Optical heterodyne signal generation and detection in cavity ringdown spectroscopy based on a rapidly swept cavity

A novel approach to cavity ringdown spectroscopy uses a continuous-wave las er and a rapidly swept optical cavity to shift the frequency of optical rad iation stored in the cavity. This frequency-shifted radiation from the ring down cavity is then combined with incident laser radiation to generate opti cal heterodyne signals, simply and efficiently. A noise-limited absorbance sensitivity of 3 x 10(-9) cm(-1) is realised, using similar to 35 muW of si ngle-mode radiation from a 1.53 mum tunable diode laser. The resonance prop erties of a swept optical cavity simplify this heterodyne-detected techniqu e by avoiding the customary need for a fast optical switch or for wavelengt h-locking of cavity length. (C) 2001 Elsevier Science B.V. All rights reser ved.