Two methods for identifying branch points from Shack–Hartmann wavefront sensor (SHWFS) measurements were studied: the circulation of phase gradients approach and the beam-spread approach. These approaches were tested using a simple optical-vortex model, with wave-optics simulations, and with experimental data. It was found that these two approaches are synergistic regarding their abilities to detect branch points. Specifically, the beam-spread approach works best when the branch point is located toward the center of the SHWFS’s lenslet pupil, whereas the circulation of phase gradients approach works best when the branch point is located toward the edge of the SHWFS’s lenslet pupil. These behaviors were observed studying the simple optical-vortex model; however, they were further corroborated with the wave-optics and experimental results. The developments presented support researchers studying high scintillation optical-turbulence environments and inform efforts in developing branch-point tolerant reconstruction algorithms.

Optical camera communication (OCC), as a rapidly developing emerging wireless optical imaging communication technology, has broad application prospects due to its low-cost and highly popular optical camera receiver. However, the received images have problems such as perspective distortion and tilt distortion, halo effect, and optical noise, making threshold decision-making in image processing difficult. Implementing an OCC system to improve data rate and transmission distance remains a problem. An image optimization demodulation scheme has been proposed. In the decoding process, the region of interest is first used to extract the image matrix to reduce the impact of halo effects. Detect for distortion in the imaging and correct it. We have designed an improved center/curround retinex image enhancement scheme based on fast scale adaptive bilateral texture filtering, which can efficiently eliminate the fluctuation of grayscale values both globally and locally. Then perform a simple threshold judgment. Use the stripe width of the received pixel for symbol judgment to complete subsequent demodulation and decoding. Obtain the original transmission data. Through experimental analysis, the improved center/surround retinex algorithm proposed in this paper solves the problem of fluctuations in grayscale values. Compared with other algorithms, whether with an increase in communication rate from 3.5 to 6.5 kbps or a change in communication distance, the algorithm proposed in this paper has the highest decoding efficiency, with an average decrease in bit error rate of about one order of magnitude and the best decoding performance.

We develop a phase reconstruction algorithm for the Shack–Hartmann wavefront sensor (SHWFS) that is tolerant to phase discontinuities, such as the ones imposed by shock waves. In practice, this algorithm identifies SHWFS locations where the resultant tilt information is affected by the shock and improves the tilt information in these locations using the local SHWFS observation-plane irradiance patterns. The algorithm was shown to work well over the range of conditions tested with both simulated and experimental data. In turn, the reconstruction algorithm will enable robust wavefront sensing in transonic, supersonic, and hypersonic environments.

We present simulation results of hypothetical satellite systems with fifth-order optical aberrations and various fractal apertures. On the image sensor side, we utilized novel metamaterial based image sensors (metaxels). Real physical systems always have some amount of optical aberrations even though electro-optical systems operating in space are generally idealized as diffraction limited in many simulations. Not only Seidel aberrations but also fifth-order aberrations are important in satellite image analyses and here we illustrate the effects of six aberration groups: spherical, linear coma, elliptical coma, oblique spherical, astigmatism-field curvature, and distortion. Our analyses yield optical diffraction and aberration effects all at once. Any combined form of aberrations could be investigated, which constitute an ample dimension for imaging chain and image quality studies. For a λ/2 aberration coefficient, quantitative comparison was provided, and for various fractal aperture cases image chain simulations are performed. Correlation coefficient values for the listed groups of aberration enabled us to quantify significance order of these fifth-order aberrations. Fifth-order distortion was the most dominant aberration affecting the image quality. In addition, we increased the fractal dimension of apertures and obtained smaller degradation in the images. The study ends with Wiener filter restoration results. Peak signal-to-noise ratio and correlation coefficient values for the restored images were higher than corresponding aperture cases as expected.

