An ring-down spectroscopy instrument comprising a ring-down cavity (RDC) and CW light source (CWLS). The CWLS produces light having components with different polarizations. The ring-down cavity is optically isolated from the light source so that light reflected from the cavity is precluded from perturbing the light source. A frequency shifter shifts a mean frequency of the first component of input light with respect to a mean frequency of the second component of input light by a frequency shift &Dgr;&ngr;. A first detector measures a signal beam with the a polarization. A second detector measures a tracking beam having a second polarization. The frequency shift &Dgr;&ngr; is equal to a difference between a resonant frequency of a first cavity mode with the first polarization and a resonant frequency of a second cavity mode having the second polarization. A threshold detector delivers a trigger pulse to the frequency shifter when an intensity of the signal beam reaches a predetermined value. The trigger pulse causes the frequency shifter to temporarily change the frequency shift &Dgr;&ngr;, thereby temporarily decoupling the first component of input light from the ring-down cavity. An embodiment of the invention includes a ring down spectroscopy method. Radiation coupled into a ring-down cavity is swept in frequency by to excite one or more resonant modes of the cavity. When a fundamental mode of the cavity reaches a predetermined threshold value a digitizer produces a ring-down decay curve. An absorption spectrum is determined by extrapolating a decay constant from a logarithm of the decay curve.