Signal-to-noise enhancement techniques for quantum cascade absorption spectrometers employing optimal filtering and other approaches |
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Authors: | RS Disselkamp JF Kelly RL Sams GA Anderson |
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Institution: | (1) Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352, USA, US |
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Abstract: | Optical feedback to the laser source in tunable diode laser spectroscopy (TDLS) is known to create intensity modulation noise
due to elatoning and optical feedback (i.e. multiplicative technical noise) that usually limits spectral signal-to-noise (S/N).
The large technical noise often limits absorption spectroscopy to noise floors 100-fold greater than the Poisson shot noise
limit due to fluctuations in the laser intensity. The high output powers generated from quantum cascade (QC) lasers, along
with their high gain, makes these injection laser systems especially susceptible to technical noise. In this article we discuss
a method of using optimal filtering to reduce technical noise. We have observed S/N enhancements ranging from ∼20% to a factor
of ∼50. The degree to which optimal filtering enhances S/N depends on the similarity between the Fourier components of the
technical noise and those of the signal, with lower S/N enhancements observed for more similar Fourier decompositions of the
signal and technical noise. We also examine the linearity of optimal filtered spectra in both time and intensity. This was
accomplished by creating a synthetic spectrum for the species being studied (CH4, N2O, CO2 and H2O in ambient air) utilizing line positions and linewidths with an assumed Voigt profile from a commercial database (HITRAN).
Agreement better than 0.036% in wavenumber and 1.64% in intensity (up to a 260-fold intensity ratio employed) was observed.
Our results suggest that rapid ex post facto digital optimal filtering can be used to enhance S/N for routine trace gas detection.
Received: 1 April 2002 / Revised version: 7 May 2002 / Published online: 21 August 2002
RID="*"
ID="*"Corresponding author. Fax: +1-509/376-6066, E-mail: robert.disselkamp@pnl.gov |
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Keywords: | PACS: 07 07 Df 42 62 Fi 42 79 -e |
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