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Abstract Fiber optic sensors have been gradually used in aerospace, petrochemical, electronic power, civil engineering, and biomedical fields because of their many advantages such as the anti-electromagnetic interference, corrosion resistance, light weight, small size, high accuracy, and easy reuse. In recent years, sensing and demodulation technologies based on microwave photonics have attracted widespread attention. Optical fiber sensing combined with microwave photonics has higher sensitivity and flexibility, which is important for the demodulation of interferometric signals. This article introduces and analyzes the principle, structure, and performance of the demodulation technology of fiber optic interferometric signals based on microwave photonics from the perspective of system structures, such as filters, oscillators, and interferometers, and discusses the future research and development directions.

Abstract Remarkable advancements have been made in the design of optical metasurfaces in recent years, particularly in compact designs. However, for their practical integration into diverse optical systems, there is a pressing need for metasurfaces to transition toward larger areas without compromising their performance. From a design perspective, efforts in the design process must focus on reducing computational costs and enhancing performance in larger areas. In this review, we introduce diverse optical analyses applicable to wide areas, including the modification of boundary conditions, fast multipole methods, coupled mode theory, and neural network–based approaches. In addition, inverse design methods based on the adjoint method or deep learning, which are suitable for large-scale designs, are described. Numerous fast and accurate simulation methods make it possible to assess optical properties over large areas at a low cost, whereas diverse inverse design methods hold promise for high performance. By concurrently addressing both the essential aspects of designing large-area metasurfaces, we comprehensively discuss various approaches to develop metasurfaces with high performance over expansive regions. Finally, we outline additional challenges and prospects for realizing mass-produced high-performance metasurfaces, unlocking their full potential for optical applications.

Abstract In the burgeoning field of quantum topological photonics, waveguide systems play a crucial role. This perspective delves into the intricate interplay between photonic waveguides and topological phenomena, underscoring the theoretical underpinnings of topological insulators and their photonic manifestations. We highlight key milestones and breakthroughs in topological photonics using waveguide systems, alongside an in-depth analysis of their fabrication techniques and tunability. The discussion includes the technological advancements and challenges, limitations of current methods, and potential strategies for improvement. This perspective also examines the quantum states of light in topological waveguides, where the confluence of topology and quantum optics promises robust avenues for quantum communication and computing. Concluding with a forward-looking view, we aim to inspire new research and innovation in quantum topological photonics, highlighting its potential for the next generation of photonic technologies.

Abstract Topological deformations from the hosting lattice have been applied to nanophotonics for the implementation of quantized topological states. This manipulation enables topological control of light at the wavelength scale, leading to strong light confinement and the excitation of unique resonant modes. In this Perspective, we discuss recent advances in the development of next-generation photonic devices based on topological deformations and present a comprehensive overview of ongoing research in this field.

Abstract Ultra-low-power consumption and high-speed integrated switches are highly desirable for future data centers and high-performance optical computers. In this study, we proposed an ultra-low-power consumption silicon electro-optic switch based on photonic crystal nanobeam cavities on a foundry platform. The proposed switch showed an ultra-low static-tuning power of 0.10 mW and a calculated dynamic switching power of 6.34 fJ/bit, with a compact footprint of 18 μm × 200 μm. Additionally, a 136-Gb/s four-level pulse amplitude modulation signal transmission experiment was carried out to verify the capability of the proposed electro-optic switch to support high-speed data transmission. The proposed device has the lowest static-tuning power consumption among silicon electro-optic switches and the highest data transmission rate. The results demonstrate the potential applications of this switch in high-performance optical computers, data center interconnects, optical neural networks, and programmable photonic circuits.

Abstract A novel multispectral smart window has been proposed, which features dynamic modulation of light transmittance and effective shielding against electromagnetic microwave radiation. This design integrates liquid crystal dynamic scattering and dye doping techniques, enabling the dual regulation of transmittance and scattering within a single-layer smart window. Additionally, the precise control of conductive film thickness ensures the attainment of robust microwave signal shielding. We present a theoretical model for ion movement in the presence of an alternating electric field, along with a novel approach to manipulate negative dielectric constant. The proposed model successfully enables a rapid transition between light transparent, absorbing and haze states, with an optimum drive frequency adjustable to approximately 300 Hz. Furthermore, the resistive design of the conductive layer effectively mitigates microwave radiation within the 2−18 GHz range. These findings offer an innovative perspective for future advancements in environmental construction.

Editorial Professor Juejun Hu was admitted by Tsinghua University as top scorer in the science college entrance examination of Fujian Province. After graduating, he went to MIT to pursue further studies, where he continued to excel and became a faculty member. Each step of his journey has been marked by extraordinary achievements, setting a standard that few can match. Today, Prof. Hu is recognized as a leading expert in integrated photonics and optical materials. His pioneering research has not only advanced the frontiers of academia but also made significant impacts on industrial applications. In this interview, we invite you to delve into Prof. Hu’s research world, exploring his unique insights into technological innovation and how he uses the power of science to shape the future.

Abstract Simultaneous lightwave information and power transfer (SLIPT), co-existing with optical wireless communication, holds an enormous potential to provide continuous charging to remote Internet of Things (IoT) devices while ensuring connectivity. Combining SLIPT with an omnidirectional receiver, we can leverage a higher power budget while maintaining a stable connection, a major challenge for optical wireless communication systems. Here, we design a multiplexed SLIPT-based system comprising an array of photodetectors (PDs) arranged in a 3 × 3 configuration. The system enables decoding information from multiple light beams while simultaneously harvesting energy. The PDs can swiftly switch between photoconductive and photovoltaic modes to maximize information transfer rates and provide on-demand energy harvesting. Additionally, we investigated the ability to decode information and harvest energy with a particular quadrant set of PDs from the array, allowing beam tracking and spatial diversity. The design was explored in a smaller version for higher data rates and a bigger one for higher power harvesting. We report a self-powering device that can achieve a gross data rate of 25.7 Mbps from a single-input single-output (SISO) and an 85.2 Mbps net data rate in a multiple-input multiple-output (MIMO) configuration. Under a standard AMT1.5 illumination, the device can harvest up to 87.33 mW, around twice the power needed to maintain the entire system. Our work paves the way for deploying autonomous IoT devices in harsh environments and their potential use in space applications.

Abstract Strict requirement of a coherent spectrum in coherent diffractive imaging (CDI) architectures poses a significant obstacle to achieving efficient photon utilization across the full spectrum. To date, nearly all broadband computational imaging experiments have relied on accurate spectroscopic measurements, as broad spectra are incompatible with conventional CDI systems. This paper presents an advanced approach to broaden the scope of CDI to ultra-broadband illumination with unknown probe spectrum, effectively addresses the key challenges encountered by existing state-of-the-art broadband diffractive imaging frameworks. This advancement eliminates the necessity for prior knowledge of probe spectrum and relaxes constraints on non-dispersive samples, resulting in a significant extension in spectral bandwidth, achieving a nearly fourfold improvement in bandlimit compared to the existing benchmark. Our method not only monochromatizes a broadband diffraction pattern from unknown illumination spectrum, but also determines the compressive sampled profile of spectrum of the diffracted radiation. This superiority is experimentally validated using both CDI and ptychography techniques on an ultra-broadband supercontinuum with relative bandwidth exceeding 40%, revealing a significantly enhanced coherence and improved reconstruction with high fidelity under ultra-broadband illumination.

