Method and apparatus for the photo-acoustic identification and quantification of analyte species in a gaseous or liquid medium
First Claim
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1. A system for identifying and quantifying one or more analyte species present in a gaseous or liquid medium, the system comprising:
- a laser that emits a laser beam;
a resonant optical cavity containing said medium and having at least two cavity mirrors, one of which is a cavity coupling mirror;
mode matching optics configured to couple the laser beam to the cavity via the cavity coupling mirror;
means for 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;
a means for detecting, at the frequency of the applied dither or modulation waveform or harmonics thereof, a varying pressure of the medium within the cavity;
a photo-detector located external to the cavity for detecting the intensity of light emerging from a cavity mirror;
an optical isolator positioned between the laser and the cavity coupling mirror, wherein the optical isolator is partially transmitting such that a portion of the light emerging from the cavity coupling mirror enters a cavity of the laser;
an element adapted to vary the optical path difference between the laser and the cavity coupling mirror; and
an intelligence module adapted to;
i) receive a time varying photo-detector output signal;
ii) determine a magnitude of an in-phase component of the time-varying photo-detector output signal at a dither frequency of the dither waveform;
iii) determine a magnitude of a quadrature component of the time-varying photo-detector output signal at the dither frequency;
iv) use the in-phase component to maintain a lock between a mean optical frequency of the laser and a resonance peak of the cavity; and
v) increase or decrease an optical path difference between the laser and the cavity coupling minor by inputting to the optical element 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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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.
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Citations
18 Claims
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1. A system for identifying and quantifying one or more analyte species present in a gaseous or liquid medium, the system comprising:
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a laser that emits a laser beam; a resonant optical cavity containing said medium and having at least two cavity mirrors, one of which is a cavity coupling mirror; mode matching optics configured to couple the laser beam to the cavity via the cavity coupling mirror; means for 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; a means for detecting, at the frequency of the applied dither or modulation waveform or harmonics thereof, a varying pressure of the medium within the cavity; a photo-detector located external to the cavity for detecting the intensity of light emerging from a cavity mirror; an optical isolator positioned between the laser and the cavity coupling mirror, wherein the optical isolator is partially transmitting such that a portion of the light emerging from the cavity coupling mirror enters a cavity of the laser; an element adapted to vary the optical path difference between the laser and the cavity coupling mirror; and an intelligence module adapted to; i) receive a time varying photo-detector output signal; ii) determine a magnitude of an in-phase component of the time-varying photo-detector output signal at a dither frequency of the dither waveform; iii) determine a magnitude of a quadrature component of the time-varying photo-detector output signal at the dither frequency; iv) use the in-phase component to maintain a lock between a mean optical frequency of the laser and a resonance peak of the cavity; and v) increase or decrease an optical path difference between the laser and the cavity coupling minor by inputting to the optical element 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. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 14, 15, 16, 17, 18)
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12. A system for identifying and quantifying one or more analyte species present in a gaseous or liquid medium, the system comprising:
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a laser that emits a laser beam; a resonant optical cavity containing said medium and having at least two cavity mirrors, one of which is a cavity coupling mirror; mode matching optics configured to couple the laser beam to the cavity via the cavity coupling mirror; means for 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; a means for detecting, at the frequency of the applied dither or modulation waveform or harmonics thereof, a varying pressure of the medium within the cavity; a means for optimizing the amplitude and shape of the applied periodic dither or modulation waveform; and an intelligence module and a photo-detector located external to the cavity for detecting the intensity of light emerging from a cavity mirror, wherein the intelligence module is adapted to optimize the amplitude and shape of the applied periodic dither or modulation waveform by repeatedly; determining 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 detection frequency. - View Dependent Claims (13)
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Specification