The extremely low quantum noise characteristics of masers play an important role as ultra-low noise preamplifiers and microwave clocks in fields such as wireless communications, deep space exploration, navigation, and time-frequency measurements. A detailed overview of the fundamental principles and developmental trends of masers is provided. Building upon the research progress of typical cryogenic ruby solid-state masers, it summarizes their advantages in low-noise microwave amplification, high-stability microwave oscillation, and ultra-sensitive nuclear magnetic resonance. To address the challenge of the traditional solid-state masers that can only operate in cryogenic environments, the novel maser formation mechanisms and gain media have been explored. It reviews recent domestic and international research efforts aimed at achieving the operation of solid-state masers at room temperature and discusses the research difficulties and future development trends of room-temperature solid-state maser.

The exploration of strong terahertz radiation sources plays a crucial role in the development of terahertz technology. Femtosecond laser-induced strong terahertz emission in a liquid line of n-octanol solution is experimentally confirmed using an alcohol solution as a terahertz excitation medium. Under the same laser excitation conditions, the terahertz radiation generated in the n-octanol solution is 3.5 times stronger than in liquid water. The terahertz emission angle in n-octanol solution is investigated, and it is shown that the intensity of the terahertz radiation generated in n-octanol solution is maximum at a detection angle of 90°, while the terahertz emission angle ranges from 45° to 135°. In addition, quantum chemical calculations on alcohols are performed to explain the physical mechanism of strong terahertz generation in alcohol solutions. The results show that the terahertz radiation generated in alcohol solutions is negatively correlated with its band gap, surface electrostatic potential, dielectric constant, and MPI polarity index.

The expression of misalignment aberration of off-axis reverse system is derived, and an efficient off-axis reverse system assembly and adjustment method is proposed, based on the vector aberration theory. Through the analysis, it is found that the off-axis system will not introduce new forms of aberration, but due to the aperture factor, compared with the coaxial system, several primary aberrations will be scaled, and the high order aperture aberration will be partially transformed into low order aperture aberration. The misalignment aberration expression is verified by the simulation of an off-axis-two-reverse system, using Zemax. The misalignment in the simulated alignment reaches the order of 10-4mm. Finally, use the method to guide the assembly and adjustment of the actual off-axis-two-reverse system beam expansion system. The primary aberration reaches the order of 10-4(=632.8nm), and the RMS value of the system reaches 0.04, which verifies the accuracy of the method.

In order to obtain a mirror with low mass, high stiffness and high surface accuracy, a topology optimization method with minimum compliance as optimization objective and mirror weight as constraint condition is proposed. Firstly, an optimization model including stiffness optimization model, mass minimization model and sensitivity analysis is constructed. Secondly, on the basis of this theoretical model, the optimization design of 450mm×290mm×60mm long strip SiC mirror is carried out. Finally,the simulation analysis of the structure optimized model is carried out, and the model is compared with the traditional design scheme. The results show that its performance is better than the traditional design scheme. This study provides an effective design method for the subsequent design of the same type mirrors.

Liquid lens has many advantages such as rapid zoom, low power consumption, low cost, etc, which determines its wide application in UAV, endoscope and other fields. The classification and working principle of liquid lens are introduced, the Gaussian theoretical calculation of liquid lens focusing is carried out, and the relationship formula between the focusing distance and the focal length of liquid lens is derived, too. A 110° objective optical system with liquid lens is designed, and the detector of the system is a 1/4 inch CMOS chip, and aberration of the optical system can meet the application requirements. Finally, experimental verification has been conducted on the physical object of this system and the results successfully meet the established design indices, as well as specified requirements for utilization in micro-lens applications.

The temperature -stress characteristics of passive devices (ion exchange planar optical waveguides) is investigated employing the self imaging theory and numerical simulation methods with the 1×5 PLC-MMI optical power splitterconsidered as an example The dependences of the insertion loss and additional loss of each port of the device on the length and refractive index change of multimode waveguide region when the device operates at 850 nm are studied. The waveguide deformation and index change due to the environmental temperature and stress are analyzed, and consequently, the splitting ratio of the MMI splitter is discussed due to the optical fiber-waveguide coupling loss and birefringence. All the results indicate that the developed 1×5 PLC-MMI optical power splitter demonstrates stable performance with excellent immunization to the influence of environment and stress.

It's difficult for all-time star sensor to avoid being affected by the atmosphere when conducting observation missions in the atmosphere. In order to conduct in-depth research on the reasons that make it difficult for all-time star sensor to observe stars during the day, it is necessary to analyze the atmospheric impact on the all-day star sensors during the imaging process. Utilizing MODTRAN software for simulating atmospheric interference, a comprehensive study was conducted on the atmospheric effects under different observational conditions, resulting in the development of a computational model for the atmospheric impact on all-time star sensor imaging. The research results show that selecting an appropriate spectral range is beneficial for daytime star observation tasks of all day star sensors. Based on the blackbody radiation law, constellations, and stellar spectral types, the calculation of stellar radiation has been revised, and a stellar target spectral radiation calculation model has been established. Finally, using the obtained atmospheric impact calculation model and the modified stellar target spectral radiation calculation model, combined with relevant detection parameters, the simulation signal-to-noise ratio and simulation star map are calculated. The correctness of the proposed model was verified by comparing the simulation results with actual experimental results.

