Morisawa and co-workers deposited a thin layer of a polymer known as Novolac onto a plastic optical fibre. This cladding swells when it is exposed to alcohol vapour and its refractive index falls from 1.6 - when no alcohol is present - to 1.45, which corresponds to saturated vapour pressure. The refractive index of the core of the optical fibre falls within this range. When the refractive index of the cladding exceeds that of the optical fibre core - that is, when alcohol levels are low - most of the light travelling through the optical fibre is refracted so that it escapes through the fibre wall. But as the concentration of alcohol rises, an increasing fraction of the light is reflected back into the fibre and travels towards a sensor. The light intensity is therefore a measure of alcohol concentration.

The team made the device even more sensitive by adding iron and sulphur to the polymer. This 'complex' expands in the same way, but it absorbs more light because it is black. As a result, very little light reaches the sensor initially. But when the complex swells and channels light along the optical fibre, the sensor records a bigger jump in the intensity of light than it does with the ordinary polymer, and this improves the sensitivity.

"The detector has many advantages such as low cost, easy handling and flexibility", says Morisawa. The highly sensitive device can also distinguish between individual gases because it responds to different types of alcohol - such as methanol, ethanol and hexane - in a characteristic way.