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Applied Optics

Applied Optics

APPLICATIONS-CENTERED RESEARCH IN OPTICS

  • Editor: Joseph N. Mait
  • Vol. 53, Iss. 3 — Jan. 20, 2014
  • pp: 356–367

Fitting of calibration-free scanned-wavelength-modulation spectroscopy spectra for determination of gas properties and absorption lineshapes

Christopher S. Goldenstein, Christopher L. Strand, Ian A. Schultz, Kai Sun, Jay B. Jeffries, and Ronald K. Hanson  »View Author Affiliations


Applied Optics, Vol. 53, Issue 3, pp. 356-367 (2014)
http://dx.doi.org/10.1364/AO.53.000356


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Abstract

The development and initial demonstration of a scanned-wavelength, first-harmonic-normalized, wavelength-modulation spectroscopy with nf detection (scanned-WMS-nf/1f) strategy for calibration-free measurements of gas conditions are presented. In this technique, the nominal wavelength of a modulated tunable diode laser (TDL) is scanned over an absorption transition to measure the corresponding scanned-WMS-nf/1f spectrum. Gas conditions are then inferred from least-squares fitting the simulated scanned-WMS-nf/1f spectrum to the measured scanned-WMS-nf/1f spectrum, in a manner that is analogous to widely used scanned-wavelength direct-absorption techniques. This scanned-WMS-nf/1f technique does not require prior knowledge of the transition linewidth for determination of gas properties. Furthermore, this technique can be used with any higher harmonic (i.e., n>1), modulation depth, and optical depth. Selection of the laser modulation index to maximize both signal strength and sensitivity to spectroscopic parameters (i.e., gas conditions), while mitigating distortion, is described. Last, this technique is demonstrated with two-color measurements in a well-characterized supersonic flow within the Stanford Expansion Tube. In this demonstration, two frequency-multiplexed telecommunication-grade TDLs near 1.4 μm were scanned at 12.5 kHz (i.e., measurement repetition rate of 25 kHz) and modulated at 637.5 and 825 kHz to determine the gas temperature, pressure, H2O mole fraction, velocity, and absorption transition lineshape. Measurements are shown to agree within uncertainty (1%–5%) of expected values.

© 2014 Optical Society of America

OCIS Codes
(120.1740) Instrumentation, measurement, and metrology : Combustion diagnostics
(120.6780) Instrumentation, measurement, and metrology : Temperature
(280.2470) Remote sensing and sensors : Flames
(280.2490) Remote sensing and sensors : Flow diagnostics
(300.1030) Spectroscopy : Absorption
(300.6380) Spectroscopy : Spectroscopy, modulation

ToC Category:
Remote Sensing and Sensors

History
Original Manuscript: August 8, 2013
Manuscript Accepted: November 14, 2013
Published: January 15, 2014

Citation
Christopher S. Goldenstein, Christopher L. Strand, Ian A. Schultz, Kai Sun, Jay B. Jeffries, and Ronald K. Hanson, "Fitting of calibration-free scanned-wavelength-modulation spectroscopy spectra for determination of gas properties and absorption lineshapes," Appl. Opt. 53, 356-367 (2014)
http://www.opticsinfobase.org/ao/abstract.cfm?URI=ao-53-3-356

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