Nowadays it is possible to determine the type of gas with sufficient accuracy when its concentration is less than {10}^{-6} (in units of ppm) fractions using spectroscopic methods (optical, radio engineering, acoustic). Along with this, the value of permissible concentrations of explosive, toxic, harmful to technology and ecology gases is practically important. Known physical experimental studies indicate only a linear dependence of the response of a laser gas sensor at ppm\gtrsim{10}^3. The research methods for ppm\lesssim{10}^3 are based on the processes of combustion, microexplosion, structural and phase transformations and are not always applicable in real practical conditions. The work is devoted to the analysis of experimentally obtained fluctuations caused by a laser beam in a gas in a photodiode (signal receiver) due to its influence not only at the atomic level, but also on the scale of clusters of nanoparticle molecules. The gas concentration is estimated by the fluctuation-dissipation ratio. It is shown that the signal correlator is saturated to a constant value when the quantum (laser photon energy) and thermal (nanoparticle temperature) factors are comparable with an increase in the concentration of the target gas. The critical values of the saturation concentration are determined by the equality of these two factors.