In recent years, vision-guided three-dimensional (3D) range-gated imaging has broken through the hardware limitations of traditional methods and brought new ideas to the field of 3D range-gated imaging. However, the existing approaches do not consider the uncertainty caused by incomplete training data, which make accuracy of the existing methods still possible for further improvement. In our work, we extend the well-known Gated2Depth framework using epistemic uncertainty by introducing Bayesian neural networks to provide uncertainty that does not exist in the input data due to incomplete training data. Finally, in the proof experiments, mean absolute error achieved 8.7% improvement on the night data and 9% improvement on the daytime data. The improvement of 3D range-gated imaging accuracy reduced the holes and blurred problems in the depth map and obtained sharper target edges.

Doppler asymmetric spatial heterodyne spectroscopy (DASH) has significant advantages in the passive measurement of wind fields. Wind speed measurement accuracy depends heavily on the phase inversion accuracy of DASH interference fringes. However, the errors in the conventional phase inversion algorithm in the DASH technique make it difficult to obtain the phase of DASH interferometric stripes with high accuracy. The error mechanism of the conventional phase inversion algorithm in the DASH technique is dissected from both theoretical derivation and numerical simulation. It is found that the effect of spectrum leakage on the inversion phase error is greater compared with the window function used for isolating spectrum features. Based on the analysis, an optimization measure by improving the recovered spectrum is proposed and verified by numerical simulation. Comparative simulation results with existing phase inversion algorithms show that our optimized processing method has significant advantages, regardless of whether dealing with single or multiple spectral lines. The research results indicate that our proposed optimization measure can effectively improve the phase inversion accuracy, obtain more accurate wind field data, and contribute to the development of the DASH technique in the field of high accuracy passive measurement of global wind fields for the middle and upper atmosphere.

Measuring the detuning angle distribution within the full aperture of a large-aperture potassium dihydrogen phosphate (KDP) crystal is of great engineering importance for improving its frequency conversion efficiency. To improve the measurement speed of the crystal detuning angle, we propose a measurement method based on a flattened Gaussian beam with a large divergence angle. Theoretical simulation shows that the detuning angle of the KDP crystal is linearly related to the vertical position of the maximum light intensity of the second-harmonic laser when using this method. Experimental results confirm our theoretical analysis and show a relative measurement error of 1.32%, which meet the requirements of engineering applications. This measurement method saves 96% of the measurement time of the detuning angle of a single point within the aperture of the KDP crystal, enabling us to measure more points in a limited time, thus improving the measurement accuracy of the detuning angles over the full aperture of the KDP crystals.

The propagation of a laser beam in an underwater environment gives rise to attenuation caused by absorption and scattering as well as fading caused by turbulence. These effects lead to a decrease in light power and fluctuations in light intensity. To account for variations in water velocity, flow direction, and distance in communication links, field measurements were conducted in rivers and ocean nearshore waters. Based on these measurements, a model for an underwater wireless optical channel was developed. Our study involves the measurement and analysis of light intensity fluctuation, loss of received light power, and the structure constant in various underwater regions. Additionally, the investigation examines the variation of the bit-error rate in relation to different signal-to-noise ratios. The results suggest that the river displays attributes of low turbulence, whereas the ocean’s nearshore waters exhibit a moderate level of turbulence under the specified circumstances. As the velocity of the water increases, the scintillation index of the received optical signal also increases, indicating a higher degree of signal instability. When transmitting horizontally, the scintillation index of the received optical signal remains unaffected by the direction of water flow. Furthermore, as the distance between the transmitter and receiver increases, the scintillation index of the received optical signal intensifies, indicating a greater level of signal instability. The presence of the external interference factors can increase turbulence intensity. Within a depth range of 60 cm underwater, the turbulence intensity at different depths remains basically unchanged.

In this research, a high-power ultraviolet-light-emitting diode was employed as a substitute for a conventional mercury lamp and found to result in a considerable reduction of the lens cementing manufacturing time from 24 h to just a few minutes. The widely recognized VGG19 architecture was employed to effectively classify images depicting cementing defects and discovered to achieve an impressive success rate of 87.26%. The training outcomes were successfully applied in subsequent experiments, which led to a marked decrease in the occurrence of cementing defects from 17.71% to a mere 1.82%. Consequently, the overall efficiency of the entire process was substantially enhanced, ultimately leading to a significant improvement in cementing lens production.

