Suppression of modal crosstalk in laser-direct-writing weakly-guiding bending multimode waveguides

Abstract

Space division multiplexing (SDM) techniques based on few-mode or multicore fibers have attracted great interest to enhance the capacity of optical networks, for which laser direct writing (LDW) is a promising approach to realize compact spatial multiplexer/demultiplexer (MUX/DEMUX). However, the modal crosstalk induced by bending multimode waveguide in LDW spatial MUXs/DEMUXs may have significant influence on the overall performance of a weakly-coupled SDM system supporting independent spatial channels. In this paper, we firstly analyze the bend-induced mode coupling behaviors for linearly-polarized (LP) modes in circularly-symmetric-step-index weakly-guiding bending multimode waveguides (WGBMW) and show that LP01 and LP11 modes will have the strongest mode coupling. Then we propose an optimal design of 90°-bend and S-bend using hybrid Euler-Circular (HEC) bend, where the length of Euler bend is optimized to be the beat length of LP01 and LP11 modes to suppress their mode coupling. Numerical simulations based on conformal mapping model and three-dimensional Beam Propagation Method are performed. The simulation results show that the optimal HEC bend can significantly suppress the mode crosstalk and loss. The crosstalk of LP01 and LP11 modes for the optimal HEC bend is below −60 dB, whereas the value for the circular bend is about −23 dB. This work provides a better understanding of bend-induced mode coupling in WGBMWs and is beneficial for the practical design of LDW devices.