Knowledge Bank

Cavity ring down spectroscopy (a.k.a. ring down cavity spectroscopy) is becoming widely used in the spectroscopic community. In this method, one determines the absorption spectrum of a sample by observation of the rate of decay of light intensity inside a very high finesse optical cavity $(Q>104typically)$. This allows for effective absorption pathlengths on the order of tens of kilometers. In the past, few nsec pulsed lasers have been used for excitation of the cavity. This allowed one to largely ignore interference effects, but resulted in very poor couping efficiency, on the order of a few parts per million, of light into the cavity on the order of a few parts per million. Earlier work by our group demonstrated that one can reach shot noise limited detection sensitivity with this method, which implies that sensitivity shuld increase with better coupling efficiency. We have begun the development of a new generation of cavity ring down spectrometer, based using a quasi-continuous wave diode laser to excite the ring down cavity. By using optical feedback locking of the diode to the ring down cavity, we have observed optical transmission of close to one despite the few kHz width of the resonance modes of the cavity. Calculations indicate that with our current parameters (which are still far from the best that can be achieved as far as mirror reflectivity), shot noise limited detection will correspond to an absorbance detection sensitivity on the order of $10-14cm-1$, ten thousand times better than has been realized using a pulsed laser. By the time of the meeting, we hope to have data to compare with this expected limit.