Narrow resonance of saturated absorption in an active cavity

We present a new scheme of laser spectrometer based on an active optical cavity containing saturated absorbing and amplifying media. Such an active cavity may ensure enhancement in the amplitude of nonlinear optical resonances of saturated absorption at least by one order of magnitude as compared with similar passive system. The efficiency of the spectrometer was demonstrated by the observation of saturated absorption resonances in methane at a wavelength of 3.39 /spl mu/m under transit-time conditions. In these conditions, the contribution of fast molecules can be greatly reduced because of a finite interaction time of particles with the radiation field. In our work with a light beam 0.6 cm in diameter, we eliminated the transit-time broadening limit by using optical selection of slow methane molecules with a transverse velocity 20 times less than the mean thermal one, and obtained sub-DoppIer resonances with a homogeneous width less than 5 kHz.