Abstract Recent advancements in wavefront shaping techniques have facilitated the study of complex structured light’s propagation with orbital angular momentum (OAM) within various media. The introduction of spiral phase modulation to the Laguerre–Gaussian (LG) beam during its paraxial propagation is facilitated by the negative gradient of the medium’s refractive index change over time, leading to a notable increase in the rate of phase twist, effectively observed as phase retardation of the OAM. This approach attains remarkable sensitivity to even the slightest variations in the medium’s refractive index (∼10 −6). The phase memory of OAM is revealed as the ability of twisted light to preserve the initial helical phase even propagating through the turbid tissue-like multiple scattering medium. The results confirm fascinating opportunities for exploiting OAM light in biomedical applications, e.g. such as non-invasive trans-cutaneous glucose diagnosis and optical communication through biological tissues and other optically dense media.

Abstract The creation of carbonaceous nanoparticles and their dynamics in hydrocarbon flames are still debated in environmental, combustion, and material sciences. In this study, we introduce single-pulse femtosecond laser sheet-compressed ultrafast photography (fsLS-CUP), an ultrafast imaging technique specifically designed to shed light on and capture ultrafast dynamics stemming from interactions between femtosecond lasers and nanoparticles in flames in a single-shot. fsLS-CUP enables the first-time real-time billion frames-per-second (Gfps) simultaneous two-dimensional (2D) imaging of laser-induced fluorescence (LIF) and laser-induced heating (LIH) that are originated from polycyclic aromatic hydrocarbons (PAHs) and soot particles, respectively. Furthermore, fsLS-CUP provides the real-time spatiotemporal map of femtosecond laser-soot interaction as elastic light scattering (ELS) at an astonishing 250 Gfps. In contrast to existing single-shot ultrafast imaging approaches, which are limited to millions of frames per second only and require multiple laser pulses, our method employs only a single pulse and captures the entire dynamics of laser-induced signals at hundreds of Gfps. Using a single pulse does not change the optical properties of nanoparticles for a following pulse, thus allowing reliable spatiotemporal mapping. Moreover, we found that particle inception and growth are derived from precursors. In essence, as an imaging modality, fsLS-CUP offers ultrafast 2D diagnostics, contributing to the fundamental understanding of nanoparticle’s inception and broader applications across different fields, such as material science and biomedical engineering.

Abstract Luminescent metal halides are attracting growing attention as scintillators for X-ray imaging in safety inspection, medical diagnosis, etc. Here we present brand-new hybrid Eu(II)-bromide scintillators, 1D type [Et4N]EuBr3·MeOH and 0D type [Me4N]6Eu5Br16·MeOH, with spin-allowed 5d-4f bandgap transition emission toward simplified carrier transport during scintillation process. The 1D/0D structures with edge/face -sharing [EuBr6]4− octahedra further contribute to lowing bandgaps and enhancing quantum confinement effect, enabling efficient scintillation performance (light yield ~73100 ± 800 Ph MeV−1, detect limit ~18.6 nGy s−1, X-ray afterglow ~ 1% @ 9.6 μs). We demonstrate the X-ray imaging with 27.3 lp mm−1 resolution by embedding Eu(II)-based scintillators into AAO film. Our results create the new family of low-dimensional rare-earth-based halides for scintillation and related optoelectronic applications.

Abstract In the era of smart agriculture, the precise labeling and recording of growth information in plants pose challenges for modern agricultural production. This study introduces strontium aluminate particles coated with H3PO4 as luminescent labels capable of spatial embedding within plants for information encoding and storage during growth. The encapsulation with H3PO4 imparts stability and enhanced luminescence to SrAl2O4:Eu2+,Dy3+ (SAO). Using SAO@H3PO4 as a low-damage luminescent label, we implement its delivery into plants through microneedles (MNs) patches. The embedded SAO@H3PO4 within plants exhibits sustained and unaltered high signal-to-noise afterglow emission, with luminous intensity remaining at approximately 78% of the original for 27 days. To cater to diverse information recording needs, MNs of various geometric shapes are designed for loading SAO@H3PO4, and the luminescent signals in different shapes can be accurately identified through a designed program, the corresponding information can be conveniently viewed on a computer. Additionally, inspired by binary information concepts, MNs patches with specific arrangements of luminescent and non-luminescent points are created, resulting in varied luminescent MNs arrays on leaves. An advanced camera system with a tailored program accurately identifies and maps the labels to the corresponding recorded information. These findings showcase the potential of low-damage luminescent labels within plants, paving the way for convenient and widespread storage of plant growth information.

Abstract Recent advances in imaging sensors and digital light projection technology have facilitated rapid progress in 3D optical sensing, enabling 3D surfaces of complex-shaped objects to be captured with high resolution and accuracy. Nevertheless, due to the inherent synchronous pattern projection and image acquisition mechanism, the temporal resolution of conventional structured light or fringe projection profilometry (FPP) based 3D imaging methods is still limited to the native detector frame rates. In this work, we demonstrate a new 3D imaging method, termed deep-learning-enabled multiplexed FPP (DLMFPP), that allows to achieve high-resolution and high-speed 3D imaging at near-one-order of magnitude-higher 3D frame rate with conventional low-speed cameras. By encoding temporal information in one multiplexed fringe pattern, DLMFPP harnesses deep neural networks embedded with Fourier transform, phase-shifting and ensemble learning to decompose the pattern and analyze separate fringes, furnishing a high signal-to-noise ratio and a ready-to-implement solution over conventional computational imaging techniques. We demonstrate this method by measuring different types of transient scenes, including rotating fan blades and bullet fired from a toy gun, at kHz using cameras of around 100 Hz. Experiential results establish that DLMFPP allows slow-scan cameras with their known advantages in terms of cost and spatial resolution to be used for high-speed 3D imaging tasks.

Abstract Ptychographic extreme ultraviolet (EUV) diffractive imaging has emerged as a promising candidate for the next generationmetrology solutions in the semiconductor industry, as it can image wafer samples in reflection geometry at the nanoscale. This technique has surged attention recently, owing to the significant progress in high-harmonic generation (HHG) EUV sources and advancements in both hardware and software for computation. In this study, a novel algorithm is introduced and tested, which enables wavelength-multiplexed reconstruction that enhances the measurement throughput and introduces data diversity, allowing the accurate characterisation of sample structures. To tackle the inherent instabilities of the HHG source, a modal approach was adopted, which represents the cross-density function of the illumination by a series of mutually incoherent and independent spatial modes. The proposed algorithm was implemented on a mainstream machine learning platform, which leverages automatic differentiation to manage the drastic growth in model complexity and expedites the computation using GPU acceleration. By optimising over 200 million parameters, we demonstrate the algorithm's capacity to accommodate experimental uncertainties and achieve a resolution approaching the diffraction limit in reflection geometry. The reconstruction of wafer samples with 20-nm high patterned gold structures on a silicon substrate highlights our ability to handle complex physical interrelations involving a multitude of parameters. These results establish ptychography as an efficient and accurate metrology tool.

