Abstract:In this paper, an organic-liquid-integrated side-hole microstructured optical fiber (SHMOF) sensor is proposed for high-sensitivity temperature measurement. The air holes of side-hole fibers are infiltrated with quinoline-dimethyl-sulfoxide mixture to excite the resonance coupling between two liquid rods and the solid fiber core. The fundamental core mode in the core region can be coupled into the liquid rod modes at specific wavelengths satisfying the phase matching condition, and the temperature-induced refractive index variation of the infiltrated materials would cause the resonance wavelength shift. By monitoring the resonance wavelength shift, high-sensitivity temperature sensing can be achieved. Further simulation results based on the finite element method are in accordance with the experimentally observed resonance shift behavior in response to the environmental temperature change. Experimental results show that the maximum sensitivity of the fabricated sensor reaches −4.88 nm/°C for the measurement range of 26.1 °C to 62 °C. Our proposed temperature sensing scheme possesses several desirable merits such as high sensitivity, compact structure and low cost, which is anticipated to find applications in various industrial as well as civil engineering areas.

Abstract: To ensure the stability of the laser communication system under complex dynamic loads, an off-axis dual-mirror optical antenna system was developed. Thermal-mechanical coupling analysis and wavefront aberration evaluation were conducted to predict the aberration under dynamic conditions and verify structural reliability. Based on D'Alembert’s principle, an acceleration-temperature gradient coupling model was constructed, and a mapping between rigid body displacement and wavefront error was established. Simulation results indicate that the maximum deformation under composite loading is 0.065 873 mm, and the root-mean-square (RMS) wavefront error increases from 0.047λ to 0.068λ, remaining within the RMS < 0.1λ design threshold. The initial wavefront RMS measured by the ZYGO interferometer is 0.052λ, deviating from the simulated value of 0.047λ by only 0.005λ. This validates the model’s accuracy and offers theoretical and engineering support for high-precision optical design in space-based laser communication systems.

Abstract:Ge p-i-n resonant-cavity-enhanced photodetectors (RCE-PDs) grown on Si-on-insulator substrate are proposed and optimized at 1 550 nm for high quantum efficiency and bandwidth. A vertical cavity is formed, consisting of a buried oxide layer as the top reflector and Si/SiO2 distributed Bragg reflector (DBR) layers as the bottom reflector, to enhance the light-matter interaction within the Ge p-i-n structure. The results demonstrate the optimized Ge RCE-PDs can achieve the quantum efficiency of 23% for the 346 nm Ge thickness (34% for the 526 nm Ge thickness) and the high bandwidth of 70 GHz (50 GHz) at 1 550 nm with 3 pairs of Si/SiO2 DBRs. These results indicate that the performance of the Ge RCE-PDs surpasses that of Ge PDs without RCE enhancement.

Abstract:The implementation of multifunctional metasurfaces through loading diodes has extremely high costs, while increasing the number of channels in the element through polarization multiplexing technology is limited. This paper proposes a dual-band five-channel (DBFC) 1-bit surface, which expands the polarization independent (PD) channels through rotating array. The polarization-independent metasurface element consists of three layers of metal, with the top layer comprising three rectangular patches oriented in the x-direction, the middle layer featuring a Jerusalem cross structure with accompanying resonators, and the bottom layer being a metal ground plane. The middle layer element can easily independently provide the required 1-bit reflection phases for two orthogonal polarizations in every frequency. The rectangular patches in the x-direction on the top layer do not contribute to the phase of y-polarization. By rotating the upper layer dielectric array 90°, the rectangular patches change to the y-direction. Under y-polarized illumination, the current distribution in the middle layer is shielded, providing a fifth set of polarization independent phases. The proposed 1-bit DBFC metasurface array has advantages in terms of structure and cost, while enhancing the utilization rate of the metasurface array. It has high application potential in microwave imaging, wireless power transmission, and other projects.

Abstract:Versatile switchable terahertz devices have important applications in the field of terahertz technology, but it is currently difficult to implement them in a single device. In order to realize the switching between slow light and absorbing functions, a slow light and absorption switchable terahertz metamaterial based on the phase transition characteristics of vanadium dioxide (VO2) is designed, which is composed of a top layer of aluminum (Al) square ring and a ring resonant unit, a middle layer of SiO2 and a bottom layer of VO2. Based on the electromagnetic field theory, the finite time domain difference (FDTD) method is used to simulate and analyze the optical properties of VO2 in two states. When VO2 is in the insulating state, the metamaterial can achieve a slow light effect with a maximum group delay of 2.85 ps, and when VO2 is in the metallic state, the absorption rate of the metamaterial can reach 88.5% at 0.287 THz and 99.95% at 0.597 THz. We simulate the temperature-controlled phase transition process of VO2 by changing the conductivity of VO2, which can achieve the switching of slow light and absorption functions. In addition, we also found that the material is polarization insensitive. The metamaterial we have designed has some value in the research of terahertz multifunctional devices.

