Patent ID: 7414727

Claim:
Gas detection method comprising the following steps of providing an initial light signal (S 0 ), by a wavelength modulated laser source ( 1 ) driven with a DC drive current; 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 ) providing a detection signal (S D ), 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 at least one measuring signal (S MI ) from said detection signal (S D , S DC1 , S DC2 ), which is a function of intensity of said initial light signal (S 0 ); 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 providing a derivated detection signal (S DA ) substantially proportional to the time derivate of said resulting light signal (S G ), multiplying said derivated detection signal (S DA ) with a second modulation reference signal (S 2f ) at twice of said initial frequency (f) and then integrated over time, whereby the second modulation reference signal (S 2f ) has a defined amplitude level and a defined phase relationship with the intensity variations of said initial light signal (S 0 ), 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 at least one measuring signal (S MI ) and thereby providing a signal relative to the presence or the concentration of a given gas, characterised by generating said second measuring signal (S MA ) by determining a first pre-measuring signal (S M0 ) when the laser source is operated at the center of the gas absorption peak, a second pre-measuring signal (S M1 ) when the laser source is operated with a DC drive current below the gas absorption peak of the gas to be detected, and a third pre-measuring signal (S M2 ) when the laser source is operated with a DC drive current above said gas absorption peak, with a difference between said DC drive currents which corresponds to the etalon fringe period determined in a calibration step before; and determining the final second measuring signal (S MA ) as the difference between the first pre-measuring signal (S M0 ) and the arithmetic mean of the second pre-measuring signal (S M1 ) and the third pre-measuring signal (S M2 ).