Spectral characteristics of radiation emitted by a YAG:Nd$^{3+}$ laser with a saturable absorber in the form of an LiF crystal containing $F_2^-$ centers

An experimental investigation was made of the spectral characteristics of radiation emitted by a YAG:Nd$^{3+}$ laser in which an LiF crystal containing $F_2^-$ centers was used as a passive switch. Single-frequency emission and mode locking operation regimes were investigated. In the single-frequency regime the minimum width of the emission line was 5$\times10^{-3}$ cm $^{-1}$ for a single pulse. The frequency selection mechanism involved spatially inhomogeneous saturation of the absorption. Mode locking took place in a ring resonator when a saturable crystal filter with an initial transmission 0.7 was used. The use of a switch containing color centers made it possible to generate picosecond pulses simply and reliably.