Optically-Pumped CW Mid-Infrared NH₃ Lasers

Abstract

<p>The work presented in this thesis concerns the development and characterization of the first optically-pumped continuous (cw) lasers in the mid-infrared. Ammonia is chosen as the active lasing medium, and this research describes experiments in which NH₃ is pumped both off- and on-resonance with a CO₂ laser using several innovative techniques. The new lasers provide efficient, powerful and line-tunable cw radiation in a region (10 to 30 μm) where such sources are scarce.</p> <p>Initial experiments were performed with several off-resonantly pumped 12-μm NH₃ transitions and, at low pumping powers, a Raman two-photon process was identified as the dominant mechanism responsible for the gain. The two-photon process results from the coherent interaction of two laser fields with a three-level system and can create gain in the absence of a population inversion. This property was essential to the successful operation of the first optically-pumped cw laser in the mid-infrared as the available pump intensity was insufficient to produce a population inversion.</p> <p>A detailed understanding of this new cw laser was achieved by probing the small-signal gain with a tunable diode laser in an amplifier cell. A theoretical model based on a density matrix formalism approach was developed and good agreement was found between experiment and theory. The tunable diode laser measurements emphasized the importance of the pump intensity in determining the magnitude of the Raman gain. Following this study, an optimized 12-μm cavity was constructed in which the pump intensity was maximized by using a small capillary tube. Quantum efficiencies as high as 45% and output powers up to 10.5 W were obtained.</p> <p>Line-tunability between 10.7 and 13.3 μm was accomplished with a novel pumping scheme. The frequency of the CO₂ laser is shifted into coincidence with the line center of an NH₃ transition using acoustooptic modulators. This on-resonance pumping enabled us to create a vibrational inversion between the NH₃ ground state and the upper vibrational level v₂=1. By adding a buffer gas, gain was obtained in the entire P- and Q-branches. A simple model which assumes thermalization among the rotational levels fully accounts for the lasing behavior.</p>