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.

The Rotation of Reference Frame Dependent Polarimetric Variables for Equidistant Fisheye Lens Projections

E. Blocker and Kenneth Voss

DOI: 10.1364/AO.519772 Received 25 Jan 2024; Accepted 03 Mar 2024; Posted 04 Mar 2024  View: PDF

Abstract: The effect of a linear polarizer is conventionally well defined when viewed along the axis normal to its face. However, even for ideal linear polarizers, the non-normal viewing geometries attainable with wide-angle lenses require further considerations. A method to describe the polarization state of light passed through a linear polarizer and observed with an equidistantfisheye lens is described.

Trapping of low and high refractive index nano-spherical particles by using highly focused Laguerre-Gaussian beam

Fatima Thabit and Mohamed Shukri

DOI: 10.1364/AO.519752 Received 22 Jan 2024; Accepted 03 Mar 2024; Posted 05 Mar 2024  View: PDF

Abstract: We have investigated the trapping force on the low and high refractive index nanoparticles using a highly focused Laguerre-Gaussian beams. It was clearly found that the increase of the beam mode 𝑝 causes a substantial increase in the number of trapping zones which frequently increase the number of particles to be captured simultaneously. Also, the decrease of 𝑝 values would increase the trap area which consequently lead to simultaneous trapping of several particles in the same area. The change of 𝑝, 𝑙, Δ𝑛 and 𝑧1 values have a direct influence on the gradient forces. The smaller values of scattering and thermal forces relative to the gradient force values represent essential conditions for the stability of the trapping performance.trapping performance.

Modelling the atmospheric refractive indexstructure parameter using macrometeorologicalobservations

Ravikiran Hegde, Anand Sarma, S Satheesh, and Krishnamoorthy K

DOI: 10.1364/AO.519025 Received 14 Jan 2024; Accepted 02 Mar 2024; Posted 04 Mar 2024  View: PDF

Abstract: The dynamic fluctuations in the atmospheric refractive index, commonly referred toas optical turbulence, causes phase distortions of the electromagnetic waves propagatingthrough the atmosphere. The consequent scintillations have large implications for free-spaceoptical communication, laser remote sensing, and directed energy applications. The refractiveindex structure parameter (𝐶2𝑛), quantifying the strength of these fluctuations, is usuallyestimated using high-frequency micrometeorological measurements, employing sonicanemometer-thermometers or scintillometers. Despite providing highly accurate information,these systems are immensely complex and costly, especially for frequent field applications andremote locations. In this study, we have developed an empirical multinomial model forestimating 𝐶2𝑛 using three-year macrometeorological data and validated it against collocatedand concurrent micrometeorological measurements, from a tropical semi-arid location. Thissimpler model would be handy for applications in remote locations having weather stationmeasurements alone.

Quantitative assessment whole-rock iron content in magnetite protolith based on terahertz time-domain spectroscopy

Mingrui Zhang, Zhiyuan Zheng, Tong Zhang, Siqi Zhang, Ren Huang, Xiaodi Zheng, Junfeng Shen, Haochong Huang, and Zili Zhang

DOI: 10.1364/AO.517400 Received 08 Jan 2024; Accepted 02 Mar 2024; Posted 04 Mar 2024  View: PDF

Abstract: The absorption coefficient and refractive index of terahertz spectra were used to establish a correlation with the whole-rock iron (TFe) content in different depth of the Bayan Obo protolith. Base on the elemental contents, three machine learning algorithms, namely partial least squares regression (PLSR), random forest (RF) and Multi-Layer Perceptron Regressor (MLP), were used to develop a quantitative analytical model for TFe content of the protolith minerals. Results show that superior to the refractive index, the absorption coefficient can be used for rapid and effective investigation of TFe content in minerals. Among the three algorithms, MLP has higher detection accuracy, with a model coefficient of determination R^2 reaching 0.944. These findings demonstrate that terahertz time-domain spectroscopy can be used to rapidly and nondestructively quantify the TFe elemental content of protolith.

Multi-mode heralded single photons based on the DLCZ

Wang Shengzhi and Mingjie Wang

DOI: 10.1364/AO.517993 Received 09 Jan 2024; Accepted 01 Mar 2024; Posted 04 Mar 2024  View: PDF

Abstract: High-quality single-photon sources are crucial for the development of simple quantum devices. Quantum communication stands at the forefront of cutting-edge technologies, promising unprecedented levels of security and efficiency. A cornerstone of this revolutionary field is the development of high-speed single photon sources, which play a pivotal role in quantum key distribution (QKD) and other quantum communication protocols. In this context, the concept of space multimode emerges as a promising avenue to propel the capabilities of single photon sources to new heights. We have spatial multiplexing technology to develop single photon sources that deliver high-speed heralded single photons in DLCZ scheme. We propose a spatial multiplexing single photon source scheme based on the DLCZ. In our experimental system, the single-photon generation rate increases sixfold by expanding from one to six spatial modes, and the second order correlation function of single photons is less than 0.5. We show that expanding the spatial degrees of freedom of the quantum storage scheme based on DLCZ does not affect the single-photon properties. The generation rate of the single photon can be significantly increased through spatial multiplexing with a feedback circuit. Our approach offers a promising path to creating a high speed photon source based on spatial multi-mode scheme.

Half-spectrum OFDM to quadruple the spectralefficiency of underwater wireless opticalcommunication with digital power divisionmultiplexing

xiaojian hong, Yuan Wang, Tianyi Zhang, Chao Fei, Jiahan Tian, Junwei Zhang, Yitong Xie, Fei Zhang, JI DU, Guowu Zhang, Gaoxuan Wang, and Sailing He

DOI: 10.1364/AO.514639 Received 30 Nov 2023; Accepted 29 Feb 2024; Posted 01 Mar 2024  View: PDF

Abstract: Improving spectrum efficiency (SE) is an effective method to further enhance the data rate of bandwidth-limited underwaterwireless optical communication (UWOC) systems. Non-orthogonal frequency division multiplexing (NOFDM) with a compressionfactor of 0.5 can save half of the bandwidth without introducing any inter-carrier-interference (ICI) only if the total number ofsubcarriers is large enough, and we termed it as half-spectrum OFDM (HS-OFDM). To the best of our knowledge, this is the firstreported work on a closed-form HS-OFDM signal in the discrete domain from the perspective of correlation matrix. Due to thespecial mathematical property, no extra complex decoding algorithm is required at the HS-OFDM receiver, making it as simple asthe conventional OFDM receiver. Compared with traditional OFDM, HS-OFDM can realize the same data rate but with a largersignal-to-noise ratio (SNR) margin. To fully use the SNR resource of the communication system, we further propose a digital powerdivision multiplexed HS-OFDM (DPDM-HS-OFDM) scheme to quadruple the SE of the conventional OFDM for the bandwidthstarved UWOCs. The experimental results show that HS-OFDM can improve the receiver sensitivity by around 4 dB as opposed tothe conventional 4QAM-OFDM with the same data rate and SE. With the help of the DPDM-HS-OFDM scheme, the data rate of themulti-user UWOC can reach up to 4.5 Gbps under the hard-decision forward error correction (HD-FEC) limit of bit error rate (BER)of 3.8 × 10-3. Although there is some performance degradation in comparison with the single-user HS-OFDM, the BER performanceof the multi-user DPDM-HS-OFDM is still superior to that of the conventional single-user 4QAM-OFDM. Both the single-user HSOFDM and multi-user DPDM-HS-OFDM successfully achieve 2-Gbps/75-m data transmission, indicating that the DPDM-HS-OFDMscheme is of great importance in bandwidth-limited UWOC systems and also has guiding significance to underwater wirelessoptical multiple access.

Down-Conversion and Frequency SpectrumConvolution of the Broadband Signal Basedon the Active Mode Locking Technology

Feng Xiong, Yibei Wang, Yalan Wang, Dangwei Wang, Runhua Liu, Pengfei Du, Jin Zhang, Anle Wang, Haixia Cong, and Xiaoniu Peng

DOI: 10.1364/AO.517148 Received 27 Dec 2023; Accepted 29 Feb 2024; Posted 01 Mar 2024  View: PDF

Abstract: A multi-function processor for broadband signal based on the active mode lockingoptoelectronic oscillator (OEO) is proposed and experimentally demonstrated. The centralfrequency down-conversion and frequency spectrum convolution of the target broadband signal(TBS) are realized by just tunning the wavelength of the optical carrier or by time domainproduct, respectively. To achieve the central frequency down-conversion of the TBS, an opticaltunable delay line (OTDL) is adopted to match the delay time of the OEO loop with therepetition period of the TBS. Then the spectrum convolution of the TBS is produced by justinjecting a lower frequency signal consistent with the free spectral range (FSR) of the OEOloop. Moreover, the frequency convolution repetition is also multiply increased by harmonicmode-locking injection. The equivalent bandwidth of the TBS is enlarged by ~ 50 timesbenefits from the frequency convolution. The central frequency conversion flexibility and thebandwidth compatibility are also discussed in detail. This work provides a multi-functionprocessor system and may have potential usage in multifunctional integrated radar systems.

An ultra-compact SOI based higher-ordermode pass wavelength demultiplexer

Jinhua Chen, WeiWeii Pan, and CHEN JI

DOI: 10.1364/AO.517258 Received 27 Dec 2023; Accepted 29 Feb 2024; Posted 01 Mar 2024  View: PDF

Abstract: We proposed an ultra-compact mode filtering wavelength demultiplexer design witha footprint of 3 × 3 𝜇𝑚2. Our device can route input 𝑇𝐸1 mode signals at 1310 nm and 1550nm to different output ports, while simultaneously blocking fundamental transverseelectric(𝑇𝐸0) mode input. Our device is designed based on the topology optimization algorithm,which results in an ultra-compact footprint combining wavelength routing and mode filteringfunctions for the first time. Our final optimized devices demonstrated insertion losses of 1.26dB and 1.47 dB for the C and O band output ports, respectively, with inter-port crosstalk as lowas -21.25 dB and -30.99 dB. The extinction ratio between 𝑇𝐸1 mode and 𝑇𝐸0 mode are 24.02dB and 30.12 dB at the 1310 nm and 1550 nm output ports. The combination of small footprint,broad transmission bandwidth, 𝑇𝐸1 to 𝑇𝐸0 mode selectively filtering, C and O band 𝑇𝐸1 modedemultiplexing functions make this a uniquely versatile device that can play an important rolein future high density mode-wavelength multiplexing systems.

