Optics: measuring and testing – For light transmission or absorption – Of fluent material
Reexamination Certificate
1999-07-30
2002-03-12
Rosenberger, Richard A. (Department: 2877)
Optics: measuring and testing
For light transmission or absorption
Of fluent material
C250S343000
Reexamination Certificate
active
06356350
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention (Technical Field):
The present invention relates to wavelength modulation spectroscopy.
2. Background Art:
Wavelength modulation spectroscopy (WMS) is a form of optical absorption spectroscopy that allows detection of small optical absorbances of gases and, thereby, measurements of gas concentrations. The technique is effective because absorption measurements are shifted from frequencies near DC, where light sources are noisy, to high frequencies where shot-noise-limited absorption measurements are possible. This shift in detection band can improve measurement sensitivity by three to five orders of magnitude.
WMS is usually implemented with continuously tunable lasers such as diode lasers. Typically, the wavelength of the light source is modulated by a small amount about an absorption feature of the target species. The modulation frequency is f. As the light beam propagates through a sample, absorption by the target species converts some of the wavelength modulation into an amplitude modulation (AM) of the light because more light is absorbed at the absorption peak wavelength. When the light impinges onto a photodetector such as a photodiode the output signal from the detector contains AC components at the modulation frequency, f, and its higher harmonics, 2f, 3f, 4f, etc. In conventional usage, one of the AC components is selected for measurement using a phase sensitive detector such as a lock-in amplifier or a mixer. This signal processing step is known as demodulation. Usually a portion of the modulation waveform is used to generate a reference waveform (local oscillator) for the demodulator. The resulting demodulated signal is related to the optical absorbance and to the intensity of the light beam.
Detailed theory describing WMS and the relationships between the absorption lines shape and demodulated line shapes is given by Silver [J. Silver, “Frequency-modulation spectroscopy for trace species detection: theory and comparison among experimental methods,” Applied Optics 31, 707-717 (1992)]. In qualitative terms, the waveform produced by slowly stepping the average laser wavelength across an absorption line while demodulating at frequency nf is similar in shape to the nth derivative of the absorption line shape and is referred to as the nf signal or nf spectrum. In the limiting case where the extent (depth) of modulation is much less than the absorption line width, theory predicts that the nf spectrum is directly proportional to the exact nth derivative of the absorption line shape.
The shape of a wavelength modulation spectrum depends strongly on the ratio of the extent of the wavelength modulation to the line width of the absorption feature. Any phenomenon that changes the absorber line width, such as variations in sample pressure or, to a lesser extent, variations in sample temperature, will change the shape and peak intensities of the corresponding wavelength modulation spectrum. Changes in absorber line width can, therefore, introduce error into quantitative applications of WMS particularly where such applications are used to measure species concentrations.
A number of methods exist that can be used to apply wavelength modulation spectra for gas sensing despite changes in the absorber line width; each of these approaches, however, has some limitation. For example, Wilson [G. V. H. Wilson, “Modulation broadening of NMR and ESR line shapes,”
J. Appl. Phys
. 34, 3276-3285 (1963)] shows that the exact shape of a wavelength modulation spectrum can be used to extract the absorber line width and, thereby, calculate the actual optical absorbance and the species concentration. Wilson's method, however, requires WMS measurements that are free of noise and background artifacts (i.e., etalons) in order to obtain accurate line widths, absorbances, and species concentrations. Wilson's numerical inversion methods do not always guarantee convergence and are subject to numerical singularities.
Goldstein et al. patented an improvement to wavelength modulation spectroscopy in which the detector signal at twice the modulation frequency (2f) is monitored while the extent of the wavelength modulation is changed [N. Goldstein, F. Bien, and L. Bernstein, “Gaseous Species Absorption Monitor,” U.S. Pat. No. 5,026,991, issued Jun. 25, 1991; N. Goldstein, S. Adler-Golden, J. Lee, and F. Bien, “Measurement of molecular concentrations and line parameters using line-locked second harmonic spectroscopy with an AlGaAs diode laser,”
Appl. Opt
. 31, 3409-3415 (1992)]. The response of the 2f signal as a function of extent of modulation is representative of the shape and width of the absorption line. Goldstein's invention is simple to implement because it requires only a minor modification to standard WMS instrumentation. The most significant limitation of the invention, however, arises because lasers often respond non-linearly to applied modulation waveforms. Both the extent (depth) of modulation and the time dependence of the output wavelength may not track well the changes in the applied modulation signal. Proper implementation of the invention may require careful calibration of the response of each laser or using customized (e.g., non-sinusoidal) modulation waveforms. The nonlinearities are particularly important when relatively large wavelength excursions are needed, such as occur for detecting absorbances from samples at atmospheric or higher pressure.
Species concentrations inferred from wavelength modulation spectra can be corrected by measuring sample temperature and pressure, and using corrections calculated from basic theory or from tabulated calibrations. The computational approach can be slow, however, and requires a significant amount of computing power; tabulating a set of corrections requires a lengthy and tedious calibration. In both cases, the instrument is made more complex and more expensive by adding pressure and temperature sensors.
Other patents discussing related technology but different from the present invention include: U.S. Pat. No. 5,640,245, to Zybin et al., entitled “Spectroscopic Method with Double Modulation;” U.S. Pat. No. 5,636,035, to Whittaker et al., entitled “Method and Apparatus for Dual Modulation Laser Spectroscopy;” U.S. Pat. No. 5,267,019, to Whittaker et al., entitled “Method and Apparatus for Reducing Fringe Interference in Laser Spectroscopy;” U.S. Pat. No. 5,498,875, to Obremsky et al., entitled “Signal Processing for Chemical Analysis of Samples;” U.S. Pat. No. 5,637,872, to Tulip, entitled “Gas Detector;” U.S. Pat. No. 5,448,071 to McCaul et al., entitled “Gas Spectroscopy;” U.S. Pat. No. 5,068,864, to Javan, entitled “Laser Frequency Stabilization;” U.S. Pat. No. 4,990,775, to Rockwood et al., entitled “Resolution Improvement in an Ion Cyclotron Resonance Mass Spectrometer,” and U.S. Pat. No. 4,468,773, to Seaton, entitled “Laser Control Apparatus and Method.”
U.S. Pat. No. 5,015,848 to Bomse et al., entitled “Mass Spectrometric Apparatus and Method,” is related to the field of mass spectrometry, but has no relation to the present invention except for the presence of common inventors, and is included here only for the sake of completeness. Co-pending Application Ser. No. 09/005,356, to Bomse, entitled “Phaseless Wavelength Modulation Spectroscopy,” is perhaps most relevant to the present invention and the disclosure therein is incorporated herein by reference. It improves wavelength modulation spectroscopy by extracting information about the line width and line shape of absorption features. The information is in the form of the relative intensities of wavelength modulation spectra acquired at a plurality of demodulated harmonics. This added information can be used to improve the accuracy of gas concentration measurements or to infer physical properties of the gas such as pressure, temperature, and chemical composition.
A key difference between “Phaseless Wavelength Modulation Spectroscopy” and the present invention is that the phaseless method uses one h
Bomse David S.
Silver Joel A.
Myers Jeffrey D.
Rosenberger Richard A.
Southwest Sciences Incorporated
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