The wide-angle and wide exit pupil of the display optics in near-eye display systems present a trade-off relationship. An optical design scheme for achieving both wide-angle and large exit-pupil sizes in near-eye display systems is proposed and discussed. For the projector design, several aspherical lenses are co-designed to improve the imaging quality and to reduce the distortion, where the exit pupils in YZ and XZ plane are separated to expand the exit pupil in one direction and increase the optical length from the projector to the exit pupil. To validate this optimization design method, a near-eye display system with full-color, a field of view of 53.6°, an exit pupil distance of 30mm, and an exit pupil diameter of 8mm is achieved. The optimized design method of the near-eye display optics provides a technical reference for the realization of wide-field near-eye display systems.

Abbe number is an important fundamental parameter in the field of optical design. Compared to solids, data of liquid Abbe number is relatively scarce. With the development of liquid optical systems, accurate measurement of liquid Abbe number is becoming increasingly important. The commonly used methods for measuring the Abbe number of transparent liquids have drawbacks such as cumbersome operation, low measurement efficiency, and unsuitable for measuring toxic and volatile liquids. Therefore, a method for measuring the Abbe number of transparent liquids based on the defocusing width of a composite liquid-core cylindrical lens is proposed. During the experiment, it is only need to rotate the coaxial filter wheel to replace the filters for red, yellow, and cyan light, and then CCD can capture the defocused images under three different light sources quickly. Based on the good linear relationship between the defocused image width and the refractive index of the liquid, the refractive index of the transparent liquid at three wavelengths can be obtained, and the Abbe number can be calculated. This method combines the advantages of simple experimental setup, easy operation, and fast measurement, which can be used to quickly expand the basic data of transparent liquids Abbe number.

Camera calibration is a crucial aspect in structured light three-dimensional reconstruction systems, and circular calibration boards are widely employed for camera calibration. During the actual calibration process, the circular calibration images are transformed into ellipses due to the perspective effect of the camera. Additionally, the blurring of edge pixels of circular markers affects the precision of camera calibration. Therefore, a camera calibration method based on subpixel edge detection and perspective projection is proposed. Firstly, the Canny operator with adaptive thresholding is employed to extract the edges of the circular features on the calibration board. Sub-pixel refinement is achieved through interpolation based on the relationships among the edge pixel characteristics, leading to the determination of the ellipse's center. Subsequently, the ellipse feature points are projected into an approximately standard circle using perspective transformation. After extracting the coordinates of the circle center, the points are inversely projected back to the original image. Finally, the Zhang's method is employed to calibrate the camera based on the coordinates of the feature points in the actual images.Experimental results demonstrate that the proposed method reduces the reprojection error by 43.8% compared to traditional methods. The root mean square error for the three-dimensional measurements of standard blocks is minimized at 0.1316mm, and the measurement error ranges between 0.1mm and 0.5mm, meeting the requirements for industrial measurements.

Indoor target positioning is a crucial technique in the intelligent logistics system, visible light positioning technique becomes a feasible solution in the intelligent logistics center because it does not need extra communication equipments. However, the reflection of walls would lead to accuracy reduction of indoor visible light positioning, in view of this problem, a hybrid neural networks based indoor visible light positioning method is proposed. Firstly, the gated recurrent unit is used to capture the dependency of the optical power value in the optical power vectors, the one dimensional convolutional layer is used to extract the local spatial features of the optical power vectors; then, these deep features are fused to enhance the ability of feature learning for the optical power vectors, so as to improve the accuracy of indoor light positioning. Simulation results show that, compared to the other neural network based indoor visible light positioning methods, the positioning error of the proposed method is lower, and the time efficiency is within a reasonable range.

The Pseudo-LiDAR algorithm is one of the general algorithms for 3D object detection with stereo cameras. Because of its low-cost stereo vision scheme and excellent adaptability, the Pseudo-LiDAR algorithm is widely used in the fields of autonomous driving, robots and so on. However, the algorithm will produce too many distortion points in the prediction of the edge of the object, resulting in a large estimation error of the transition region between the object and the background. The depth estimation network adopts a four-branch structure, and the large amount of calculation affects the running speed. To solve the above problems, the Spatial Pyramid Pooling (SPP) four-branch structure within the Pseudo-LiDAR algorithm has been optimized to a single-branch structure to improve the running speed. Innovatively construct a Gaussian confidence filtering module to filter out data with large errors in the transition region; PointVoxel-RCNN (PV-RCNN) is introduced as a 3D object detection network in the Pseudo-LiDAR algorithm to improve the detection accuracy. The experimental results on the KITTI dataset show that compared with the original algorithm, the average accuracy of 3D detection of car objects under simple, medium and difficult levels is improved by 22.36%, 26.48% and 28.55%, respectively

