The design of the fiber-based 4-LP mode multi/demultiplexer includes the following parts. Firstly, the appropriate distance between the transmitting core and the surrounding parts are below 15μm, 12μm and 10μm for multi/demultiplexing of LP11, LP21 and LP02 mode, respectively, which can guarantee the coupling loss is less than 2%. Then we design the structural parameters (core diameter and refractive index) of single-mode coupling cores to achieve phase matching between LP01 (single-mode coupling cores) and LP11, LP21, LP02 (transmitting core), respectively. The appropriate core diameter and refractive index for the coupling cores of the multi/demultiplexing parts are 5μm and 1.465703 (LP01-LP11), 2μm and 1.479793 (LP01-LP21), 1.5μm and 1.488404 (LP01-LP02), respectively, all of which can ensure that the coupling efficiency are more than 90% in the C+L band. Afterwards, by discussing the coupling crosstalk of each multi/demultiplexing part, we find that LP02-LP01 mode multi/demultiplexing part does not need any assistant core, LP21-LP01 and LP11-LP01 mode multi/multiplexing parts need to add 1 or 2 assistant single mode core as required to decrease the coupling crosstalk. Finally by adjusting the relative position of the three multi/demultiplexing parts and the transmitting core (ensure the performance of the whole system is not affected), we can ensure that the 4 mode can be multi/demultiplexed at the same time.
Above all, we put forward a fiber-based 4-LP-mode multi/demultiplexer which excites or separates 4-LP mode simultaneously with high multi/demultiplexing efficiency, low coupling loss, and low coupling crosstalk.