A dual-parameter optical sensor fabricated by gradient axial doping of an optical fibre

There is a general need for optical sensors that respond to multiple substances or physical parameters. Multiple-parameter sensing is not only more efficient, but also permits interacting or interdependent parameters to be individually determined. In this paper, we describe a novel approach to the fabrication of an optical sensor, sensitive to two separate atmospheric conditions, but made from a single fibre optic. The optical fibre is drawn from polycarbonate, with a temperature-sensitive phosphor (La2O2S:Eu3+) incorporated directly into the core. Thus, the light-guiding portion of the fibre is responsible for determining the first parameter of measurement, ambient temperature. A thin fibre cladding is subsequently added to the temperature-sensitive core and serves as a chemically sensitive component. This cladding is made from Nafion®, and is doped with rhodamine 800. Fluorescence at 750 nm from the rhodamine 800 is shown to be enhanced by the presence of atmospheric moisture, and is used in conjunction with a ratiometric means of measuring temperature provided by the phosphorescence from the fibre core. This scheme provides a simple and potentially inexpensive way to manufacture fibre-optic sensors capable of multicomponent determinations. In addition, the temperature-sensitive core material provides a built-in normalization factor for the temperature-dependent response of the chemically sensitive cladding. The developed dual sensor was evaluated over the temperature range from 20 to 95 °C and demonstrated better than 1% relative standard deviation (RSD). The humidity sensor component was evaluated over the range of relative humidity (RH) from 0 to 20% RH over temperatures up to 56 °C. The detection limits for the humidity sensor were 0.17 and 2.35% RH at 20 and 56 °C, respectively.