385 µmD2O laser linewidth measurements to -60 dB

First linewidth measurements over a 60 dB dynamic range of a pulsed, high-power, optically pumped far infrared laser are presented. These measurements were made possible by using a 385 μmD
2
O laser with an N
2
O absorption filter and a sensitive heterodyne receiver. Studying a 385 μmD
2
O laser oscillator we find that the low-level linewidth (<-20 dB) can be explained by the ac Stark effect due to the high-power pump field. Also, we have not observed any frequency pulling of the main Raman emission frequency due to the strong pump and FIR laser fields.