Abstract Switching of light polarization on the sub-picosecond timescale is a crucial functionality for applications in a variety of contexts, including telecommunications, biology and chemistry. The ability to control polarization at ultrafast speed would pave the way for the development of unprecedented free-space optical links and of novel techniques for probing dynamical processes in complex systems, as chiral molecules. Such high switching speeds can only be reached with an all-optical paradigm, i.e., engineering active platforms capable of controlling light polarization via ultrashort laser pulses. Here we demonstrate giant modulation of dichroism and birefringence in an all-dielectric metasurface, achieved at low fluences of the optical control beam. This performance, which leverages the many degrees of freedom offered by all-dielectric active metasurfaces, is obtained by combining a high-quality factor nonlocal resonance with the giant third-order optical nonlinearity dictated by photogenerated hot carriers at the semiconductor band edge.

Abstract Thermochromic hydrogels exhibit a smart capacity for regulating solar spectrum transmission, enabling automatically change their transmissivity in response to the ambient temperature change. This has great importance for energy conservation purposes. Military and civilian emergency thermochromic applications require rapid visible-light stealth (VLS); however, concurrent smart solar transmission and rapid VLS is yet to be realized. Inspired by squid-skin, we propose a micropatterned thermochromic hydrogel (MTH) to realize the concurrent control of smart solar transmittance and rapid VLS at all-working temperatures. The MTH possesses two optical regulation mechanisms: optical property regulation and optical scattering, controlled by temperature and pressure, respectively. The introduced surface micropattern strategy can arbitrarily switch between normal and diffuse transmission, and the VLS response time is within 1 s compared with previous ~180 s. The MTH also has a high solar-transmission regulation range of 61%. Further, the MTH preparation method is scalable and cost-effective. This novel regulation mechanism opens a new pathway towards applications with multifunctional optical requirements.

Abstract Optical parametric oscillation (OPO) in Kerr microresonators can efficiently transfer near-infrared laser light into the visible spectrum. To date, however, chromatic dispersion has mostly limited output wavelengths to >560 nm, and robust access to the whole green light spectrum has not been demonstrated. In fact, wavelengths between 532 nm and 633 nm, commonly referred to as the “green gap”, are especially challenging to produce with conventional laser gain. Hence, there is motivation to extend the Kerr OPO wavelength range and develop reliable device designs. Here, we experimentally show how to robustly access the entire green gap with Kerr OPO in silicon nitride microrings pumped near 780 nm. Our microring geometries are optimized for green-gap emission; in particular, we introduce a dispersion engineering technique, based on partially undercutting the microring, which not only expands wavelength access but also proves robust to variations in resonator dimensions. Using just four devices, we generate >150 wavelengths evenly distributed throughout the green gap, as predicted by our dispersion simulations. Moreover, we establish the usefulness of Kerr OPO to coherent applications by demonstrating continuous frequency tuning (>50 GHz) and narrow optical linewidths (

Abstract Optical Kerr effect, in which input light intensity linearly alters the refractive index, has enabled the generation of optical solitons, supercontinuum spectra, and frequency combs, playing vital roles in the on-chip devices, fiber communications, and quantum manipulations. Especially, terahertz Kerr effect, featuring fascinating prospects in future high-rate computing, artificial intelligence, and cloud-based technologies, encounters a great challenge due to the rather low power density and feeble Kerr response. Here, we demonstrate a giant terahertz frequency Kerr nonlinearity mediated by stimulated phonon polaritons. Under the influences of the giant Kerr nonlinearity, the power-dependent refractive index change would result in a frequency shift in the microcavity, which was experimentally demonstrated via the measurement of the resonant mode of a chip-scale lithium niobate Fabry-Pérot microcavity. Attributed to the existence of stimulated phonon polaritons, the nonlinear coefficient extracted from the frequency shifts is orders of magnitude larger than that of visible and infrared light, which is also theoretically demonstrated by nonlinear Huang equations. This work opens an avenue for many rich and fruitful terahertz Kerr effect based physical, chemical, and biological systems that have terahertz fingerprints.

Abstract Isolated attosecond pulse (IAP) generation usually involves the use of short-medium gas cells operated at high pressures. In contrast, long-medium schemes at low pressures are commonly perceived as inherently unsuitable for IAP generation due to the nonlinear phenomena that challenge favourable phase-matching conditions. Here we provide clear experimental evidence on the generation of isolated extreme-ultraviolet attosecond pulses in a semi-infinite gas cell, demonstrating the use of extended-medium geometries for effective production of IAPs. To gain a deeper understanding we develop a simulation method for high-order harmonic generation (HHG), which combines nonlinear propagation with macroscopic HHG solving the 3D time-dependent Schrödinger equation at the single-atom level. Our simulations reveal that the nonlinear spatio-temporal reshaping of the driving field, observed in the experiment as a bright plasma channel, acts as a self-regulating mechanism boosting the phase-matching conditions for the generation of IAPs.

Abstract Traditional optical anti-counterfeiting (AC) is achieved by static printed images, which makes them susceptible to lower levels of security and easier replication. Therefore, it is essential to develop AC device with dynamic modulation for higher security. Electrophoretic display (EPD) has the advantages of low power consumption, high ambient contrast ratio, and capability of showing dynamic images which is suitable for dynamic AC applications. Herein, we prepared a dynamical AC device based on a fluorescent EPD, and achieving the image switch between black, white, and green fluorescence states under the dual-mode driving (electronic field and UV light). We loaded perovskite quantum dots (CsPbBr3) onto the TiO2 particles and further prepared fluorescent electrophoretic particles TiO2/CsPbBr3-3-PLMA (TiO/CPB-3) by grafting and polymerizing method. In addition, we fabricated the AC devices based on the fluorescent EPD, which exhibits the multifunctional AC, where the fluorescent EPD has a fast response time of 350 ms, a high contrast ratio of 17, and bright green fluorescence. This prototype demonstrates a new way for future dynamic AC and identification.

Abstract Non-invasive characterization of powders may take one of two approaches: imaging and counting individual particles; or relying on scattered light to estimate the particle size distribution (PSD) of the ensemble. The former approach runs into practical difficulties, as the system must conform to the working distance and other restrictions of the imaging optics. The latter approach requires an inverse map from the speckle autocorrelation to the particle sizes. The principle relies on the pupil function determining the basic sidelobe shape, whereas the particle size spread modulates the sidelobe intensity. We recently showed that it is feasible to invert the speckle autocorrelation and obtain the PSD using a neural network, trained efficiently through a physics-informed semi-generative approach. In this work, we eliminate one of the most time-consuming steps of our previous method by engineering the pupil function. By judiciously blocking portions of the pupil, we sacrifice some photons but in return we achieve much enhanced sidelobes and, hence, higher sensitivity to the change of the size distribution. The result is a 60 × reduction in total acquisition and processing time, or 0.25 seconds per frame in our implementation. Almost real-time operation in our system is not only more appealing toward rapid industrial adoption, it also paves the way for quantitative characterization of complex spatial or temporal dynamics in drying, blending, and other chemical and pharmaceutical manufacturing processes.

