Absorption measurements in microfluidic devices using ring-down spectroscopy

Abstract

When monitoring separation events in microfluidic devices, one frequently needs to detect small amounts of analyte in picolitre sized volumes with a time response of milliseconds. Fluorescence detection is typically the method of choice due to its very high sensitivity and fast response. However, since many analytes are not naturally fluorescent, labelling protocols may have to be introduced and thereby increase the complexity of the analysis. Here, we present an alternative method that is based on optical absorption, or more specifically on the ring-down time of an optical signal in a cavity or loop made of waveguide material. This optical decay constant changes as small liquid samples containing absorbing species are introduced into a fiber-optic loop. It is demonstrated that one can obtain the optical decay constant using a continuous wave laser beam that is intensity modulated and then coupled into an optical fiber loop. The inherent exponential decay in the fiber loop introduces a phase shift of the light emitted from the loop with respect to the pumping beam. By measuring this phase shift, one can readily determine the concentration of the analyte introduced between the two fiber ends and a model is established to describe the relationship. It is demonstrated that this technique, dubbed "phase-shift fiber-loop ring-down spectroscopy" (PS-FLRDS), is well suited as an absorption detector for any flow system in which the optical absorption path is limited by the instrument architecture.

Simultaneous and Continuous Multiple Wavelength Absorption Spectroscopy on Nanoliter Volumes Based on Frequency-Division Multiplexing Fiber-Loop Cavity Ring-Down Spectroscopy

“…A cavity of microliter dimensions may be made by reducing the distance between the mirrors to millimeters. − Alternatively, one can insert either a cuvette or flow cell − or a liquid film , into a larger two-mirror cavity. Another possibility is to use total internal reflection as a third cavity mirror and probe the sample with the evanescent wave. − Even smaller volumes of much less than 1 μL can be interrogated by using a fiber optic waveguide as the cavity medium. ,, We have demonstrated such measurements in several versions at various wavelengths and with different fluid delivery designs. ,− …”

Absorption detection using optical waveguide cavities

“…33 In one device, a commercial microcross was used to achieve fair coupling of light between the two ends of the fiber loop, which were fixed at a distance of 5-50 mm from each other. 34 Two capillaries were inserted orthogonal to the optical fibers and used to flow samples of cyanine dyes in water between the fiber ends ( Fig. 3).…”

Novel passive fibre-cavity design for ring-down experiments using a multimode optical waveguide

“…To overcome the drawback of surface roughness due to imperfect polishing of the fibre ends, fibre bragg gratings were used as reflective elements and applied to refractive index sensing 6 and microfluid monitoring. 7,8 Another promising approach, using fibre-loops (fibre-ring cavities) instead of reflective layers, have been the subject of many publications to date, being applied to numerous sensing purposes such as refractive index measurements, 9 trace gas detection, 10 pressure, 11, 12 strain, 13 temperature, 14 and force sensing 15 as well as for fluid monitoring. 16,17 A major drawback that all the above approaches have in common is an almost 100 coupling loss when launching light into the cavity, either through the reflective layers (R>99.9 ) or by using couplers of high split ratios (e.g.…”