Lengthening of ultrashort-pulse trains in a Nd:YAG laser with passive negative feedback

The parameters of two optical elements that are used with a solid-state laser to generate long trains of ultrashort pulses are analyzed and compared with experiment. The first of these provides passive mode locking, the second negative feedback based on the mechanism of two-photon absorption via direct gap transitions in a semiconductor. In the experiments we used a Nd:YAG laser with a modified Sagnac interferometer. Using a nonlinear absorber based on a solution of the dye 3274-U (the first element) and a negative-feedback element based on a film of GaAs (the second), we were able to obtain a tenfold lengthening of the envelope of the pulse train (with respect to its initial value) with a twofold decrease in the duration of individual pulses in the train.