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.
Fiber Logix http://www.fiberlogix.com
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
For more technical
developments see www.eurekamagazine.co.uk