A new technique to improve the sensitivity of frequency-modulation laser absorption spectroscopy is reported. Theoretical analysis and experimental demonstration show that the application of two harmonically related, appropriately phase-shifted rf waves to the electro-optic modulator can reduce the residual amplitude-modulation background signal in the photodetector output current without reducing the absorption-induced signal. When the new method was used, the residual amplitude-modulation background was reduced by a factor of 4. Our results suggest that shot-noise-limited detection sensitivity may eventually be achieved without using dual-beam compensation or dc-biased modulators.

© 1988 Optical Society of America

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