Highly Enhancement Photoluminescence of Monolayer MoS2 in Hybrid Plasmonic Systems

Yan Huang, Ruoying Kanyang, cizhe fang, Xiaoxi Li, Xiangyu Zeng, yibo wang, Liu Yan, Yue Hao, and Genquan Han

DOI: 10.1364/AO.517514 Received 02 Jan 2024; Accepted 29 Feb 2024; Posted 01 Mar 2024  View: PDF

Abstract: Monolayer molybdenum disulfide (MoS2) has weak light-matter interaction due to ultrathin thickness, which limits its potential application in lasing action. In this study, we propose a hybrid structure consisting of nanocavity and Au nanoparticles to enhance the photon emission efficiency of monolayer MoS2. Numerical simulations show that photoluminescence (PL) emission is significantly enhanced by introducing localized surface plasmon resonance (LSPR) to the proposed structure. Furthermore, an exciton energy band system is proposed to elucidate the physical mechanism of the PL process. By optimizing the spacer thickness, a high Purcell enhancement factor of 95 can be achieved. The results provided by this work pave the way to improve the PL efficiency of two-dimensional (2D) material, which constitutes a significant step towards the development of nanodevices such as nanolasers and sensors.

Description and reconstruction of typical structured light beams with vector spherical wave functions

Yiyu Shi, zhiwei cui, Zhanfei Liu, Shuaishuai Ren, and Fuping Wu

DOI: 10.1364/AO.518878 Received 12 Jan 2024; Accepted 29 Feb 2024; Posted 01 Mar 2024  View: PDF

Abstract: It is well known that the generalized Lorenz-Mie theory (GLMT) is a rigorous analytical method for dealing with the interaction between light beams and spherical particles, which involves the description and reconstruction of the light beams with vector spherical wave functions (VSWFs). In this paper, a detailed study on the description and reconstruction of the typical structured light beams with VSWFs is reported. We first systematically derive the so-called beam shape coefficients (BSCs) of typical structured light beams, including the fundamental Gaussian beams, Hermite-Gaussian beams, Laguerre-Gaussian beams, Bessel beams, and Airy beams, with the aid of angular spectrum decomposition method (ASDM). Then based on the derived BSCs, we reconstruct these structured light beams using VSWFs and compare the results of reconstructed beams with that of original beams. Our results will be useful to the study of typical structured light beams interaction with spherical particles in the framework of GLMT.

Transport of Intensity and Phase: Applications to Digital Holography

Nouf Alanazi, Austin Scott, hammid al-ghezi, Muhammad Faryad, Akhlesh Lakhtakia, and Partha Banerjee

DOI: 10.1364/AO.519170 Received 15 Jan 2024; Accepted 29 Feb 2024; Posted 01 Mar 2024  View: PDF

Abstract: We first review transport of intensity and phase and show their use as aconvenient tool to directly determine the unwrapped phase of an imaged object, eitherthrough conventional imaging or using digital holography. For both cases, either thetraditional transport of intensity and phase, or with a modification, viz., electricallycontrollable transport of intensity and phase, can be used. The use of digital holographywith transport of intensity for 3D topographic mapping of fingermarks coated withcolumnar thin films is shown as an illustrative application of this versatile technique.

Magneto-optical fiber-based orbital angular momentum mode converters

Geng Chen, Bao-Jian Wu, Qi-Quan Wang, Feng Wen, and Kun Qiu

DOI: 10.1364/AO.510563 Received 27 Oct 2023; Accepted 29 Feb 2024; Posted 01 Mar 2024  View: PDF

Abstract: Orbital angular momentum (OAM) mode division multiplexing (MDM) systems can support large-capacity and high-speed rate information transmission, in which the OAM mode conversion devices play an important role. In this paper, the mode conversion principle of magneto-optical fiber-based long period grating (MOF-LPG) is analyzed for further developing new magneto-optical (MO) OAM mode converters, including three types of CP01 to OAM±1,1, OAM±1,1 to OAM±2,1 and OAM±1,1 to CP02. It is shown that, the magnetic tunability of the mode converters through the propagation constants of the eigenmodes is useful for compensating for process errors and increasing the operating wavelength range.

Impact of different receiver geometry on reconfigurable intelligent surface assisted multi-cell VLC system in presence of light path blockage

Deblina Sabui, Sourish Chatterjee, and Gufran Khan

DOI: 10.1364/AO.516475 Received 21 Dec 2023; Accepted 28 Feb 2024; Posted 01 Mar 2024  View: PDF

Abstract: We report the effect of integrating metasurface-aided reconfigurable intelligent surfaces (RIS) on the signal-to-interference-plus-noise ratio (SINR) and data rate of a multi-cell visible light communication (VLC) system. RIS has been deployed in the channel between transmitter and receiver to redirect the reflected light in the desired directions, even in the absence of line-of-sight (LoS) links. Results show that the introduction of RIS has improved average SINR but reduced average illumination level compared to no-RIS system. As the quantity of RIS increases, a discernible improvement in the maximum SINR value is observed. Here, three different receiver geometries, namely photodiode (PD), freeform diversity receiver (FDR), and modified FDR (MFDR), have been adopted. The impact of individual receivers has been reported in the presence of light path blockage. MFDR geometry is found to be most suitable with more coverage probability compared to the other two receivers. With (40cm x 24cm) RIS area, during blockage, MFDR maintains an average SINR 21.95 dB, which is 97.29% and 14.24 % greater than PD and FDR, respectively.

Optical Information Hiding for Different SurfaceImages

Zhaoke Mi, Yusi Zhu, Zhu Yupeng, Tianhao Zhang, Zhengzhong Huang, Fengming Wu, Changjun Ke, Siqin Ge, Lu Rong, and Yishi Shi

DOI: 10.1364/AO.517913 Received 08 Jan 2024; Accepted 28 Feb 2024; Posted 01 Mar 2024  View: PDF

Abstract: Optical hiding often requires the selection of specific artificial optical components as carriers, which results in poorversatility of the carriers and high costs for the hiding system. To conceal secret information on different surfaces such as metal,wood and paper, we propose an optical information hiding method. In this method, we use images of surfaces, whose grayscalehistograms have the characteristic of symmetric distribution. Based on this characteristic, we first scramble the surface image,and then adjust part of the gray value of the surface image to the complementary value to embed the secret information into ascrambled surface image to generate a key image. In the extraction process, a projector is used to reproduce the scrambled surfaceimage and the key image, which are then incoherently superimposed to extract the secret information using the human visualsystem. The extraction process does not require complex optical knowledge and is simple and feasible. Simulation experimentsand optical experiments indicate that this method is applicable in practice and possesses good security and imperceptibility.Furthermore, we prove the reliability of this method by embedding secret information in different surface images, demonstratingthe potential application of more surface images in the field of optical information hiding. Finally, we discuss the applicability ofsurface information images and analyze the imperceptibility of key images.

Surface hardness determination of Laser Cladding using laser-induced Breakdown Spectroscopy and Machine Learning(PLSR, CNN, ResNet and DRSN)

Jiacheng Yang, Linghua Kong, and Ye Hongji

DOI: 10.1364/AO.516603 Received 21 Dec 2023; Accepted 28 Feb 2024; Posted 01 Mar 2024  View: PDF

Abstract: In this study, we employed laser-induced breakdown spectroscopy (LIBS) along with machinelearning algorithms, which encompass partial least squares regression(PLSR), deep convolutionalneural network(CNN), deep residual neural network(ResNet), and deep residual shrinkage neuralnetwork(DRSN), to estimate the surface hardness of laser cladding layers( The layers wereproduced using Fe316L, FeCrNiCu, Ni25, FeCrNiB, and Fe313 powders, with 45 steel and Q 5serving as substrates ). The research findings indicate that both linear and nonlinear modelscan effectively fit the relationship between LIBS spectra and surface hardness. Particularly, themodel derived from the ResNet exhibits superior performance with an 𝑅2value as high as 0.9967.We hypothesize that the inclusion of numerous noises in the LIBS spectra contributes to the enhanced predictive capability for surface hardness, thereby leading to the superior performance of the ResNet compared to the DRSN.

Phase screen prediction using deep phase network for FSO links

Ming Li, Zhigeng Wu, Tianyi Wang, Pengxin Zhang, and Milorad Cvijetic

DOI: 10.1364/AO.517151 Received 28 Dec 2023; Accepted 28 Feb 2024; Posted 01 Mar 2024  View: PDF

Abstract: Due to the presence of air turbulence in free-space optical (FSO) links, randomfluctuations in both wavefront phase and amplitude of the optical signal are reduced afterit propagates through the air channel, which degrades the performance of free-space opticalcommunication (FSOC) systems. Phase screen reflects the phase distortions resulting from airturbulence. Accordingly, accurate prediction with respect to phase screen is of significance forthe FSOC. In this paper we propose a phase screen prediction method based on the deep phasenetwork (DPN). The advantages of the proposed method include strong robustness against airturbulence, low model depth, few number of parameters as well as low complexity. The resultsreveal that our DPN enables desired inference accuracy and faster inference speed comparedto the existed models by combining the mean square deviation loss function with the pixelpenalty terms. More concretely, the accuracy of phase screen prediction can reach up to 95%,the average time consumed to predict the phase screen is the order of millisecond only undervarious turbulence conditions. Also, our DPN outperforms the traditional Gerchberg-Saxtonalgorithm in convergence speed.

Investigation of Hohlraum Fields with Monoenergetic Proton Radiography at OMEGA

Jacob Pearcy, Graeme Sutcliffe, Timothy Johnson, Benjamin Reichelt, Skylar Dannhoff, Yousef Lawrence, Johan Frenje, Maria Gatu-Johnson, Richard Petrasso, and Chikang Li

DOI: 10.1364/AO.506985 Received 26 Sep 2023; Accepted 28 Feb 2024; Posted 01 Mar 2024  View: PDF

Abstract: A more complete understanding of laser-driven hohlraum plasmas is critical for the continued development and improvement of ICF experiments. In these hohlraums, self-generated electric and magnetic fields play an important role in determining plasma dynamics and evolution; however, it has largely been uncertain whether electric fields or magnetic fields dominate these systems. To explore this question, we conducted several experiments at the OMEGA laser facility, using monoenergetic proton radiography to probe laser-driven vacuum-filled gold and plastic hohlraums. To facilitate this analysis, we utilized reconstructive methods ­to recover information about proton deflections, and developed new techniques for interpreting these radiographs to infer structure of electromagnetic fields in the hohlraum, as well as to quantify the relative magnitudes of proton deflections due to electric and magnetic fields respectively. This work was supported in part by the U.S. DOE, NLUF, and LLE.

