Impact of random birefringence on nonlinear ultrashort pulse propagation in multimode fibers

Nonlinear pulse propagation in multimode fibers (MMFs) has attracted considerable attention in recent years, owing to its rich spatiotemporal nonlinear dynamics and diverse potential applications. In practical scenarios, random birefringence in MMFs cannot be neglected, affecting the polarization-dependent nonlinear pulse propagation. This paper explores the impact of random birefringence in MMFs on the nonlinear ultrashort pulse propagation employing the generalized multimode nonlinear Schrödinger equation combined with random birefringence. Two scenarios, multimode soliton propagation and spatial beam self-cleaning, are specifically examined. It is found that while random birefringence typically weakens nonlinearity in MMFs, certain nonlinear processes may exhibit a complex relationship with random birefringence. For the soliton self-frequency shift induced by the intra-pulse Raman effect, we found that as the randomness of the birefringence effect increases, the general trend of the frequency shift is first to decrease and then increase. The study also reveals that beam self-cleaning can endure random birefringence at high input peak powers. This research provides guidance for practical applications that utilize the nonlinear transmission of ultrashort pulses in MMFs.