Study On Adaptive Polarization Control Technology Of Fiber Amplifier

Fiber lasers have found of high conversion efficiency, simplicity in thermalmanagement, excel beam quality, and compact configuration. Their great characters inhigh-energy laser systems draw many an interest from militarists and researchers, andhad been widely applied in industry, medicine and defense. Thepolarization-maintaining (PM) YDFAs are more attractive, thanks to their polarizationstability and high extinction ratio (ER). But it is costly under high-power operationsituations. Compared with the PM YDFAs, the non-PM YDFAs can reduce the cost ofthe system. However, the polarization fluctuation of non-PM YDFAs due to thermal andmechanical disturbance limits their use. In order to expand the use of non-PM YDFAsand reduce the system cost, it is necessary to solve the problem of polarizationfluctuation in non-PM YDFAs. Recently, the researchers have taken some polarizationcompensated methods to solve this problem. In the present paper, adaptive polarizationcontrol technique of non-polarization-maintaining fiber amplifier based on stochasticparallel gradient decent (SPGD) algorithm is studied systematically and somemeaningful results are achieved as follows:1. The paper reviews the history of high power fiber lasers in the near decade.Then the current development state domestic high power fiber lasers and its drivingcauses of are summarized. And the high cost of PM YDFAs drive the development ofadaptive polarization control technique in non-PM YDFAs, so the paper review the aseries of achievements on the polarization control of the non-PM YDFAs.2. We review the history of polarization optics, and analyze the polarizationcharacters in single mode fiber. Comparison and analysis of pre-polarizationcompensation technique and end-polarization compensation technique is presented. Bycontrast, we point out the different demands and supplies of the two systems. Adaptivepolarization control is studied theoretically。3. The system is analyzed from noise source, cost function, polarization controllerand control algorithm. Based on the all-round analyzed of the adaptive polarizationcontrol system, we point out the pre-polarization compensation technique is morepotential and applied in the fiber amplifier system.4. We demonstrated the necessarily of adaptive polarization control on fiberamplifiers by the measure of the extinction ratio. An experimental platform for adaptivepolarization control is established. In the experiment, we demonstrate the adaptivepolarization control of non-polarization-maintaining fiber amplifier based on stochasticparallel gradient decent (SPGD) algorithm by using dynamic polarization controller. Wefind that the results are different by choosing different cost functions. We present theplatform of coherent beam combining (CBC) of a polarization-maintaining fiber amplifier and a non-polarization-maintaining fiber amplifier. The experiment is basedon adaptive polarization control and active phase control. The stochastic parallelgradient decent (SPGD) algorithm is used for the adaptive polarization control in thenon-polarization-maintaining fiber amplifier and the active phase control is realized bysingle frequency dithering (SFD) algorithm. When the adaptive polarization controlsystem and the phase control system go into closed loop, the fringe contrast of far-fieldintensity pattern is improved to more than87.2%.5. The multi-mode non-polarization-maintain fiber amplifiers platform isestablished. Though the polarization control technique of multi-modenon-polarization-maintain fiber amplifiers, we point out the shortages of the currentscheme.