Modal selectivity at 850 nm employing standard single-mode couplers: Theory and experimental demonstration

In this paper, we evaluate experimentally and analyze theoretically and by simulation the demultiplexing of LP01 and LP11 modes in a 850 nm few mode optical link when a standard single-mode coupler theoretically designed for 1550 nm operation is used. The simulation analysis indicates a feasible conversion efficiency higher than 98% for the LP01 mode and 100% efficiency for LP11 mode. These results are experimentally validated considering different off-the-shelf optical couplers configured in 99:1, 90:10, 80:20 and 60:40 coupling ratios. Experimental analysis of the optical couplers demonstrate adequate functionality in correspondence with the theoretical analysis, exhibiting a good near-field pattern and modal profile for the LP01 and LP11 modes. The mode purity observed was 98.4% for the LP01 mode, and higher than 90% for the LP11 mode. The 90:10 optical coupler configuration shows best mode purity with the minimal insertion losses.