An external cavity tunable semiconductor laser is designed, which is based on a traditional Littman-Metcalf structure. The semiconductor laser with a wide gain spectrum is served as the seed source of the internal cavity. Meanwhile, the external cavity feedback unit is composed of a blazed grating and a tuned mirror. The experimental results of no mode-hopping, high precision such as ±1.38 pm, high side-mode suppression ratio such as 84.34 dB and high stability such as power ±0.008 83 dB/1h, wavelength ±0.25 pm/1h, with tunable wavelength range of 1 479.96~1 599.96 nm are realized. The effective cavity length of the designed external cavity tunable semiconductor laser is about 50 mm, the maximum output power is 15.19 dBm, and the minimum output spectral linewidth is 18.89 kHz. The research is conductive to promoting the research of new ultra-high resolution spectral analysis systems and providing autonomous and controllable laser signal generation source.

The traditional photoelectric detection equipment can only obtain the intensity information of the target, while the polarization detection technology can increase the information dimension of the detection by obtaining the polarization information, significantly enhance the contrast between the target and the background, and solve the problem of target detection under complex background. At first, the technical connotation and development history of polarization detection is briefly introduced. And then, the application of polarization detection technology in target detection is introduced in detail. At last, the development trend of polarization detection technology in the future is prospected.

Infrared polarization imaging technology is a new detection technology which combines two dimensions of infrared imaging and polarization imaging. IR polarization imaging simulation technology can generate a large amount of data to train IR polarization imaging system, which can effectively improve the system platform performance, shorten the development time and reduce the investment cost. In recent years, scholars at home and abroad have carried out a lot of research work on infrared polarization imaging simulation, bi-directional reflection distribution function model and infrared polarization properties of target materials. The research background, basic principles and development history of infrared polarization imaging simulation technology for target scenes at home and abroad are introduced, and the application value and development trend of this key technology are outlooked.

All-solid-state single-frequency continuous-wave (CW) tunable titanium sapphire (Ti:S) lasers act as the important source in atomic physics, quantum information, Raman spectroscopy and other research fields owing to its advantages of narrow linewidth, perfect beam quality, high stability, wide wavelength tuning range of 650~1 030 nm, which can cover the transition lines of many kinds of atoms. Especially, with the development of precision measurement, quantum simulation and other new research fields in recent years, researchers have focused not only on wide tuning range, but also on conversion efficiency, output power, continuous tuning capability, stability and noise properties of titanium sapphire laser. The important research progress of all-solid-state single-frequency CW Ti:sapphire lasers in recent years are summarized.

In severe weather conditions, the scattering particles in the environment can significantly degrade of the image obtained by imaging equipments, which reduces the application value of the image greatly. Polarization imaging technology can obtain the multi-dimensional polarization information of the object as well as the intensity information of the object. By restoring this multi-dimensional information, the image details and visibility can be improved. A time-sharing polarization imaging system is developed in which the polarizer is rotated by a gear driven by a motor. The upper computer controls the rotation angle of the polarizer to obtain the polarization image of any wave band. At the same time, the obtained images are processed by using the polarization imaging algorithm. Since the polarization imaging algorithm causes obvious difference of light and shade in different positions of the reconstructed image, the adaptive image enhancement algorithm is used to reconstruct part of the overprocessed image, so that the overall contrast of the image is balanced, and it can provide more detailed information ability, which has important practical application value.

Polarization detection technology is a scene multi-dimensional optical information acquisition technology. It can be used in the detection of small targets, and can overcome the shortcomings of traditional optical detection technology that are difficult to detect in low-light scenes and harsh weather conditions. However, the target size in the polarized image is small and the response is weak, and the distribution characteristics of background clutter and image noise are different from the intensity image, which are the problems that the detection algorithm needs to solve. At first, the principle of polarization imaging is analyzed, and the characteristics of polarization image and traditional intensity image are compared. And then, the intensity image and polarization image are fused and enhanced. At last, according to the local contrast saliency theory, a double-neighborhood gradient based on polarization fusion image target detection algorithm is proposed. Experimental results show that compared with the traditional algorithm, the algorithm makes full use of the characteristics of polarized images, and can more effectively detect small targets.

Underwater imaging technology has great research value in the field of underwater target detection and recognition, but the traditional underwater polarization imaging method often cannot meet the imaging quality and real-time performance simultaneously. In order to solve the problems of large amount of data and long processing time caused by traditional underwater polarization descattering imaging, a fast and real-time descattering and sharpness polarization imaging method for underwater targets is proposed. This method makes use of the difference between the polarization degree of the background light and the polarization degree of the target light, which is used to obtain the background scattered light area adaptively and quickly. At the same time, it continues the assumption that the overall polarization degree of the target is consistent. By adjusting the dynamic range of the image gray level, the weighted histogram homogenization method is used to weight the gray levels of different frequencies, so as to obtain an image that is more in line with the visual effect of human eyes. Experimental results show that the fast underwater polarization descattering technology can improve the imaging quality and effectively increase the processing speed, so it has a good application prospect in underwater work.

In order to better monitor the underwater environment and seafood in marine pastures， an underwater polarization imaging system is integrated by combining a 532 nm laser light source with wave sheet modulation， and the polarization image is processed by Miller matrix method. The experiment takes sea cucumber as the target and simulates the water environment of Marine pasture. After obtaining the polarization image through the imaging system， image fusion is performed according to the derived Miller matrix formula. By comparing the contrast and information entropy between the fused Miller matrix image and the original image， it is proved that the Miller matrix method can effectively eliminate the influence of background scattered light to improve the imaging contrast and quality.

Underwater polarization imaging technology can enhance the contrast of underwater images. But due to the limitation of detector′s dynamic range, it is difficult to obtain accurate polarization information, which decreases the final enhancement results. An underwater image denoising method based on three images with arbitrary polarization analyzing angles is proposed. The enhancement results with different polarization analyzing angles are studied. The results showed that for active linear polarization illumination, it′s better to choose three angles near the orthogonal polarization direction of the incident light. For non-polarized light illumination, it′s better to choose three angles near 45° away from the polarization angle of backscattered light. The results can support the underwater active polarization imaging enhancement technology in highly turbid media, which may expand the application of underwater active polarization imaging technology.

A high resolution Stokes spectrum reconstruction method for channeled spectropolarimetry is proposed. A forward model matrix is constructed according to the principle of the interferometer. The elements are arranged using the phase terms of the two retarders and a solving matrix is obtained. The polarization spectra are recovered by the interferogram and the solving matrix using the least squares method. In addition, the forward model matrix can be replaced by a random matrix for a wider application. The new method does not separate the spectrum by windowing, avoiding the aliasing among channels.

Due to the complex and variable underwater environment, turbulent flow, camera shake, occlusion of suspended particles and light absorption and propagation will lead to the problems of motion blur, colour distortion and low contrast of the underwater video. To solve these problems, an improved underwater video super-resolution algorithm based on BasicVSR is proposed to enhance the details in reconstructed images, and at the same time improve the phenomenon of blue and green underwater image. At first, a convolutional neural network is used to fit the parameters of the underwater image degradation model and obtain the underwater image features. And then, the optical flow values between consecutive frames are computed using the low-resolution video frames as input. At last, the optical flow information is used for bidirectional propagation on the feature maps to reconstruct each frame. Experimental results show that the underwater images processed by the algorithm exhibit better conformity to human visual characteristics and significantly improve image quality evaluation metrics compared with other algorithms, which better meet the requirements of underwater video quality for underwater advanced visual tasks.