Hign Power Study On Mode Field Control With Optimization Algorithm Based On Adaptive Optics Of Fiber Laser

Fiber lasers have wide potential for industrial and military applications due to their essential characters such as high conversion efficiency, excellent beam quality, robust performance and compact configuration. However, the ultimate output power of single fiber is limited by nonlinear effect, thermal effect and so on. Beam cleanup technique has been widely applied in high-power solid state laser systems to improve beam quality; therefore it would be an effective approach for achieving high brightness lasers if the beam cleanup technique could be employed in high power laser systems. In the present paper, the new method that control the mode field of high power fiber lases based on adaptive optics is presented and analyzed systematically, and some meaningful results are achieved as follows.1、Reviewed and analysis on various theniques of large mode area(LMA) fibers is presented, the result indicates that it is hard to achieve high brightness by means of fiber design. The idea that applies beam cleanup technique to increase high power fiber laser beam quality is proposed while the history of high power solid state laser technology and high power fiber laser technology are compared. The new method actualizes high power and high beam qualitr by master oscillator power amplifier (MOPA) and beam cleanup respectively, which get out of the traditionary thought in high brightness fiber laser technoly.2、Using beam propagation method and mode analysis method calculate the beam field of LMA fiber lasers based on the configuration of MOPA and multi-MOPA combine respectively. The characteristic of two kind beam fields are analyzed and the essential consistency of beam fields illuminates the feasibility of fiber laser beam cleanup. The system model of mode field control of high power fiber laser based on adaptive optics is build up and is analyzed in detail. Optimization algorithm based on adaptive optics (OABAO) is confirmed for the beam cleanup system. OABAO employs Stochastic Parallel Gradient Descent (SPGD) algorithm and Power in the bucket (PIB) is selected as the quality evaluation function. Simulation software with graphics-user-interface used for modeling and simulation of mode field control of high power fiber laser based on adaptive optics is developed.3、Beam cleanup of single LMA fiber is investigated in theory and experiment. Starting with single high order mode, the influence of system resolution、phase noise、 power fluctuation on the result of beam cleanup is theoretically analyzed. The conclusion indicates that PIB, as the quality evaluation function can eliminate the the influence of power fluctuation and the toleration of standard deviation of phase noise could be π/2when the frequency of noise and algorithm updating rate is approximate. A LP11mode transform is demonstrate in experiment. The system transforms the LP11 mode into a nearly Gaussian beam and the power in the diffraction-limited bucket in the far-field is increased by more than a factor of3.6. The beam cleanup of multimode fiber laser is demonstrated in experiment and the power in the diffraction-limited bucket in the far-field is increased from0.08to0.7that the feasibility of multimode fiber laser cleanup is proved.4、Beam cleanup of multi-MOPA combine fiber laser is investigated in theory and experiment. The theory of phase-locking and beam cleanup multi-MOPA combine fiber laser is analyzed. The influence of error axial on enegy coupling and mode composing is analyzed with the parameters of multimode fiber is105/125μm, NA=0.22. A two-channel fiber combine is demonstrated in experiment. After phase-lockingl and cleanup, the power in the diffraction-limited bucket in the far-field is increased from0.2to0.71, which the feasibility of multi-MOPA combine fiber laser beam cleanup is proved.