Abstract Label-free detection techniques for single particles and molecules play an important role in basic science, disease diagnostics, and nanomaterial investigations. While fluorescence-based methods are tools for single molecule detection and imaging, they are limited by available molecular probes and photoblinking and photobleaching. Photothermal microscopy has emerged as a label-free imaging technique capable of detecting individual nanoabsorbers with high sensitivity. Whispering gallery mode (WGM) microresonators can confine light in a small volume for enhanced light-matter interaction and thus are a promising ultra-sensitive photothermal microscopy platform. Previously, microtoroid optical resonators were combined with photothermal microscopy to detect 250 nm long gold nanorods and 100 nm long polymers. Here, we combine microtoroids with photothermal microscopy to spatially detect single 5 nm diameter quantum dots (QDs) with a signal-to-noise ratio exceeding 104. Photothermal images were generated by point-by-point scanning of the pump laser. Single particle detection was confirmed for 18 nm QDs by high sensitivity fluorescence imaging and for 5 nm QDs via comparison with theory. Our system demonstrates the capability to detect a minimum heat dissipation of 0.75 pW. To achieve this, we integrated our microtoroid based photothermal microscopy setup with a low amplitude modulated pump laser and utilized the proportional-integral-derivative controller output as the photothermal signal source to reduce noise and enhance signal stability. The heat dissipation of these QDs is below that from single dye molecules. We anticipate that our work will have application in a wide variety of fields, including the biological sciences, nanotechnology, materials science, chemistry, and medicine.

Abstract To reduce system complexity and bridge the interface between electronic and photonic circuits, there is a high demand for a non-volatile memory that can be accessed both electrically and optically. However, practical solutions are still lacking when considering the potential for large-scale complementary metal-oxide semiconductor compatible integration. Here, we present an experimental demonstration of a non-volatile photonic-electronic memory based on a 3-dimensional monolithic integrated ferroelectric-silicon ring resonator. We successfully demonstrate programming and erasing the memory using both electrical and optical methods, assisted by optical-to-electrical-to-optical conversion. The memory cell exhibits a high optical extinction ratio of 6.6 dB at a low working voltage of 5 V and an endurance of 4 × 104 cycles. Furthermore, the multi-level storage capability is analyzed in detail, revealing stable performance with a raw bit-error-rate smaller than 5.9 × 10−2. This ground-breaking work could be a key technology enabler for future hybrid electronic-photonic systems, targeting a wide range of applications such as photonic interconnect, high-speed data communication, and neuromorphic computing.

Abstract Individual optical addressing in chains of trapped atomic ions requires the generation of many small, closely spaced beams with low cross-talk. Furthermore, implementing parallel operations necessitates phase, frequency, and amplitude control of each individual beam. Here, we present a scalable method for achieving all of these capabilities using a high-performance integrated photonic chip coupled to a network of optical fibre components. The chip design results in very low cross-talk between neighbouring channels even at the micrometre-scale spacing by implementing a very high refractive index contrast between the channel core and cladding. Furthermore, the photonic chip manufacturing procedure is highly flexible, allowing for the creation of devices with an arbitrary number of channels as well as non-uniform channel spacing at the chip output. We present the system used to integrate the chip within our ion trap apparatus and characterise the performance of the full individual addressing setup using a single trapped ion as a light-field sensor. Our measurements showed intensity cross-talk below ~10–3 across the chip, with minimum observed cross-talk as low as ~10–5.

Abstract In recent years, two-dimensional (2D) van der Waals materials have emerged as a focal point in materials research, drawing increasing attention due to their potential for isolating and synergistically combining diverse atomic layers. Atomically thin transition metal dichalcogenides (TMDs) are one of the most alluring van der Waals materials owing to their exceptional electronic and optical properties. The tightly bound excitons with giant oscillator strength render TMDs an ideal platform to investigate strong light-matter coupling when they are integrated with optical cavities, providing a wide range of possibilities for exploring novel polaritonic physics and devices. In this review, we focused on recent advances in TMD-based strong light-matter coupling. In the foremost position, we discuss the various optical structures strongly coupled to TMD materials, such as Fabry-Perot cavities, photonic crystals, and plasmonic nanocavities. We then present several intriguing properties and relevant device applications of TMD polaritons. In the end, we delineate promising future directions for the study of strong light-matter coupling in van der Waals materials.

Abstract Restricted by the lighting conditions, the images captured at night tend to suffer from color aberration, noise, and other unfavorable factors, making it difficult for subsequent vision-based applications. To solve this problem, we propose a two-stage size-controllable low-light enhancement method, named Dual Fusion Enhancement Net (DFEN). The whole algorithm is built on a double U-Net structure, implementing brightness adjustment and detail revision respectively. A dual branch feature fusion module is adopted to enhance its ability of feature extraction and aggregation. We also design a learnable regularized attention module to balance the enhancement effect on different regions. Besides, we introduce a cosine training strategy to smooth the transition of the training target from the brightness adjustment stage to the detail revision stage during the training process. The proposed DFEN is tested on several low-light datasets, and the experimental results demonstrate that the algorithm achieves superior enhancement results with the similar parameters. It is worth noting that the lightest DFEN model reaches 11 FPS for image size of 1224×1024 in an RTX 3090 GPU. Graphical Abstract

Abstract A Mueller matrix covers all the polarization information of the measured sample, however the combination of its 16 elements is sometimes not intuitive enough to describe and identify the key characteristics of polarization changes. Within the Poincaré sphere system, this study achieves a spatial representation of the Mueller matrix: the Global-Polarization Stokes Ellipsoid (GPSE). With the help of Monte Carlo simulations combined with anisotropic tissue models, three basic characteristic parameters of GPSE are proposed and explained, where the V parameter represents polarization maintenance ability, and the E and D † parameters represent the degree of anisotropy. Furthermore, based on GPSE system, a dynamic analysis of skeletal muscle dehydration process demonstrates the monitoring effect of GPSE from an application perspective, while confirming its robustness and accuracy.

Abstract Surface plasmon resonance (SPR) sensors are based on photon-excited surface charge density oscillations confined at metal-dielectric interfaces, which makes them highly sensitive to biological or chemical molecular bindings to functional metallic surfaces. Metal nanostructures further concentrate surface plasmons into a smaller area than the diffraction limit, thus strengthening photon-sample interactions. However, plasmonic sensors based on intensity detection provide limited resolution with long acquisition time owing to their high vulnerability to environmental and instrumental noises. Here, we demonstrate fast and precise detection of noble gas dynamics at single molecular resolution via frequency-comb-referenced plasmonic phase spectroscopy. The photon-sample interaction was enhanced by a factor of 3,852 than the physical sample thickness owing to plasmon resonance and thermophoresis-assisted optical confinement effects. By utilizing a sharp plasmonic phase slope and a high heterodyne information carrier, a small atomic-density modulation was clearly resolved at 5 Hz with a resolution of 0.06 Ar atoms per nano-hole (in 10–11 RIU) in Allan deviation at 0.2 s; a faster motion up to 200 Hz was clearly resolved. This fast and precise sensing technique can enable the in-depth analysis of fast fluid dynamics with the utmost resolution for a better understanding of biomedical, chemical, and physical events and interactions.

