Spontaneous narrowing of the emission spectrum (spectral "collapse") in neodymium lasers with Q switching by LiF:F2– crystals

An investigation was made of how the output energy, its variance, and the width of the emission spectrum were affected by the number of pulses emitted from pulsed neodymium glass (GLS 24P) and YAG:Nd lasers with passive Q switching by LiF:F₂^– crystals. The results indicated that after 100–200 pulses the variance of the output energy increased strongly from 0.2 to 5% and the emission spectrum became narrower (in the case of the GLS 24P glass laser the width of the emission spectrum decreased from 9 cm^–1 at the frequency of the second harmonic right down to 1 or 2 longitudinal modes). The effects observed were explained by assuming that high-power laser radiation destroyed the LiF:F₂^– color centers and created a structure with spectrally selective properties inside the crystal switch.