A Inter-module Gap Filling Method for PhotonCounting Detectors Based on Dual Acquisition

ZHUO CHEN, Xiaoqi Xi, Yu Han, Siyu Tan, Lei Li, Xuejing Lu, and Bin Yan

DOI: 10.1364/AO.508066 Received 27 Oct 2023; Accepted 28 Feb 2024; Posted 01 Mar 2024  View: PDF

Abstract: The use of photon-counting detectors in X-ray imaging missions can effectively improvesignal-to-noise ratio and image resolution. However, the stitching of photon counting detector modulesleads to large-size localized information loss in the acquired projected image, which seriously affects theregional observation. In this paper, we propose a method to fill the inter-module Gap based on dualacquisition, referred to as the GFDA algorithm, which is divided into three main steps: (i) Acquire themain projection by short-exposure scanning, and then scan again by vertically moving the carrier tableto acquire the reference projection. (ii) Use the alignment method to locate the projected region ofinterest. (iii) Use image stitching and image fusion to recover the missing information. We analyzed thegray value of the region of interest of the Siemens star projection and the reconstructed conch slice data,and proved that the proposed method can recover the information more smoothly and perfectly. TheGFDA algorithm is able to achieve better image restoration effect without additional scanning time, andbetter retain image details. In addition, the GFDA algorithm is scalable, which is demonstrated in thetask of filling the stitching of multiple types of photonic technology detectors.

Design of optical system of dynamic target simulator with large FOV and long exit pupil distance

Tuo Cao and Lingyun Wang

DOI: 10.1364/AO.510301 Received 25 Oct 2023; Accepted 28 Feb 2024; Posted 01 Mar 2024  View: PDF

Abstract: According to the new generation missile test and detection requirements, it isnecessary to introduce the Hardware in the Loop (HIL) simulation technology to realize theperformance test of the seeker. This paper introduces the concept of secondary imaging andprovides the method of focal distance allocation, so that the optical system field of view (FOV)1can still be greater than 6° under the condition that the exit pupil distance is larger than 800mm,and a high-quality lighting system is designed. The design results show that when the exit pupildistance of the system is 1000mm and the FOV is 6.4°, the MTF of the system is 60lp/mm>0.4,and the distortion is less than 0.1%. The uniformity of the illumination system is greater than90%, meeting the requirements of the design index.

Microwave Photonics Frequency MeasurementWith Improved Accuracy Based on ArtificialNeural Network

Xin An, Zhangyi Yang, Zuoheng Liu, Youdi Zhang, and Wei Dong

DOI: 10.1364/AO.519402 Received 18 Jan 2024; Accepted 28 Feb 2024; Posted 01 Mar 2024  View: PDF

Abstract: Photonics-assisted techniques for microwave frequency measurement (MFM) showgreat potential for overcoming electronic bottlenecks, with wild applications in radar andcommunication. The MFM system based on stimulated Brillouin scattering (SBS) effect canmeasure the frequency of multiple high-frequency and wide-band signals. However, theaccuracy of the MFM system in multi-tone frequency measurement is constrained by the SBSbandwidth and the nonlinearity of the system. To resolve this problem, a method based onartificial neural network (ANN) is raised, which can establish a nonlinear mapping between themeasured two-tone signal spectra and the theoretical frequencies. Through simulationverification, the ANN optimized frequencies within the range of (0.5, 27) GHz of the MFMsystem show 79%, 76%, 70%, 44% reduction in errors separately under 20 dB, 15 dB, 10 dB,0 dB four spectral signal-to-noise ratios (SNR) conditions, and the frequency resolution isimproved from 30 MHz to 10 MHz.

Optical design of visible/short-wave infraredcommon-aperture optical system with longfocal length and wide field of view

Aqi Yan, Weining Chen, Qianxi Li, min guo, and HAO WANG

DOI: 10.1364/AO.517643 Received 03 Jan 2024; Accepted 27 Feb 2024; Posted 27 Feb 2024  View: PDF

Abstract: Addressing the urgent need for long-distance dim target detection with a wide fieldof view and high sensitivity, this paper proposes a visible and short-infrared dual-bandcommon-aperture optical system characterized by a broad field and extended focal length. Toachieve system miniaturization and high sensitivity target detection, the visible and infraredoptical systems share a Ritchey-Chretien primary and secondary mirror. The primary opticalpath is segmented into visible light (0.45μm to 0.75μm) and short-wave infrared (SWIR) (2μmto 3μm) bands by a dichroic spectral splitter prism. The SWIR optical system utilizes fourshort-wave cooled infrared detectors, and wide-field stitching is achieved using a field-ofview divider. While ensuring high cold shield efficiency of cooled infrared detectors, thiscommon-aperture optical system delivers visible and SWIR dual-band images with expansivefields, elongated focal lengths and sizable apertures. The visible-light optical system has afocal length of 277mm, a field of view of 2.3°×2.3°, and an entrance pupil diameter of130mm. Meanwhile, the SWIR optical system features a focal length of 480mm, a field ofview of 2.26°×1.8°, and an entrance pupil diameter of 160mm. Design outcomes suggest thatthe imaging quality of the optical system approaches the diffraction limit. This visible/SWIRcommon-aperture optical system exhibits high sensitivity, a large field of view, compactstructure, and excellent imaging quality, thereby meeting the requirements for long-distancedim target detection and imaging.

The Influence of Diffraction Distance on Image Restoration in Deep Learning Networks

SHIHONG HUANG, YANXU YANG, Jiayang Wu, and CAIHOU LIN

DOI: 10.1364/AO.506951 Received 27 Sep 2023; Accepted 26 Feb 2024; Posted 01 Mar 2024  View: PDF

Abstract: In recent years, significant advancements have been made in the field of computational imaging,particularly due to the application of deep learning methods to imaging problems. However, only a fewstudies related to deep learning have examined the impact of diffraction distance on image restoration.In this paper, the effect of diffraction distance on image restoration is investigated based on thePhysenNet neural network. A theoretical framework for diffraction images at various diffractiondistances is provided along with the applicable propagators. In the experiment, the PhysenNet networkis selected to train on diffraction images with different distances and the impact of using differentpropagators on network performance is studied. Optimal propagators required to recover images atdifferent diffraction distances are determined. Insights obtained through these experiments can expandthe scope of neural networks in computational imaging.

Uncertainty of digital image correlation undervideo compression and DSP optimization

Xiaowen Li, Xinqiao Tang, Yinuo Xu, Wei Sun, and Zhenning Chen

DOI: 10.1364/AO.511691 Received 09 Nov 2023; Accepted 26 Feb 2024; Posted 01 Mar 2024  View: PDF

Abstract: The storage and transmission of videos at high spatial resolution remains a greatchallenge in image-based optical techniques. The uncertainty of digital image correlation (DIC)was assessed following speckle video compression under High Efficiency Video Coding(HEVC/H.265). First, the evaluation criterion for the DIC accuracy affected by compressionwas provided. The stability of H.265 video compression in DIC was studied consideringdifferent compressed frames under different target quantization parameters (QPs) andcompression ratios (CRs). The deformation uncertainty of the DIC itself as affected by H.265video compression was further investigated through uniform translation and non-uniformsinusoidal deformation performance. Moreover, the optimized digital speckle pattern (DSP)was re-evaluated considering video compression-induced uncertainty. DSPs with parametersof different diameters and randomness were compressed using various QPs and CRs. Inaddition, DSP evaluation was performed under both translation and non-homogeneousdeformation conditions. The feasibility of the re-optimized DSP under H.265 videocompression was validated using a defective bending beam, and DSP videos with a speckle sizeof 8 pixels reached a high CR within an acceptable margin of error.

Pixel-wise calibration method for telecentric structured light system

Yuchen Yang and Song Zhang

DOI: 10.1364/AO.509535 Received 19 Oct 2023; Accepted 26 Feb 2024; Posted 27 Feb 2024  View: PDF

Abstract: This paper introduces a pixel-wise calibration method designed for a structured light system utilizing a camera attached with a telecentric lens. In the calibration process, a white flat surface and another flat surface with circle dots serve as the calibration targets. After deriving the properties of the pinhole projector through conventional camera calibration method using circle dots and determining the camera's attributes via 3D feature points estimation through iterative optimizations, the white surface calibration target was positioned at various poses and reconstructed with initial camera and projector calibration data. Each 3D reconstructions was fitted with an ideal virtual ideal plane that was further used to create the pixel-wise phase-to-coordinate mapping. To optimize the calibration accuracy, various angled poses of the calibration target are employed to refine the initial results. Experimental findings validate that the proposed approach offers high calibration accuracy for a structured light system using a telecentric lens.

Three-dimensional Image Authentication from Multi-view Images

Zhen Leng, Jing Chen, and Bo Liu

DOI: 10.1364/AO.514144 Received 30 Nov 2023; Accepted 26 Feb 2024; Posted 27 Feb 2024  View: PDF

Abstract: Three-dimensional (3D) optical authentication is important for modern information security. Existing 3D optical authentication methods rely on integral imaging devices, necessitating meticulous calibration and incurring high transmission overhead. To streamline the acquisition of 3D information, this paper introduces a novel 3D optical authentication approach based on the construction of 3D data from multi-view images. The proposed method simplifies 3D projection by generating fixed-viewpoint elemental images, eliminating the need for additional viewpoint information during transmission and authentication. Compressed sensing is used for compression during transmission, and a deep learning network is designed for 3D reconstruction, enhancing the recovery.Experimental outcomes confirm the efficiency of our proposed approach for 3D authentication across diverse datasets.

Miniaturizing a coherent beam combining system into a compact laser diode module

Christos Mourikis, Alexander Sahm, Philipp Hildenstein, Katrin Paschke, and Guenther Traenkle

DOI: 10.1364/AO.516029 Received 14 Dec 2023; Accepted 26 Feb 2024; Posted 27 Feb 2024  View: PDF

Abstract: We present a laser module with dimensions of 76 x 43 x 15 mm3that for the first time realizes a coherent beam combination in such a compact device, using two taperedamplifiers seeded by a distributed Bragg reflector ridge waveguide laser diode operating at761 nm in single longitudinal mode. The generated combined optical power is up to 5 Wcontinuous wave with a combing efficiency of 85 %. The phase of the system is controlled bythe current in the ridge waveguide section of one of the tapered amplifiers. The phasestabilization process is automated using a reverse hill-climbing algorithm and an ATmega328Pmicrocontroller.

