Research On Laser Mode Decomposition Based On Iterative Reconstruction Of Complex Amplitude

Laser mode is the spatial field distribution in the optical cavity,and it is the key factor that determines the laser beam quality and transmission characteristics.In the fields of high-power fiber lasers,fiber communications,and the like,being able to accurately resolve the laser mode is an important prerequisite for mode control and application.Among the current mode decomposition methods,the numerical analysis method has attracted much attention because of the simple experimental device,but its algorithm is greatly affected by the initial value and the convergence result is uncertain.In this thesis,the laser mode decomposition technology based on iterative reconstruction of complex amplitude was studied.The main research work includes,The laser complex amplitude reconstruction method based on multi-spot iterative operation was studied,and the complex amplitude reconstruction model based on the iterative operation of variable-position and variable-focus spots was established.The simulation and experimental results verified the accuracy of the complex amplitude reconstruction of single-and multi-mode lasers.The similarity between the intensities of the reconstructed spot and the actual spot was 96%,and the error of the spot diameter was 11%.In order to improve the laser beam quality(M2)measurement speed of the variable-focus fitting method,a fast M2 factor measurement method based on liquid lens and variable-focus iterative retrieval(VFIR)was proposed.A model for calculating M2 with the reconstructed complex amplitude was established.The measurements of single-and multi-mode lasers were conducted,and the measurement error was less than 5%,compared with the measurement results of the commercial M2-200s analyzer.Without moving parts,this method can achieve M2 fast measurement accurately with three variable-focus spots.In order to solve the problems of initial-value sensitivity,mode-number limitation,and uncertainty of convergence results in numerical analysis method,a modified Stochastic Parallel Gradient Descent algorithm based on iterative phase retrieval(IPR-SPGD)was proposed.The iteratively reconstructed laser complex amplitude was projected to each eigenmode to obtain the initial value of the complex coefficients of each mode.Then through the optimization of the SPGD algorithm,the mode decomposition was realized.The simulation and experimental results verified the accuracy and stability of this method.The mode decomposition of the output laser beam from a few-mode fiber(supporting the propagation of 8 modes,the core diameter is 9 μm,Normalized frequency V is 5.36)was conducted.The intensity similarity of the reconstructed and measured spots was 98%,the spot diameter error was 5%,and the M2 factor error was 5%.The experimental device of this method is simple and the calculation result is stable.The projection calculation results of the reconstructed complex amplitude were used as the initial value,which improves the upper limit of the mode number and the measurement accuracy of the numerical analysis method.Finally,the parameter settings of the multi-spot iterative algorithm and the IPR-SPGD algorithm,and the effect of system aberrations were studied.The optimal values of parameters such as the number of spots,lens focal length,perturbation amplitude,and gain coefficient are obtained under the experimental conditions.The stability of the calculation results of IPR-SPGD algorithm under lower-order aberration were discussed.Then the beam quality calculation errors of were analyzed,and the reconstructed spot was compared with the spot measured by CCD at different propagating positions.The influence of eigenmode deviation was discussed,and it was pointed out that the modified eigenmode and eigenmode radius search algorithms can improve the accuracy of mode decomposition.