We have performed cavity ring-down spectroscopy by locking a high-fine
sse resonator to the probe laser. We have obtained combination overton
e spectra of water vapor in the ambient environment with a baseline no
ise of 5x10(-9) cm(-1) for decay constants (R = 99.93% reflectors) of
1 mu s. This cavity-locked approach ensures single transverse mode exc
itation, reduces shot-to-shot fluctuations in the decay constant to 4
x 10(-3), and eliminates oscillations in spectral backgrounds. This ap
proach also allows ring-down decay acquisition rates limited only by t
he ring-down and buildup constants of the resonator, and holds the pro
mise of offering truly shot-noise-limited cavity ring-down spectroscop
y measurements. (C) 1998 American Institute of Physics.