Theory of solid-state laser mode locking by coherent semiconductor quantum-well absorbers

Abstract

Solid-state laser mode locking by semiconductor quantum-well absorbers (QWA’s) is investigated, with the coherent polarization response of the absorbers taken into account. It is shown that femtosecond pulses with durations below the response time and the dephasing time can be generated that are stabilized by Rabi flopping of the absorber population. The shortest pulses are found when the effects of self-phase modulation (SPM) and group-velocity dispersion (GVD) are not present, whereby highly stable 2π pulses are formed inside the absorber. In the presence of SPM and GVD the pulse area deviates from 2π; nevertheless, the absorber’s coherent response results in incomplete Rabi flopping of the population. In agreement with experimental observations, we found limited femtosecond pulse operation regions for the pump rate and the GVD. These parameter regions are bounded by Q-switching instabilities and multiple-pulse operation. Further, the influence of inhomogeneous broadening of the carriers, dynamic bandgap renormalization owing to Coulomb exchange, and excitation-dependent dephasing of highly excited semiconductors is studied. The key absorber parameters are predicted that should permit a reduction of the shortest possible pulse durations.