Self-phase modulation of nanosecond pulses in fiber amplifiers with gain saturation

SPM is an important limitation in some fiber lasers and amplifier systems. In this paper, the influence of gain saturation on the SPM-induced spectral broadening for coherent and incoherent nanosecond pulses is discussed. The models of SPM-induced spectral broadening for coherent and incoherent nanosecond pulses are generalized to the amplification with gain saturation. Experiments are conducted to validate our theoretical analysis for incoherent nanosecond pulses in a super-luminescent diode seeded cascaded fiber amplifier system where pulses with high peak power and high pulse energy are generated and gain saturation occurs. Excellent agreements between theoretical analysis and experimental results are obtained, and the influence of higher order nonlinear terms on the SPM effect is observed and analyzed .