Multi-dimensional Mode Control Of Large-mode Area Fiber Laser Based On A Photonic Lantern

High power lasers with adjustable modes have wide applications in material processing,optical detection and national defense.Among the mainstream fibers currently selected for high-power lasers,large-mode area fibers can increase the threshold of nonlinear effects,but also face the challenge of mode control at high power.Photonic lantern is a coherent combiner with mode selection functionality.Combined with the adaptive optics,the photonic lantern is expected to solve the problems of mode-control difficulty and beam-quality degradation in large-mode area fibers.In this thesis,we carry out simulations and experiments on the adaptive mode control system based on photonic lantern,from the perspective of structural design and control strategy.The main works are as follows:1.Based on the beam propagation method,the mode propagation within certain structures of the photonic lantern is studied.According to the design rules of photonic lantern proposed for communication,the transmission matrices of 3×1 and 6×1 photonic lantern are obtained by simulation;we further optimize the geometrical structure parameters of 7×1 photonic lantern and its transmission matrix is also calculated,which provides a good basis for the subsequent experiments.2.We optimize the transmission-matrix inferred control（TMIC）strategy using the structural information of the photonic lantern,which significantly improves the convergence speed and accuracy of mode control.Combining the characteristics of the output optical field and the transmission matrix obtained from the simulation,the initial amplitude parameters of the input optical field are calculated.For the six low-order LP modes,we simulate and compare the control effects of the control strategies using random amplitude control,equal amplitude control and TMIC,which verifies the advantages of TMIC.For the evaluation function of the power in the bucket,the simulation compares the effect of minimizing the power in the center bucket and maximizing the power in the eccentric bucket for controlling the LP₁₁ modes.3.Based on the 7×1 photonic lantern,an experimental platform is built using master oscillation power amplification at 100W level.We realize stable output LP₀₁mode and LP₁₁ mode in a large mode field fiber with an output core diameter of 30μm（NA=0.08）,and the mode percentages of LP₀₁ mode and LP₁₁ mode reach 96.4%and89.8%,respectively.