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P. Martin-Mateos, J. Hayden, P. Acedo, B. Lendl:
"Heterodyne Phase-Sensitive Dispersion Spectroscopy in the Mid-Infrared with a Quantum Cascade Laser";
Analytical Chemistry, 89 (2017), 11; S. 5916 - 5922.

Molecular dispersion spectroscopy encompasses a group
of spectroscopic techniques for gas analysis that retrieve the
characteristics of the sample from the measurement of the profile of
its refractive index in the vicinity of molecular resonances. This
approach, which is in clear contrast to traditional methods based on the
detection of absorption, provides inherent immunity to power
fluctuations, calibration-free operation, and an output that is linearly
dependent on gas concentration. Heterodyne phase-sensitive dispersion
spectroscopy (HPSDS) is a very recently proposed technique
for molecular dispersion spectroscopy based on tunable lasers that is
characterized by a very simple architecture in which data processing
and concentration retrieval are straightforward. Different HPSDS
implementations have been experimentally validated in the near-IR.
Here, we present the first demonstration of HPSDS in the mid-IR
using a directly modulated quantum cascade laser for the measurement of CO. The setup is put under test to characterize its
response to changing concentrations, pressures, and levels of optical intensity on the detector, and the limit of detection is
estimated. Besides this, an experimental comparison with wavelength modulation spectroscopy with second-harmonic detection
(2f-WMS) is performed and discussed in detail in order to offer a clear view of the benefits and drawbacks that HPSDS can
provide over what we could consider the reference method for gas analysis based on tunable laser spectroscopy.

http://dx.doi.org/10.1021/acs.analchem.7b00303