Nonlocal Microcapillary Fibre Sensing Platform

The resonant optofluidic sensing platform based on monitoring the spectrum of whispering gallery modes (WGMs) propagating along thin-wall microcapillary fibre allows accurate monitoring of chemical, biological and physical processes at the fluid-fibre surface interfaces [1]–[3]. The setup for the microfluidic sensing originally proposed in [4] and used in multiple follow-up publications [3], [5] is illustrated in Fig. 1(a). To enable the evanescent wave sensing of fluids, the wall of a microcapillary silica fibre is ~2-7 μm. The microcapillary is placed in contact with a transverse fibre taper with micron-diameter waist (microfibre) contacting the microcapillary. The microfibre is connected to the optical spectrum analyser (OSA) and serves as an input-output waveguide exciting WGMs in the microcapillary. Changes in the WGMs spectrum detected by the OSA measure variations of refractive index inside the microcapillary at the microfibre location and can be used for microfluidic sensing. Here we extend the axial sensing region by using a slow cooked microresonator [3], which makes it possible to perform microfluidic sensing nonlocally along the full length of the microresonator, using a single input-output microfibre, whose position is fixed within the microresonator length. Fig. 1 (a) local and (b) nonlocal microcapillary fibre sensing platforms. (c) Spectrograms
$P(\lambda,z)$
of a microresonator fabricated at the surface of the microcapillary, with experiment and simulation results shown in (c1) and (c2) respectively. (d) Temporal spectrograms
$\mathrm{P}(\lambda,z_{0},t)$
measured at a fixed axial position
$Z$
0
at the microresonator centre whilst water fills the empty microcapillary fibre with experiment and simulation results shown in (d1) and (d2).