Cavity ring-down spectroscopy with a continuous wave laser

Most CRDS experiments used pulsed lasers as a light source because the pulsed laser is a convenient source of high-intensity photon bursts, even though the energy coupling efficiency is small. CRDS with a CW laser was successfully demonstrated by Romanini et al. (1997). They used an acousto-optic modulator to interrupt the CW laser beam introduced into the cavity. We demonstrate a new simple CRDS setup that yields photon decay signals only with a PZT instead of using an acousto-optic modulator as an optical switch. We also numerically calculate the coupling efficiency of a CW CRDS system dependent on the laser linewidth, the scanning speed of the cavity, and the finesse of the cavity.