High-intensity far-infrared laser with buffer gases

Abstract

Analytic expressions are given, using a four-level rate-equation model, for the dependence of the far-infrared (FIR) small-signal gain, of the saturation intensity of the FIR laser transition, of the output power of the FIR laser, and of the absorption coefficient of the pump beam on buffer-gas pressure. The model of Henningsen and Jensen [ IEEE J. Quantum Electron.QE-11, 248 ( 1975)] is modified by the inclusion of induced emission at the pump transition and by the inclusion of power broadening of the levels. The pressure behavior of the small-signal gain is described correctly by this modified model without anomalies in the large- and small-pressure limits, and, by using the expressions for the evaluation of experimental data, the rotational and the vibrational relaxation rates can be deduced for different laser and buffer gases. An increase in the output power of the FIR laser is expected only if the pump rate is larger than the vibrational deexcitation rate. In this case the absorption is proportional to the increase of the vibrational deexcitation rate caused by the collision with the buffer-gas molecules. The output power of the FIR laser increases only as a result of the increased absorption. Adding buffer gas to the laser gas causes the gain of the material to decrease in both weak and strong pumping regimes.