Abstract
The article is devoted to a current topic in science and technology - determining the type and concentration of gas using a laser. At close values of green laser radiation intensity and bias voltage, interference patterns of the time series of the photodiode output signal were obtained. The degree of coherence (more than ~0.1) made it possible to distinguish between the types of gases and their concentrations. Signal coherence was controlled by Allan deviation values. The novelty of the research method is the choice of the harmonic of the electrical network as the main modulation frequency. It is shown that the correlation functions and the corresponding power spectra are sensitive to low-frequency fluctuations of molecules and their clumps. This allows the results and methods of this work to be used in specific cases instead of large-sized and expensive complexes of optical instruments at room pressure and temperature.