Since its start in 2008, the PHOSFOS project funded by the European Commission's 7th Framework Programme has created a new paradigm for flexible optical sensors integrated with electronic modules and control circuitry. Its aim was to develop a generic technology that offers an integrated solution to embedded optoelectronic circuits. The PHOSFOS project is now completed and several breakthroughs in the field of optical sensing, flexible materials, embedding technologies and integration concepts have been achieved which may be used in a wide range of applications.

A first highlight of the research involves the development of a new pressure sensitive and temperature insensitive sensor. The pressure sensitivity of the new sensor exceeds the state-of-the-art by a factor of 20, whilst being truly temperature insensitive. The technology enables accurate pressure measurements in the presence of temperature gradients as required in the field of oil and gas exploration. The pressure sensitivity comes from the inherent properties of the fibre rather than any external mechanical housing which means that the sensor is very compact.

The sensor is based on a novel design of a highly birefringent microstructured optical glass fibre that features a high pressure sensitivity a negligible temperature sensitivity and that is compatible with conventional ultraviolet Bragg grating inscription setups. This fibre is known as the "Butterfly" fibre owing to the shape of the layout of air holes in its cross section (see inset). The temperature insensitivity was achieved by carefully tailoring the design of the doped region in the core of the microstructure.

The sensor has also been used as an embedded sensor in composite materials. The measurement capabilities exceeded previously demonstrated transverse strain sensitivities by an order of magnitude. The sensor can therefore contribute vital information about the structural health of composite materials by following the mechanical strain in its most vulnerable direction as required in the field of aeronautics.

A second highlight involves Bragg grating sensors in polymer optical fibres. Prior to the commencement of PHOSFOS, gratings in polymer optical fibre (POF) only existed in the 1550 nm spectral region where the large fibre loss only permitted very short fibre lengths to be used and the devices had to be butt coupled to a silica fibre lead on the optical bench. The PHOSFOS consortium has developed a means for reliably splicing POF to silica fibre and produced the first gratings in the 800 nm spectral region where losses are almost 2 orders of magnitude less than at 1550 nm. These developments have allowed POF grating sensors to be used outside the laboratory for the first time.

Fibre Bragg grating sensors in POF have potential advantages over their silica counterparts in applications that require very large strains (> 5%) to be monitored. Also POF sensors are beneficial where the structure to be monitored is very compliant and the silica fibre would simply reinforce the structure. When embedded in flexible tubing for example the strain transfer from the flexible outer tube is higher for POF than with silica fibres. Also, the sensor is safer than one containing glass fibre. This is especially important for medical applications since should any breaks occur all of the material used is biocompatible.

One of the limitations on the volume of commercial uptake of fibre Bragg grating (FBG) technology is cost. The PHOSFOS consortium developed a new low cost POF sensor interrogator designed to work with polymer optical fibres. The sensor interrogator has been designed to operate at a wavelength around 850 nm to match the low loss transmission window of POF and to significantly reduce component costs.  It is designed to monitor a multimode fibre which increases the optical power of the signal coming back from the sensors.

Using this technology a new polymer multipoint FBG sensor that can measure the pressure in various medical applications has been demonstrated.  A POF sensor array has been embedded in a biocompatible polymer tube that can be used to measure pressure within the body, for example in the throat during swallowing. The system has been designed to give a graphical representation of pressure to make interpretation of the data straightforward for clinicians

More PHOSFOS results and contact information can be obtained on the website of the project www.phosfos.eu. An introductory video about the project is available on YouTube at http://www.youtube.com/watch?v=pGpL_icFn1c.

Contact:

Prof. Francis Berghmans, Vrije Universiteit Brussel, Brussels, Belgium fberghma@vub.ac.be, tel. +32 2 6293453
                 

Figure: Schematic representation of a cross section of highly birefringent microstructured optical glass fibre.