Dynamics of Optically-Pumped Pulsed Mid-Infrared Ammonia Lasers.

An extensive theoretical and experimental study of the dynamics of optically-pumped pulsed NH(,3) lasers is presented. The results completely characterize the small-signal gain of NH(,3) lasers operating on rotational -vibrational transitions in the mid-infrared wavelength range from 10 to 21 (mu)m. Two gain mechanisms are identified from the experiments: single-photon inversion gain and two-photon Raman gain. Raman gain is described by a density-matrix model as a coherent interaction of two radiation fields with three molecular levels. Raman emission is offset in frequency from the line center of the emitting transition, and is the dominant process both at large pump offsets and in pure NH(,3). The Raman process and the ac Stark effect, which shifts the optimum gain frequency, are always present in NH(,3) lasers and affect all transitions which have one level in common with the pumped transition. Significant population transfer produces inversion gain which is the dominant gain process in mixtures of NH(,3) with a buffer gas. A rate-equation model is developed based on the assumption of rapid rotational thermalization in all vibrational levels within, but not between, ortho -NH(,3) (K = 3n) and para-NH(,3) (K = 3n(+OR-)1). The model accounts for the observed distributions and magnitudes of inversion gain for a wide range of pumping conditions. The thermalization concept makes it possible to characterize a gain distribution by the ratio of vibrational populations. Another important consequence of rotational thermalization is the prediction and demonstration of a new line-tunable NH(,3) laser. A two-step pumping process produces lasing from 16 to 21 (mu)m on many ortho-NH(,3) 2(nu)(,2) transitions. A final interesting prediction of the rate-equation model is that a 10-atm NH(,3) laser is feasible. At this pressure, there is a potential for tuning the laser emission across 36% of the wavelength range from 10.5 to 13.8 (mu)m, and for producing pulses as short as 10 ps.