Measurements of density fluctuations by modulation spectroscopy

Theoretical and experimental results describing the application of wavelength modulation spectroscopy to density fluctuations are given. The effects of concurrent amplitude modulation, which often occurs when such experiments are performed with diode lasers are accounted for. It is shown that the characteristics of the signal magnitude at line center as a function of density include the expected increase with density, n, in the Doppler regime. In the effective collision broadened regime the signal magnitude at line center falls approximately as n(-N), where N is the order of the harmonic at which detection is performed. This result incorporates the well-known result for direct absorption (N=0) in which the signal magnitude at line center stays constant while the linewidth increases in the collision broadened regime. It is shown that the sensitivity to density fluctuations measured by wavelength modulation spectroscopy around any ambient value depends strongly on the order of the detection harmonic employed, and that in many instances this sensitivity can be increased by using detection harmonics of order greater than the commonly used second order. (C) 1996 American Institute of Physics.