All-fiber mode selective couplers for mode-division-multiplexed optical transmission

Abstract

All-fiber mode selective coupler (MSC) is comprised of a few mode fiber (FMF) and a single mode fiber (SMF), coupling the LP 01 mode of the SMF to a specific higher-order mode (HOM) of the FMF. In order to achieve high coupling ratio and low insertion loss, phase-matching condition between the LP 01 mode of SMF arm and the HOM of FMF arm should be satisfied. A polished-type MSC is made by getting their cores into intimate contact. Prism coupling with a polished coupler block can measure the effective refractive index of the mode accurately. We propose and demonstrate three kinds of allfiber mode multiplexer that is composed of consecutive MSCs. 4-mode multiplexer can multiplex 4 modes of LP 01 , LP 11 , LP 21 , and LP 02 by cascading LP 11 , LP 21 , and LP 02 MSCs. It is used for MDM transmission of three modes with 120 Gb/s DP-QPSK signals. In order to enhance the signal transmission performance by receiving degenerate LP modes simultaneously, a mode multiplexer to utilize two-fold degenerate LP 11 modes is proposed. It is composed of two consecutive LP 11 MSCs that allows the multiplexing of LP01 mode and two orthogonal LP 11 modes. We demonstrates WDM transmission of 30 wavelength channels with 33.3 GHz spacing, each carrying 3 modes, over 560 km of FMF. 6- mode multiplexer can multiplex 6 modes of LP 01 , LP 11a , LP 11b , LP 21a , LP 21b , LP 02 modes. We demonstrated WDM-MDM transmission with the all-fiber 6-mode multiplexer. In this paper, the manufacturing method and the recent advancements of the all-fiber mode multiplexer based on the MSCs are reviewed. Long-distance mode division multiplexing (MDM) optical signal transmissions with the all-fiber mode multiplexer are experimentally demonstrated.