Speciality and microstructured polymer optical FBG sensors

Abstract

In light of commercialisations of polymer optical fibre Bragg grating (POFBG) sensors, high performance and stable operation of the gratings are key requirements. Traditionally towards the development such sensors, the polymer optical fibres (POFs) were annealed in conventional ovens before or after the grating inscriptions. In conventional ovens humidity is not controlled thus humidity significantly decreases as the temperature rises to the annealing temperature. This technique of thermal annealing did not allow obtaining high performance POFBGs sensors, particularly those based on poly-methylmethacrylate (PMMA). During the PhD project it has been discovered that humidity plays a significant role in the annealing of PMMA POFBGs and the later sensor responsivity to environmental measurands. The investigation revealed that, PMMA POFBGs annealed at high level of humidity (I) provided a superior performance in humidity responsivity over a wide temperature range over the one annealed at low humidity (II) enabled tuning the Bragg wavelength by more than 200 nm which provide a way to produce gratings at short wavelengths and multiplexing several grating in a single fibre using only a single phase mask. The described humidity assisted annealing was performed in a climate chamber which was not cost effective despite it provided superior result. However, to combat this, new method of annealing at room temperature was proposed and investigated which only requires methanol-water solution but delivered a comparable result. The most commonly used polymer material for POFs fabrication is PMMA. Although this polymer is ideal for humidity sensors development due to its high water absorption capability, the maximum operating temperature limit of PMMA POFBGs is limited to only 75 °C when they are operated at high humidity, for instance 90 % relative humidity (RH). In addition when PMMA POFBGs are applied for strain and temperature measurements, the high RH sensitivity pose a problem. Thus it was important to develop POFs that can mitigate the aforementioned problems. One of the objectives of the project involved exploring and bringing specialty polymers which have different features than PMMA for fibre Bragg gratings (FBGs) based sensing applications. The new polymers that were explored and exploited for the fabrication of solid core microstructured polymer optical fibres (mPOFs) were Polycarbonate (PC) and Zeonex 480R. In addition, a combination of Topas 5013S-04 and Zeonex 480R has been used for step index fibre realisation. PC has also moisture absorbing capability but not as strong as PMMA and it has excellent mechanical property. While Topas 5013S-04 and Zeonex 480R have a very low moisture absorbing capability. All of them have a glass transition temperature (Tg) greater than PMMA has and transmittance as good as PMMA but lower. Therefore, grating fabricated from POFs produced from these polymers utilised to sense temperature above PMMA can function. Not only this but also the strong effect of humidity cross sensitivity that were seen when temperature and strain were measured with PMMA mPOFBGs was avoided using these new polymers. PC POFBGs allowed sensing temperature up to 125 °C and this is a record temperature measured with currently existing POFBGs. In addition, with PC POFBGs it was possible to measure RH up to 90% at a temperature of 100 °C which was limited to only up to 75 °C by PMMA based gratings. FBG strain sensors with wide range, hysteresis free and stable operation were also realised from fibres fabricated from these polymers. Prior to the PhD Topas grade 8007F-04 and 5013S-04 mPOFs have been fabricated for humidity insensitive temperature and strain measurements. However, each of them has their own limitation. Topas 8007F-04 has a Tg 26 °C smaller than that of PMMA so the temperature sensing range is very narrow. Whereas Topas 5013S-04 has high flowability thus difficult to draw microstructured fibre from it despite it has a Tg 28 °C higher than that of PMMA and suitable for high temperature sensing. Therefore, to fought back these challenges two solutions were proposed: to fabricate a step index Topas 5013S-04 fibre but yet single mode as the microstructured one or to explore another polymer which has low affinity to water as Topas but friendly to draw. Both of these solutions have been addressed and realised during the research period. Zeonex 480R was the ideal candidate to replace Topas 5013S-04 as they share most properties in common but the former is easily drawable. Using these two polymers and by combing casting, drilling and injection moulding techniques a step index fibre was made. Heat and draw method was used to fabricate the fibre. The fabricated fibre was the first humidity insensitive and high operating temperature single mode SIPOF. The employed fibre production method was very efficient, flexible and cost effective as no doping was required. The fibre also presented grating inscription time shorter than PMMA mPOFs and much shorter that Topas mPOFs. Further, the 4.8 µm core size of the fibre was compatible and good for optimum coupling with standard silica fibre which is single mode in the 850 nm region. However, small core size caused high scattering loss which increases the overall loss of the fibre, and this was the main limitation. The second proposed solution was to develop a Zeonex 480R mPOF as its flowability is suitable for mPOF drawing. This grade of Zeonex showed a superior drawablity over Topas 5013S-04 with wide range of drawing temperature. The realised Zeonex 480R mPOF was not only drawing friendly but also has low loss and presented a very good compatibility with PMMA for co-drawing applications compared to Topas 5013S-04. Further, grating realised in it provided high sensitivity to temperature. The co-drawability feature of Zeonex 480R with PMMA enabled to draw a hybrid Zeonex 480R-PMMA mPOF for the development of the first fully polymeric thermo-hygrometer. The fibre had Zeonex 480R as core and cladding and PMMA as an over cladding. The thermo-hygrometer was developed based on dual in line gratings and operated in the range 20 °C-80 °C and 10-90 % RH, with a root means square error of 0.6 °C and 0.8 RH %. The device key advantages are being easy to fabricate, cost effective, fully polymeric and mechanically stable.