A system and method for spectroscopic detection of loss in a resonator cavity is disclosed. The system can include a number of components. A tunable laser source can generate a laser beam. A frequency locking system can lock the frequency of the laser beam to a resonance of the resonator cavity or lock the length of the cavity to the frequency of the laser beam. The first modulation element can modulate the laser beam at a first modulation frequency to generate a modulated laser beam. The input coupler can direct the modulated laser beam into the resonator cavity. The first directing element can direct a first portion of light reflected from the input coupler to a first photodetector. The first demodulator can demodulate the first modulation signal to generate a first error signal which is a function of the loss in the resonator cavity.