A type of Laser spectroscopy technique for highly sensitive spectroscopic measurements. The substance to be evaluated is placed within the Resonator of a laser; once turned on, the laser will oscillate on many Resonator Modes simultaneously, and eventually, the Spectrum of the generated Light will strongly concentrate on the spectral region with the highest gain. A measurement of the spectrum is done after Switching on the laser; this time should be long enough to allow for strong special features to develop, but short enough to prevent too strong narrowing of the spectrum caused by the finite gain bandwidth.