Published: October 13, 2011 r2011 American Chemical Society 8492 dx.doi.org/10.1021/ac201775f | Anal. Chem. 2011, 83, 8492–8500 ARTICLE pubs.acs.org/ac Top Notch Design for Fiber-Loop Cavity Ring-Down Spectroscopy Cathy M. Rushworth, Dean James, Jason W. L. Lee, and Claire Vallance* Department of Chemistry, University of Oxford, Chemistry Research Laboratory, 12 Mansﬁeld Road, Oxford, OX1 3TA, U.K. ’ INTRODUCTION Cavity ring-down spectroscopy (CRDS) is a highly sensitive spectroscopic technique most commonly used for measuring weak absorptions of trace gas species. 1 Light is trapped within an optical cavity usually formed from two highly reﬂective super- mirrors, gaseous sample is introduced between the two mirrors, and the increase in cavity losses is measured by comparing the decay time constant for the cavity (the “ring-down time”) in the absence and presence of sample. 2 CRDS gains its sensitivity from two sources: ﬁrst, as the light is trapped within a high ﬁnesse cavity, the path length through the sample is vastly increased relative to a single-pass experiment; second, the decay time is independent of the initial light intensity, so the measured signal is essentially immune to ﬂuctuations in the intensity of the light source. The use of two-mirror cavities has reached maturity over the past few decades. However, interesting cavity innovations continue to emerge, particularly in the area of small volume liquid sample analysis. 3,4 Fiber-optic-based variants of CRDS are parti- cularly attractive for small volume liquid analysis since the small size of optical ﬁbers (typically <500 μm core diameter) means that picoliter to nanoliter sample volumes can easily be inter- rogated. There are two types of ﬁber-optic-based cavities, employing linear and loop conﬁgurations, respectively. Linear ﬁber CRDS measurements use a length of ﬁber with either mirrors 5À7 or ﬁber Bragg gratings 8 at either end to conﬁne the light in the cavity, in a direct analogue to the two-mirror approach. In this case, a region of the ﬁber cladding is often stripped and the evanescent ﬁeld from the surface of the ﬁber core is absorbed by sample in contact with the ﬁber surface. In a potentially simpler cavity arrangement, ﬁber-loop CRDS, 9 the cavity comprises a loop of optical ﬁber formed by join- ing together two ends of a length of optical ﬁber. Light is side-coupled into and out of the ﬁber-loop cavity, and sample can be introduced to the circulating light simply by separating the two ﬁber ends by a short distance and injecting liquid sample between the ﬁber ends. The ﬁrst ﬁber-loop CRDS measurements were reported by Stewart and co-workers, 10 who constructed a cavity from standard ﬁber-optical components and inserted a 5 cm microoptical gas sample cell. The sample region in this case (even without an absorbing species) introduced a loss of around 20% per pass, with further losses from the couplers. A ﬁber ampliﬁer was required in order to oﬀset the loop losses, and the sensitivity of the measurements thus became dependent on the ampliﬁer’s stability. Passive (nonampliﬁed) ﬁber-loop CRDS has been developed by Loock and co-workers. 11,12 In the ﬁrst pulsed CRDS experiments, light from a Nd:YAG-pumped dye laser was coupled into and out of the ﬁber loop by bending the optical ﬁber at two diﬀerent positions and exploiting the resulting macro- bending losses. Bend coupling is inherently ineﬃcient: Loock’s group reported a coupling fraction of 10 À8 when coupling light into the loop and 10 À4 when coupling out using 3 cm bends in a loop of 50 μm core diameter optical ﬁber, 11 and the coupling losses themselves were signiﬁcantly higher, since light was lost in all directions from the bend. Coupling eﬃciency can be improved by using smaller bends, but at the cost of higher round-trip losses. An alternative coupling strategy is to use commercially available couplers, although these are also relatively ineﬃcient, and often occasion higher insertion losses than expected. Lehmann and co- workers used two 99%/1% split ratio input/output couplers and found round-trip losses of around 13%, attributed to losses from Received: July 10, 2011 Accepted: October 13, 2011 ABSTRACT: Fiber-loop cavity ring-down spectroscopy (CRDS) is a highly sensitive spectroscopic absorption technique which has shown considerable promise for the analysis of small-volume liquid samples. We have developed a new light coupling method for ﬁber-loop CRDS, which overcomes two disadvantages of the technique: low eﬃciency light coupling into the cavity and high loss per pass. The coupler is based on a 45° reﬂective notch polished between 10 and 30 μm into the core of a large-core-diameter (365 μm) optical ﬁber, and allows for nearly 100% light coupling into the cavity, with a low loss per pass (<4%). The coupler has the additional advantage that the input and output light is spatially separated on opposite sides of the ﬁber. The detection sensitivity of a ﬁber-loop CRD spectrometer employing the new coupling method is established from ring-down measurements on aqueous rhodamine 6G (Rh6G) at 532 nm. The results are compared with data obtained using the same light source and detector, but a conventional bend-coupled small-core-diameter (50 μm) optical ﬁber loop. With our new coupler, a detection limit of 0.11 cm À1 is found, which corresponds to detection of 0.93 μM Rh6G in a volume of only 19 nL. This is an improvement of over an order of magnitude on our bend-coupled small-core optical ﬁber results, in which a detection limit of 5.3 cm À1 was found, corresponding to a detection of 43 μM Rh6G in a volume of 20 pL.