Theoretical analysis of contributions of self-phase modulation and group-velocity dispersion to femtosecond pulse generation in passive mode-locked dye lasers.

The influence of self-phase modulation (SPM) and group-velocity dispersion (GVD) on pulse development in a passive mode-locked dye laser is investigated numerically by using fast Fourier transformations. The situation of positive SPM is considered. Four regions of laser performance may be distinguished: (i) In the positive GVD region pulse broadening by GVD must be balanced by the pulse-shortening effect of the saturable absorber. (ii) In a region around zero GVD the laser is periodically self-quenching and the temporal and spectral pulse shapes change periodically similar to higher-order solitons. (iii) It follows a negative GVD region where stable pulses of smooth temporal and spectral shapes are generated similar to fundamental solitons. In this region the pulse duration is practically independent of the saturable absorber concentration, and the saturable absorber is needed mainly for background suppression. (iv) Further increasing the negative GVD, pulse broadening must be balanced by the saturable absor ber pulse shortening.