Dynamics of the line-tunable 12-µm continuous-wave NH3 laser as measured with a tunable-diode laser

A CW CO2 laser operating on the R(30) 9-micron transition is downshifted by 180 MHz in a pair of acousto-optic modulators to give an exact coincidence with the sR(5,0) transition in NH3. The downshifted radiation optically pumps mixtures of NH3 in N2 and creates vibrational inversion in the nu-2 mode of NH3. This inversion is monitored with a tunable-diode laser operating in the 11-12-micron region. Gain coefficients as large as 3.4 percent/cm are measured in the P branch of the nu-2 band, and the measured gain coefficients are found to be in good agreement with a rate-equation model based on rotational thermalization. Detailed measurements of gain coefficients are presented as a function of NH3 transition, pump power, and gas mixture and pressure. These results will aid in the optimization of CW NH3 lasers operating in the 11-13-micron region.