Influence of Phase Accuracy on Mode Compensation Using Progressive Phase Conjugation

In recent years, the transmission capacity of single-mode fibers has reached its theoretical limit owing to the limitations of optical power density in the fibers. However, capacity constraints must be overcome to support future transmission demands; therefore, transmission systems using multi-mode fibers (MMF) have recently attracted attention [1]. In MMFs, a beam can be transmitted in various modes, and each mode is treated as a separate transmission channel for light. However, factors such as fiber bending and nonlinear interaction introduce coupling between light waves propagating in the MMF, and multiple modes with different propagation constants are superimposed [2]. Owing to mode coupling, even if a signal pulse is excited as a single mode, it tends to couple with other modes during MMF transmission, which can cause mode dispersion at the receiver. Mode dispersion causes considerable crosstalk during signal detection, resulting in a lower transmission rate and a restricted transmission range of the communication system. Thus, the spatial mode beam must be compensated to restore the dispersed spatial mode beam to the fundamental mode.