

DOI: 10.1364/AO.452166 Received 28 Dec 2021; Accepted 17 May 2022; Posted 08 Jun 2022  View: PDF

Abstract: A photon transfer curve (PTC) is used to determine the fundamental detector noise parameters such as read noise, conversion gain, and fixed pattern noise. Here, the method for determining PTC is expanded to include 3D noise parameters. A 3D Noise PTC provides more insight into detector noise and is a treated as the next logical step to the classical PTC. However, it induces several new challenges in analyzing the results, specifically the fitting of seven, or more, variance curves compared to the one (total variance) or two (temporal and fixed pattern variance) prior. Therefore, a general measurement model is created which provides a new method to separate out all the classical terms, such as DSNU and PRNU, but can also handle high gain cameras with a noise factor. This method is then verified using Monte-Carlo simulations and applied to a commercial machine vision camera. In addition, the effects of lens vignetting and Non Uniformity Correction (NUC) are explored, along with a comparison of the single pixel PTC.

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

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

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

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

This is a test submission

Dan McDonold and Keith Jackson

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

Abstract: This is a test submission