POGS · Polymer Grating Sensors

Polymer Grating Sensors

Polymer Optical Grating Sensors (POGS) has been a project focused on the improvement of technology in the field of polymer optical fibre (POF) gratings for sensing applications. It has included the fabrication of microstructured POF based on poly(methl methacrylate) (PMMA), the design and construction of a new advanced grating inscription system, the realisation of POF grating sensor connectorization, the development of a new POF cleaver and the study of the POF photosensitivity and long-term grating stability. All these contributions are key to enabling applications of polymer grating sensors as well as their commercial exploitation. The first part of the project was focussed on training, with the Fellow learning to use commercial software (mode solutions from Lumerical) to model and design few moded and single mode microstructured POF (mPOF). Later, he spent 4 months at Denmark Technical University (DTU) where learnt techniques enabling him to manufacture a range of doped and undoped mPOF based on PMMA; see Table 1 in the attachment for information on all the fibres produced. The fibre was used for grating sensor inscription throughout the rest of the project. During this period we can highlight that the fellow fabricated the first mPOF doped with Benzyl Dimethyl Ketal (BDK). We found that to dope mPOF is actually simpler than doping step-index POF since it is not necessary to include extra dopants in the core to compensate the refractive index variation introduced by the photosensitising dopant. Figure 1 illustrates the manufacturing process of the doped fibre while figure 2 shows close up microscope images of the doped cane and fibre. Following his return from DTU, the Fellow designed and mounted a new advanced facility to inscribe fibre Bragg gratings (FBGs) in mPOF. In this system the gratings are fabricated scanning the POF horizontally with a UV laser rather than vertically through a phase mask. This configuration have several advantages with respect to the previous system, such as straightforward alignment, longer grating lengths, simple fabrication of complex optical devices such as Fabry-Perot interferometers, strain control during the inscription and control of the gap between the phase mask and mPOF with a precision of 10 microns. Once the inscription system was mounted the photosensitivity of the fibre was studied for both doped and undoped fibre. For the first time, gratings were inscribed in mPOF doped with BDK which showed high photosensitivity under UV radiation at 325 nm; only 13 minutes were necessary to obtain a grating reflectivity of 99 %; see figure 3. The results have been published in the journal Optics Letters. Second, the photochemical mechanisms responsible for the photosensitivity of undoped mPOF based on PMMA were researched. It was found that the photosensitivity increased with the inscription strain (see figure 4) which according to the literature means that the photosensitivity should be related with the photo-degradation of the material; however, the refractive index change under UV illumination was positive, indicating some relation with photo-polymerization. From this it was proposed that the photosensitivity took the form of a competitive process between photo-degradation and photo-polymerization in which, depending on the experimental characteristics (laser intensity, exposure time, fibre strain, etc…), one mechanism would prevail over the other. This results has also been published in the journal Optics Letters. After identifying the main photochemical reactions responsible for the photosensitivity of the undoped PMMA, the long-term stability of the fabricated gratings was studied. Gratings were monitored during 100 days after inscription for two different resonance wavelength (1562 nm, 827 nm); during the measurements the grating strengths decreased rapidly to zero, then, after some hours, the grating strength increased slowly reaching an approximately steady-state after around 50 days. These results are significant for the fabrication of FBG sensors since this long stabilization considerably lengthens the fabrication process. These unexpected results have never been observed before and therefore we will be submitting them to Optics letters as soon as possible. Further investigations will be done in the next month to reduce the stabilization time. Additionally, gratings were embedded in epoxy glue just after the inscription to monitor the behavior; in this case the stabilization time was similar however the wavelength and reflectivity variation were smaller. Due to the viscoelastic nature of the material, cleaving polymer optical fibre is significantly more difficult than cleaving silica optical fibre. As a consequence the fabrication of a portable and low-cost POF cleaver is a challenge. Currently, POF cleaving nvolves the control of the temperature of both blade and the fibre as well as the blade velocity; fulfilling these requirements necessitates a set of electronic components (motor and temperature controllers, power supply, etc...) which make the cleaver not portable outside of the lab and expensive. The Fellow has developed a new method to cleave single mode POF which does not require the fibre or the blade to be heated. It allows the construction of a much more portable cleaver, approaching the simplicity of a silica optical fibre cleaver. The method was implemented in a prototype cleaver, which allowed the Fellow to find the best working parameters. An additional advantage is that the cleaver also simplifies the connectorisation of POF, avoiding the necessity to polish the ends of the connector. Aston University is currently considering the patenting of the invention. Once the university takes a decision, a journal paper will also be submitted to Optics Letters. The impact of the research occurs in two areas. The more fundamental studies are important for researchers in the field and represent a significant contribution to our understanding of the inscription process - important for optimising the sensor fabrication and exploiting the technology. The work on cleaving is more in the engineering field and closer to market. Negotiations are taking place with a company for the commercialisation of this technology. Marie Curie Fellow: Dr David Saez Rodriguez Postdoctoral research (Programa Juan de la cierva) Ed.8G Acceso A planta 1 Instituto de telecomunicaciones y aplicaciones multimedia (ITEAM). Camino de Vera s/n Valencia, 46022, SPAIN Ext 88144 dasaerod@upv.es Scientist in charge: David J Webb Professor of Photonics Deputy Director, Aston Institute of Photonic Technologies Room N322 School of Engineering and Applied Science. Aston Triangle Birmingham, B4 7ET, UK 0121 204 3541 d.j.webb@aston.ac.uk

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