Patent ID: 8109128

Claim:
A photo acoustic trace gas detector for detecting a concentration of a trace gas in a gas mixture, the photo acoustic trace gas detector comprising: a light source for producing a light beam; a light modulator for modulating the light beam into a series of light pulses at a chopping frequency for generating sound waves in the gas mixture, an amplitude of the sound waves being a measure of the concentration; an optical cavity for containing the gas mixture and for amplification of a light intensity of the light pulses; a transducer for converting the sound waves into electrical signals; and a feedback loop for regulating a ratio of a length of the optical cavity and a wavelength of the light beam for the amplification of the light intensity of the light pulses in the optical cavity, the feedback loop comprising: ratio modulating means for modulating the ratio at a modulation frequency; a photo detector for measuring the light intensity of the light pulses; and adjusting means, coupled to the photo detector and to the ratio modulating means for, in dependence of the measured light intensity, adjusting an average of the ratio, wherein the chopping frequency is higher than the modulation frequency.