Constructing Ten-Mode Linearly Polarized Mode Sorters With Seven Masks Imprinted on a Spatial Light Modulator

Multi-plane Light Conversion (MPLC) based mode sorters stand out for high mode count, high mode purity, low crosstalk, and flexibility, but generally require number of phase masks 1∼2 times of the mode count. There are few exceptions that the number of masks is significantly reduced in sorting Hermite-Gaussian (HG) or orbital angular momentum (OAM) beams thanks to the special mode structure and the corresponding geometric input layout. However, linearly-polarized (LP) modes in fiber are out of these exceptions for the more complex mode structure compared to that of HG or OAM modes. In this paper, we explore the possibility to construct LP mode sorters with fewer phase masks. We used trial-and-error approaches from multiple aspects to improve the performance of LP mode sorters, such as optimizing the geometric input layout to account for phase shift characteristics of liquid crystal, modifying algorithm to obtain smooth phase patterns, and improving beam alignment to reduce performance degradation. Finally, we experimentally demonstrate a 10-LP mode sorter with 7 phase masks using a liquid crystal on silicon (LCoS) spatial light modulator (SLM), achieving insertion loss of -3.6 dB, mode-dependent loss of -5.6 dB and 1-dB bandwidth of 50 nm. The results indicate that LP-mode sorters with moderate performance can be effectively constructed by LCoS SLMs. LCoS SLMs are preferred in laboratory use for their flexibility and ease of rapidly constructing MPLC-based mode sorters, which can adapt to fibers with various refractive index profiles.