Eye-Safe Extracavity Raman Laser: a Passive Way of Eliminating Optical Feedback with Double-Pass Pumping

A passive way of eliminating optical feedback between a pump laser and an extracavity two-pass pumped Raman laser in a compact arrangement was demonstrated. Feedback was suppressed by arranging the circuit elements in a configuration where the pump-laser 100% mirror and the Raman-laser output mirror formed an unstable cavity characterized by a substantially higher threshold for generating the fundamental radiation as compared with the stable pump-laser cavity. The method was demonstrated using an eye-safe KGd(WO4)2 Raman laser pumped by a pulsed KGd(WO4)2:Nd laser with the working transition $$ {}^{{}^4}F_{3/2}\to {}^4I_{13/2} $$ as an example. The Raman laser emitted pulses of energy ~11.5 mJ at an electrical pump energy of ~10 J, corresponding to a conversion efficiency of ~31%. The Stokes pulse width (~17 ns) was comparable to the pump pulse width (~20 ns) owing to the extracavity location of the Raman laser.