Abstract Refractive index (RI) sensors play an important role in various applications including biomedical analysis and food processing industries. However, developing RI sensors with both high resolution and wide linear range remains a great challenge due to the tradeoff between quality (Q) factor and free spectral range (FSR) of resonance mode. Herein, the optical steelyard principle is presented to address this challenge by synergizing resonances from the Fabry–Perot (FP) cavity and metasurface, integrated in a hybrid configuration form on the end facet of optical fibers. Specifically, the FP resonance acting like the scale beam, offers high resolution while the plasmonic resonance acting like the weight, provides a wide linear range. Featuring asymmetric Fano spectrum due to modal coupling between these two resonances, a high Q value (~ 3829 in liquid) and a sensing resolution (figure of merit) of 2664 RIU−1 are experimentally demonstrated. Meanwhile, a wide RI sensing range (1.330–1.430 in the simulation and 1.3403–1.3757 in the experiment) is realized, corresponding to a spectral shift across several FSRs (four and two FSRs in the simulation and experiment, respectively). The proposed steelyard RI sensing strategy is promising in versatile monitoring applications, e.g., water salinity/turbidity and biomedical reaction process, and could be extended to other types of sensors calling for both high resolution and wide linear range.

Abstract The issue of brightness in strong ambient light conditions is one of the critical obstacles restricting the application of augmented reality (AR) and mixed reality (MR). Gallium nitride (GaN)-based micro-LEDs, renowned for their exceptional brightness and stability, are considered the foremost contenders for AR applications. Nevertheless, conventional heteroepitaxial growth micro-LED devices confront formidable challenges, including substantial wavelength shifts and efficiency droop. In this paper, we firstly demonstrated the high-quality homoepitaxial GaN-on-GaN micro-LEDs micro-display, and thoroughly analyzed the possible benefits for free-standing GaN substrate from the material-level characterization to device optoelectronic properties and micro-display application compared with sapphire substrate. The GaN-on-GaN structure exhibits a superior crystal quality with ultra-low threading dislocation densities (TDDs) of ~ 105 cm−2, which is three orders of magnitude lower than that of GaN-on-Sapphire. Through an in-depth size-dependent optoelectronic analysis of blue/green emission GaN-on-GaN/ Sapphire micro-LEDs from 100 × 100 shrink to 3 × 3 μm2, real that a lower forward voltage and series resistance, a consistent emission wavelength (1.21 nm for blue and 4.79 nm for green @ 500 A/cm2), coupled with a notable reduction in efficiency droop ratios (15.6% for blue and 28.5% for green @ 500 A/cm2) and expanded color gamut (103.57% over Rec. 2020) within GaN-on-GaN 10 μm micro-LEDs. Last but not least, the GaN-on-GaN micro-display with 3000 pixels per inch (PPI) showcased enhanced display uniformity and higher luminance in comparison to its GaN-on-Sapphire counterpart, demonstrating significant potentials for high-brightness AR/MR applications under strong ambient light.

Abstract Shack–Hartmann wavefront sensors measure the local slopes of an incoming wavefront based on the displacement of focal spots created by a lenslet array, serving as key components for adaptive optics for astronomical and biomedical imaging. Traditionally, the challenges in increasing the density and the curvature of the lenslet have limited the use of such wavefront sensors in characterizing slowly varying wavefront structures. Here, we develop a metasurface-enhanced Shack–Hartmann wavefront sensor (meta SHWFS) to break this limit, considering the interplay between the lenslet parameters and the performance of SHWFS. We experimentally validate the meta SHWFS with a sampling density of 5963 per mm2 and a maximum acceptance angle of 8° which outperforms the traditional SFWFS by an order of magnitude. Furthermore, to the best of our knowledge, we demonstrate the first use of a wavefront sensing scheme in single-shot phase imaging of highly complex patterns, including biological tissue patterns. The proposed approach opens up new opportunities in incorporating exceptional light manipulation capabilities of the metasurface platform in complex wavefront characterization.

Abstract Achieving high-luminescence organic light-emitting devices (OLEDs) with narrowband emission and high color purity is important in various optoelectronic fields. Laser displays exhibit outstanding advantages in next-generation display technologies owing to their ultimate visual experience, but this remains a great challenge. Here, we develop a novel OLED based organic single crystals. By strongly coupling the organic exciton state to an optical microcavity, we obtain polariton electroluminescent (EL) emission from the polariton OLEDs (OPLEDs) with high luminance, narrow-band emission, high color purity, high polarization as well as excellent optically pumped polariton laser. Further, we evaluate the potential for electrically pumped polariton laser through theoretical analysis and provide possible solutions. This work provides a powerful strategy with a material–device combination that paves the way for electrically driven organic single-crystal-based polariton luminescent devices and possibly lasers.

Abstract High-power terahertz (THz) quantum cascade laser, as an emerging THz solid-state radiation source, is attracting attention for numerous applications including medicine, sensing, and communication. However, due to the sub-wavelength confinement of the waveguide structure, direct beam brightness upscaling with device area remains elusive due to several mode competition and external optical lens is normally used to enhance the THz beam brightness. Here, we propose a metallic THz photonic crystal resonator with a phase-engineered design for single mode surface emission over a broad area. The quantum cascade surface-emitting laser is capable of delivering an output peak power over 185 mW with a narrow beam divergence of 4.4° × 4.4° at 3.88 THz. A high beam brightness of 1.6 × 107 W sr−1m−2 with near-diffraction-limited M2 factors of 1.4 in both vertical and lateral directions is achieved from a large device area of 1.6 × 1.6 mm2 without using any optical lenses. The adjustable phase shift between the lattices enables a stable and high-intensity surface emission over a broad device area, which makes it an ideal light extractor for large-scale THz emitters. Our research paves the way to high brightness solid-state THz lasers and facilitates new applications in standoff THz imaging, detection, and diagnosis.

Abstract Imaging through dynamic scattering media is one of the most challenging yet fascinating problems in optics, with applications spanning from biological detection to remote sensing. In this study, we propose a comprehensive learning-based technique that facilitates real-time, non-invasive, incoherent imaging of real-world objects through dense and dynamic scattering media. We conduct extensive experiments, demonstrating the capability of our technique to see through turbid water and natural fog. The experimental results indicate that the proposed technique surpasses existing approaches in numerous aspects and holds significant potential for imaging applications across a broad spectrum of disciplines.

Abstract Materials for radiation detection are critically important and urgently demanded in diverse fields, starting from fundamental scientific research to medical diagnostics, homeland security, and environmental monitoring. Low-dimensional halides (LDHs) exhibiting efficient self-trapped exciton (STE) emission with high photoluminescence quantum yield (PLQY) have recently shown a great potential as scintillators. However, an overlooked issue of exciton-exciton interaction in LDHs under ionizing radiation hinders the broadening of its radiation detection applications. Here, we demonstrate an exceptional enhancement of exciton-harvesting efficiency in zero-dimensional (0D) Cs3Cu2I5:Tl halide single crystals by forming strongly localized Tl-bound excitons. Because of the suppression of non-radiative exciton-exciton interaction, an excellent α/β pulse-shape-discrimination (PSD) figure-of-merit (FoM) factor of 2.64, a superior rejection ratio of 10−9, and a high scintillation yield of 26 000 photons MeV−1 under 5.49 MeV α-ray are achieved in Cs3Cu2I5:Tl single crystals, outperforming the commercial ZnS:Ag/PVT composites for charged particle detection applications. Furthermore, a radiation detector prototype based on Cs3Cu2I5:Tl single crystal demonstrates the capability of identifying radioactive 220Rn gas for environmental radiation monitoring applications. We believe that the exciton-harvesting strategy proposed here can greatly boost the applications of LDHs materials.

