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Applied Optics

Applied Optics


  • Editor: Joseph N. Mait
  • Vol. 48, Iss. 29 — Oct. 10, 2009
  • pp: 5546–5560

Calibration-free wavelength-modulation spectroscopy for measurements of gas temperature and concentration in harsh environments

Gregory B. Rieker, Jay B. Jeffries, and Ronald K. Hanson  »View Author Affiliations

Applied Optics, Vol. 48, Issue 29, pp. 5546-5560 (2009)

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We present a practical implementation of calibration-free wavelength-modulation spectroscopy with second harmonic detection (WMS- 2 f ) for measurements of gas temperature and concentration in harsh environments. The method is applicable to measurements using lasers with synchronous wavelength and intensity modulation (such as injection current-tuned diode lasers). The key factors that enable mea surements without the on-site calibration normally associated with WMS are (1) normalization of the WMS- 2 f signal by the first harmonic ( 1 f ) signal to account for laser intensity, and (2) the inclusion of laser-specific tuning characteristics in the spectral-absorption model that is used to compare with measured 1 f -normalized, WMS- 2 f signals to infer gas properties. The uncertainties associated with the calibration-free WMS method are discussed, with particular emphasis on the influence of pressure and optical depth on the WMS signals. Many of these uncertainties are also applicable to calibrated WMS measurements. An example experimental setup that combines six tunable diode laser sources between 1.3 and 2.0 μm into one probe beam for measurements of temperature, H 2 O , and CO 2 is shown. A hybrid combination of wavelength and frequency demultiplexing is used to distinguish among the laser signals, and the optimal set of laser-modulation waveforms is presented. The system is demonstrated in the harsh environment of a ground-test scramjet combustor. A comparison of direct absorption and 1 f -normalized, WMS- 2 f shows a factor of 4 increase in signal-to-noise ratio with the WMS technique for measurements of CO 2 in the supersonic flow. Multidimensional computational fluid-dynamics (CFD) calculations are compared with measurements of temperature and H 2 O using a simple method that accounts for the influence of line-of-sight (LOS) nonuniformity on the absorption measurements. The comparisons show the ability of the LOS calibration-free technique to gain useful information about multidimensional CFD models.

© 2009 Optical Society of America

OCIS Codes
(120.1740) Instrumentation, measurement, and metrology : Combustion diagnostics
(120.6780) Instrumentation, measurement, and metrology : Temperature
(300.1030) Spectroscopy : Absorption
(300.6280) Spectroscopy : Spectroscopy, fluorescence and luminescence
(300.6380) Spectroscopy : Spectroscopy, modulation

ToC Category:

Original Manuscript: June 15, 2009
Revised Manuscript: September 1, 2009
Manuscript Accepted: September 3, 2009
Published: October 5, 2009

Gregory B. Rieker, Jay B. Jeffries, and Ronald K. Hanson, "Calibration-free wavelength-modulation spectroscopy for measurements of gas temperature and concentration in harsh environments," Appl. Opt. 48, 5546-5560 (2009)

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