Abstract
Recovery of absolute gas absorption
line shapes from first harmonic residual AM (RAM)
signals in tunable diode laser spectroscopy with
wavelength modulation (TDLS-WM) offers significant
advantages in terms of measurement accuracy (for gas
concentration and pressure), freedom from the need for
calibration and resilience to errors, or drift in system
parameters/scaling factors. However, the signal strength
and SNR are compromised somewhat relative to
conventional WM spectroscopy (WMS) by the signal
dependence on the laser's intensity modulation amplitude
rather than on the direct intensity, and by the need to
operate at low modulation index, ${m}\ (< 0.75)$, in the previously reported study. In part 1
of this two-part publication, we report a more universal
approach to the analysis of recovered RAM signals and
absolute absorption line shapes. This new approach
extends the use of RAM techniques to arbitrary m values up to 2.2.
In addition, it provides the basis for a comparison of
signal strength between the RAM signals recovered by the
phasor decomposition approach and conventional first and
second harmonic TDLS-WM signals. The experimental study
reported here validates the new model and demonstrates
the use of the RAM techniques for accurate recovery of
absolute gas absorption line shapes to ${m} = 2.2$ and above. Furthermore, it demonstrates that
the RAM signal strengths can be increased significantly
by increasing the modulation frequency and defines
regimes of operation such that the directly recovered
RAM signals are comparable to or even greater than the
widely used conventional second harmonic TDLS-WM signal.
Finally, a critique of the RAM techniques relative to
the conventional approaches is given.
© 2011 IEEE
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