Phase mask localization for multi-pass cavity and mode conversion

Multiple plane light conversion (MPLC) has great potential in mode manipulation but is highly sensitive to misalignment. To cope with this challenge, this work proposes an image-processing-based phase mask localization method to realize high alignment accuracy between the phase mask and the optical beam. Sub-pixel precision in locating the beam center can be obtained by directly analyzing the interaction between the lateral shifts of the phase mask and the correlation coefficient of the beam mode, without requiring complex optical setups or advanced algorithms. The feasibility of the proposed scheme has been experimentally verified with several typical modes (LP₀₁, LP_11a, and LP_21a), where the phase mask localization time is all below 3 min, while maintaining a similarity > 90% compared to the theoretical results. All results show that this method offers a practical and effective solution for enhancing the performance of multi-pass cavity systems with high efficiency and robustness.