DSA Volume 1 Issue 4 December 2010 - Defence Science and ...

Tiny light-based glass fibre
sensors with a big future
DSTO is collaborating with the Institute of Photonics & Advanced Sensing (IPAS) to
develop new ways of sensing the composition and condition of materials with laser
light guided in ‘microstructured’ optical fibres.
IPAS is a recently established research
institute that builds on the Centre of
Expertise in Photonics (CoEP) established
at the University of Adelaide in 2005. The
institute was set up with support from DSTO
along with several other organisations.
CoEP researchers have for a number of years
been developing techniques to make optical
fibres using non-silica ‘soft glasses’. These
glasses, unlike the hard kind, have the ability
to transmit light in the mid-infrared frequency
range, making it possible to develop several
important applications of interest to Defence.
Additionally, because soft glasses melt at
lower temperatures, they are amenable to
production methods that enable the creation
of fibres with arrays of air holes running the
length of the fibre, known as microstructures.
Microstructure features provide a new
way of containing light inside a fibre, without
which, it would quickly dissipate through the
sides. The previous method of containment
required using two kinds of compatible
glasses with different refractive indices for
core and cladding.
The advent of microstructure designs thus
enables optical fibres to be made with just
one kind of glass, greatly increasing the range
of materials that can be transformed into
optical fibres.
Moreover, microstructure fabrication
techniques have advanced to the point where
structures can be produced with features
as small as 20 nanometers – significantly
smaller than the wavelength of light. This
opens up extraordinary new possibilities for
optical fibre use.
Biological and chemical signal sensors
One such use is for sensors that can
analyse liquids or gasses. IPAS is
investigating the possibility of developing
various real-world applications.
“The new optical fibres we’ve developed
use a suspended glass nanorail to guide
light. This nanorail serves as the sensing
platform, allowing light to interact with the
environment in which it is embedded, or into
which it is dipped,” explains IPAS Director
Professor Tanya Monro.
“Because a significant proportion of the
light guided by the fibre falls outside of
the glass, this can be used as a means for
sensing conditions in the immediate locality
by observing how it interacts with particular
kinds of matter present.”
The sensing apparatus consists of a laser
light source, a length of fibre optic cable
with one or more small sensing regions, and
a detector. A sensing region can be exposed
to the external environment, or alternatively,
the material to be sensed can be loaded into
holes within the fibre.
The IPAS work on sensing for defence
applications currently involves the use
of sensors with surfaces prepared in ways
that react only to molecules associated
with a certain type of material – such as an
explosive, biological agent or contaminant –
to determine whether that material is
present or not.
“When particular molecules of interest are
present, they bind to the specially prepared
surfaces, and under stimulation by laser light,
fluorescence occurs, which the system then
detects,” says Professor Monro.
The fluorescent light further provides a
measure of the relative quantity of such
molecules present, as revealed by the light
intensity levels detected.
High-tech dipstick
Liquid samples can be assayed in such a
way using an optical fibre with a central core
surrounded by air holes. The liquid is drawn
into the fibre by capillary action or forced up
into the holes by pressure.
The volume needed for accurate
sampling is extremely small, just nanolitres
(10 -9 litres). Concentrations of chemicals and
biomolecules as low as 0.2 nanomoles can be
detected even when assaying sample volumes
this small, and further improvement in the
detection limit is expected in the future.
The fact that the fluorescing light travels
in both directions means it can be observed
at either end of the fibre. This makes possible
the development of a continual automated
monitoring application in the form of a
‘dip-sensor’.
The system involves a length of optical fibre,
with laser light being sent at one end, a liquid
sample entered into it at the other end, and
a detector for fluorescence mounted at the
same end as the laser light source.
The dip-sensor, involving no moving parts
and no electronics, need only contact the
liquid surface to facilitate a reading. Being
very small and robust, such devices could
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