Cavity ringdown heterodyne spectroscopy: High sensitivity with microwatt light power - art. no. 061802

We present an ac heterodyne technique in cavity ringdown spectroscopy that permits 1 X 10(-10) absorption sensitivity with microwatt-level light power . Two cavity modes, one probing the empty cavity and the other probing intr acavity absorption, are excited simultaneously but with their intensities t emporally out of phase, with one mode decaying and the other rising. Hetero dyne detection between the two modes reveals the dynamic time constants ass ociated with the empty cavity and the additional intracavity gas absorption . The method offers a quick comparison between the on- and off-resonance in formation, a prerequisite to reaching the fundamental shot-noise limit. Thi s simple and yet important improvement of cavity ringdown spectroscopy shou ld lead to the enhanced performance of a wide range of applications.