Patent ID: 8441644

Claim:
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.