Absorption measurements in liquid core waveguides using cavity ring-down spectroscopy.

Short liquid core waveguides (LCWs) were included into a fiber-loop cavity ring-down absorption spectrometer to reduce the detection limit over, both, single pass absorption in a LCW and cavity-enhanced absorption using a conventional fiber-loop cavity. LCWs of 5 and 10 cm length were interfaced with a pressure-flow system and a multimode fiber-loop cavity using concave fiber lenses with matching numerical apertures and diameters. Two red dyes, Allura Red AC and Congo Red, were detected with a 532 nm pulsed laser at a 5 nM limit of detection in a detection volume of less than 1 µL, corresponding to a minimal detectable absorbance of less than 4 × 10(-4) cm(-1) and a minimal detectable change in absorption cross section, σ(min) = V(det) × Îµ × C(LOD), of about 14 µm(2) (Allura Red AC) and 37 µm(2) (Congo Red).

Fabrication and modeling of multimode fiber lenses.

We report on the fabrication, modeling, and experimental verification of the emission of fiber lenses fabricated on multimode fibers in different media. Concave fiber lenses with a radius of 150 µm were fabricated onto a multimode silica fiber (100 µm core) by grinding and polishing against a ruby sphere template. In our theoretical model we assume that the fiber guides light from a Lambertian light source and that the emission cone is governed solely by the range of permitted emission angles. We investigate concave and convex lenses at 532 nm with different radii and in a variety of surrounding media from air (n(0)=1.00) to sapphire (n(0)=1.77). It was found that noticeable focusing or defocusing effects of a silica fiber lens in ethanol (n(0)=1.36) and dimethyl sulfoxide (DMSO) (n(0)=1.48) are only observed when the fiber lens radius was less than the fiber diameter.

405 nm absorption detection in nanoliter volumes.

Analytical UV absorption detection for microfluidic devices, capillary electrophoresis, and even high-performance liquid chromatography is hampered by the small detection volumes, short absorption paths, and the need to sample at a high rate with a stable background and low noise. Fiber-loop ring-down spectroscopy (FLRDS) permits absorption detection of dilute liquid samples in volumes as small as a few nanoliters, while being insensitive to light source fluctuations and permitting a millisecond temporal resolution. We demonstrate a FLRDS based detection scheme that is compatible in dimensions (<200 microm absorption path, 6.0 nL detection volume) and optical design (405 nm detection wavelength, fiber coupled) with existing separation systems. An optical/fluidic interface has been built that allows injection of laser light into the loop while also permitting delivery of the sample. The detection limit of tartrazine was determined to be 5 microM (30 fmol) corresponding to an absorption of 0.11 cm(-1). Equivalent results were obtained when detecting myoglobin, a heterocyclic pharmaceutical ingredient, and 5.17 microm diameter polystyrene beads.