ROTATIONAL RELAXATION AND GAIN SATURATION IN CO2 LASER AMPLIFIERS

Amplifiers are frequently used in conjunction with Q-switched laser oscillators for the generation of high-power pulses. The time dependence of gain saturation in CO2 laser amplifiers has been studied. The rate of change of amplifier gain at power levels near 104 W cm-2 is generally determined by relaxation processes between rotational levels of the 001 and 100 vibrational states. A theoretical model which takes rotational relaxation into account is presented and compared with experimental data on the gain of an amplifier both during and after an oscillator pulse. Good agreement between theory and experiment is obtained, from which an average value of the rotational relaxation time can be deduced. The variation of the rotational relaxation time with partial pressure of helium and nitrogen is also described. The implications of these results in the design of CO2 laser systems to produce high-power pulses are pointed out.