Abstract:Cu2ZnSn(S,Se)4 (CZTSSe) thin films were prepared using the sol-gel method, and the crystal morphology of the CZTSSe films was improved by Mg doping. The prepared films were characterized using techniques such as X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and ultraviolet-visible-near infrared (UV-Vis-NIR) spectroscopy. The results showed that Mg replaced Zn in the CZTSSe lattice, forming the Cu2Zn1-xMgxSn(S,Se)4 (CMZTSSe) phase. As the Mg doping concentration increased, the grain size initially increased and then decreased. After Mg doping, no additional impurities are produced. When the Mg doping concentration was 0.1, the film exhibited the optimal crystal morphology, the narrowest peak width, the largest grain size, the best light absorption properties, the smoothest and most compact surface, which is favorable for use as an absorber layer in solar cells.

Abstract:The stimulated Brillouin scattering (SBS) of heavy germania-doped few-mode fiber (HG-FMF) up to 98 mol% and its dependence on temperature, strain, and bending are studied in this paper, respectively. Two widely located individual Brillouin gain spectra (BGS), whose central peaks are 8.151 GHz and 8.862 GHz, are found in HG-FMF, respectively. These two BGS are generated by the interaction between the fundamental and higher-order acoustic wave modes, which match well with the numerical simulation. The heavy germania-doping induced a large refractive index difference between the core and cladding that strongly confined the optical field in the fiber core and modified its Brillouin parameter. This fact also leads to the bending resistance of HG-FMF, in the bending radii from 0.3 cm to 2 cm, in contrast to traditional FMFs, and suppressed the temperature and strain sensitivity of the two BGS to 213 kHz/℃, 342 kHz/℃ and 20.5 kHz/μɛ, 21.4 kHz/μɛ, respectively. These advances of HG-FMF could be potentially used for bending-resistant distributed multi-parameter sensing.

Abstract:Nonlinear distortions from atmospheric turbulence and scattering critically degrade free-space optical (FSO) communication performance. We propose a hybrid Vol_LSTM framework combining the Volterra model’s nonlinear characterization with long short-term memory (LSTM) temporal dynamic modeling. This synergistic approach adaptively compensates atmospheric-induced distortions through parallel processing of instantaneous nonlinear effects and time-varying channel dynamics. Simulations and experiments demonstrate a 32 dB power budget with 31.53% performance improvement and 21% computational complexity reduction compared to conventional methods, alongside enhanced disturbance resilience. The framework’s dual mechanism of model-based Volterra filtering and data-driven LSTM adaptation provides a practical solution for atmospheric-challenged FSO systems, advancing robust optical communication design.

Abstract:This paper presents a performance enhancement for a 40 Gbit/s intensity-modulated direct-detected (IM/DD) optical orthogonal frequency-division multiplexing (OFDM) system, focusing on minimizing the peak to average power ratio (PAPR) using a selective mapping (SLM) scheme. The analysis evaluates key performance metrics, including launched power, optical signal to noise ratio (OSNR), propagation length, power spectral density (PSD), and bit error rate (BER). The implementation of the SLM technique significantly reduces the PAPR from 10.4 dB to 5.55 dB at a complementary cumulative distribution function (CCDF) of 10-3, achieving a 4.85 dB reduction. These results demonstrate the effectiveness of SLM in mitigating high PAPR in our system, which can improve tolerance to system nonlinearities while maintaining manageable power efficiency. Furthermore, the analysis suggests that additional PAPR improvements are achievable by increasing the number of SLM partition blocks. The SLM method outperforms the partial transmit sequence (PTS) in terms of PAPR reduction, PSD, and BER.

Abstract:Recently the depth estimation methods based on deep learning (DL) retain challenging to estimate a high-precision depth map in fringe projection structured light three-dimensional (3D) measurement with limited information from a single-frame fringe pattern. In this letter, we proposed a FDSUNet++ convolutional neural network (CNN), which consists of a UNet++ base model, an improved squeeze-and-excitation (ISE) block, a Fourier transform (FT) data preprocessing block, and a discrete wavelet transform (DWT) block. The proposed ISE block can improve the ability of feature extraction and the designed FT data preprocessing block preserves the key features of the fringe pattern by FT. The introduced DWT block reduces the complexity and training cost of the model. By integrating these three blocks into the UNet++, it can better achieve depth estimation. Experimental results from two structured light datasets demonstrate that the proposed FDSUNet++ outperforms the state-of-the-art networks, achieving the best performance in both qualitative and quantitative evaluation.

Abstract:To address the difficulty in recognizing subtle differences in facial biomarkers in children with autism, a learnable positional encoding enhancement (LPEE) module was combined with the adaptive token aggregation (ATA) module. The vision transformer with learnable positional encoding and adaptive token aggregation (ViT-LPATA), a predictive model for autism, was proposed. The model leverages the LPEE module to dynamically capture facial geometric deformation features and integrates the ATA module to enhance the feature representation capability of pathological regions, thereby establishing precise mappings of biomarker differences. Experiments on a publicly available autism facial dataset demonstrated that the ViT-LPATA achieved optimal performance, with 99.2% accuracy and an area under the curve (AUC) value of 0.940.