Abstract
The principles and implementation of a phasor decomposition method for analyzing
signals in tunable diode laser spectroscopy with wavelength modulation are described.
This new technique enables recovery of the isolated and normalized residual amplitude
modulation (RAM) signal from measured first harmonic signals at any chosen fundamental
modulation frequency. Like the previously reported RAM technique, this new approach is
absolute, yielding gas absorption line shape functions, concentrations and pressures
without the need for calibration, under certain defined operating conditions. It
represents an advancement of the RAM technique in that it obviates the need to operate
at a specific high frequency to achieve phase quadrature between the RAM and derivative
signals: the signals may be recovered at their maximum levels at any frequency.
Measurements of the 1650.96 nm and the 1666.2 nm rotation/vibration absorption line
shape functions for 1% and 10% methane in nitrogen at various pressures are compared to
theoretical predictions derived from HITRAN data. The excellent agreement confirms the
validity of the new technique. Further measurements of concentration and pressure
confirm the efficacy of the technique for determining concentration in industrial
process environments where the pressure may be unknown and changing. With the above
features this new method is particularly suited to stand alone instrumentation for
on-line deployment in industrial processes where the calibration factors in the
conventional approach would present significant difficulties.
© 2008 IEEE
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