Narrowing of the emission spectrum of a solid-state laser with a slow-response saturable absorber and negative feedback

A strong (by a factor of almost 100) narrowing of the spectrum was observed in quasi-stady operation of a solid-state laser with neodymium-doped potassium gadolinium tungstate when the cavity contained a negative feedback loop and a slow-response saturable absorber. The role of a nonlinear narrow-band selector, which reduced the width of the spectrum, was played by a periodic saturated-absorption grating formed by a standing intracavity laser radiation wave in a lithium fluoride crystal containing $F_2^-$ colour centres and used as a saturable absorber.