  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.
