Abstract: The invention relates to the field of analytical measuring technology and can be used for the detection of harmful substances in the atmosphere, for the process control of chemical gas phase reactions and for analyzing burnt off products, for example in laser surgery. The determination of the concentration of the substance to be detected ensues from the change in the modulation depth of the intensity of the irradiation of a laser operating according to the longitudinal two-mode-operation upon passage thereof through the gaseous medium to be analyzed.