Fibre optic cables detect chemicals

Tom Shelley reports on a technology that allows optical fibre systems to be used for chemical as well as temperature, pressure and strain measurement

A combination of nano particles and fibre optic sensing technology permits various sensing devices to be integrated into a single length of optical fibre. A series of sensors ‘etched’ along the length of the fibre could detect a range of chemicals – as well as measuring temperature, pressure and strain. The devices are extremely light in weight, immune to electrical interference and intrinsically safe. This could simplify the assembly and reduce the cost of sensing systems in factories, process plants, the oil and gas industries, aircraft – and even medical endoscopes. The sensing platform is the brainchild of a small, specialist optical fibre device company, Fiber Logix, based in Watford. The company already manufactures speciality fibres and devices for use in optical fibre sensors, switching systems and fibre optic gyroscopes. Thanks to a 2003 DTI Smart Award and a collaboration with researchers at Queen Mary College, London it has now added chemical sensing abilities to its repertoire of optical fibre technologies. “It has the potential to replace all of today’s conventional electronic sensors with a single strand of fibre,” says Fiberlogix director Saeed Rehman. “A number of companies are able to provide small parts of the solution, but until now, nobody has been able to provide it all.” The working principle relies on the fact that light propagates along the fibre core, which is about 10 microns across, surrounded by 125 microns of cladding. According to Dr Rehman, the key to this is the manipulation of the tiny fraction of light in the cladding within a few microns of the core called the evanescent field. The laws of physics dictate that some energy is always carried in this cladding. The principle of evanescent sensing is to micro-machine the fibre cladding to create a pit in the side of the fibre. Light travelling through the fibre does not notice the change and continues its journey uninterrupted. But if a material with a high refractive index – which is also sensitive to chemicals – is moved close enough to overlap with the evanescent field, light can be manipulated inside the core. The interaction of the light and the chemical sensing material can be precisely controlled by changing the distance of the high index material from the core. The exact location at which this is happening on a fibre can be established using a technique called OTDR, or Optical Time Domain Reflectometry. The patented process invented by FiberLogix is to use a laser to excavate a roughly rectangular cavity in the side of the fibre. This creates a window that allows access to the evanescent wave. Into one of the cavities is inserted an Interpenetrating Polymer Network (IPN) – two or more networks that are partially interlaced on the molecular scale but not covalently bonded to each other. The networks cannot be separated unless chemical bonds are broken. The refractive index is tailored to be similar to that of the core of the fibre by combining the two polymers in appropriate proportions. To make it chemically sensitive, the IPN is impregnated with suitable particles – normally only a few tens of microns across – such as titanium oxide, palladium (to sense hydrogen), platinum or rhodium. In the presence of a chemical species being measured, there is a change in refractive index, which changes the tail of the light distribution curve across the fibre. “We are using the tail to wag the dog,” says Dr Saeed. By filling adjacent cavities with material combinations that respond to different chemical substances, it is possible to use the same length of fibre to respond to different chemical substances. It is also possible to build in temperature sensing using a dual core fibre. One core is used for the chemical sensors and the other has a series of Bragg gratings written onto it by a laser. Changes in temperature cause changes in the spacings of the bars – which can also be measured very precisely. It is then possible to apply temperature compensation to the changes in light transmission caused by the presence of measured chemicals. The cable could be produced using automated manufacturing techniques that are less complex than those used for chip sensor manufacture. All the component technologies are mature and well known to work. Applications in process plant control systems are fairly obvious, but the technology also lends itself to biohazard detection by incorporating nano particles that respond to the presence of particular species. The Bragg gratings that measure temperature can also measure bending, allowing the fabrication of chemical sensors for medical endoscopes that measure the direction in which the tips are travelling. As well as sensing, the excavated cavities can also leak out small quantities of light, which could apply very localised heating in micro-surgery for cancer tumour treatment. Aircraft companies are interested in using the technology for distributed sensing in aircraft. Representatives of “a major avionics and aircraft manufacturing company” have said that, by 2011, they would like all their sensing technology to be optical fibre based. Oil and gas companies are also interested in using the technology both for offshore work and down hole – an environment in which conventional sensors find it difficult to survive. Pressure and strain are already measured along distributed lengths of optical fibre at the same time as temperature, and the new technology allows the addition of chemical technology. Dr Saeed believes that his optical sensor will fill “a big gap in the market”. Being a small technology oriented company, Fiber Logix is looking for support for further research and development, partnership opportunities and licensing arrangements, rather than trying to mass-produce the devices by itself. FiberLogix Eureka says: This technology could transform the whole sensor industry, replacing multiple devices with a single length of fibre Pointers * Different chemical or biological species can be detected using a single length of fibre * By using fibres with two cores, it is possible to use one core for chemical sensing and the other for temperature, strain and pressure sensing * Initial target markets are aerospace and oil and gas but if mass manufactured, the devices could be used across industry