Manufacturing of Infrared Polarization Imaging Optical System Based on Micro-scanning

Division of focal plane polarimeters can obtain transient polarization imaging information, which is a research hotspot in the field of infrared polarization imaging technology. However, there are also some shortcomings. In the measurement process, the instantaneous field of view error will be produced and the image resolution will be reduced. The method of combining micro-scanning technology and infrared polarization imaging technology can make up for the above shortcomings. The focusing lens in the infrared imaging system is used as the micro-scanning lens and fixed on the two-dimensional micro-scanning platform which adopts 2 x 2 mode to implement periodic scanning. Finally, four sequential images of the same scene with one pixel shift can be obtained, to obtain the target's polarized light intensity data in 4 different directions, and then calculate the Stokes parameters. The polarization imaging optical system based on lens micro-scanning requires that the displacement of the micro-scanning lens does not reduce the imaging quality, that is, the tolerances such as the coaxiality, , true position, and scanning displacement of the micro-scanning lens are not sensitive, so higher requirements are proposed for the design and assembly of the optical system.The article first discusses the relevant theoretical basis of polarization based on Stokes vector method , introduces the representation of Stokes vector and the calculation formulas of polarization degree and polarization angle, and then introduces the division of focal plane polarimeters based on micro-scanning technology in detail, including the distance of each displacement, the movement path of the micro-scanning lens. What's more, the infrared imaging system is designed. The catadioptric system is selected as the initial structure, the aspherical secondary mirror is simplified to a plane mirror only for reflecting the light path, and then the aspherical rear lens group is used to correct the aberration. The infrared system has four lenses, and using the last lens as a micro-scanning lens to realize the orthogonal displacement of 2 x 2 mode. The wavelength of the system is 3 similar to 5 mu m, the F-number is 2, the optical field angle is +/- 2 degrees, the focal length is 176 mm and the aperture is not less than phi 40 mm. After completing the optical design, the optical transfer function and spot diagram are performed. The optical transfer function of each field is higher than 0.47@17 lp/mm and the RMS radius of each field is less than 11 mu m. The results show that the optical system meets the requirements for use. Then the tolerance analysis is completed. From the result of the Ment-Karol simulation, the probability of MTF value greater than 0.2@17 lp/mm is over 90%. The influence of the coaxiality, , true position, and scanning displacement of the micro-scanning lens on the imaging quality is also analyzed. The system MTF value changes with the decenter and tilt of the micro-scanning lens in the X and Y directions are provided. The results show that the optical system can still ensure good imaging quality in the range of decenter +/- 200 mu m and tilt +/- 0.4 degrees , and it is not sensitive to tolerances. In addition, structural design is completed. The entire imaging system includes the optical system and the mechanical structure supporting the optical system. Among them, the support frame is made of titanium alloy material to improve rigidity and the rest of the structure is made of aluminum alloy material. The main reflector is the core of the catadioptric optical system, and its surface shape accuracy determines the imaging quality of the system. Therefore, a stress isolation groove is used to realize a flexible connection with the support frame. All mirrors and lenses are machined using a single point diamond turning method. Finally, a polarization imaging experiment is carried out with the developed system, and the results show that infrared polarization imaging images have higher contrast, clearer target contours, and better identification of different materials compared with infrared intensity imaging.