Phase retrieval algorithm applied to high-energyultrafast lasers

Jikai Wang, Abdolnaser Ghazagh, Sonam Ravi, Stefan Baumbach, Benjamin Dannecker, Michael Scharun, Dominik Bauer, Stefan Nolte, and Daniel Flamm

DOI: 10.1364/AO.516820 Received 21 Dec 2023; Accepted 26 Feb 2024; Posted 27 Feb 2024  View: PDF

Abstract: A standardized phase retrieval algorithm is presentedand applied to an industry-grade high-energy ultrashortpulsed laser to uncover its spatial phase distribution.We describe in detail how to modify the well-knownalgorithm in order to characterize particularly stronglight sources from intensity measurements only. Withcomplete information about the optical field of the unknown light source at hand, virtual back propagationcan reveal weak points in the light path such as apertures or damaged components.

SCAPSM : A Novel Attenuated Phase-Shift Mask Structure for EUV Lithography

chen li, Lisong Dong, and Yayi Wei

DOI: 10.1364/AO.517264 Received 27 Dec 2023; Accepted 26 Feb 2024; Posted 27 Feb 2024  View: PDF

Abstract: The Attenuated Phase-Shift Mask (Att. PSM) is proven to be a promising resolution enhancement technology (RET) to improve the imaging performance in the extreme ultraviolet (EUV) lithography. However, due to the reflective nature of mask structure, the serious shadowing effect can affect the diffraction near field of mask intensely, and further impact the lithography imaging. With the purpose of improving the contrast of lithography imaging, a novel structure of Att. PSM is proposed in this paper. By introducing an absorbent sidewall along the edge of mask absorber, the diffraction and shadowing effects can be mitigated. By applying Kirchhoff approximation of mask diffraction, the ability of the novel structure on improving imaging performance is theoretically analyzed. And these analyses are confirmed by rigorous lithography simulations. The simulation results demonstrate that the proposed mask structure can improve the imaging contrast of EUV lithography, which has the potential usage in advanced integrated circuit (IC) manufacturing.

In situ calibration of shear ratio forquadriwave lateral shearing interferometryusing double-slit interference

Xiaotian Zhang, Xiting Han, Tao Wang, Hui Zhong, Shuhao Liu, Yanqiu Li, and Ke Liu

DOI: 10.1364/AO.519203 Received 25 Jan 2024; Accepted 26 Feb 2024; Posted 27 Feb 2024  View: PDF

Abstract: A new method based on double-slit (DS) interference is proposed to accuratelyestimate the shear ratio of the system, when plane wave or spherical wave incidence. Existingshear ratio calibration methods, designed primarily for lateral shearing interferometry (LSI)with plane wave incidence, are not applicable to LSIs directly testing divergent or convergentspherical waves. Equations for calculating the shear ratio using the fringe spacing of the DSinterferogram and the NA of the incident spherical wave are derived in this paper. Thesimulation result shows that the relative error of the shear ratio value is about 0.3%, when theshear ratio is 0.1. In the experiment, the quadriwave LSI is designed with a plug-in feature.The shear ratio at integer multiples of 1/6 Talbot distance from the modified Hartmann maskwas calibrated using a DS, and the results were in good agreement with theoretical values,confirming the accuracy of the method. Subsequently, with the assistance of an inductancemicrometer, the shear ratio was calibrated at intervals of 0.5mm, the results closely matchedthe theoretical variation of shear ratio caused by displacement, confirming the high precisionof the method.

Temperature dependent index of refraction Sellmeier modelfor crystalline and polycrystalline materials

Michael Thomas

DOI: 10.1364/AO.511188 Received 05 Nov 2023; Accepted 26 Feb 2024; Posted 27 Feb 2024  View: PDF

Abstract: The temperature dependence of optical window materials remains an important issue for a variety ofapplications from spacecraft, laser components to high speed aircraft. Concerning the refractive index in regions oftransparency, and current models are empirically based polynomial fits for the Sellmeier model strength and modelocation parameters. These polynomial fit functions limit the ability to accurately extrapolate beyond theexperimental range used to develop the fit functions. Thus, the development of a physics based model as a functionof temperature is an important goal for these critical materials. Such a model will allow extrapolation to higher andlower temperatures as long as the physical mechanisms don’t change. For vibrational modes, a thermal average ofthe anharmonically shifted energy levels is investigated and compared to experimental data. The first anharmonicterm can be estimated using the Morse potential based on a multiphonon absorption model. Experimentally, thesemodes red shift, and this is consistent with the developed temperature dependent index of refraction Sellmeiermodel. This red-shifting phenomena can also be applied to electronic transition shifts. In addition, the temperaturedependent oscillator number density can be obtained from known expansion coefficient models and experimentaldata. Other model parameters, in particular the electronic and vibrational mode polarizability, still needexperimental grounding for a given material. The method is incorporated into a modified Sellmeier model format.

Multiple HAPS-based Space-Air-Ground Network withFSO Communication: A Performance Analysis

Deepshikha Singh, Swaminathan R, and Anh Pham

DOI: 10.1364/AO.515707 Received 11 Dec 2023; Accepted 26 Feb 2024; Posted 27 Feb 2024  View: PDF

Abstract: Due to the fact that the existing generation of wireless communication cannot possibly keep up with thecurrent traffic explosion and emerging applications, research and development on next-generation (i.e. sixthgeneration 6G) wireless technologies is being carriedout worldwide. In this regard, it is anticipated that thespace-air-ground (SAG) network with free space optics(FSO) communication can provide terabits per secondthroughput necessary to sustain various potential 6Gapplications. However, FSO communications are susceptible to atmospheric turbulence, pointing errors, andbeam scintillation effects. To remedy the severe atmospheric effects, we propose a multiple high-altitude platform station (HAPS)-based SAG network with HAPSselection scheme. For the proposed system, we have derived the closed-form expressions for outage probability,average symbol error rate (SER), ergodic capacity, andoutage capacity over Málaga distribution with pointingerrors. Further, the asymptotic expressions for outageprobability, average SER and outage capacity were derived to enhance the comprehension of the system froma practical standpoint. It is observed from the numerical results that the multiple HAPS-based FSO systemperforms better than the existing HAPS-based FSO systems.

A scheme for WDM-PON with colorless opticalnetwork units based on flat optical frequency comb

Yian Cao and Shibao Wu

DOI: 10.1364/AO.516098 Received 17 Dec 2023; Accepted 26 Feb 2024; Posted 27 Feb 2024  View: PDF

Abstract: A new scheme for wavelength division multiplexing passive optical network (WDM-PON) based on flat optical frequencycomb (OFC) is proposed. Using OFC as the optical source of the system can realize the colorlessness of optical networkunits (ONUs), and the direct detection of the downlink data further simplifies the ONU structure. The coherentdemodulation of the uplink data improves the system performance due to the coherence of the comb lines. In thisresearch, the proposed system is studied for its performance and power budgeting. The results show the flexibility,effectiveness and practicability of the proposed scheme, which can be applied to future high-capacity optical accessnetworks.

DMSO synergized with sugars optical clearingagents modulation of Optical CoherenceTomography of skin tissues

Yuelin Wang, Wenhui Fang, Kai Zhang, and zhiwei men

DOI: 10.1364/AO.516082 Received 18 Dec 2023; Accepted 25 Feb 2024; Posted 26 Feb 2024  View: PDF

Abstract: The stratum corneum of the outermost skin is an important barrier impedingtransdermal permeation, and permeation enhancers can reduce the barrier resistance of thestratum corneum and enhance the permeation of drugs in tissues. The optical imaging depth,signal intensity, and scattering coefficient variation rules of skin tissues in time dimension areobtained by using Optical Coherence Tomography (OCT). The effect on OCT imaging ofoptical clearing agents (OCAs) is obtained by quantitatively analyzing the changes in theoptical properties of tissues. D-fructose, one of the monosaccharides, and sucrose, one of thedisaccharides, were selected for the ex vivo optical clearing experiments on pig skin tissuesutilizing the dimethyl sulfoxide (DMSO) carrier effect. We find that DMSO synergized withsugars applied to skin tissue has a more significant increase in the optical imaging depth andsignal intensity and a reduction in the scattering coefficient with the increasing concentrationof DMSO. DMSO with high concentration and D-fructose with saturated concentration (10:1;v/v) effectively reduce light attenuation in OCT imaging and improve image quality. Thisoperation will also shorten the application time to minimize skin damage from hyperosmoticagents.

Fourier domain filtering analysis for color polarization camera demosaicking

Nathan Hagen, Thijs Stockmans, Yukitoshi Otani, and Prathan Buranasiri

DOI: 10.1364/AO.516696 Received 28 Dec 2023; Accepted 24 Feb 2024; Posted 26 Feb 2024  View: PDF

Abstract: We review Fourier-domain methods for demosaicking Bayer-filter color cameras and monochrome polarization cameras, and then generalize the approach for the quad-Bayer filter mosaic, and for color polarization cameras. For each of these four mosaic filter types, we provide theoretical expressions for the sampling functions, the Fourier-domain channels, and the linear combination of reconstructed channels (the demosaicking algorithm) needed to estimate the input (pre-sampled) image. A useful advantage of the Fourier-domain approach is that it provides a direct means of visualizing and quantifying when aliasing is likely or unlikely to be present. For the Bayer and quad-Bayer filter types, we provide simulated images, while for the polarization camera types we provide experimental images and videos to illustrate the algorithm.

Laser cleaning of RTV coating on the insulatorsurface by using millisecond pulse lasers

Jialin Liu, Xin Chen, Yongqian Chen, yinghao cui, Shirui Guo, Xiwang Wu, and Lujun Cui

DOI: 10.1364/AO.515713 Received 17 Dec 2023; Accepted 24 Feb 2024; Posted 26 Feb 2024  View: PDF

Abstract: High-efficiency and high-quality removal of sulfurized siliconerubber from insulator surfaces is paramount for high-voltagepower systems. To address this issue, millisecond laser cleaningtechnology was selected in this study, aiming to achieve preciseand non-destructive cleaning of room temperature vulcanized(RTV) coatings. Successful and efficient cleaning of the RTVcoating was performed by adjusting laser parameters.Characterization techniques, including scanning electronmicroscopy (SEM), energy-dispersive X-ray spectroscopy(EDS), and confocal microscopy, were employed tocomprehensively assess the cleaning effects and ensure theintegrity of the substrate surface. The results indicate that byadjusting the scanning power combination of the high powerof the millisecond pulse laser to 200 W and the low power of150 W, the glass substrate surface maintains excellentroughness and micro-morphological features after lasercleaning, providing optimal conditions for subsequentprocessing and utilization. This research contributes anefficient and cost-effective solution to the insulation treatmentprocess in high-voltage power systems.

