Observation of undamped pulsations in a low-pressure, far-infrared laser and comparison with a simple theoretical model

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Abstract
Experimental details of undamped pulsations are presented for a low pressure, far-infrared (FIR) laser optically pumped by a single-frequency CO2 laser in a standing-wave configuration for both the pumping and the FIR lasing action. Spontaneous pulsing in the single-mode HCOOH FIR laser at 742 μm was observed when the cavity was tuned to the FIR resonance and when the CO2 laser of high intensity was sufficiently detuned from the resonance of the pump transition to excite two distinct velocity groups of molecules to contribute to the FIR gain. Theoretical results, based on studies of a model for a laser with two distinct resonant species forming the gain medium, provide a basis for understanding the origin of these instabilities. The model compares satisfactorily with the observed FIR instabilities including the variations in pulsing frequencies observed experimentally as the CO2 laser is detuned. Additional predictions from the model include details of transitions from cw to pulsing behavior and thresholds for deterministic chaos that may be tested in future experiments.