Coupled-cavity ring-down spectroscopy technique

We present a technique called coupled-cavity ring-down spectroscopy (CC-RDS) for controlling the finesse of an optical resonator. Applications include extending the sensitivity and dynamic range of a cavity-enhanced spectrometer as well as widening the useful spectral region of high-reflectivity mirrors. CC-RDS uses controlled feedback of the probe laser beam to a ring-down cavity, which leads to interference between the internally circulating light and that which is fed back through a cavity mirror port. Using a 74 cm long ring-down cavity and a feedback cavity with a finesse of 16, we demonstrate that this effect increases the decay time constant from 210 mu s to 280 mu s, corresponding to an increase of finesse from 2.7 x 10(5) to 3.6 x 10(5). Finally, we show that with the addition of a second feedback cavity, we observe ring-down times as long as similar to 0.5 ms, which is equivalent to (1 - R) approximate to 4.9 x 10(-6), where R is the effective mirror reflectivity. (C) 2012 Optical Society of America