Frequency characteristics of an ultrasonic varifocal liquid crystal lens

Yuma Kuroda, Yuki Harada, Akira Emoto, Mami Matsukawa, and Daisuke Koyama

DOI: 10.1364/AO.515888 Received 14 Dec 2023; Accepted 23 Feb 2024; Posted 26 Feb 2024  View: PDF

Abstract: Compound lens systems with mechanical actuators are used to focus objects at near to far distances. The focal length of ultrasound varifocal liquid crystal (LC) lenses can be controlled by modulating the refractive index spatial distribution of the medium through the acoustic radiation force, resulting in thin and fast-response varifocal lenses. The frequency characteristics of such a lens was evaluated in this paper, and several axisymmetric resonant vibration modes over 20 kHz were observed. The effective lens aperture decreased with the wavelength of the resonant flexural vibration generated on the lens, meaning that this parameter can be controlled with the driving frequency.

Optical cavity spectroscopy using heterodynedetection with optical feedback laser frequencylocking

Marianne Beaumont, Irène Ventrillard, and Daniele Romanini

DOI: 10.1364/AO.518338 Received 11 Jan 2024; Accepted 23 Feb 2024; Posted 26 Feb 2024  View: PDF

Abstract: We demonstrate an accurate high sensitivity method for cavity spectroscopy. Wemeasure the frequency intervals of transverse electromagnetic modes relative to a fundamentalmode in a high finesse optical resonator, and attribute their mode numbers unambiguously. Alaser is frequency locked to a fundamental TEM00 cavity mode by optical feedback, and phasemodulation is used to obtain frequency side bands, which may come to resonance with othertransverse cavity modes as the radio-frequency of the modulation is tuned. At these resonances,transmission of the side bands is sensitively detected by heterodyning with the carrier. We alsoanalyse the transverse spatial profile of the heterodyne signal for identification of mode numbers.The adjustment of the Gaussian cavity model to the measured frequency intervals yields accuratevalues of cavity length, mirrors radius of curvature and mirrors ellipticity, to the ppm level.

Design of a compact multispectral telescopeconsisting of a Cassegrain- telescope and aconcave elliptical grating

NAJMA maasoomi, Abdolnasser Zakery, and Marzieh Afkhami

DOI: 10.1364/AO.502424 Received 03 Aug 2023; Accepted 23 Feb 2024; Posted 26 Feb 2024  View: PDF

Abstract: In this paper we design a high-performance multispectral telescope with a concaveelliptical grating for a field of view of 3° in the VNIR spectral range of 0.48-0.82 micrometersat an altitude of 760 km from the ground, with total length of 140 mm, which has a smallvolume and a simple structure. The paper reports on the MTF, spot, and field curvaturediagrams, which show that it can achieve spectral and spatial resolutions of 25 nm and 5.5 mrespectively with good image quality (MTF value for all wavelengths is higher than 0.2 atNyquist frequency of 217 cycles per mm) and has the least possible aberrations, without theneed for any lenses.

Diffuse photon remission associated with the center-illuminated-area-detection geometry. III. Perspectives on the patterns of saturation

Daqing Daching Piao, Tengfei Sun, and Nafiseh Farahzadi

DOI: 10.1364/AO.506514 Received 20 Sep 2023; Accepted 22 Feb 2024; Posted 26 Feb 2024  View: PDF

Abstract: Understanding scattering-insensitiveness in diffuse reflectance spectroscopy (DRS) will be useful to enhance the spectral specificity to absorption. In DRS based on center-illuminated-area-detection (CIAD), the scattering response can saturate as the relative strength of scattering with respect to the collection-size, represented by a dimensionless reduced scattering, increases over a threshold. However, the formation of saturation versus the same range of dimensionless reduced scattering may differ between a fixed reduced-scattering over an increasing collection-size (case 1) and an increasing reduced scattering over a fixed collection-size (case 2), due to the absorption. Part III demonstrates the differences of the scattering saturation as well as the effect of absorption on it in the CIAD geometry between the two cases while assessed over the same range of the dimensionless reduced scattering. A model allows predicting the absorption-dependent levels of saturation and the corner parameters of saturation transition. When assessed for the absorption coefficient to vary over [0.001, 0.01, 0.1, 1] mm-1, the model-predicted levels of saturation agree with MC results with ≤2.2% error in both cases. In comparison, the model-predicted corner parameters of saturation show much different agreements with MC results in the two cases, suggesting that the saturation pattern is much better formed in one than in the other. Experiments conforming to the CIAD geometry support the discrepancy of the saturating patterns between the two cases.

Airborne atmospheric carbon dioxidemeasurement using 1.5 µm laser double-pulseIPDA lidar over desert area

Fan Can, Juxin yang, Jiqiao Liu, lingbing bu, Qin Wang, Chong Wei, Yang Zhang, Xiaopeng Zhu, shiguang Li, Huaguo Zang, and Weibiao Chen

DOI: 10.1364/AO.507905 Received 01 Nov 2023; Accepted 22 Feb 2024; Posted 26 Feb 2024  View: PDF

Abstract: An integrated path differential absorption (IPDA) lidar can accurately measureregional CO2 weighted column average concentration (XCO2), which are crucial forunderstanding the carbon cycle in climate change studies. To verify the performance and datainversion methods of space-borne IPDA lidar, in July 2021, we conducted an airborne lidarvalidation experiment in Dunhuang, Gansu Province, China. An aircraft was equipped with alidar system developed to measure XCO2 and an in situ greenhouse gas analyzer (GGA). Tominimize measurement errors, energy monitoring was optimized. The system bias error of theDAOD was determined by changing the laser output mode from the off/on to the on/on mode.The XCO2 inversion results obtained through comparing the schemes of averaging signalsbefore “log (logarithm)” and averaging after “log” indicate that the former performs better. TheIPDA lidar measured XCO2 over the validation site at 405.57 ppm, and both the IPDA lidarand GGA measured sudden changes in the CO2 concentration. The assimilation data showed asimilar trend according to altitude to the data measured by the in situ instrument. A comparisonof the mean XCO2 derived from the GGA results and assimilation data with the IPDA lidarmeasurements showed biases of 0.80 ppm and 1.12 ppm, respectively.

Two-Dimensional Binary Phase Gratings forZero-Order and High-Order DiffractionSuppression

Yifan Chen, Kun Zhang, Xinjie Lv, Gang Zhao, and Shining Zhu

DOI: 10.1364/AO.516119 Received 26 Dec 2023; Accepted 22 Feb 2024; Posted 26 Feb 2024  View: PDF

Abstract: A two-dimensional binary phase grating is proposed in this paper. Unlikeconventional transmission grating, in theory, proposed phase grating can simultaneouslyeliminate the zero- and high-order diffraction along certain axis on the image plane, forming apure sinusoidal transmission modulation which leaves only the first-order diffraction. The firstever theoretical model for achieving sinusoidal transmission modulation is raised in this paper,then the theoretical calculation and experiment results are displayed to investigate the physicalmechanism of proposed grating. Moreover, the manipulation on the arrangement of gratingdesign can disperse or concentrate the diffraction energy at specific axis. Finally, almost firstorder-only diffraction is achieved on single axis by introducing random changes to certaingeometrical parameters of the two-dimensional binary phase grating. Our work providespotential applications in optical science and engineering fields.

Improving grating duty cycle uniformity:Amplitude-splitting flat-top beam laser interferencelithography

Dongbai Xue, xiao deng, Xiong Dun, Jun Wang, Zhanshan Wang, and Xinbin Cheng

DOI: 10.1364/AO.513766 Received 20 Nov 2023; Accepted 22 Feb 2024; Posted 22 Feb 2024  View: PDF

Abstract: Laser interference lithography is an effective approach for grating fabrication. As a key parameter of thegrating profile, the duty cycle determines the diffraction characteristics and is associated with the irradianceof the exposure beam. In this study, we developed a fabrication technique amplitude-splitting flat-topbeam interference lithography to improve duty cycle uniformity. The relationship between the duty cycleuniformity and irradiance of the exposure beam is analyzed, and the results indicate that when the beamirradiance nonuniformity is less than 20%, the grating duty cycle nonuniformity is maintained below ±2%.Moreover, an experimental amplitude-splitting flat-top beam interference lithography system is developedto realize an incident beam irradiance nonuniformity of 21%. The full-aperture duty cycle nonuniformityof the fabricated grating is less than ±3%. Amplitude-splitting flat-top beam interference lithographyimproves duty cycle uniformity, greatly reduces energy loss compared to conventional apodization, and ismore suitable for manufacturing highly uniform gratings over large areas.

Subwavelength Bessel beams array with high-uniformity based on metasurface

Chenyang Wu, Xuanlun Huang, Yipeng Ji, Jiaxing Wang, and Connie Chang-Hasnain

DOI: 10.1364/AO.519840 Received 23 Jan 2024; Accepted 22 Feb 2024; Posted 23 Feb 2024  View: PDF

Abstract: Bessel beam arrays are highly attractive due to non-diffraction properties, parallel processing and large capacity capabilities. However, conventional approaches of generating Bessel beams, such as spatial light modulators, axicons, and diffraction optical elements, suffer from various limitations of system complexity and bulkiness, low uniformity, and limited numerical aperture (NA). The limited NA imposes constraints on achieving minimal Full Width at Half Maximum (FWHM) of the Bessel beam, ultimately compromising the resolution of the beam. In this study, we demonstrate a novel method for generating Bessel beam arrays with regular and random patterns via ultra-compact metasurface. This approach integrates the phase profile of an optimized beam splitter with a meta-axicon. The Bessel beam arrays exhibit subwavelength dimensions of FWHM (590nm, ~0.9λ) and relatively high uniformity of 90% for NA=0.2 and 69% for NA=0.4. Furthermore, the method achieves effective suppression of background noise and 0th order intensity compared to methods based on Dammann gratings (DGs) based metasurface. The proposed method highlights potential applications of Bessel beam arrays in various fields, such as laser machining, optical communication and biomedical imaging.

