Quantitative Investigations Of Passively Mode-Locked Lasers

A passively mode-locked TEM-00 Nd:YAG laser is investigated theoretically and experimentally in detail. The numerical calculations base on the refined Letokhov model, taking into account the spontaneous emission and the time dependent pumping light. The statistical noise pulses of the spontaneous emission are approximated by an average noise power and two single pulses which develop into the mode locking pulse and its satellite. By starting at t = 0 with all atoms in the ground state, the temporal behaviour of the following quantities is pursued: main pulse energy and width, energy of the satellite pulse, average noise power, and their dependence on absorber transmission, pumping rate, and resonator losses. Special importance is attached to a realistic model of the nonlinear absorber which compresses the mode-locking pulse and determines the resulting pulse width in conjunction with the amplifier. The gaussian structure of the pulse in time and space is taken into account. The numerical results were checked experimentally. Pulse width measurements were carried out with two photon fluorescence and a fast streak-camera. The results confirm the theory and demonstrate that the refined Letokhov-model is suitable to describe mode-locking quantitatively.