Witnessing the pulse birth—transient dynamics in a passively mode-locked femtosecond laser

High temporal resolution measurements of the output of an Yb:KYW femtosecond laser, recorded immediately after opening the laser cavity, give an insight into the transient laser dynamics. The evolution of the light intensity measured with linear and nonlinear detectors, together with the time-resolved laser spectrum were measured and the process of the mode-locking onset was investigated. A phenomenological model of the laser dynamics has been developed that reproduces the experimental results. (letter)