Frequency modulation spectroscopy with a CO_2 laser: results and implications for ultrasensitive point monitoring of the atmosphere

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Abstract
In a demonstration of frequency modulation spectroscopy with a CO2 laser, sidebands at 1 GHz were generated using a CdTe electrooptic phase modulator driven either by 5-kW pulses from a microwave cavity oscilllator or by 10 W of cw power from a solid-state microwave amplifier. Frequency modulation signals resulting from sideband absorption by Fabry-Perot resonances were measured using a room-temperature 1-GHz bandwidth HgCdTe detector. Signal-to-noise ratios for the conditions of our experiments were ∼200:1 and limited by rf pickup noise in the detection electronics. Substantial improvements in SNR can be made by providing better rf shielding for the detection electronics and by using a liquid–nitrogen-cooled detector in conjunction with improved modulator designs.