Atmospheric pollution measuring system by incoherent heterodyne detection using a tunable CO2 laser

The paper examines the realization of heterodyne detection for incoherent light by using a tunable CO2 laser. The input signal, the SNR, and the temperature resolution of an incoherent heterodyne radiometer are analyzed. A double-synchronous detection method to eliminate the shot noise due to the signal light is proposed along with a correction method to compensate the errors due to the nonlinearity and the excess noise of the semiconductor photodetector used. The described system is applied to the detection of air pollution. It is shown that the incoherent heterodyne detection technique is suitable for measuring the temperature distribution and for detecting sensitive gases such as SF2 and CCl2F2 by measuring their absorption spectra.