Abstract: A semiconductor laser HL has an external resonator including a reflector R which reflects the emitted laser beam back into the front facet of the laser. The light emitted from the rear facet of the laser falls on a photodiode PD which generates a signal s which is evaluated by an evaluation unit AE. By differentiation or other processing of the shapes of pulses in the signal s, it is possible to determine the direction of change of any of several operating parameters of the laser system, such as the length L between laser HL and reflector R, the tilt angle of reflector R with respect to the optical axis of the laser or the refractive index n of the medium in the space N defined between the laser and the reflector. For example, one can detect when gas G is fed into space N to change the medium from ordinary air to carbon dioxide, or the reverse.