On-chip cavity-enhanced absorption spectroscopy using a white light-emitting diode and polymer mirrors.

We have developed a disposable microfluidic chip with integrated cavity mirrors comprised of two pieces of 3M Vikuiti™ enhanced specular reflector II (ESRII) film, for performing cavity-enhanced absorption spectroscopy with a white light-emitting diode (LED). Compared to measurements made with a chip without cavity mirrors, the absorption path length is enhanced by a maximum factor of 28 at 544 nm, and the sensitivity is enhanced by approximately 5 times, enabling micromolar range detection limits to be achieved in an optical path length of only 50 µm.

Top notch design for fiber-loop cavity ring-down spectroscopy.

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.

Broadband cavity-enhanced absorption spectroscopy for real time, in situ spectral analysis of microfluidic droplets.

Broadband cavity-enhanced absorption spectroscopy has been used to record, in real time, the absorption spectrum of microlitre volume aqueous phase droplets within a microfluidic chip assembly. Using supercontinuum radiation and broadband coated external mirrors, the full visible spectrum (430 nm < λ < 700 nm) of each passing droplet is acquired in situ at high repetition rates (273 Hz/3.66 ms acquisition time) and high sensitivity (α(min) < 10(-2) cm(-1)). The possibilities for further improvements in sensitivity and acquisition rate using custom designed chips are discussed.

Fabrication of an optical fiber reflective notch coupler.

A method of fabricating a reflective notch coupler in an optical fiber has been developed. The coupler consists of a 45° microprism that penetrates into the core of a multimode optical fiber. One face, at 90° to the fiber axis, is nonreflective, and one face, at 45° to the fiber axis, is reflective. Our method of fabricating a notch and selectively mirroring only the 45° face is low-cost, precise, and easily scalable. The coupler allows near-100% coupling of light into an optical fiber from the side, while allowing coupling of any desired fraction of light out from the core at a 90° angle on the opposite side of the fiber.