Evolution of the shape and spectrum of ultrashort pulses upon active mode locking

Time sweeps of the autocorrelation function and the emission spectrum of an actively mode-locked Nd:YAG laser are recorded during the development of quasi-continuous pre-lasing maintained by a negative feedback loop. It is found that higher Hermitian–Gaussian supermodes are present at the transient stage of radiation, and the ultrashort pulse shortening is accompanied by the shift of the lasing spectrum to the red wing of the gain band of the active medium.