Fast and slow self-phase modulation induced pulse shaping of subpicosecond pulses in semiconductor laser amplifiers

Self- and cross-phase modulation (SPM, XPM) of picosecond pulses due to gain saturation in semiconductor laser amplifiers has been investigated, both theoretically and experimentally.1,2,3 Because of the large value of the so-called linewidth enhancement factor α, these effects can result in considerable nonlinear sweep and spectral broadening. The spectrally broadened pulses can be partially compressed1,2 by providing dispersion externally. It has also been shown3 that the quadratic shape of the gain spectrum can result in considerable positive second-order phase dispersion. For pulse durations of about 1 ps or shorter (and input energies in the 0.1-1 pj range) the fast n2 must also be considered. The gain saturation induced “slow” self phase modulation (SSPM) is an integrating, energy dependent effect. On the other hand, the fast n2 responds to the instantaneous intensity of the pulse, resulting in “fast” self phase modulation (FSPM). The interplay of saturation, two kinds of SPM and dispersion will result in complicated pulse shaping. In this paper, we investigate the evolution of subpicosecond pulses in semiconductor amplifiers using a differential equation similar to one used for studying saturated dye laser amplifiers.4 Under certain conditions, soliton-like compression effects are predicted.