Abstract The surge in interest regarding the next generation of optical fiber transmission has stimulated the development of digital signal processing (DSP) schemes that are highly cost-effective with both high performance and low complexity. As benchmarks for nonlinear compensation methods, however, traditional DSP designed with block-by-block modules for linear compensations, could exhibit residual linear effects after compensation, limiting the nonlinear compensation performance. Here we propose a high-efficient design thought for DSP based on the learnable perspectivity, called learnable DSP (LDSP). LDSP reuses the traditional DSP modules, regarding the whole DSP as a deep learning framework and optimizing the DSP parameters adaptively based on backpropagation algorithm from a global scale. This method not only establishes new standards in linear DSP performance but also serves as a critical benchmark for nonlinear DSP designs. In comparison to traditional DSP with hyperparameter optimization, a notable enhancement of approximately 1.21 dB in the Q factor for 400 Gb/s signal after 1600 km fiber transmission is experimentally demonstrated by combining LDSP and perturbation-based nonlinear compensation algorithm. Benefiting from the learnable model, LDSP can learn the best configuration adaptively with low complexity, reducing dependence on initial parameters. The proposed approach implements a symbol-rate DSP with a small bit error rate (BER) cost in exchange for a 48% complexity reduction compared to the conventional 2 samples/symbol processing. We believe that LDSP represents a new and highly efficient paradigm for DSP design, which is poised to attract considerable attention across various domains of optical communications.

Abstract Augmented reality (AR) displays, heralded as the next-generation platform for spatial computing, metaverse, and digital twins, empower users to perceive digital images overlaid with real-world environment, fostering a deeper level of human-digital interactions. With the rapid evolution of couplers, waveguide-based AR displays have streamlined the entire system, boasting a slim form factor and high optical performance. However, challenges persist in the waveguide combiner, including low optical efficiency and poor image uniformity, significantly hindering the long-term usage and user experience. In this paper, we first analyze the root causes of the low optical efficiency and poor uniformity in waveguide-based AR displays. We then discover and elucidate an anomalous polarization conversion phenomenon inherent to polarization volume gratings (PVGs) when the incident light direction does not satisfy the Bragg condition. This new property is effectively leveraged to circumvent the tradeoff between in-coupling efficiency and eyebox uniformity. Through feasibility demonstration experiments, we measure the light leakage in multiple PVGs with varying thicknesses using a laser source and a liquid-crystal-on-silicon light engine. The experiment corroborates the polarization conversion phenomenon, and the results align with simulation well. To explore the potential of such a polarization conversion phenomenon further, we design and simulate a waveguide display with a 50° field of view. Through achieving first-order polarization conversion in a PVG, the in-coupling efficiency and uniformity are improved by 2 times and 2.3 times, respectively, compared to conventional couplers. This groundbreaking discovery holds immense potential for revolutionizing next-generation waveguide-based AR displays, promising a higher efficiency and superior image uniformity.

Abstract Non-radiative optical modes attracted enormous attention in optics due to strong light confinement and giant Q-factor at its spectral position. The destructive interference of multipoles leads to zero net-radiation and strong field trapping. Such radiationless states disappear in the far-field, localize enhanced near-field and can be excited in nano-structures. On the other hand, the optical modes turn out to be completely confined due to no losses at discrete point in the radiation continuum, such states result in infinite Q-factor and lifetime. The radiationless states provide a suitable platform for enhanced light matter interaction, lasing, and boost nonlinear processes at the state regime. These modes are widely investigated in different material configurations for various applications in both linear and nonlinear metasurfaces which are briefly discussed in this review.

【Objective】With the continuous emergence of information-based bandwidth-consuming services such as 5th Generation Mobile Communication Technology (5G), the Internet of Things, and cloud computing, people’s demand for high-speed information transmission has increased dramatically. However, due to the inherent nonlinear effects, the transmission capacity of single-mode optical fiber has approached the Shannon limit, and it will no longer be able to meet people’s needs for ultra-high-speed and large-capacity transmission. Solving the transmission capacity problem has become a top priority. In order to solve the needs of large-capacity communication systems and long-distance high-speed transmission problems, we have built a two-mode single-channel C-band Few Mode Fiber (FMF) transmission system.【Methods】At the transmitting end, an arbitrary waveform generator is used to convert the digital signal into an electrical signal and drive the In-phase and Quadrature (IQ) modulator to modulate the optical carrier. The modulated signal is transmitted simultaneously using two multiplexing technologies: Mode Division Multiplexing (MDM) and Polarization Division Multiplexing (PDM). In order to achieve long-distance transmission of dual-mode signals of 1 000 km, we construct a dual-circulation loop system. Each time the signal passes through the loop, it will pass through a 50 km FMF. After being transmitted to the target distance, the coupler outputs the signal to the demultiplexing module, and the coherent optical receiver performs homodyne detection on the demultiplexed modulated signal. Finally, the transmitted signal is stored in an oscilloscope for offline Digital Signal Processing (DSP). The signal is sequentially subjected to frequency domain dispersion compensation, downsampling, clock recovery, and least mean square algorithm to restore the original signal.【Results】It was found that within the range of Optical Signal-to-Noise Ratio (OSNR) of each channel in the experiment, the Bit Error Rate (BER) under low Signal to Noise Ratio (SNR) is close to the theoretical channel result. Under high SNR condition, the BER is 1×10-2, which is 2.5 dB away from the theoretical value. We test the BER of LP11a and LP11b modes at Back-To-Back (BTB) and 250, 500, 750 and 1 000 km transmission cases respectively. The BER at all distances are lower than the Low-Density Parity-Check (LDPC) soft decision threshold with 28% redundancy (5.2×10-2 Soft Decision - Forward Error Correction (SD-FEC)). The BER after 1 000 km transmission in the two modes are 1.7×10-2 and 1.8×10-2 respectively, and the total net transmission rate is 400 Gbit/s.【Conclusion】This article demonstrates the transmission of a 32 Gbaud MDM-PDM-16 Quadrature Amplitude Modulation (QAM) C-band signal in a 1 000 km two-mode single-channel FMF system. At the receiving end, the advanced Multiple Input Multiple Output (MIMO)-DSP algorithm is used for channel equalization, and the obtained two-mode BER of 1.7×10-2 and 1.8×10-2 are both lower than the LDPC SD-FEC threshold with 28% redundancy. The result reachs a domestic record of 400 Gbit/s net transmission rate based on FMF transmission, and highlight the potential of FMF in large-capacity long-distance transmission.

【Objective】Single-photon detection and single-photon ranging systems have widespread applications in three-dimensional imaging and long-distance spatial remote sensing. However, device limitations and background noise from the sun light limit the application to only nighttime conditions.【Methods】We have improved traditional laser radar systems by implementing all-fiber optics systems with ultra-narrowband fiber optic filtering, leading to significantly improved system stability. By implementing mechanical motion control, we are able to attain single-photon imaging systems with ultra-wide scanning and ultra-high resolution. The systems can operate in optical environments that are typically challenging, such as daytime or foggy weather. Moreover, the laser radar system is designed based on the 1 550 nm wavelength which has good atmospheric penetration and low transmission loss, enhancing its ability to work in foggy and rainy conditions.【Results】In a foggy condition with a visibility of about 500 m, the system achieves three-dimensional imaging of objects 1.6 km away, which is a distance 3 times longer than the visibility at the time. The distance resolution can be significantly enhanced by optimizing the intensity and distance information of the histogram using the minimized negative logarithmic likelihood function, with the assistance of a high-resolution time/digital converter and the model constructed by the system response function of the laser radar. Compared to the maximum value method with a distance resolution of 0.05 m, this system achieves a distance resolution of 0.006 m via the minimized negative logarithmic likelihood function method. Additionally, the image normalization processing is performed to suppress fluctuations in photon technology and eliminate noise in the image. The 1 550 nm all-fiber laser radar system has strong mechanical stability and can work in adverse weather conditions.【Conclusion】It has significant application value in remote sensing and mapping, ground and vehicle-mounted laser radar, and other fields.

