Wavefront aberration, which caused by atmospheric turbulence, needs to be measured in the free space optical communication. The existing sensors of wavefront aberration measurement are mainly divided into two classes, wavefront sensors and image-based sensors. Wavefront sensors , such as Hartmann sensor and shearing interferometry, measure wavefront slope to calculate wavefront aberration. However, wavefront sensors always need most of the laser energy, which means it is hard to use wavefront sensors in free space optical communication in the daytime. Image-based sensors usually requires iteration, which means poor real-time and locally optimal solution. No existing method can measure wavefront aberrations in real time in free space optical communication in the daytime. In this article, a new method of measuring wavefront aberration with CNN is proposed, which can be used in free space optical communication in the daytime and have good real-time performance. We made some modifications in VGG to make it can be used to fitting the Zernike coefficients. The input to the network was the PSF of focal plane and defocus plane and the output was the initial estimate of the Zernike coefficients. 22000 pairs of images were collected in the experiment, which produced by liquid crystal and the wavefront was built by 64 Zernike coefficients when atmospheric coherent length(r0) is 5cm. 20000 pairs of images were used as training sets and the other were used as testing sets. The root-mean-square(RMS) wavefront errors of VGG is on average within 0.0487 waves and the time it needs is 11-12ms. We use RMS wavefront error less than 0.1 waves as the correct standard and the correct rate is 98.75% , while other RMS wavefront errors were properly close to 0.1 waves.
Photoelectric shaft encoder is a kind of high-precision angle measuring device ,its core angle measuring component is encoder disc. In this paper, a single-ring absolute coding method combining Huffman coding is proposed. The Huffman coding algorithm commonly used for teletext compression transmission is applied to the single-ring photoelectric axis encoder to realize the new single-ring coding method of the coder disc. The method constructs a special Huffman binary tree and designs a special traversal sequence, which is traversed from top to bottom in this order. The code obtained at the leaf node is the shift sequence code required by the code disc, which can be used to engrave a single-circle absolute code disc. This method can improve the coding speed and reduce the complexity of the algorithm under the premise of high-order coding. It provides technical support for the research of small high-resolution photoelectric shaft encoder and accelerates its development speed.
In the liquid crystal spatial light modulator, creating overdriving look-up table is a cumbersome process. A large number of experiments are needed to measure the driving time between different target phases and the final phase, and the overdrive look-up table occupies excessive memory space that slow down the response speed of the liquid crystal. Aiming at the problems above, an establishment method of overdriving look-up table is put forward. The method uses PWM driving and curve fitting, only two response time curves measured will complete the calculation of the entire drive table, which solves the problem of the complicated experiment of overdriving table construction. Finally, choosing the appropriate quantization wavelength to optimize the overdriving table, effectively improving the accuracy of the drive table and the maintainability and scalability of the system, and it is more suitable for engineering applications.
ATP(Acquisition, pointing and tracking)technology is one of the key technologies and an important guarantee for establishing reliable links in the field of space laser communication. Liquid crystal phased array( LCOPA)is generally used as the core phase shifter for laser beam precise deflection. The classical precise deflection control method can only realize sparse and uneven concentric circle distribution in the deflection range, and the diffraction efficiency decreases seriously with the increase of deflection angles. Therefore, we propose a liquid crystal beam control method based on planar phased array radar model. The beam direction is controlled by changing phase difference between the electrodes. This method can realize two dimensional continuous beam deflection and uniform distribution. The deflection precision are analyzed and the theoretical simulation and experimental analysis are carried out. The results show that the precision is less than 14 μrad of x direction and 12 μrad of y direction within the range of 2.1 mrad beam deflection. Finally, the effect of rotation angle on two-dimensional deflection precision is analyzed.
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