Fundamental noise sources in a high-sensitivity two-tone frequency modulation spectrometer and detection of CO2 at 1.6 mu m and 2 mu m

A high-sensitivity diode laser spectrometer around 1.6 mu m, based on two-tone frequency modulation spectroscopy in conjunction with a balanced homodyne detection scheme, has been implemented. The noise contributions to the final signal have been analyzed, and excess amplitude noise and residual amplitude modulation have been reduced below the quantum noise level, thanks to a rejection ratio in excess of 30 dB. The spectrometer has been tested on weak molecular absorptions of CO2, and its sensitivity is 7(2) x 10(-8) in a kHz bandwidth. The corresponding minimum detectable concentration of CO2 in air has been estimated to be 1 ppm . m. This opens the possibility of a detection at ppb levels at 2 mu m, where a two orders of magnitude increase in the CO2 absorption signal is demonstrated.