Gain broadening and mode-locking in overcoupled second harmonic Q-switched microsecond pulses

An intracavity frequency doubled, Q-switched Nd:YLF emitting at a wavelength of 527 nm was designed with the goal to temporally stretch the Q-switched pulses up to some microseconds at pulse energies of several millijoules. With different resonator configurations pulse durations between 12 μs and 3 μs with energies of 1 mJ-4.5 mJ have been achieved, which is demanded for an application in ophthalmology. For tighter intracavity foci and high pump power, however, strong power modulations by trains of picosecond pulses on the rear flank of the microsecond pulses were observed, indicating the occurrence of cascading nonlinearities and mode-locking. Simultaneously a significant increase of the fundamental spectrum up to 5 nm was found. A similar effect, which is referred to as gain broadening, has previously been observed by using ppKTP for intracavity second harmonic generation. This is, to the best of our knowledge, the first observation of this effect with unpoled second harmonic media.