Design and Fabrication of Asymmetric Microstructure for Stray Light Suppression in Optical System

Objective This paper proposes a novel asymmetric microstructure different from the existing light- trapping structures. By reducing the specular reflection, the microstructure can improve the stray light suppression performance at a small angle of incidence. The microstructure can be installed in the internal structure of the optical system to effectively reduce the mass and size of the optical system and improve the stray light suppression performance of hoodless optical systems. Methods Comparing the theoretical stray light suppression performance of microstructures with different angles between the front reflective surface and the baseline of the substrate surface, this paper designs asymmetric microstructures whose angle between the front and back reflective surfaces is 90 degrees and angle beta between the front reflective surface and the baseline of the substrate surface is smaller than 45 degrees. To fabricate the asymmetrical microstructures, this paper also proposes a laser galvanometer processing system for tilting machining. Subsequently, the intensity distribution of the focused laser is obtained by drawing on the research on the action range of the focused light spot under different tilt angles and applying the phase recovery technique (Fig. 4). When the tilt angle of the laser is 60 degrees, the intensity distribution of the focused light spot is in a shape similar to that of the microstructure shown in Fig. 3. Then, a new high-speed laser processing platform is designed and utilized to process the surface of aluminum alloy samples. The three-dimensional morphology of the processed sample surfaces is measured by confocal laser scanning microscopy (CLSM). The formation mechanism of the microstructure surface under different scanning velocities is preliminarily investigated, and the appropriate processing parameters are obtained. Furthermore, the specular reflection test experiment and the integrated simulation experiment are designed to evaluate the performance of the samples. Results and Discussions The investigation of the surface morphology of the microstructures processed at different scanning velocities shows that when the processing scanning velocity is 1600 mm/s, the average angle between the front and back reflective surfaces of the microstructure is 93. 5 degrees, which is close to the designed angle of 90 degrees shown in Fig. 6(d). In the specular reflection test experiment, the ability of the microstructure to suppress specular reflection is verified [Fig. 8(d)]. Then, in the integrated simulation experiment simulating the influence of off-axis collimated stray light on the optical system, the angle of incidence is set to 15 degrees, and the illumination light source is 650 nm laser. The relative reflectivity of the microstructure surface is 10% that of the conventional anodized surface. Only visible light sources (520 nm and 650 nm) are used as test light sources in this paper, and the performance of the proposed microstructure in the infrared wavelength range will be tested in the follow-up research. In addition, the processing parameters will be further optimized, and the mechanism of tilting laser on the formation of the microstructure will be investigated to improve the manufacturing accuracy of the microstructure and thereby improve the stray light suppression performance of the microstructure surface. Conclusions A novel microstructure with asymmetric characteristics is designed. In this microstructure, multiple reflective surfaces are periodically arranged on the substrate surface. The off-axis stray light is suppressed by increasing the reflection angle of the stray light and changing the reflection direction. The angle of incidence is set to 15 degrees, and the illumination light source is 650 nm laser. The stray light suppression performance of the microstructure is 10 times higher than that of the conventional anodized surface, and its relative reflectivity is only 0. 008%. No light-absorbing coating is added to the surface of the tested microstructure sample. It is believed that a microstructure surface with better performance can be obtained by adding a light-absorbing coating to the surface.