A two-wavelength oscillation CO2 laser for differential absorption measurements of atmospheric trace molecules

A two-wavelength oscillation C0₂ laser was constructed for the differential absorption measurement of low concentrations of atmospheric trace molecules and pollution molecules. A diffraction grating was alternately angle-modulated for the two-branch oscillation using a single C0₂ laser. The output power of the two wavelengths was balanced to be equal. With these characteristics, the phase-sensitive detection technique can be applied to give highly sensitive detection of the received power difference, which is proportional to the absorption in the optical path of the transmitted beams. A differential absorption spectrometer was used to demonstrate the usefulness of this two-wavelength laser oscillation method incorporating the output power balancing technique. The concentration of ethylene molecules was detected in a calibration cell and was also measured in real time in the exhaust of a combustion engine. From these results it has been shown that this differential absorption scheme offers a minimum detectable absorption of approximately 0.8%.