Fiber optics detects drunk drivers

17 Jun 2002

Fuel leaks, pollution and even the alcohol in a driver's breath could soon be traced using optical fibers.

Story courtesy of PhysicsWeb

Chemicals that contain alcohols - many of which are toxic or explosive - play a central role in industry, and the efficient detection of leaks is crucial for the safety of workers and the environment. With this in mind, Masayuki Morisawa of Yamanashi University, Japan, and colleagues have developed a quick and sensitive way of detecting alcohol leaks by harnessing the "swelling phenomenon" exhibited by an alcohol-absorbing polymer, (Measurement Science and Technology, 12 877 2001).

Morisawa and co-workers deposited a thin layer of a polymer known as Novolac onto a plastic optical fiber. They discovered that when exposed to alcohol vapor, the polymer cladding swells, forcing a change in its refractive index. At low alcohol levels the refractive index of the cladding exceeds that of the optical fiber core, and most of the light travelling through the optical fiber is refracted so that it escapes through the fiber wall. However, as alcohol concentrations rise, an increasing fraction of the light gets reflected back into the fiber 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 fiber, 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 such as methanol, ethanol and hexane.