Acta Photonica Sinica, Volume. 53, Issue 2, 0223001(2024)

Boundary Layer Effect Simulation Based on Liquid Crystal Spatial Light Modulator

Gang JING1, Zhi LIU2、*, Peng LIN2, Shutong LIU3, and Qingfang JIANG3
Author Affiliations
  • 1School of Optoelectronic Engineering,Changchun University of Science and Technology,Changchun 130022,China
  • 2National and Local Joint Engineering Research Center of Space Optoelectronic Technology,Changchun University of Science and Technology,Changchun 130022,China
  • 3School of Electronic Information Engineering,Changchun University of Science and Technology,Changchun 130022,China
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    With the advancement of flight conditions for aerospace platforms, the impact of the boundary layer effect on space laser communication transmission is also growing. In order to reduce the influence of the boundary layer effect, corresponding correction methods must be studied. To provide verification conditions for the boundary layer effect correction system of laser communication on aerospace platforms, a simulator based on the Liquid Crystal Spatial Light Modulator (LC-SLM) was designed to simulate the boundary layer effect. This article first analyzes it from the perspective of geometric optics. It is assumed that the boundary layer is a thin layer (the inner and outer diameters are equal). The refractive index of the free flow is different from the refractive index of the boundary layer. Substituting them into the focal length formula of the lens makes it equivalent to a negative lens. Under different flight conditions, the focal length of the equivalent lens is different. Then use computer software to perform numerical analysis. Setting the wall curvature radius of the aerodynamic platform as 190 mm and the flight Mach number range as 0~5 Ma, in the case of troposphere (0~11 km), lower stratosphere (11~20 km), and upper stratosphere (20~32 km), the relationship curves are obtained between the Mach number, flight altitude, and the equivalent focal length of the negative lens representing the boundary layer effect. Research shows that as the Mach number increases, the equivalent focal length of the negative lens decreases, indicating a greater impact on the communication beam. Especially when the Mach number is small, the equivalent lens focal length changes more significantly with the Mach number. After that, the changes stabilized. In terms of flight altitude, the higher the flight altitude, the thinner the air and the lower the temperature, the boundary layer effect decreases accordingly until the equivalent lens focal length value approaches infinity, at which point it can basically be ignored.Based on the above analysis, the following is an introduction to the boundary layer simulation equipment. The core device of this simulator is the LC-SLM. The phase modulation grayscale image is obtained from the phase distribution function of the lens. The grayscale information controls the applied voltage of the LC-SLM, thereby controlling the deflection of the liquid crystal molecules in the LC-SLM, thereby affecting the response to light phase modulation. Load phase grayscale images of different focal lengths into the LC-SLM to realize the function of the zoom lens, and change the focal length of the simulated zoom lens to simulate the boundary layer effect under different flight conditions of the aviation platform. Due to the LC-SLM modulation depth is not an ideal 2π, and the relationship between the modulation depth and the loaded gray value is not an ideal mapping, the designed focal length value of the simulated zoom lens is inconsistent with the actual focal length value. Therefore, it is necessary to design and build an experimental platform to verify the accuracy of its simulation. This experiment uses a 1 550 nm laser as the light source, and emits parallel light after passing through the collimated beam expansion system. Since LC-SLM can only modulate linearly polarized light, a polarizer is added in front, and then enters the LC-SLM. The phase grayscale images of the lens focal lengths corresponding to the ten types of boundary layer effects in flight states are loaded into the LC-SLM respectively, and finally an infrared camera is used to receive and collect light spots at a close distance. There is a lot of noise in the original spot image, which will affect the accuracy of the spot size analysis. Therefore, the obtained original spot image is sequentially subjected to brightness adjustment, median filtering, threshold segmentation, morphological processing and edge coordinate extraction, and Hough transform circle fitting is used to obtain the spot centroid position and spot radius. Due to limitations of computer memory and computing power, the calculation accuracy is 0.05 pixel values. Then make an error line chart between the actual spot radius value of these ten sets of data and the theoretically calculated spot radius value. The calculated root mean square error is 0.043 75. This experiment verified the feasibility and effectiveness of the proposed boundary layer simulation method.


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    Gang JING, Zhi LIU, Peng LIN, Shutong LIU, Qingfang JIANG. Boundary Layer Effect Simulation Based on Liquid Crystal Spatial Light Modulator[J]. Acta Photonica Sinica, 2024, 53(2): 0223001

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    Paper Information

    Category: Optical Device

    Received: Jul. 10, 2023

    Accepted: Aug. 31, 2023

    Published Online: Mar. 28, 2024

    The Author Email: LIU Zhi (