Study On Technology Of Tilt-tip Wavefront Control In Coherent Beam Combining Of Fiber Lasers

Coherent beam combination (CBC) is an effective way to obtain high power andhigh beam quality output of laser. In CBC, the filling of aperture, tilt-tip wavefront andpiston phase of laser array have direct impacts on the combined beam’s quality. In fact,because of the impacts from assemblage error, pointing error, vibration of mechanismand atmosphere perturbation, etc., the optical axis of each beamlet in CBC array maydeviate from the ideal one, which will result in the beams being separate on the targetand therefore the serious degradation of CBC’s effect. The tilt-tip wavefront must becontrolled effectively in order to further improve the combined beam’s quality andsystem’s practicability in CBC. In this paper, the theoretical and experimental researchon tilt-tip control technology in CBC has been studied, employing the conformal laserarray and stochastic parallel gradient descent (SPGD) algorithm.Firstly, the paper begins with reviewing the achievements in field of tilt-tipwavefront control in CBC and summarizing the current state of tilt-tip controltechnology, then introduces the basic structure of tilt-tip wavefront control system inCBC, and comparatively analyzes the common devices of tilt-tip wavefront control andphotonic detection. The algorithm controllers are compared and chosen, and theadvantages and disadvantages of varied metric functions are also discussed. Theschemes and steps of different algorithms in optimizing control are stated. Finally, inthis paper’s study on tilt-tip wavefront control in CBC, the adaptive fiber-opticscollimator (AFOC) is chosen as the main device of tilt-tip control in experiment,high-speed camera and photonic detector (PD) with a pinhole diaphragm are used asphotonic detecting devices, SPGD algorithm is employed as the control algorithm.Secondly, the theoretical model of tilt-tip control in CBC is built up, and the tilt-tipwavefront errors are corrected effectively using SPGD algorithm, employing thenumerical simulation through computer programme. By large quantity of numericalsimulation, the paper comparatively analyzes some factors’ impacts on tilt-tip controleffect, such as metric function, parameters of control algorithm, tilt-tip mean squareerror (MSE), tilt-tip errors’ frequency and number of combined laser beams.The resultsreveal that the power-in-the-bucket (PIB) has the advantages such as less calculationwork, better convergence effect and practicability, so PIB can be chosen as the metricfunction in experiments in practice. The tilt-tip control effect degrades evidently whentilt-tip MSE, tilt-tip frequency and the number of lasers increase. The bandwidth oftilt-tip dynamic control is analyzed by simulation. The results demonstrate that thecontrol bandwidth decreases obviously as the number of lasers, tilt-tip MSE andfrequency increase.Furthermore, the numerical simulation on tilt-tip control effect in long distance CBC system using delayed stochastic parallel gradient descent (D-SPGD) algorithm isperformed. The simulation analyzes the impacts of tilt-tip MSE, laser number and delayparameter on the system’s control effect and control bandwidth. The numericalsimulation result reveals that, compared with SPGD algorithm, D-SPGD algorithm canevidently improve the tilt-tip control iteration rate in long distance CBC system, and theincrease of laser number has almost little influence on the convergence rate of thesystem.Then, based on the theoretical analysis, the fiber laser array systems of cascadedand simultaneous tilt-tip and phase-locking control are set up, and the control programis compiled. In cascaded control system, the experimental research on pistonphase-locking and tilt-tip wavefront control of2and4lasers are performed, respectively.In simultaneous control system, the experimental research of4and6lasers areperformed respectively. The experimental results reveal that, in both systems, tilt-tipcontrol can make the separate beams superposed on each other well, and phase-lockingcontrol can compensate the piston phase errors effectively. Cascaded control system canbe applied in the quasi-static compensation of tilt-tip errors in CBC, and simultaneoussystem is proper to be employed in the dynamic compensation of tilt-tip errors.In the last part, a CBC system with total power of350W, composed by2fiberamplifiers, is set up, and the experimental study on simultaneous control of tilt-tip andphase-locking is performed, the system’s dynamic control bandwidth is analyzed. Theresults conclude that the high power tilt-tip control in CBC using AFOC can effectivelycorrect the tilt-tip errors. The system’s dynamic control bandwidth is about5.40Hzwhen the tilt-tip MSE is10μrad.