Spatiotemporal deterioration in nonlinear ultrafast fiber amplifiers

Pulse coherence and spatial beam quality are important physical parameters to evaluate the quality of ultrafast lasers. In this paper, spatiotemporal deterioration (STD) of multidimensional light field is proposed to depict the degradation of coherence in both temporal and spatial domains in nonlinear ultrafast fiber amplifiers. Simulation results uncover that in gain-managed nonlinearity (GMN) pulse amplifiers, once the STD threshold is reached, beam quality quickly degrades, Raman noise intensifies, pulsed coherence decreases, and pulse compression becomes challenging. The findings demonstrate that stimulated Raman scattering is the primary cause of STD, which triggers a rapid energy transfer of the blue-wavelength portion of fundamental mode to higher-order modes when phase matching is satisfied by the strong Kerr effect in the GMN regime. Moreover, gain redshift that dominates the redshift of the pulse spectrum can considerably drive the energy transfer and, hence, accelerates STD in the nonlinear regime, while suppressing STD in the linear regime. Finally, the effects of seed characteristics and pump wavelength on STD are also studied, and methods to suppress STD in GMN amplifiers are presented. These findings we believe can provide insight into high-energy ultrafast fiber amplifiers for generating high-coherent, high-beam-quality, sub-50 fs, and μJ level pulses at 1 μm and other complex multimode gain systems.