Study of Optically Pumped NH₃ Mid-Infrared Lasers

Abstract

<p>The work described in this thesis concerns the investigation of the fundamental mechanism involved in off-resonant low power pumped NH₃ mid-infrared lasers. Our theoretical model shows two different contributions to the mid-infrared gain: a Raman-type term and an inversion term. Even if both gain contributions can be present in a laser medium, one process usually dominates over the other. These two processes are characterized by very different properties with respect to the lasing frequency, the magnitude of the gain and the pump absorption. Strong experimental evidence, based on the measurements of these various characteristics, has demonstrated that the Raman-type two quantum transition was responsible for lasing in the NH₃ lasers investigated.</p> <p>One consequence of Raman lasing is the possibility of obtaining pumped MIR lasers with very low pumping powers. We report here pulsed threshold powers as low as 20 Watts peak which represents an improvement by a factor of 100 over the lowest MIR threshold previously reported. Furthermore, the main parameter influencing the pump threshold value, the offset between the pump frequency and the NH₃ absorption line center, was varied during our experiment. It is shown that a decrease of this pump offset frequency generally reduces the pump threshold, but a very close pump coincidence produces the opposite effect. Consequently, we have estimated that the optimum pump offset for minimum pump power threshold is ~ 100 MHz, for NH₃ in the 12 µm region (at room temperature).</p> <p>One important extension of this work concerns the possibility of developing an optically pumped cw MIR laser. The low pump power thresholds and long MIR pulse lengths (~ 500 µsec) observed during this work indicate that cw MIR lasing is feasible.</p>