Laser Absorption Spectroscopy

The Oxigraf oxygen sensor uses absorption spectroscopy similar to the absorption spectroscopy method used to measure CO2. Absorption spectroscopy for oxygen occurs in a region of the visible spectrum near 760 nm where no interferences by other gases are known. The emission line width of the laser diode source used by Oxigraf and the absorption width of the individual electronic-rotational lines of O2 are very narrow, both less than 0.01nm, compared to about 100 nm for the CO2 absorption band. This makes the specificity of laser diode spectroscopy for oxygen much greater than the black-body and filter technique of infrared spectroscopy used today to measure CO2. To measure oxygen the spectrally pure laser is tuned thermally and electronically to an oxygen absorption line. As the oxygen concentration increases in the sample chamber, the light intensity, measured with a photodiode detector, is attenuated by the energy absorbed by oxygen molecules. The oxygen analyzer response varies linearly with oxygen concentration.