Beam quality evaluation of 20/400 µm large-mode-area fiber based on mode decomposition and reconstruction

Abstract

We implement a fast beam quality factor evaluation of a large-mode-area 20/400 µm fiber based on mode decomposition and reconstruction. Each transverse mode power weight factor and relative phase difference at the fiber output facet are estimated by the stochastic parallel gradient descent algorithm based simply on the instantaneous acquired charge coupled device near-field beam profile. The beam quality factor M2 in two orthogonal directions can be analyzed in consideration of the complex amplitude of each transmission mode. The performance of our method is verified by applying it to the beam quality measurement of a 217.4 W high-power fiber laser. The calculated M2 in two directions are 1.25 and 1.18, respectively, which is in good agreement with the measured results of 1.29 and 1.24. The proposed method could be helpful for beam quality evaluation of high-power fiber lasers and amplifiers, where the rapid change of the beam quality factor is difficult to detect due to mode instability.