【Objective】To improve the linear fit between the reflected spectral center wavelength and external environmental variables in Fiber Bragg Grating (FBG) sensing system, this paper proposes to use particle swarm optimization of Gaussian process regression model to the field of FBG stress sensing.【Methods】For the reflectance spectral characteristics of the FBG, the paper studies the impact of the linear fit in the spectral fitting of FBG sensing system. The particle swarm algorithm is used to search for the optimal hyperparameters in the Gaussian process regression model in order to enhance the predictive performance of the reflectance spectral wavelength of the center. A FBG stress sensing experimental platform was built, and the FBG was laid on the strength beam. Different weights were applied to one end of the equal strength beam to produce axial strain on the FBG, and the reflectance spectral data were collected by the spectrometer and analyzed by linear fitting with the studied model. The results obtained by the unoptimized Gaussian process regression model, the maximum value method, the Gaussian fitting method, and the center of mass method were used as the control group.【Results】The results show that under the conditions of erbium-doped fiber amplifier output power of 10 dBm, transmission fiber distance of 50 m, and the number of sampling points of the spectrometer of 501, the linear fit between the reflected spectral center wavelength and the mass of the weights is better than that of the control group. The linear fit of the studied model can reach up to 0.951 9, which is improved compared with that of the control group. Under the conditions of 501, 251, 167 and 126 spectral sampling points, the studied model can improve the linear fit of the system to 0.990 0, which is a maximum improvement of 0.258 7 compared with the maximum value method.【Conclusion】The analysis results show that the Gaussian process regression model optimized by the particle swarm is able to effectively improve the linear fit of the FBG stress sensing system.

【Objective】The optical communication system working at the traditional telecommunication band is faced with "capacity crunch" with the rapidly development of optical communication technology. The acceptable loss of 0.2 dB/km (@2 μm) from the hollow-core photonic bandgap fibers as well as the high optical amplification gains (30 dB@2 μm) of thulium-doped fiber amplifiers (CTFA type) provides the potential for the 2 μm wavelength band to alleviate the communication "capacity crunch". As a result, the needs of the 2 μm silicon-based photodetectors is also raised due to its mature and convenient manufacture process. The main solutions of the silicon-based photodetectors include the usage of Ⅲ-Ⅴ compounds with tunable band gap, the introduction of low band gap width materials as absorption regions, the use of new absorption mechanism in optical absorbed material at the 2 μm wavelength band and so on. The Ge material has a high Two Photon Absorption (TPA) coefficient (1 225 GW/cm@2 μm). And the Ge-on-Silicon-On-Insulator (SOI) photodetector is an excellent method to realize 2 μm optical signal detection, which is compatible with the standard silicon photonics device manufacturing process that has the advantages of low production difficulty and cost. The purpose of this paper is to verify the feasibility of the Ge-on-SOI photodetector to achieve 2 μm wavelength band photoelectric detection by using the special optical absorption mechanism of TPA.【Methods】In this paper, the photoelectric detection at the 2 μm wavelength band based on the physical absorption mechanism of current generated from high TPA of Ge material is realized. The quantification of the photocurrent generated by Ge material through TPA effect is analyzed and discussed in this paper. In the experimental test, we first apply a high power 2 μm wavelength band input light source magnified by thulium-doped fiber amplifiers to the input port (grating coupler) of the commercial waveguide-type Ge-on-SOI photodetector in the on-chip active silicon photonics device test system. Then we adjust the alignment of the optical fiber with the grating coupler and the input light polarization to reduce the optical transmission loss from the input optical fiber to the Ge absorption region. Finally, the TPA photocurrent of the on-chip photodetector is obtained by a probe.【Results】A net photogenerated current up to 651 nA and an estimated value of responsivity greater than 10 mA/W is obtained experimentally under a 2 μm wavelength optical input power of 21.9 dBm.【Conclusion】The work of this paper verifies the scientific feasibility of 2 μm waveband photodetector through the TPA effect of Ge material, and provides experimentally support for the design of the 2 μm wavelength band Ge-on-SOI photodetector based on TPA.

【Objective】In order to solve the problem of cross sensitivity of temperature and strain in Fiber Bragg Grating (FBG) sensing, the response characteristics of temperature and strain in each mode of Few Mode Fiber (FMF)-FBG were studied in this paper. A four-mode FBG temperature and strain dual parameter sensor was also proposed.【Methods】The basic mode and a few higher modes can be transmitted simultaneously in the low-mode fiber, taking into account the advantages of low mode dispersion of Single Mode Fiber (SMF) and low nonlinearity of multi-mode fiber, which can be used for sensing multiple physical quantities at the same time. By analyzing the different optical power sensitivity of the LP01 mode in the FMF-FBG at different temperatures, and considering the good linearity of wavelength, temperature and strain under this mode. Thus, the temperature and strain dual parameter sensing of FMF-FBG is realized.【Results】The results show that the sensor can better solve the problem of cross sensitivity of temperature and strain.【Conclusion】Compared with SMF-FBG, the FMF-FBG has several reflection peaks of different modes at the same time, which can not only solve the cross-sensitivity problem of temperature and strain, but also greatly improve the accuracy and stability of sensing, which may have a good application prospect in the field of new type of sensing.

【Objective】The light source of fiber grating sensing system provides energy for the system. The stable and continuous light source is crucial for the application of the whole demodulation system. When the Modulated Grating Y-branch (MG-Y) laser is used as the light source of the fiber grating demodulation system, it is necessary to solve the problem that it is difficult for the MG-Y laser to quickly obtain the stable and continuous tuning within the required specific wavelength range.【Methods】In order to obtain the stable and continuous wavelengths quickly for MG-Y lasers, the K-Means (KMeans) clustering model is proposed and applied to obtain high-quality current-wavelength Look Up Table (LUT) for MG-Y lasers.【Results】By combining the tuning characteristics of the MG-Y laser with the KMeans clustering model method, the centroid of the optimal tuning parameter curve in the phase tuning region of the LUT can be quickly obtained with time of about 18.26 s. Then, according to the tuning characteristics of the left, right and phase of the MG-Y laser itself, the fine tuning of the MG-Y laser wavelength can be achieved by uniform interpolation method. The LUT with the full tuning range is obtained from the KMeans clustering model, and the target LUT is selected at a certain interval. According to the output wavelength issued by the laser according to the target LUT, the mean value of the measured wavelength and the absolute value of the error of the target wavelength is 0.18 pm. The standard deviation of the absolute value of the error is 0.52 pm. Therefore, the MG-Y laser can be used as a stable light source for the fiber grating sensing system.【Conclusion】It is verified by experiments that the traditional manual search for each phase tuning region needs at least 30 min to find the centroid point. The same operation can be completed by KMeans clustering model method, which can greatly shorten the time to find the centroid point. Based on this method, the stable wavelength of quasi-continuous tuning can be achieved by controlling the MG-Y laser.

