  We report on a novel approach to cavity-ring-down spectroscopy with the
sample gas in saturated-absorption regime. This technique allows to decouple
and simultaneously retrieve empty-cavity background and absorption signal, by
means of a theoretical model that we developed and tested. The high sensitivity
and frequency precision for spectroscopic applications are exploited to
measure, for the first time, the hyperfine structure of an excited vibrational
state of $^{17}$O$^{12}$C$^{16}$O in natural abundance with an accuracy of a
few parts in $10^{-11}$.