A switching multifunctional metasurface with wideband absorption, cross-polarization conversion, and wideband line-to-circle in the terahertz frequency band is designed based on the phase transition characteristic of vanadium dioxide (VO₂). The simulation results show that when the VO₂ is in the metallic state, the structure has the function of broadband absorption; the absorption rate reaches more than 90% in the range of 3.32 to 8.04 THz, and it has the characteristics of wide-angle incidence and polarization insensitivity. When the VO₂ is in the insulated state, the designed metasurface can realize cross-polarization conversion and broadband line to circular polarization conversion. The linear polarization of 1.85 to 2.76 THz is transformed into cross-polarization, the polarization conversion efficiency is >90 % , and the ellipticity is >90 % for 1.61 to 1.75 THz and 3.06 to 8.37 THz ranges, which can convert line polarization to circular polarization and maintain good efficiency in the polarization conversion function within acceptable incidence and polarization angles. The proposed optical metasurface has great potential in terahertz fields, such as stealth technology, polarization conversion, and radar communication.

The optical modulation format conversion scheme of converting star quadrature amplitude modulation signals to four-stage pulse amplitude modulation signals is theoretically realized by combining phase-sensitive amplification in a silicon-p-toluene sulphonate hybrid waveguide. By optimizing the waveguide structure, a phase mismatch bandwidth from 1310 to 1890 nm and an ultra-high nonlinear coefficient of 6.966 × 10³ m ^{− 1} W ^{− 1} are obtained. Constellation plots, error vector magnitude, and bit error rate (BER) are used to evaluate the conversion scheme. Simulation results show that the scheme not only ensures the integrity of the signal information before and after conversion but also achieves nearly 12 dB performance improvement in the converted signal with a BER threshold of 10 ^{− 3}. It is further shown that the scheme has great potential for applications such as long-short distance interconnections and hierarchical modulation of advanced format signals.

Fractional arrays of vortex beams with different topological charges (TCs) can potentially be used to improve the efficiency of optical communication. However, when the incident light is non-monochromatic, high-order diffraction of vortex beam arrays, which is naturally generated by traditional two-dimensional fork gratings, always superimposes on the useful first order, resulting in the complexity of the subsequent spectrum decoding. Herein, we propose a planar crossed-fork gratings (PCFGs) that can effectively eliminate high-order diffraction. Compared to sinusoidal fork gratings, the PCFGs is a binarized structure and features sinusoidal transmittance, bypassing the need to fabricate a complex three-dimensional structure to achieve sinusoidal transmittance, and therefore its fabrication is compatible with semiconductor processes. Numerical simulations and experimental results consistently show that the PCFGs can effectively suppress second-order and high-order diffractions, retaining only ±1st -order vortex beams array symmetrically distributed around the 0’th order. Moreover, its helical phase structure with multiple TCs has also been experimentally verified.

Metasurface is a two-dimensional form of metamaterial that is ultra-thin in volume and light in weight and capable of controlling the polarization state of electromagnetic waves; nevertheless, it has a very small bandwidth. We offer a terahertz (THz) metamaterial with several transmission-mode-operating dielectric layers that may, over a wide frequency range, convert polarized light. The suggested polarization quarter wave plate (WP) is effective throughout a broad frequency range, including 0.85 to 2.45 THz, with a low insertion loss of 2.57 dB and an ultrabroad bandwidth approaching 1600 GHz when the angle of polarized incidence is 45 deg. Regarding the impacts of shapes, elliptical pillar geometries, and the quantity of dielectric layers, the effectiveness of polarization conversion are also investigated. The proposed WPs are highly suited for use in THz optical systems due to their ultrathin subwavelength thickness. Furthermore, the relationship between the number of layers and bandwidth based on the multi-layer elliptic cylinder results in novel transmission phase properties that increase the likelihood of polarization control of incident waves over a wider bandwidth.

