Resonant optical cavity system with optical feedback, suitable for detecting traces of gas by Raman spectroscopy

The invention relates to a method for analysing gas by Raman spectroscopy, the method comprising steps of generating, with a laser source (100), a laser beam that scans a frequency range including a plurality of resonant modes of an optical cavity (180) including gases to be analysed; delivering the laser beam to the cavity; extracting, from the cavity, a feedback beam that is sent, amplitude- and phase-adjusted, to the source; during the scan of the frequency range, detecting light-intensity peaks (221-229) in the laser beam in the cavity, the phase of the feedback beam being adjusted to decrease asymmetries in the shapes of the peaks, the amplitude of the feedback beam being adjusted to decrease zero-intensity intervals between the peaks; and carrying out a spectral analysis on light inelastically scattered in the cavity, in order to determine the composition of the gases to be analysed.