Optics: measuring and testing – For light transmission or absorption
Reexamination Certificate
2001-02-12
2003-09-09
Lee, Michael G. (Department: 2876)
Optics: measuring and testing
For light transmission or absorption
C356S437000, C250S339090
Reexamination Certificate
active
06618148
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention (Technical Field)
The present invention relates to high-sensitivity detection of contaminants in gases by optical techniques generally termed photoacoustic spectroscopy (PAS) or optoacoustic spectroscopy.
2. Background Art
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
Photoacoustic spectroscopy is a known technique for high sensitivity detection of trace gases. Absorption by the target species of incident optical energy results in a transient heating of the gas. If the incident optical energy is modulated then the gas is periodically heated which creates a time-varying pressure wave or sound. The sound can be measured with a microphone.
PAS is often enhanced through the use of acoustically resonant gas cells. These cells build up sound intensity at the resonance frequencies. Depending on the type of noise dominant in the system, resonant cells can dramatically improve signal-to-noise ratios and thereby, the measurement sensitivity. Individual and combination acoustic resonance modes including longitudinal, radial, and azimuthal are often utilized. Unfortunately, resonance frequencies depend on the local speed of sound which can change with temperature and gas composition. In addition, changes in cell dimensions due to mechanical stress can change these resonance frequencies. Thus, for a practical resonance-based photoacoustic spectrometer, it is necessary to maintain the modulation frequency at the acoustic resonance frequency.
The ability to maintain the optical source modulation frequency or its harmonics on an acoustic resonance frequency of a photoacoustic cell will hereafter be referred to as resonance frequency locking. In field measurements, microphone noise is not usually a photoacoustic instrument's sensitivity-limiting noise source. When microphone noise is not the limiting noise source, operation on an acoustic resonance enhances the signal-to-noise ratio.
Optoacoustic Spectroscopy
and Detection, Y-H Pao, ed. (Academic Press, New York, 1977), pps. 20-22. Because there is a 1/f dependence of the photoacoustic signal on frequency, operation at a resonance frequency represents a compromise between the enhancement, or Q, of the resonance cell and higher frequency operation. Thus, resonant operation is usually desirable provided the resonance frequency is not so high that the 1/f penalty outweighs the cavity Q. For example, a cell with a resonance at 8000 Hz with a Q of 200 would generate the same signal, all other parameters being equal, as a non-resonant cell operating at 40 Hz. In many systems, there is less background acoustic noise at 8000 Hz than at 40 Hz; resonant operation at 8000 Hz then provides a better signal-to-noise ratio.
Acoustic resonances may have narrow bandwidths. As the amplification factor (or resonance cavity Q) increases for a given frequency, the bandwidth gets narrower. Thus, as sensitivity is increased by improving the acoustic quality of the photoacoustic cell, the need for a method to maintain the acoustic modulation frequency on resonance increases proportionately. For example, a cell with a Q of 200 at 8000 Hz will have a bandwidth of 40 Hz. Thus, a change of the resonance frequency of only 20 Hz will reduce the signal by a factor of 2. A change of 20 Hz can be caused by a temperature change of less than 1 degree C at room temperature.
The present invention achieves continuous, real-time acoustic resonance frequency locking by sweeping the optical source modulation frequency or its harmonics across a cell acoustic resonance. Because the frequency of the modulation is swept over the cell resonance, the amplitude of the acoustic signal is modulated. The acoustic resonance converts the frequency modulation into an amplitude modulation. This effect is similar to frequency or wavelength modulation where a laser is swept across a molecular absorption feature where there is a maximum attenuation of the beam on the peak of the molecular absorption. With the present invention, there is a maximum amplification or enhancement on the peak of the acoustic resonance feature.
With analogy to wavelength modulation, if the acoustic modulation frequency is swept equally to either side of the peak of the acoustic resonance, a sweep frequency of &ohgr; will result in an amplitude modulation at a frequency of 2&ohgr; since the sweep will cross the resonance center twice during every sweep cycle. When the frequency sweep is symmetric about the acoustic resonance line center, the carrier, the 2&ohgr; and higher even harmonic signals will be at maximum and the 1&ohgr; and higher odd harmonic signals will have zero crossings. Thus, an odd harmonic of the frequency modulation sweep rate can be used as an error signal for adjusting the frequency modulation carrier frequency. As the acoustic resonance frequency drifts with temperature, gas composition, etc., the acoustic modulation frequency will be continually updated and maintained to match the cell acoustic resonance frequency.
The present invention for acoustic resonance frequency locking can be used to equal effectiveness regardless of the method of producing photoacoustic signals. In traditional PAS the optical source radiation is amplitude modulated (AM). The modulation can be achieved by means of a mechanical chopper, a shutter, an acousto-optic modulator, or modulation of a (e.g., semiconductor) pump waveform. Other methods for achieving an amplitude modulated optical source are contemplated by and fall within the invention. In addition, Southwest Sciences, Inc. has implemented wavelength modulation spectroscopy (WMS) with PAS detection, as described in U.S. patent application Ser. No. 09/687,408. With WMS, the optical radiation source is modulated in wavelength, not amplitude (if WMS is implemented with injection current modulation of a diode laser, AM results only as a side-effect). Nevertheless, WMS produces a synchronous amplitude modulated pressure wave at the microphone. Because the present acoustic resonance frequency locking mechanism depends on features of the cell acoustic resonance and not the source of the sound, it is equally applicable to AM and WMS-based PAS.
A source of acoustic power independent of PAS generation (a speaker) has previously been used to implement an acoustic frequency locking mechanism. M. W. Sigrist and coworkers generated sound at a resonance frequency of a PAS cell with a speaker whose frequency was locked to a cell resonance via the microphone's detection phase at the resonance frequency. G. Z. Angeli, et al., “Design and characterization of a windowless resonant photoacoustic chamber equipped with resonance locking circuitry”
Rev. Sci. Instrum
. 62, 810 (1991). The locked resonance frequency was used to generate an amplitude modulated optical frequency for PAS generation The optical source modulation operated at a separate cell resonance that was a constant fraction of the frequency used for resonance locking. Several disadvantages of this approach are readily apparent. The method requires a separate acoustic source independent of the PAS generation source. The method introduces sound at frequencies other than that where the PAS signal occurs. This sound must be attenuated in order to prevent overloading of the detection microphone. Depending on the acoustic source spectral purity, noise may be induced at the PAS detection frequency. The method relies on the PAS resonance frequency and the acoustic source generated locking frequency changing in the same way in a dynamic environment. If the cell geometry changes differently for the two frequencies (due, for example, to a mechanical stress), the frequency ratio will not
Bomse David S.
Pilgrim Jeffrey S.
Silver Joel A.
Fureman Jared J.
Lee Michael G.
Myers Jeffrey D.
Southwest Sciences Incorporated
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