【Objective】In the production test process of 50 Gbit/s Small Form Pluggable (SFP) 56 optical module, the number of instruments and equipment to be matched for the test is large. Therefore, the test instruments are used in isolation, and the test process is complex. These issues make the optical module test longer, and the test efficiency is relatively low. It brings a lot of inconvenience to the test personnel, and can not guarantee the consistency of the test products. The purpose of this paper is to enhance debugging and testing efficiency while alleviating testers' workload.【Methods】Python language and its robust third-party libraries, including the VISA and pyserial libraries, are utilized to develop an automatic testing system for SFP56 optical modules compatible with the Windows operating system.【Results】The system can debug and test the optical module automatically. The test instruments can be used in parallel, reducing the waste of time and resources. According to the test requirements, the optical module can also be tested step by step, which greatly improves the efficiency of the debugging and testing of the optical module in the production line. It can also reduce the requirements of the tester. The verification shows that compared with the traditional optical module test, the test time of each optical module is saved by about 70 s, which greatly improves the test efficiency of the optical module. In the optical module test, the result deviation caused by human operation is avoided, and the product consistency can be guaranteed.【Conclusion】In comparison to Labview-based automatic optical module testing systems, which are currently prevalent in the market, the system designed in this study is more scalable and convenient for upgrades and maintenance. Moreover, it can be expanded to support optical module testing at speeds of 100, 200, 400, and 800 Gbit/s, thus laying a technical foundation for future high-speed optical module testing.

【Objective】E/W band millimeter-wave frequency covers 60~110 GHz, with abundant frequency band resources and potential for high-capacity transmission. Compared with lasers, E/W band millimeter-wave beam is wider, with advantages of directionality, anti-interference and ease of alignment, making it a promising frequency band for long-distance wireless communication at sea. However, the characteristics of near-sea surface channels such as atmospheric absorption and multipath at the sea surface limit the transmission distance of E/W-band millimeter waves, resulting in bottlenecks in E/W-band millimeter wave transmission near-sea surface. This paper focuses on the direction of long-distance millimeter-wave wireless transmission at sea, using microwave photonics technology to propose an ultra-wideband millimeter-wave modulation and demodulation system architecture to improve the millimeter-wave frequency conversion and modulation performance. We then study the optimal high-order modulation format in near-sea surface long-distance transmission systems to achieve high-capacity long-distance transmission.【Methods】The power amplifier in long-distance communication systems can introduce nonlinear distortion to the signal. It analyzes the factors affecting the performance of high-order modulation formats such as Amplitude Phase Shift Keying (APSK) and Quadrature Amplitude Modulation (QAM) due to the nonlinear characteristics of solid-state power amplifiers. The optimal distribution, Euclidean distance and inner and outer radius ratio of 16APSK signals have been studied. By the equivalent power experiments, the nonlinear resistance of 16QAM and 16APSK signals are compared and analyzed.【Results】The millimeter-wave long-distance transmission experiment is carried out for performance investigation. It proves that the APSK modulation format is more suitable for long-distance transmission system than QAM format.【Conclusion】Based on the 16APSK modulation format, the performance of 16 Gbit/s, equivalent 34 km long-distance transmission are achieved in the experiment, providing high-value technical means for long-distance high-capacity transmission at sea.

【Objective】In order to solve the problem of the limited bandwidth of polarization insensitive optical power splitter, an ultra-broadband polarization insensitive optical power splitter based on Silicon Nitride (Si3N4) waveguide is proposed.【Methods】The ultra-broadband polarization insensitive optical power splitter is designed based on adiabatic coupling Si3N4 waveguide.Optical bandwidth of 300 nm (from 1 400 ~ 1 700 nm) is demonstrated for both Transverse Electric (TE) and Transverse Magnetic (TM) modes.【Results】Over the entire optical bandwidth, the losses for TE and TM mode are less than 0.10 and 0.15 dB, respectively.The footprint of the proposed ultra-broadband polarization insensitive optical power splitter is only 60.00 μm×2.75 μm, which means that the optical power splitter can be used for large-scale integration.Furthermore, the relationship between the performance and the structure parameters of the proposed optical power splitter is studied.【Conclusions】The results show that the performance of the optical power splitter maintains almost unchanged by varying the structure parameters with ±50 nm.In other words, the optical power splitter possesses large fabrication tolerance which can be easily fabricated.

【Objective】In this paper, machine learning method is applied to 30 Tbit/s (60 × 500 Gbit/s) Nyquist Dual Polarization-16 Quadrature Amplitude Modulation (DP-16QAM) system after 6 300 km transmission in G. 654E optical fiber. Nonlinear channel equalization is used to reduce the transmission Bit Error Rate (BER).【Methods】Referring to the "receptive field" mechanism of convolution neural network, the size of "convolution core" is designed, and the data set is constructed according to the divided sampling data. The artificial neural network is constructed by optimizing the parameters. The one-to-one data corresponding to the transmission and reception of different wavelengths, different optical signal-to-noise ratios, and different fiber input powers in the C-band are collected. Refer to the classic full-connection neural network structure, the neural network is constructed according to the data structure of the data set. The network fitting is carried out for the real part and the imaginary part respectively. After training stage, the test data is sent into the network, and the performances are compared with the traditional methods.【Results】Two kinds of neural networks are used to fit the transmission BER under 60 different wavelength transmission conditions of C band frequency from 191.562 5 to 195.987 5 THz. Compared with Maximum Likelihood Sequence Estimation (MLSE), Network 1 has an average reduction of 23% in BER, and Network 2 has an average reduction of 41% in BER. A frequency of 193.812 5 THz is then selected for the calculation of the fiber input power ranging from 14 to 19 dBm. The average improvement in network 1 and network 2 are 32% and 52%, respectively. Under different optical signal-to-noise ratios, Network 1 has an average improvement of 30%, and Network 2 has an average improvement of 57%.【Conclusion】The two neural networks have excellent performance in nonlinear equalization of coherent transmission systems. At the same time, the number of network layers and nodes will jointly affect the fitting results. Increasing the number of layers and nodes can obtain better fitting results, but the corresponding parameters, training time and the required space will also increase. Therefore, in the application, the actual situation should be considered to choose between the fitting performances and the model attributes.

【Objective】The significance of wireless network equipment troubleshooting can be used to solve the network connection problems, improving user experience, and maintaining network stability and reliability. However, the accuracy of existing methods needs to be further improved.【Methods】To address this problem, this paper quotes the attention mechanism in the Convolutional Neural Networks(CNN)- Gated Recurrent Unit (GRU) network structure and proposes a wireless network equipment fault diagnosis method based on the CNN-GRU-Attention network structure.【Results】Compared with other methods in the simulation experiment, the method in this paper improved the accuracy of the model from 91% to 98%.【Conclusion】Through the research of this article, the quality and efficiency of wireless network equipment operation have been improved. The network capacity, functionality and reliability have also been improved, and people's growing communication needs for wireless networks have been met.