Improvement of output stability of CO 2 laser pumped FIR lasers by using an optical isolator

Simultaneously oscillated 48-μm and 57-μm CH3OD lasers pumped by a 9R(8) CO2 laser have been developed for fusion plasma diagnostics. A back reflection from an optically pumped far-infrared (FIR) laser cavity affects not only the FIR laser stability but also the pump CO2 laser stability. The back reflection has been reduced by an optical isolator using a quarter-wave plate and an absorbing thin-film reflector. As a result, we have achieved output stabilities of ±1.4 %/h at the 48-μm laser and ±1.3 %/h at the 57-μm laser.