A Joint Estimation Model for FSO Channel Parametersand Performance Evaluation Based on CNN

DAN CHEN, RUI WANG, CHENHAO WANG, YUE GAO, and HAOYA CHEN

DOI: 10.1364/AO.514064 Received 21 Nov 2023; Accepted 22 Feb 2024; Posted 26 Feb 2024  View: PDF

Abstract: Free space optical (FSO) communication systems experience turbulence-induced fading. As a possible solution, adaptivetransmission which adjusts transmitter parameters based on instantaneous channel state information (CSI) can be used.Most of the existing channel estimation methods ignore the impact of detection noise at the receiver which will lead toadditional estimation errors. In this paper, a joint estimation model based on convolutional neural networks (CNN) isproposed to estimate detection noise and turbulence fading parameters. We obtained turbulence channel simulationdata sets considering the background of detection noise based on the edge probability distribution function of thereceive signal. The training of CNN estimator is carried out through maximum pooling, adaptive learning rate andregularization, ultimately accurately estimating channel characteristics based on the optimal output results of thenetwork. The simulation results show that the proposed CNN joint estimator performs better in high detection noiseenvironments compared with traditional maximum likelihood estimatior, and it has better generalization ability indifferent real atmospheric environments.

Highly Sensitive Optical Fiber Pressure SensorBased on FPI and Vernier Effect via FemtosecondLaser Plane-by-Plane Writing Technology

Hu Xixi, Dan Su, and Xueguang Qiao

DOI: 10.1364/AO.516751 Received 21 Dec 2023; Accepted 21 Feb 2024; Posted 22 Feb 2024  View: PDF

Abstract: In this paper, a highly sensitive pressure sensor based on fiber-optic Fabry-Pérotinterferometers (FPI) and the Vernier effect (VE) is proposed and experimentally demonstrated.For the sensing cavity, we employ a closed capillary-based FPI𝑠, and for the reference cavity, we usean FPI𝑟 created through femtosecond laser refractive index modulation, which remains imperviousto pressure changes. Connecting these two FPIs in series produces a VE-based cascaded sensorwith a clear spectral envelope. The femtosecond laser micromachining technique provides precisecontrol over the length of FPI𝑟 and facilitates adjustments to the VE’s amplification degree.Experimental results reveal significant pressure sensitivities of -795.96 pm/MPa and -3219.91pm/MPa, respectively, representing a 20-fold and 80-fold improvement compared to FPI𝑠 (-39.80pm/MPa). This type of sensor has good sensitivity amplification and, due to its all-silica structure,can be a promising candidate for high-temperature and high-pressure sensing, especially in harsh environments.

Analysis and implementation of volumereflection gratings in photorefractive lithiumniobate for edge enhancement

Austin Scott and Partha Banerjee

DOI: 10.1364/AO.512442 Received 15 Nov 2023; Accepted 21 Feb 2024; Posted 27 Feb 2024  View: PDF

Abstract: Diffraction from volume reflection gratings written in bulk photorefractive lithiumniobate is modeled for the case of longitudinally varying index modulation depths. Numericalsolutions to the Helmholtz equation are found, in the spatial frequency domain, leading totransfer functions for the volume reflection grating. These transfer functions are then used toshow the spatial frequency filtering effect of the volume reflection grating on input light fieldscontaining 2D spatial information. It is shown, first through simulations and then byexperiment, that the 0th order transmitted beam undergoes 2D edge enhancement.

High-uniformity and high-performance waveguide Ge photodetectors for O-band and C-band

xinyu li, qiang xu, Ruogu Song, Jinyu Wang, Shuxiao Wang, Wencheng Yue, Wei Wang, Yan Cai, and Mingbin Yu

DOI: 10.1364/AO.518425 Received 10 Jan 2024; Accepted 20 Feb 2024; Posted 22 Feb 2024  View: PDF

Abstract: This article presents test results for the high-performance and high-uniformity waveguide silicon-based germanium (Ge) photodetectors (PDs) for O-band and C-band. Both wafer-scale and chip-scale test results are provided. The fabricated lateral p-i-n (LPIN) PDs exhibit a responsivity of 0.97A/W at the bias of -2V, a bandwidth of 60GHz, and a No-Return-to-Zero (NRZ) eye diagram rate of 53.125Gbps. Additionally, an average dark current of 22.4nA was obtained in the vertical p-i-n (VPIN) PDs at -2V by optimizing the doping process and the device can reach an average responsivity of 0.9A/W in O-band. The standard deviation in a wafer of dark current and responsivity is as low as 7.77nA and 0.03A/W at -2V, respectively.

Viewing zone enlargement method for holographic displays based on slanted pixel arrangement of spatial light modulator

Yuta Yamaguchi, Masato Miura, Ryo Higashida, Ken-Ichi Aoshima, and Kenji Machida

DOI: 10.1364/AO.506449 Received 11 Oct 2023; Accepted 20 Feb 2024; Posted 22 Feb 2024  View: PDF

Abstract: This study proposes a method for enlarging the viewing zone of holographic displays using the slanted arrangement of pixels on a spatial light modulator (SLM). The pixel arrangement equivalently reduces the horizontal pixel pitch, thereby the horizontal viewing zone of displays is enlarged. Computer-generated holograms (CGHs) were calculated using an asymmetric band-limit filter corresponding to the asymmetric bandwidth of the SLM with slanted pixels. The proposed methods were evaluated through an optical reconstruction experiment using static holograms with a pixel size of 1 × 1 μm, fabricated via electron-beam lithography. The enlarged horizontal viewing zone angle was found to be 41.6°.

Photonic Integrated Coherent Beam Combiner based on MMI and SPGD Algorithm

Jifang Qiu, Suping Jiao, Guojun Zhao, and Jian Wu

DOI: 10.1364/AO.515697 Received 11 Dec 2023; Accepted 20 Feb 2024; Posted 22 Feb 2024  View: PDF

Abstract: Atmospheric turbulence severely degrades the optical wavefront of a propagating beam, which greatly reduces the coupling efficiency of Free-space optical (FSO) receivers. Among the various methods to mitigate the effects, the use of a multi-channel receiver is more convenient and economical. After passing through the multi-channel receiver, multiple single-mode fibers (SMF) are outputted and need to be combined. In this paper, we propose photonic integrated coherent beam combiners based on multimode interference (MMI) and the stochastic parallel gradient descent (SPGD) algorithm, which avoids detecting the light out of each channel and adding the data signal in the electrical domain. Firstly, we propose a 4-channel coherent beam combiner based on a 4 × 1 MM, about 21 iterations of the SPGD algorithm are required to enhanced the combined optical power to a maximum of 96%. Furthermore, we demonstrate a combination of 16 beams using five 4 × 1 MMIs, which requires 140 iterations to enhance the combined power to 89%. This study would offer theoretical insights to enhance the integration of FSO communication system.

Imaging-Based Measurement of Lunar Dust Velocity and Particle Size

YiDan Dai, Yiyi Zhao, JinYou Tao, bin xue, and Jianfeng Yang

DOI: 10.1364/AO.516801 Received 21 Dec 2023; Accepted 20 Feb 2024; Posted 22 Feb 2024  View: PDF

Abstract: This paper introduces an innovative optical-mechanical system designed for the dynamic detection and analysis of lunar dust, typically characterized as particles under 20 micrometers on the lunar surface. The system's design is both compact and lightweight, aligning with the payload constraints of lunar exploration missions. It is capable of real-time tracking and recording the motion of lunar dust at various altitudes, a crucial capability for understanding the environmental dynamics of the lunar surface. By capturing images and applying sophisticated algorithms, the system accurately measures the velocity and size of dust particles. This approach significantly advances the quantitative analysis of lunar dust, especially during agitation events, filling a critical gap in our current understanding of lunar surface phenomena. The insights gained from this study are not only pivotal for developing theoretical models of lunar surface air flow disturbances and dust movement but also instrumental in designing effective dust mitigation and hazard avoidance strategies for future lunar missions, thereby enhancing both scientific knowledge and engineering applications in lunar exploration.

A Methane Gas Sensor Based on Direct Absorption Spectroscopy and Laser Self-Heating Effect

Qi Wu, Yuechun Shi, siqi Sun, yuanjin Yang, yang Xu, wenlong Wang, chao Men, and bingxiong Yang

DOI: 10.1364/AO.517016 Received 26 Dec 2023; Accepted 19 Feb 2024; Posted 22 Feb 2024  View: PDF

Abstract: In this paper, a methane detection sensor based on direct absorption spectroscopy and the self-heating effect of lasers is proposed, which abandons the traditional method of relying on a thermoelectric cooler (TEC) to ensure stable gas concentration detection. The sensor can achieve stable concentration measurement in the temperature range of -10 to 40 °C without the need for a TEC, which greatly simplifies the structure of the sensor and reduces the cost. The results of gas concentration calibration experiments show that the sensor has a good linear correlation (R2=0.9993). Long-term continuous detection experiments show that the sensor maintains a relative detection error between -2.667% and 4.3% over the full test temperature range. In addition, signal-to-noise ratio analysis experiments further determine that the minimum detection limit of the sensor for methane gas is 27.33 ppm·m (1σ). Given its advantages of simple structure, low cost, high accuracy, and stability, this methane detection sensor is well suited for natural gas leakage monitoring in home environments and can also be widely used in industrial safety detection and environmental monitoring applications. This technology provides a new, cost-effective solution for domestic and industrial methane detection.

Numerical simulation of ultra-wideband supercontinuum generation covering 2-20 μm waveband in cascaded all-soft-glass fiber

Yiqi Chu, Zhiyu Xu, Chuchen Li, Chenyun Jiang, Zhongbin Zhao, Tianhao Hu, Zhou Yong, Xiaohui Ma, WENTAN FANG, Wei Zhang, Xiaolin Chen, Song Huang, Ronghua Yu, Meisong Liao, and Weiqing Gao

DOI: 10.1364/AO.517865 Received 09 Jan 2024; Accepted 19 Feb 2024; Posted 22 Feb 2024  View: PDF

Abstract: We design a cascaded all-soft-glass fiber structure and simulate mid-infrared 2-20 μm ultra-wideband supercontinuum (SC) generation numerically. The cascaded fiber structure consists of a 1.5 m InF3 fiber, a 0.2 m chalcogenide photonic crystal fiber and a 0.2 m tellurium-based chalcogenide photonic crystal fiber. Using 2 μm pulse pumping this cascaded structure, the generated SC covering the wavelengths longer than 20 μm has been demonstrated theoretically. The 30 dB bandwidth reaches 20.87 μm from 1.44 to 22.31 μm. The affect of different pulse widths on SC generation is considered. With the increase of peak power and the decrease of pulse width, the energy of SC in 15-20 μm waveband increases gradually. The mechanism of SC broadening process has also been analyzed. The SC generation more than 20 μm in this cascade structure is caused by the self-phase modulation (SPM), soliton effects, four-wave mixing (FWM) and red-shifted dispersive wave (RDW). This method demonstrates the possibility of generating ultra-wide bandwidth SCs up to 20 μm waveband by commercial 2 μm pump source and all-fiber structure.

