Wavelength modulation diode laser absorption spectroscopy for high-pressure gas sensing
A general model for 1f-normalized wavelength modulation absorption spectroscopy with nf detection (i.e., WMS-nf) is presented that considers the performance of injection-current-tuned diode lasers and the reflective interference produced by other optical components on the line-of-sight (LOS) transmission intensity. This model explores the optimization of sensitive detection of optical absorption by species with structured spectra at elevated pressures. Predictions have been validated by comparison with measurements of the 1f-normalized WMS-nf (for n = 2–6) lineshape of the R(11) transition in the 1st overtone band of CO near 2.3 μm at four different pressures ranging from 5 to 20 atm, all at room temperature. The CO mole fractions measured by 1f-normalized WMS-2f, 3f, and 4f techniques agree with calibrated mixtures within 2.0 %. At conditions where absorption features are significantly broadened and large modulation depths are required, uncertainties in the WMS background signals due to reflective interference in the optical path can produce significant error in gas mole fraction measurements by 1f-normalized WMS-2f. However, such potential errors can be greatly reduced by using the higher harmonics, i.e., 1f-normalized WMS-nf with n > 2. In addition, less interference from pressure-broadened neighboring transitions has been observed for WMS with higher harmonics than for WMS-2f.
KeywordsBackground Signal Modulation Depth Tunable Diode Laser Wavelength Modulation Spectroscopy Reflective Interference
This work was supported by the National Energy Technology Laboratory of the Department of Energy, with Dr. Susan Maley as the contractor monitor; and by the Air Force Office of Scientific Research, with Dr. Chiping Li as the contract monitor.
- 4.J. Wolfrum, Proc Combust Inst 27, 1–41 (1998)Google Scholar
- 9.J.B. Jeffries, A. Fahrland, W. Min, R.K. Hanson, D. Sweeney, D. Wagner, K.J. Whitty, 2009 Int. Pittsburgh Coal Conference, Pittsburgh (2009)Google Scholar
- 10.X. Chao, J.B. Jeffries, R.K. Hanson, Measurement Science and Technology 20, 115201 (9 pp) (2009)Google Scholar