Method and apparatus for the photo-acoustic identification and quantification of analyte species in a gaseous or liquid medium
First Claim
1. A method for the identification and quantification of one or more analyte species present in a gaseous or liquid medium utilizing a laser that outputs a laser beam and a resonant optical cavity containing said medium and having at least two cavity mirrors, one of which is a cavity coupling mirror, the method comprising:
- coupling the laser beam to the cavity via the cavity coupling mirror using mode matching optics;
applying a periodic dither or modulation waveform to the optical frequency of the laser beam or to the laser itself to thereby induce modulation of the intracavity optical power;
detecting, at the frequency of the applied dither or modulation waveform or harmonics thereof, a signal representing a varying pressure of the medium within the cavity;
obtaining an electronic waveform signal proportional to the intracavity optical power by detecting an intensity of light emerging from a cavity mirror;
calculating a ratio of a magnitude of the varying pressure signal to a magnitude of the electronic waveform signal at the detection frequency; and
simultaneously sampling the varying pressure signal and the electronic waveform signal for use in reducing noise in the varying pressure signal caused by noise in the optical signal of the incident laser beam.
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Abstract
A method and apparatus for the photo-acoustic identification and quantification of one or more analyte species present in a gaseous or liquid medium in low concentration utilizing a laser and a resonant optical cavity containing the medium and having within the cavity at least two partially transparent mirrors, one of which is a cavity coupling mirror and one of which is moveably mounted on an assembly responsive to an input signal.
41 Citations
18 Claims
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1. A method for the identification and quantification of one or more analyte species present in a gaseous or liquid medium utilizing a laser that outputs a laser beam and a resonant optical cavity containing said medium and having at least two cavity mirrors, one of which is a cavity coupling mirror, the method comprising:
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coupling the laser beam to the cavity via the cavity coupling mirror using mode matching optics; applying a periodic dither or modulation waveform to the optical frequency of the laser beam or to the laser itself to thereby induce modulation of the intracavity optical power; detecting, at the frequency of the applied dither or modulation waveform or harmonics thereof, a signal representing a varying pressure of the medium within the cavity; obtaining an electronic waveform signal proportional to the intracavity optical power by detecting an intensity of light emerging from a cavity mirror; calculating a ratio of a magnitude of the varying pressure signal to a magnitude of the electronic waveform signal at the detection frequency; and simultaneously sampling the varying pressure signal and the electronic waveform signal for use in reducing noise in the varying pressure signal caused by noise in the optical signal of the incident laser beam. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15)
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16. A method for the identification and quantification of one or more analyte species present in a gaseous or liquid medium utilizing a laser that outputs a laser beam and a resonant optical cavity containing said medium and having at least two cavity mirrors, one of which is a cavity coupling mirror, the method comprising:
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coupling the laser beam to the cavity via the cavity coupling mirror using mode matching optics; applying a periodic dither or modulation waveform to the optical frequency of the laser beam or to the laser itself to thereby induce modulation of the intracavity optical power; detecting, at the frequency of the applied dither or modulation waveform or harmonics thereof, a signal representing a varying pressure of the medium within the cavity, wherein a portion of the light emerging from the cavity coupling mirror reenters a cavity of the laser while maintaining an optical phase that results in periodic optical feedback locking whereby a mean optical frequency of the laser matches a resonance peak of the cavity, the method further comprising; i) obtaining a time varying photo-detector output signal proportional to the intracavity optical power using a photo-detector located external to the cavity; ii) obtaining a magnitude of an in-phase component of the time-varying photo-detector output signal at a dither frequency of the dither waveform; iii) obtaining a magnitude of a quadrature component of the time-varying photo-detector output signal at the dither frequency; iv) using the in-phase component to maintain a lock between the mean optical frequency of the laser and the resonance peak of the cavity; and v) increasing or decreasing an optical path difference between the laser and the cavity coupling mirror by inputting to an element adapted to vary the optical path difference between the laser and the cavity coupling mirror a value proportional to the magnitude of the quadrature component of the time varying photo-detector output signal at the dither frequency until the magnitude of the quadrature component zeroes down.
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17. A method for the identification and quantification of one or more analyte species present in a gaseous or liquid medium utilizing a laser that outputs a laser beam and a resonant optical cavity containing said medium and having at least two cavity mirrors, one of which is a cavity coupling mirror, the method comprising:
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coupling the laser beam to the cavity via the cavity coupling mirror using mode matching optics; applying a periodic dither or modulation waveform to the optical frequency of the laser beam or to the laser itself to thereby induce modulation of the intracavity optical power; detecting, at the frequency of the applied dither or modulation waveform or harmonics thereof, a signal representing a varying pressure of the medium within the cavity; and optimizing the amplitude and shape of the applied periodic dither or modulation waveform, wherein optimizing includes; monitoring the intracavity optical power using a photo-detector located external to the cavity; and repeatedly; obtaining a magnitude of in-phase and quadrature components of an output signal of the photo-detector at the second harmonic of a dither frequency of the dither waveform; determining a phase of second harmonic sine and cosine reference functions that zeroes out the quadrature component, and setting a constant phase relative to the phase of the dither waveform to this determined phase; and determining an optimum laser wavelength dither amplitude that results in a maximum value of the in-phase component of the photo-detector signal at the second harmonic of the dither frequency. - View Dependent Claims (18)
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Specification