Abstract
Tunable diode laser spectroscopy is extremely important for gas detection
in a wide variety of industrial, safety and environmental monitoring applications.
Of particular interest is the development of calibration-free, stand-alone
systems and instrumentation which can operate in high-temperature or high-pressure
environments (such as in fuel cells, or gas turbine engines) where continuous
and simultaneous monitoring of pressure, temperature and concentration of
gases may be required. Here, in Part 1 of this two-part paper, we present
the full theoretical basis and range of techniques for calibration-free line-shape
recovery to allow simultaneous measurements of concentration and pressure/temperature
for a wide range of potential applications. Firstly, on the basis of Fourier
analysis, we present the general signal components that arise with both intensity
and frequency modulation of diode lasers and identify the issues and difficulties
associated with accurate line-shape recovery in conventional wavelength modulation
spectroscopy (WMS). We then show how line-shape recovery may be effectively
performed using first harmonic signals and, by use of a general correction
function from Fourier coefficients, we extend the techniques previously reported
to include arbitrary large modulation indices, different line-shape profiles
and high gas concentration with non-linear absorption. Previous approximate
techniques based on Taylor series expansions are included as a special case
of the Fourier analysis for low modulation indices. We also show that the
signal amplitudes obtained in this way can be comparable to, or even exceed,
that of conventional WMS by appropriate choice of the modulation index and
frequency.
© 2011 IEEE
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