Refractive index specific optical fibre detector and spectrophotometer for liquid droplets

Abstract

A twisted pair of optical fibre waveguides made from unclad plain silica or glass are used to detect the arrival of liquid droplets. The drop spreads in the inter-fibre cuspid region and permits refractive transfer of light within a defined range of incident angle from the source fibre, which is filled with light, to the receiver waveguide which transmits light to a detector. Drop volumes of 0.5 µl give an easily detected response if the liquid has a refractive index which falls within a narrow window defined by the refractive indices of the waveguides.

Use of a liquid-filled tubular receiver waveguide permits wider applicability of the technique, but with increased complexity.

Absorption spectra of the droplet can be obtained with an effective path length of around 0.7 mm for a 0.5-µl drop on 200-µm diameter twisted fibres, with a path length in excess of 1 mm possible on larger diameter fibres.