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We present a new cavity enhanced, continuous wave spectroscopic technique for the detection of weak atomic and molecular transitions. Differential Cavity Mode Spectroscopy (DCMS) measures the difference in absorption between two adjacent cavity longitudinal modes to yield a highly sensitive, yet relatively simple, cavity enhanced spectroscopic technique. In addition this relative absorption measurement is, to first order, independent of both laser frequency noise and cavity acoustic noise. Here we present both a theoretical description of this new technique and an initial experimental demonstration.

A simplified “quantum speed meter” interferometer configuration, a candidate for future gravitational-wave (GW) detectors, was constructed and locked using a radio frequency (RF) locking scheme for length control. Employing a signal-laser–injection technique, we obtain the experimental frequency response, which shows good agreement with theoretical modeling.