We present a theoretical description of the ultrasensitive cavity-enhanced spectroscopic technique called noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE OHMS) for the case of transitions described by a Voigt line shape. The two levels of modulation used in NICE OHMS are treated with the standard theory for frequency modulation spectroscopy and a Fourier description of wavelength modulation spectroscopy. We compare predicted line shapes with experimental results for pressure-broadened transitions in molecular oxygen and show that our description can be used to determine the spectroscopic parameters. A key aspect of this research is the application of NICE OHMS to broad absorption features across a range of wavelengths, and etalon effects are shown to limit the detection sensitivity.
© 2004 Optical Society of America
(020.0020) Atomic and molecular physics : Atomic and molecular physics
(020.3690) Atomic and molecular physics : Line shapes and shifts
(300.0300) Spectroscopy : Spectroscopy
(300.6380) Spectroscopy : Spectroscopy, modulation
Nicola J. van Leeuwen and Andrew C. Wilson, "Measurement of pressure-broadened, ultraweak transitions with noise-immune cavity-enhanced optical heterodyne molecular spectroscopy," J. Opt. Soc. Am. B 21, 1713-1721 (2004)