All-dielectric high-NA achromatic metalenses in the mid-infrared band based on subregions

ZHIHENG WANG, Weiguo Liu, Yechuan Zhu, and SUN ZHOU

DOI: 10.1364/AO.510342 Received 27 Oct 2023; Accepted 19 Feb 2024; Posted 22 Feb 2024  View: PDF

Abstract: For conventional refractive lenses, chromatic aberration inevitably occurs due to the refractive index variation of the lens material with the incident wavelength, leading to axial aberrations and lower imaging system quality. Achromatic metalenses have demonstrated a great capability to solve this problem and been extensively investigated. However, the metalens achromatic method involves construction of a unit structure satisfying a phase distribution greater than 0-2𝜋 or phase compensation. Although this design method can obtain a good achromatic effect, finding a unit that satisfies a linear distribution during design is difficult. In this paper, we use subregion discrete wavelength modulation to achieve broadband achromatism. The total number of structural units in each region is optimized for different incident wavelengths, and the internal and external ring unit structures are also optimized. This achromatic metalens exhibits a large aperture and a high numerical aperture in the 4.2-4.7 μm mid-infrared band (NA=0.83). Our research has strong potential and application prospects in ultracompact imaging and laser beam shaping.

Focal Length Switchable Metalens Based on Vanadium Dioxide

Junhao Niu, Renfang Tian, Wei Mo, Chunhui Li, Qianyu Yao, and Aijun Zhu

DOI: 10.1364/AO.512470 Received 16 Nov 2023; Accepted 19 Feb 2024; Posted 22 Feb 2024  View: PDF

Abstract: A metalens is a flat lens that can control the phase of light so that dispersed light can be reconcentrated. This study devised a tunable metalens with a switchable focal length based on the phase transition properties of vanadium dioxide (VO2). The unit structure comprises three layers from bottom to top: gold, polyimide, and two square resonant rings. The metalens can not only transform incident x-polarized waves into y-polarized waves but also achieve beam focusing simultaneously. The designed metalens achieve polarization conversion efficiency at an operating frequency of 0.8 THz. In the insulating state of VO2, the beam focal point is at L = 1914 μm; in the metallic state, the wave converges at L = 982 μm, closely aligning with the predetermined focal length. By controlling external temperature, focal point switching can be achieved, making it highly versatile in practical applications.

Analysis of the interaction-length dependence of frequency stability in an iodine-stabilized Nd: YAG laser

Feng-Lei Hong, Shogo Matsunaga, Rei Kato, Mayuko Yoshiki, and Daisuke Akamatsu

DOI: 10.1364/AO.515683 Received 11 Dec 2023; Accepted 19 Feb 2024; Posted 22 Feb 2024  View: PDF

Abstract: We report a numerical simulation and an experimental study on the interaction-length dependence of frequency stability in an iodine-stabilized Nd: YAG laser. A saturation spectroscopy model was used in the simulation to calculate the interaction-length dependence of the linewidth and signal-to-noise ratio of the iodine saturation spectrum. We determined that 2 m was the optimal interaction length for laser frequency stabilization. We confirmed the simulation results by performing modulation transfer spectroscopy and laser frequency stabilization using 45-cm- and 2-m-long iodine cells and multipass configurations. The results of this study are useful for designing compact and highly stable iodine-stabilized lasers.

Multifunctional metamaterial device based on VO2 and the equivalent diode

Zelong Wang, Xin Wang, Junlin Wang, Shengjie Sun, Huizhong Pang, Xingyu Pei, and Kaixuan Shi

DOI: 10.1364/AO.506094 Received 19 Sep 2023; Accepted 19 Feb 2024; Posted 22 Feb 2024  View: PDF

Abstract: This paper proposes a switchable multifunctional metamaterial device operating in the terahertz (THz) band. The device is loaded with an equivalent diode and utilizes vanadium dioxide (VO2). The middle layer of the whole device, a metal layer, divides the device into the I side and the II side. When the diode is ON, the I side can achieve dual-band absorption at 1.975 THz and 4.345 THz. When the diode is OFF, the I side can achieve single-band absorption at 4.28 THz. In the case of VO2 being insulating, the II side can achieve linear-to-linear (LTL) polarization conversion at 2.342-4.18 THz. In the case of VO2 being conductive, the II side can realize linear-to-circular (LTC) polarization conversion at 2.105-3.283 THz. The device provides a new strategy for the subsequent combination of multiple functions. The device can be used in electromagnetic stealth, intelligent applications, radiometers, and sensors and has relatively large application potential in miniaturized multifunctional metamaterials and THz band research.

Genetic-algorithm-based waveguide displaysystem with multiplexed volume holographicgrating

Qibin Feng, jiahao cai, Yusong Guo, Min Guo, Wang Zi, and Guoqiang Lv

DOI: 10.1364/AO.515405 Received 07 Dec 2023; Accepted 16 Feb 2024; Posted 16 Feb 2024  View: PDF

Abstract: Most of the current holographic waveguide display systems are designed based onthe center beam. When the incident beam consists of the rays with different angles, the field ofview and optical efficiency would greatly reduce. The heavy angular dependence of volumeholographic grating and the back-coupling loss are two main reasons. The paper proposes adesign method of waveguide display system with multiplexed VHG, which is based on geneticalgorithm to optimize and calculate the parameters both of VHG and the waveguide. Thesimulation results show that the diagonal field of view of the holographic waveguide system isincreased to 28° and its optical efficiency is improved by 30%. The design method of thewaveguide system with the multiplexed grating proposed in this paper can effectively expandthe field of view and improve the optical efficiency.

Stitching interferometry for long X-ray mirrors with lateral multi-shift based absolute calibration

Guang Zhou, Jiezhuo Wang, Weizheng Lei, Xiaohao Dong, and jie wang

DOI: 10.1364/AO.516190 Received 19 Dec 2023; Accepted 15 Feb 2024; Posted 15 Feb 2024  View: PDF

Abstract: To eliminate the reference mirror (REF) error of the Fizeau interferometer for measuring X-ray mirrors, the reference calibration method of lateral multi-shift measurements with hundreds-micrometer pixel level is presented. Because of the high aspect ratio of long X-ray mirrors, by shifting the surface under test (SUT) along the tangential direction with integer multiple pixels, we extend the calibration method by using the difference between multiple shifted measurements to build an augmented multi-matrix for extracting two-dimensional (2D) absolute surface. The method can be applied to arbitrary measurement regions of the test optics, and the measurement for both the benchmark sub-aperture and calibration of the REF is accomplished in a single measuring process. Furthermore, by adjusting the shift to the millimeter scale, reference-subtracted sub-apertures can be stitched to obtain the absolute 2D map of the X-ray mirror. Experiment results show that all the 2D discrepancies reach sub-nanometer repeatability, which indicates that the proposed method meets the requirements for X-ray mirror measurements.

Compact, remote optical waveguide magnetic fieldsensing using a double pass Faraday rotation inducedoptical attenuation

Yunlong Guo, John Canning, Zenon Chaczko, and GangDing Peng

DOI: 10.1364/AO.513826 Received 15 Dec 2023; Accepted 15 Feb 2024; Posted 15 Feb 2024  View: PDF

Abstract: Compact, magnetic field, B, sensing is proposed and demonstrated by combining the two Faraday rotation elements andbeam displacement crystals within a micro-optical fiber circulator with a fiber reflector and ferromagnets to allow highcontrast attenuation in an optical fiber arm. Low optical noise sensing is measured at  = 1550 nm as a change inattenuation, , of optical light propagating through the rotators and back. The circulator’s double pass configuration,using a gold mirror as the device reflector, achieves a magnetic field sensitivity s = Δα/ΔB = (0.26 ± 0.02) dB/mT with aresolution of  = 0.01 mT, over a detection range B = 0 to 89 mT. The circulator as a platform provides direct connectivityto the internet, allowing remote sensing to occur. The method described here is amenable to multi-sensor combinations,including with other sensor technologies, particularly in future integrated waveguide Faraday optical circuits and devices,extending its utility beyond point magnetic field sensing applications.

Heimdallr, Baldr and Solarstein: designing the next generation of VLTI instruments in the Asgard suite

Adam Taras, Gordon Robertson, Benjamin Courtney-Barrer, Josh Carter, Fred Crous, Michael Ireland, and Peter Tuthill

DOI: 10.1364/AO.514831 Received 01 Dec 2023; Accepted 13 Feb 2024; Posted 13 Feb 2024  View: PDF

Abstract: High angular resolution imaging is an increasingly important capability in contemporary astrophysics. Of particular relevance to emerging fields such as the characterisation of exoplanetary systems, imaging at the required spatial scales and contrast levels results in forbidding challengesin the correction of atmospheric phase errors, which in turn drives demanding requirements for precise wavefront sensing. Asgard is the next-generation instrument suite at the European Southern Observatory’s Very Large Telescope Interferometer (VLTI), targeting advances in sensitivity, spectral resolution and nulling interferometry. In this paper, we describe the requirements and designs of three core modules: Heimdallr, a beam combiner for fringe tracking, low order wavefront correction and visibility science; Baldr, a Zernike wavefront sensor to correct high order atmospheric aberrations; and Solarstein, an alignment and calibration unit. In addition,we draw generalisable insights for designing such system and discuss integration plans.

Design and fabrication of liquid crystal polarization grating for mid- and far-infrared wavelengths

Moritsugu Sakamoto, Sora Ohara, Yohei Mitsuboshi, Kohei Noda, masato suzuki, Tomoyuki Sasaki, Nobuhiro Kawatsuki, and Hiroshi Ono

DOI: 10.1364/AO.518070 Received 14 Jan 2024; Accepted 13 Feb 2024; Posted 13 Feb 2024  View: PDF

Abstract: A lot of research on liquid crystal polarization gratings (LCPGs) that can separate circularly polarized light with 100\,\% diffraction efficiency has been conducted in the visible and near-infrared wavelength regions. In this paper, we tried to design and fabricate the LCPGs that are available for use in the mid- and far-infrared (MIR and FIR) wavelength regions. The materials for making LCPGs were selected in view of low absorption characteristics measured by the use of a Fourier-transform infrared (FT-IR) spectrometer. The LCPGs designed for 3.8\,$\mu$m and 9.5-10.6\,$\mu$m are respectively fabricated, and evaluated their diffraction properties experimentally. The MIR and FIR LCPG should open new application fields of LC technologies including polarimetry, spectroscopy, and beam steering.

