Abstract

The temporal shape of laser pulses emitted from an unstable laser resonator was investigated [1, 2]. The resonator was Q-switched by means of stimulated Brillouin scattering (SBS) in a cell filled with acetone. Besides a change in pulse length from 40 ns to 2-3 ns by a variation of the resonator configuration, sub-GHz-modulations of the 40 ns-pulses were observed (fig. 1a)-c)). In a set of 150 laser shots 28 were strongly modulated, which were taken for analysis in fig.2. Only modulation frequencies which are harmonic multiples of the resonator roundtrip frequency (100 MHz) appeared whereas frequencies of 400 MHz and above 600 MHz were missing. It is remarkable, that the measured frequencies belong to a subharmonic of the Brillouin shift (in acetone 3000 MHz at 1064 nm), whereas the missing frequencies are no subhamtonics of the Brillouin shift. It turns out, that the modulations are caused by a not too low Q-value before the Q-switching of the laser, which enables prelasing. Based on long prelasing pulses a spontaneaous mode locking takes place, which is responsible for the statistical behavior of the modulation frequencies. If the modulation frequency of this mode locking is matched to the Brillouin shift, a four wave mixing process is initiated in the SBS-cell causing a Q-switch of the resonator. In analogy to Brillouin enhanced four wave mixing (BEFWM) this process may be called resonator enhanced four wave mixing (REFWM).

© 1996 IEEE