Method and apparatus for the photo-acoustic identification and quantification of analyte species in a gaseous or liquid medium

A method for the photo-acoustic identification and quantification of one or more analyte species present in a gaseous or liquid medium utilizing a laser and a resonant optical cavity containing said 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, comprising the steps of:

• a) selecting the cavity parameters to cause the cavity resonance width to be equal to or larger than the frequency bandwidth of a laser beam directed into said optical cavity,
• b) converting the spatial distribution of the beam to match the spatial distribution of the fundamental TEM00 mode of the cavity and directing said converted beam into the cavity through said coupling cavity mirror,
• c) positioning a first photo-detector in the path of the beam emerging from a cavity mirror other than the coupling mirror and connecting said first photo-detector to a DPCU,
• d) applying a periodic dither waveform to the frequency of the laser beam with the initial peak to peak optical frequency deviation being equal to said cavity resonance width, if there is no stored value for this deviation, or equal to the stored value
• e) moving the moveably mounted cavity mirror until the output signal of the first photo-detector in the form of a sequence of peaks and valleys thereby bringing the laser dither frequency midpoint close to the cavity resonance peak,
• f) obtaining the magnitude of the in-phase and quadrature components of the first photo-detector output signal at the dither frequency by multiplying said first photo-detector output signal by a sine and a cosine reference function having the same frequency as the dither frequency and a constant phase relative to the phase of the dither waveform,
• g) locking the cavity resonance to the laser with a feedback loop by directing to the movable cavity mirror assembly an electrical input signal which is proportional to said in-phase component and has a polarity that provides negative feedback,
• h) adjusting the phase of the sine and cosine reference function to minimize the value of the quadrature component and storing the found phase value in order to utilize said phase value as the constant phase in subsequent operating cycles,
• i) optimizing the dither amplitude by repeatedly
• obtaining the magnitude of the in-phase and quadrature components of the first photo-detector output signal at the second harmonic of the dither frequency
• determining the phase of the second harmonic sine and cosine reference functions that zeroes out the quadrature signal, and setting said constant phase relative to the phase of the dither waveform to this determined phase value,
• determining the optimum laser wavelength dither amplitude that results in the maximum value of the in-phase component of the first photo-detector time-varying signal at the second harmonic of the laser wavelength dither frequency
• j) storing the value of the optimum dither amplitude and the maximum magnitude of the in-phase component of the second harmonic of the time varying first photo-detector output signal in order to utilize said optimum dither amplitude in step d) for subsequent operating cycles,
• k) periodically determining the magnitude of the in-phase and quadrature components of the photo-acoustic signal from a photo-acoustic transducer situated inside the cavity at the second harmonic of the dither frequency,
• l) calculating the ratio of the photo-acoustic signal to the magnitude of the in-phase component of the first photo-detector signal at the second harmonic of the dither frequency to thereby determine a normalized photo-acoustic signal proportional to the optical absorption of said medium,
• m) determining the optical absorption of said medium at the laser signal wavelength by multiplying said normalized photo-acoustic signal by a pre-determined calibration coefficient
• n) incrementally scanning the laser operating wavelength and for each wavelength repeating steps f) through m),
• o) maintaining the position of the movable cavity mirror within its displacement range, and if the mirror's position approaches either of its range limits, resetting the mirror's position back by one cavity free spectral range,
• p) ensuring that the cavity maintains a locked position relative to the laser by measuring the magnitude of the in-phase component of the second harmonic of the time varying first photo-detector signal, comparing it with its value obtained in step i) and stored in step j) and if the ratio of these two values decreases below a pre-set threshold re-establishing the lock by repeating step e).