  We propose a non-standard spectroscopic technique that uses a feedback
control of the input probe field parameters to significantly increase the
contrast and quality factor of the atomic resonances. In particular, to apply
this technique for the dark resonances we sustain the fluorescence intensity at
a fixed constant level while taking the spectra process. Our method, unlike the
conventional spectroscopy, does not require an optically dense medium.
Theoretical analysis has been experimentally confirmed in spectroscopy of
atomic rubidium vapor in which a considerable increase (one-two order) of the
resonance amplitude and a 3-fold decrease of the width have been observed in
optically thin medium. As a result, the quality factor of the dark resonance is
increased by two orders of magnitude and its contrast reaches a record level of
260%. Different schemes, including magneto-optical Hanle spectroscopy and
Doppler-free spectroscopy have also showed a performance enhancement by using
the proposed technique.
