Bandwidth dependence of pulse duration in intra-cavity frequency-doubled lasers: Theory and experiment

A model is presented of pulse evolution in broadband intra-cavity frequency-doubled lasers. The model utilizes normalized coupled rate equations for each mode, including terms that represent the loss due to nonlinear mixing between longitudinal modes. The pulse energy, shape, peak power and duration are calculated by numerical solution of these equations. The model shows that the pulse duration depends not only on the initial population inversion, photon lifetime and the effective nonlinear coupling coefficient, as is the case for narrowband lasers, but also on the fundamental bandwidth. A gain-switched Ti:sapphire laser, pumped by a Q -switched Nd-doped yttrium aluminium garnet laser at 532nm, was frequency doubled using an intra-cavity β-barium borate crystal. The bandwidth was reduced from about 25nm to about 1.5nm in two steps using a series of prisms, and the resulting changes in experimental pulse durations and energies agree well with the model.