We propose a fiber-based 4-LP mode (LP
01, LP
11, LP
21 and LP
02) multi/demultiplexer, which excites or separates 4-LP mode simultaneously in the integrated structure of a single optical fiber. The structure contains a 4-mode transmitting core in the center of the fiber and the other three multi/demulitplexing parts (consist of a coupling core and several assistant cores or not) which are deployed around the transmitting core with 120 degrees between each other.
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 LP
11, LP
21 and LP
02 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 LP
01 (single-mode coupling cores) and LP
11, LP
21, LP
02 (transmitting core), respectively. The appropriate core diameter and refractive index for the coupling cores of the multi/demultiplexing parts are 5μm and 1.465703 (LP
01-LP
11), 2μm and 1.479793 (LP
01-LP
21), 1.5μm and 1.488404 (LP
01-LP
02), 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 LP
02-LP
01 mode multi/demultiplexing part does not need any assistant core, LP
21-LP
01 and LP
11-LP
01 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.