Fiber-loop ring-down spectroscopy

Abstract

Pulsed, visible and near-infrared laser light is coupled into an optical fiber, which is wound into a loop using a fiber splice connector. The light pulses traveling through the fiber-loop are detected using a photomultiplier detector. It is found that once the light is coupled into the fiber it experiences very little loss and the light pulses do a large number of round trips before their intensity is below the detection threshold. Measurements of the loss-per-pass and of the ring-down time allow for characterization of the different loss mechanisms of the light pulses in the fiber and splice connector. This method resembles “cavity ring-down absorption spectroscopy” and is well suited to characterize low-loss processes in fiber optic transmission independent from power fluctuations of the light source. It is demonstrated that by measuring the ring-down times one can accurately determine the absolute transmission of an optical fiber and of the fiber connector. In addition it is demonstrated that the technique is useful as an absorption spectroscopic technique of very small sample volumes. A solution of an organic dye was placed between the fiber ends instead of the usual index matching fluid, and an absorption spectrum of 7×10−15 mol of the dye 1,1′-diethyl-4,4′-dicarbocyanine iodide in 7×10−12 L of dimethylsulfoxide was recorded.

Absolute surface coverage measurement using a vibrational overtone

“…18 -21 Here, we exploit the sensitivity and absolute measurement capability of CRDS to determine absolute sub-monolayer surface number densities using a vibrational overtone. While most CRDS studies have probed gas-phase species using a conventional linear optical resonator, novel extensions of the technique have included studies of a submonolayer adsorbate, [22][23][24][25] thin films, 26 -33 a thin liquid channel, 34 bulk liquids, 35,36 fibers, [37][38][39][40] and nanoparticles, 41 often employing unconventional resonator configurations. Guided by the Fresnel equations, some configurations have utilized the Brewster angle 24,26,27,36,41 or total-internal reflection ͑TIR͒ [22][23][24][25]39,42 to admit intracavity elements or interfaces that facilitate sampling while maintaining a low intrinsic loss.…”

Liquid-Phase and Evanescent-Wave Cavity Ring-Down Spectroscopy in Analytical Chemistry

“…Because the absorption measurement is based on the phase-shift of this integrated light, phase-shift CRDS is less sensitive to light-source-intensity fluctuations than is CEAS. LODs using the phase-shift fiber-loop CRDS are on the order of several micromolars for a strong absorber-a 45-fold improvement over the pulsed fiber-loop CRDS setup, where a flow-cell with a 10-times-shorter optical pathlength was used (77,79).…”

“…can easily be inserted into this cavity via a commercially available splice connector; the indexmatching fluid of the splice connector is replaced by the sample to be measured (77). Because the capillaries commonly used in capillary electrophoresis (CE) have much smaller volumes than do LC columns, their detection cell volumes are subject to even more severe constraints.…”

Cavity-Enhanced Spectroscopy in Condensed Phases: Recent Literature and Remaining Challenges

“…Another instrumental variant of CES is to conduct the experiment within a linear section or loop of optical fiber [25,[27][28][29][30][31][32][33][34]. This implementation of CES is attractive because measurements on sub-nL volumes of the sample can be conducted due to the small diameters of optical fibers used.…”