We theoretically analyze the performances of the anti-PT-symmetric gyroscope near the exceptional point (EP). The results show that the frequency splitting caused by rotation is proportional to the square root of the fraction of power coupled from the bus to the ring. To fully exploit the potential of the EP, one needs to overcome two aspects of challenges. The first one is the preparation of sensitive unit, to detect the angular velocity of 10⁴ deg / h with a micron-scale sensor; the allowable variation in the ratio of cavities’ radii is within the order of ^{10 − 16}, which is exceedingly challenging in current nano fabrication. The other is controlling environmental disturbances. A temperature variation of 0.002°C would make the minimum detectable rotation rate up to 10¹⁰ deg / h, which is far beyond practically applications. Our analysis points out the difficulties in realizing this gyroscope, which has significant referential values to future work.

An optical controlled AlGaN/GaN semiconductor power transistor with a low on-resistance normally off high electron mobility two-dimensional electron gas (2DEG) channel is numerically proposed. The device consists of a p-GaN region sandwiched between undoped GaN over which n-AlGaN (Al 20%) is formed. The 2DEG channel is formed at the n-AlGaN/GaN heterojunction due to photogenerated electron jumps from the valance band to the conduction band, when a beam of light having energy greater than the band gap (3.4 eV) of GaN at a wavelength of 350 nm falls on the SiO₂ and TiO₂ Bi-layers antireflecting structure and light penetrates deeper into the p-GaN region and generates e-h pairs. The device current can be optically controlled by varying the power intensity of the incident beam of light; this device exhibits very low (<1 Ω) on-resistance, which yields a very low conduction loss. The switching characteristics of the device are also investigated, and the device attains low rise and fall times.

A compact and broadband polarization beam splitter (PBS) based on silicon (Si) nitride (SiN)-on-Si-on-insulator multilayer platform with vertical asymmetrical directional coupler (ADC) is designed and analyzed. The vertical ADC is consisted of two waveguides, which are placed in the bottom Si layer and upper SiN layer separately. It is found that by properly choosing the values of the structure parameters, especially taking advantages of the extra freedom of the relative location between these two waveguides, the performance of the PBS can be significantly improved. By incorporating tapered structures into the coupling region in the further step, bandwidth of 294 nm, which is determined by the insertion loss of <1 dB and the extinction ratio of >20 dB in both polarizations, is realized within a coupling length as compact as 6.9 μm. The proposed device has a good potential to be applied in three-dimensional photonic integration, where higher integration density or more on-chip functions can be realized.

10 gigabit passive optical network (XGPON) is considered to be an efficient and energy-saving solution for bandwidth resource allocation in mobile fronthaul (MFH), as it significantly reduces delay and jitter caused by signal transmission in the MFH network. In the upstream direction of passive optical networks, optical network units (ONUs) report their load conditions to the optical line terminal (OLT), and based on these conditions, the OLT schedules and allocates network resources using dynamic bandwidth allocation (DBA) algorithm. However, DBA based on the request/grant mechanism fails to meet the low delay requirements of 5G networks. Therefore, we propose a machine learning-based DBA algorithm that utilizes extreme learning machine (ELM) to predict the future load conditions of ONUs. ELM is widely popular in machine learning due to its fast learning speed and excellent generalization performance. It achieves this by randomly initializing the weights and biases of hidden layer neurons and then solving the output weights directly using linear equations. According to simulation results, the proposed ELM-DBA algorithm exhibits outstanding performance in terms of delay and jitter. Specifically, the algorithm achieves low delay of 125 μs, meeting the 3GPP requirements for delay below 250 μs, and the jitter remains below 80 μs consistently. Furthermore, ELM demonstrates exceptional learning speed, with training speed improved by at least 600 times compared to gate recurrent unit and long short term memory.

Erratum corrects a value in Table 1.