Beam Stabilization Research On The SPGD Adaptive Optics Beam Cleanup System

The stochastic parallel gradient descent (SPGD) adaptive optical (AO) system which uses power in bucket (PIB) as the metric of beam quality have found instability problems for the measurement of the metric. Beam direction stabilizing technology is studied in this thesis in order to use the SPGD AO efficiently in a beam cleanup system.According to the requirements of the practical applications and the jitter characteristics of the laser beam direction, a control system structure is first designed for the beam direction stabilization, and the control performance metrics are determined. In the hardware aspect of the control system, the basic configurations, operating principles and selection requirements is briefly discussed for the PSD detector, A/D acquisition card, D/A card and tip/tilt mirror. The using method of D/A and A/D cards in VC++ 6.0 development environment are studied, and the reason for which the output performance of the D/A port of NI-6229 is analyzed.The digital controller is the core of the beam stabilization system. The basic principle of a PID digital controller is first introduced, and the shaping and confirming method of its parameters is discussed. Each parameter value of the digital controller is then obtained, which provides the theoretical basis for the analysis of the system performance and lays the technical ground for later experimental verification.With respect to the open-loop band-width limit of the high-speed tip/tilt mirror, an experiment is designed to measure its frequency response curve, its exact transfer function is then obtained by curve fitting and a digital compensation controller is finally designed to boost the open-loop bandwidth of the tip/tilt mirror. Simulation results show that the open-loop band-width of the tip/tilt mirror is efficiently improved after the application of the digital compensation controller.At last, an experimental system of laser beam stabilization is set up. The closed-loop results show that the system can correct the static deviation as well as dynamic jittering of the beam direction, which verifies the feasibility of the system design. The correction accuracy of the static beam direction deviation is 0.15 arcsecond, and the control bandwidth for dynamic beam direction jittering reaches about 20Hz, which satisfies the design requirements and provides good foundation for later SPGD AO beam cleanup experiments.