The phase measuring deflectometry fails to measure the double surfaces of transparent objects, because the fringes reflected from both the front and rear surfaces are superimposed in the captured images. Therefore, accurately extracting the phase from the coupled fringe patterns is considered as one of the main problems in PMD. A fringe decoupling method based on continuous wavelet transform is proposed to eliminate parasitic reflections. It proposes a method based on modular maximum and image segmentation, which extracts the best multi-region ridge points in time-frequency domain and obtain the principal components of different wavelet ridges in amplitude spectrum. Then the fringe phase of the corresponding position of the wavelet ridge is extracted. The proposed method successfully attains a curvature radius mean value of 0.038mm for 135.800~515.090mm radius of curvature lens in the experiment. The root mean square of the surface shape error is 1.4m. The proposed algorithm can eliminate parasitic reflections successfully which eventually realizes high-precision measurement of transparent objects.

The absolute liquid surface inspection uses a liquid metal plane as the basis for calibrating the flatness of optical plane crystal. The high reflectivity of liquid metal leads to an inability to match light intensity, making it challenging to obtain high-quality interference images. To address this issue, a method of attenuating light intensity by using a metal mesh has been adopted. This method reduces the light intensity by exploiting the diffraction effect of the metal mesh and achieve intensity matching. Further analysis on the selection of the mesh number of the metal mesh and how to suppress the introduced error has also been conducted. Derive the relationship between zero-order diffraction efficiency and the number of sieve mesh, and verify it experimentally in a Zygo GPI 4-inch interferometer. The results show that the selection range for the mesh count of the screen is between 120 and 200. In the experiment, a 150-mesh sieve was selected, yielding high-quality interference patterns. An uneven distribution of the metal screen mesh can introduce wavefront errors. In the simulation process, a random wire error in a fixed direction was given, the resulting wavefront errors are distributed in a grid shape. These are mid-to-high frequency errors that can be suppressed using low-pass filtering. The liquid metal calibration experiment was carried out with a 300mm aperture Fizeau interferometer. The face shape is consistent with the static liquid face shape distribution, the PV value deviation is 0.007, and the RMS value deviation is 0.0036. It is compared with the measurement results of water and 0.65Cst silicon oil as the liquid surface, and the three face shape distributions are basically the same. The experiment proves that the use of a metal screen can effectively solve the problem of high reflectivity of liquid metal, and the introduced error can be suppressed by using a low-pass filter.

To improving the accuracy of Brillouin Optical Time Domain Reflectometer (BOTDR) in long-distance temperature sensing, the detection optical power optimization and wavelet threshold denoising for long-distance BOTDR temperature sensing are proposed. By analyzing the SNRE at the end of the timing curve under the intrinsic Brillouin frequency shift, the detection optical power optimization of BOTDR is achieved, while combining wavelet threshold denoising algorithm to improve the temperature measurement accuracy. The experimental results demonstrate that the range of Brillouin frequency shift fluctuation is decreased from 22MHz to 10MHz in the temperature region by the wavelet threshold denoising. Moreover, the temperature fluctuation of end temperature region is between ±5.3℃. In repeated measurement, the Root Mean Square Error (RMSE) of temperature fluctuation is less than 1.68℃ and the RMSE of localization is less than 1.78m. The detection optimal power and Brillouin frequency shift accuracy at the end of the fiber were explored under different spatial resolutions. This research results indicate that the accuracy of long-range BOTDR systems can be improved by wavelet threshold noise reduction combined with detection optical power optimization.

To address irregular contours and border ambiguity in skeletal fluorosis X-ray images, A segmentation method that combines dynamic snake convolution and graph convolution attention is proposed. The omni-dimensional dynamic snake convolution module is introduced in the encoder to compensate for the lack of multi-scale feature perception in single convolutions, reducing feature loss. The graph convolution attention module in the decoder captures long-range dependencies and focuses on key lesion areas through channel attention. Additionally, the ghost convolution block enhances performance and reduces parameters. By eliminating the skip connection with the first two layers of features in the U-Net decoding structure, generalization performance improves. Validation on a skeletal fluorosis X-ray image segmentation dataset shows DSC 0.77, IoU 0.64, Accuracy 0.85, and Recall 0.75. Experimental results demonstrate the method's superiority in segmenting skeletal fluorosis small osteogenic lesions compared to current mainstream medical image segmentation methods.

Oral health detection is closely related to people's lives.Existing methods for oral health examination have limitations such as X-ray radiation and the need for professional operation, making it difficult to apply to large-scale population screening.With the advancement of optical technologies, techniques like near-infrared imaging can more easily and accurately detect oral health conditions. It summarizes the detection of dental caries and demineralization using near-infrared light and optical coherence omography (OCT), demineralization and dental plaque detection using fluorescence, research progress on demineralization using Raman spectroscopy, and the application of multispectral endoscopy in dentistry.It discusses the advantages and disadvantages of infrared, OCT, fluorescence, Raman, and endoscopy in oral health detection, as well as providing an outlook on the development of optical instruments for oral examination.