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Training & Research Involving Polymer Optical Devices

Final Report Summary - TRIPOD (Training & Research Involving Polymer Optical Devices)

This ITN TRIPOD (Training & Research Involving Polymer Optical Devices; had two aims. It sought to bring a new interdisciplinary technology to a level of maturity to enable commercialisation whilst simultaneously training a cohort of early stage researchers who will then be able to further develop and exploit the technology, either in industry or in research institutions.

The project is located in the field of optical fibre sensors - an area where Europe has developed internationally competitive research and commercial activity based on silica (glass) optical fibre. The aim is to significantly extend the range of application of optical fibre grating sensors by developing a mature version of the technology in polymer optical fibres and thereby increase European competitiveness.

Polymer fibres offer some key advantages over silica, the two most important perhaps being the ability to sense much higher strains and the considerably reduced stiffness of the plastic compared to the glass fibre. Polymers are however complex materials and the properties of a sensor in this material are dependent on all stages of the sensor fabrication process, from initial preform production, through fibre drawing to grating fabrication.
TRIPOD encompassed an interdisciplinary scientific team with expertise covering all aspects of the sensor fabrication path needed to develop a full understanding of the process and thereby produce optimised grating sensors, efficiently, repeatably and reliably. Integral to the programme were end-user companies providing direction on sensor development and training to the researchers on business issues, as well as familiarising themselves with the new technology. In addition, we included technology innovators to open up further applications and potential markets.

Significant progress has been achieved in all areas, resulting in more than 100 journal and conference publications from the 11 recruited researchers.
New polymers have been studied along with means of doping to increase photosensitivity. The potential for various preform fabrication techniques has been explored, including drilling, stack-and-draw, casting and 3D printing and we have optimised the process chain for the production of good quality microstructured polymer optical fibre (mPOF) from high temperature TOPAS, Zeonex and polycarbonate. We have shown that these materials are also photosensitive, allowing the laser inscription of Bragg grating senors in these fibres. Prior to the start of TRIPD, the highest temperature at which polymer fibre Bragg grating sensors could be used was around 80 °C, whereas these new fibre materials allow sensors to be used at in excess of 125 °C. Furthermore, unlike the traditional poly (methyl methacrylate) fibre material, Zeonex is not appreciably sensitive to humidity.

Bragg gratings have been produced in mPOF in the visible spectral region for the first time, allowing operation in the fibre’s relatively low loss spectral region. Bragg gratings have also been inscribed in the low loss perfluorinated fibre (CYTOP) using a fs laser. This low loss fibre permits grating sensor arrays to be distributed over many tens of metres, significantly opening up the range of potential applications. Indeed this led within TRIPOD to a collaboration developing with a geostructural engineering company, which needs a high-strain monitoring capability.

Techniques have been developed to realise stable and repeatable sensor fabrication and as a spin off of this work, we have developed the ability to easily anneal the Bragg gratings achieve either a lower or a higher Bragg wavelength than was originally produced.

An unexpected breakthrough was the discovery of the possibility to produce strong Bragg gratings using a single short laser pulse, which opens up the possibility of draw tower fabrication of POFBGs, which would in turn lower the cost of production enhancing the commercial attractiveness of the technology. The effect of radiation on POF and POFBGs has been studied suggesting the possibility of distributed dosimetry and showing that POFBGs can survive the levels of radiation associated with sterilisation techniques.

The project also sought to develop technology associated with the use of POFBgs. Demountable connector losses have been reduced from 8 to 1dB. POFBGs sensors have been demonstrated in a range of applications from fuel level sensing and erythrocyte concentration measurement to temperature insensitive hysteresis free humidity monitoring. POFBGs and POFBG based interferometers have been characterised for photoacoustic imaging applications. We have developed fabrication techniques for the realisation of compound parabolic fibre tips to enhance the collection of light from fibre based glucose sensors. We have developed a novel spectrometer signal processing algorithm allowing the tracking of POFBGs in multimode fibre and built a prototype Digital Micromirror Device (DMD) based interrogator for POFBGs and other spectroscopic based devices. Finally we have investigated the integration of POFBG sensors in 3D printed structure to create bespoke smart structures.