Distortion measurement of lithographyprojection lens based on multichannel gratinglateral shearing interferometry

Yisha Cao, YUNJUN LU, Peng Feng, Xiaoyue Qiao, Sotero Ordones, Rong Su, and Xiangzhao Wang

DOI: 10.1364/AO.513688 Received 23 Nov 2023; Accepted 12 Feb 2024; Posted 12 Feb 2024  View: PDF

Abstract: The optical distortion of the lithographic projection lens can reduce the imagingquality and cause overlay errors in lithography, thus preventing the miniaturization of theprinted patterns. In this paper, we propose a technique to measure the optical distortion of alithographic projection lens by sensing the wavefront aberrations of the lens. A multichanneldual-grating lateral shearing interferometer is used to measure the wavefront aberrations atseveral field points in the pupil plane simultaneously. Then, the distortion at these field pointsis derived according to the proportional relationship between the Z2 and Z3 Zernike terms (thetilt terms) and the image position shifts. Without the need for additional devices, our approachcan simultaneously retrieve both the wavefront aberrations and the image distortioninformation. Consequently, it improves not only the measurement speed and accuracy but alsoenables accounting for displacement stage positioning error. Experiments were conducted on alithographic projection lens with a numerical aperture of 0.57 to verify the feasibility of theproposed method.

"Hidden phase" in two-wavelength adaptive optics

Milo Hyde, Jack McCrae, Matthew Kalensky, and Mark Spencer

DOI: 10.1364/AO.516039 Received 14 Dec 2023; Accepted 10 Feb 2024; Posted 12 Feb 2024  View: PDF

Abstract: Two-wavelength adaptive optics (AO), where sensing and correcting (from a beacon) is performed at one wavelength $\lambda_\text{B}$ and compensation and observation (after transmission through the atmosphere) is performed at another $\lambda_\text{T}$, has historically been analyzed and practiced assuming negligible irradiance fluctuations (i.e., weak scintillation). Under these conditions, the phase corrections measured at $\lambda_\text{B}$ are robust over a relatively large range of wavelengths, resulting in a negligible decrease in AO performance. In weak-to-moderate scintillation conditions, which result from distributed-volume atmospheric aberrations, the pupil-phase function becomes discontinuous, producing what Fried called the ``hidden phase'' because it is not sensed by traditional least-squares phase reconstructors or unwrappers. Neglecting the hidden phase has a significant negative impact on AO performance even with perfect least-squares phase compensation.To the authors' knowledge, the hidden phase has not been studied in the context of two-wavelength AO. In particular, how does the hidden phase sensed at $\lambda_\text{B}$ relate to the compensation (or observation) wavelength $\lambda_\text{T}$? If the hidden phase is highly correlated across $\lambda_\text{B}$ and $\lambda_\text{T}$, like the least-squares phase, it is worth sensing and correcting; otherwise, it is not.Through a series of wave optics simulations, we find an approximate expression for the hidden-phase correlation coefficient as a function of $\lambda_\text{B}$, $\lambda_\text{T}$, and the scintillation strength. In contrast to the least-squares phase, we determine that the hidden phase (when present) is correlated over a small band of wavelengths centered on $\lambda_{\text{T}}$. Over the range $\lambda_\text{B},\lambda_\text{T} \in \left[1,3\right] \text{ } \mu\text{m}$ and in weak-to-moderate scintillation conditions (spherical-wave log-amplitude variance $\sigma_\chi^2 \in \left[0.1,0.5\right]$), we find the average hidden-phase correlation linewidth to be approximately $\text{0.35} \text{ } \mu\text{m}$. Consequently, for $\left|\lambda_\text{B}-\lambda_\text{T}\right|$ greater than this linewidth, including the hidden phase does not significantly improve AO performance over least-squares phase compensation.

Angularly stable Terahertz multiband LiNbO3-polymer hybrid metamaterial for microfluidicrefractive index sensing

Tao Ma, Linxing Su, Yabo Fan, Wenqian Wang, and Heng Liu

DOI: 10.1364/AO.508021 Received 16 Oct 2023; Accepted 08 Feb 2024; Posted 08 Feb 2024  View: PDF

Abstract: A terahertz (THz) LiNbO3-polymer hybrid metamaterial (LPHM) consisting ofthree-layers Au patterns and two medium interval layers is demonstrated, and the bulkrefractive index (RI) sensing performances are also studied. The parameter optimizations andsensing performances of the LPHM are simulated by finite element method (FEM). The resultsshow that the reflection or absorption spectrum of the LPHM has four peaks in the 1-10 THzband, and the peaks move towards the lower frequency when the period (P) of the LPHM orthe side length (a) of the notched square frame increases, but shift to the higher frequency whenw1 or w2 increases. Moreover, the LPHM has a wide angular stability and good structuralstability. The sensing performances show that the LPHM can achieve a RI sensitivity of 11.5μm/RIU with a detection limit (DL) of 2.9×10-4 RIU. The LPHM has potential applications inpharmacological biodevices, THz immunosensing, modern medical and clinical practices, anddetection of thin films and biochemical substances, and it can be expected to realize multiphysical parameter measurements.

Target intensity correction method based on incidenceangle and distance for pulsed Lidar system

Baoling Qi, Guohui Yang, yu zhang, and Chunhui Wang

DOI: 10.1364/AO.505690 Received 14 Sep 2023; Accepted 08 Feb 2024; Posted 09 Feb 2024  View: PDF

Abstract: The pulse light detecting and ranging (Lidar) is capable of acquiring comprehensive targetinformation within a single pulse, including distance and intensity data. Intensity data reflects the target'sbackscattered intensity and is commonly regarded as a crucial observational parameter associated withtarget reflectivity information. Multiple studies have indicated the potential of intensity data in variousapplications within pulsed Lidar contexts. However, the intensity data is influenced by the incident angleand distance, hence it cannot directly manifest target characteristics. Consequently, a prerequisite for itsusage is the implementation of intensity calibration. This paper presents a novel target intensitycorrection method based on an improved tail model, designed for preprocessing intensity data. Firstly,the pulse echo signal equation is derived by incorporating the improved tail model with the detectedtarget. On this foundation, a target echo intensity correction model is established to correct the intensitiesat various incident angles to those at the normal direction. Lastly, the derived approach is validatedthrough simulation analysis, and practical experiments are conducted on a constructed pulsed Lidarsystem. These experiments meticulously investigate the influences of incident angle and distance, twoprominent factors, on echo intensity. In the context of incident angle correction experiments, the meanabsolute error (MAE) in calibrated values for diverse targets all remain within 0.04V. Prior to correction,the maximum MAE for the cystosepiment is 0.505V, after the correction it is reduced to merely 0.02V,indicating a 96% reduction in error. Furthermore, all discrepancies exhibit an error standard deviation(ESD) of 0.03V or less, showcasing favorable stability. For distance correction, under normal incidenceconditions, a diverse set of targets are measured at different distances to achieve corrected MAE andESD within 0.05V. Consequently, the proposed method effectively achieves intensity correctionconcerning incident angles and distances. To achieve this, a reflectivity lookup table for the relevanttargets was established. Combining this with the corrected intensity information enabled targetidentification in the three-dimensional imaging of pulse Lidar.

Research on diopter detection method based onoptical imaging

Kaitong Ren, Yong Liu, Wangmeiyu Xing, Jingshu Ni, Yuanzhi Zhang, Meili Dong, Yang Zhang, yikun wang, yao huang, and Mingming xu

DOI: 10.1364/AO.509341 Received 13 Oct 2023; Accepted 03 Feb 2024; Posted 22 Feb 2024  View: PDF

Abstract: Peripheral retinal refractive state plays an important role in eye growth anddevelopment and is closely related to the development of myopia. Existing methods formeasuring peripheral retinal refractive state are cumbersome and can only detect in a limitedrange. To address the above shortcomings, this paper proposes a retinal refractive statedetection method using optical refractive compensation imaging. Firstly, a series of defocusimages are captured using an optical system, and then the images are enhanced and filtered .Subsequently, the Sobel function is applied to calculate sharpness, and the AsymmetricGaussian (AG) model is employed for peak fitting, allowing for the determination of the fundusretina's overall refractive compensation value. We performed consistency analysis on thecentral and peripheral diopters with autorefractor KR-8900 (Topcon, Japan) and WAM-5500(Grand Seiko, Japan), respectively. The intraclass correlation coefficients(ICC) are all greaterthan 0.9, showing good consistency. This is a promising alternative to the current techniquesfor assessing the refraction of the peripheral retina.

Optical experimental results using Singer product apertures

Paul Shutler and Kevin Byard

DOI: 10.1364/AO.514108 Received 29 Nov 2023; Accepted 01 Feb 2024; Posted 22 Feb 2024  View: PDF

Abstract: We present the first optical experimental results obtained using the recently developed Singerproduct apertures. We also show that Fenimore and Cannon’s fine sampling and delta decoding techniquescan be combined with the fast direct vector decoding algorithm for Singer product apertures. We demonstrateresolutions and decoding speeds comparable to, or better than, those currently reported in the opticalliterature. Taken together these make possible coded aperture video in the optical domain.

Laser welding of fiber array units

Stephan Logunov, Mark Quesada, Leonard Dabich, and John Nord

DOI: 10.1364/AO.515224 Received 05 Dec 2023; Accepted 26 Jan 2024; Posted 26 Feb 2024  View: PDF

Abstract: We report the results of fabricating fiber array unit (FAU) connectors using a nearIR laser welding process, locking fibers in proper position on planar glass substrates, formingstrong glass-to-glass bonds, followed by final assembly using lower coefficient of thermalexpansion (CTE) epoxies. A thin metal film deposited on the glass substrate provides theabsorption required to attain interfacial temperatures suitable for glass-to-glass bonding. Thismethod allows the elimination of dedicated expensive V-groove plates while still maintainingvery good fiber placement accuracy. The use of epoxy is minimized to simply securing macropackaging components, and protecting fibers from environmental pressure, temperature, andhumidity variation. The thermal expansion properties of the epoxy used were essential for thelong-term FAU reliability.