ABSTRACT
A. N. Dharamsi and A. M. Bullock; App. Phys. Lett.; 69,
22, 1996.
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 linecenter 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 linecenter 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 linecenter
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.
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Modulation Spectroscopy