Patent ID: 7183553

Claim:
Gas detection method comprising the following steps of providing an initial light signal (S 0 ), by a wavelength modulated laser source ( 1 ); providing an AC modulation signal at an initial frequency for wavelength modulation of said initial light signal (S 0 ) at said initial frequency (f) symmetrically around an absorption line of a gas the concentration or presence of which is to be determined; passing said initial light signal (S 0 ) having intensity variations over the time resulting from an alternative scanning around said gas absorption line through a gas detection region ( 4 ) intended for receiving at least one of said gases; receiving a resulting light signal (S G ) exciting said gas detection region ( 4 ) by detection means ( 8 ), said resulting light signal (S G ) comprises changes in the intensity of the initial light signal (S 0 ) due to the gas concentration in the detection region ( 4 ); generating a first modulation reference signal (S 2f0 ) at twice of said initial frequency and then integrated over time, whereby said first modulation reference signal (S 2f0 ) has a 45° phase angle to said initial light signal (S 0 ), and oscillates at an amplitude level being different from the amplitude level of said second modulation reference signal (S 2f1 ) between amplitude levels 1 and 0; generating a second modulation reference signal (S 2f , S 2f1 ) at twice of said initial frequency (f) and then integrated over time, whereby the second modulation reference signal (S 2f , S 2f1 ) has a defined amplitude level and a defined phase relationship with the intensity variations of said initial light signal (S 0 ); generating a first measuring signal (S MI ), which is a function of intensity of said initial light signal (S 0 ), said first measuring signal (S MI ) is generated by multiplying a detection signal (S D0 ) directly received from said resulting light signal (S G ) with said first modulation reference signal (S 2f0 ); generating a second measuring signal (S MA ), which is a function of the gas absorption and substantially independent of an intensity modulation of said initial light signal at said initial frequency (f), said second measuring signal (S MA ) is generated by multiplying a second detection signal (S D0 , S D1 ) received from said resulting light signal (S G ) with said second modulation reference signal (S 2f , S 2f1 ); providing a final measuring signal being independent from the intensity of light incident onto the detection means ( 8 ) by dividing said second measuring signal (S MA ) by said first measuring signal (S MI ) and thereby providing a signal relative to the presence or the concentration of a given gas.