title: Top notch design for fiber-loop cavity ring-down spectroscopy.
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(-1) 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(-1) was found, corresponding to a detection of 43 μM Rh6G in a volume of 20 pL.
authors: Department of Chemistry, University of Oxford , Chemistry Research Laboratory, Oxford, UK.
authors: Cathy M
authors: CM
authors: Rushworth
authors: Dean
authors: D
authors: James
authors: Jason W L
authors: JW
authors: Lee
authors: Claire
authors: C
authors: Vallance
mesh_terms: D004867:Equipment Design; D012235:Rhodamines / Q000032:analysis* / Q000737:chemistry; D013057:Spectrum Analysis / Q000295:instrumentation* / Q000379:methods
chemical_list: D012235:Rhodamines; C026188:rhodamine 6G