Patent ID: 8322190

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 first light that is continuous and has an intensity that varies between a non-zero lower light intensity level and a higher light intensity level 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 amplifying the continuous light; a transducer for converting the sound waves into electrical signals; and a feedback loop for regulating the amplification of the continuous light in the optical cavity, the feedback loop comprising a photo detector for measuring the continuous light that passes through the optical cavity, wherein the light modulator is arranged for modulating light with a first polarization while not modulating light with a second polarization; and wherein the feedback loop comprises means for only providing the light with the second polarization to the photo detector.