Dediyagala, Nithila (2019) Optical Fibre Bragg Grating Analysis Through FEA and its Application to Pressure Sensing. PhD thesis, Victoria University.

Abstract

The focus of this thesis is developing optical fibre Bragg grating (FBG) pressure sensors with enhanced sensitivity for use in a low (gauge) pressure range (0 - 50 kPa) together with understanding observed non-linear behaviour. To appreciate the behaviour of FBG sensor spectra, it is necessary to understand geometrical and material properties of FBGs. The thesis is an in-depth investigation of the behaviour of FBGs including their manufacturing and fabrication process details. A new computational approach has been introduced to simulate FBG structures based on how the FBG fabrication process produces changes in refractive index. There are various numerical analysis methods existing for analysing fibre Bragg grating structures and their spectral properties. Although computation design and simulations are used extensively in engineering problems, current computational approaches do not combine FBGs formation and their resultant spectra. In this study, this has been addressed by developing a simple Finite Element Analysis (2-D) model using the Wave Optics module in COMSOL Multiphysics simulation software. The 2-D model was developed considering the phase mask method commonly used to fabricate FBGs. It simulates a complex grating structure which is useful for manufacturers and researchers. The 2-D model then creates a unit cell of a grating structure which is able to be implemented within an optical fibre. The model allows users to decide the length of the grating by selecting the number of unit cells required. By changing the geometrical parameter of a unit cell of the 2-D phase mask structure, it was possible to demonstrate formation of complex grating structures. There have been many studies reported for ideal gratings; however, much less attention and research has been given to the spectra produced by these complex FBG structures. Therefore, this study specially focuses on complex grating structures and their spectral behaviour. The developed 2-D model successfully reproduced observed complex grating structures arising with the use of multiple phase masks orders, with theoretically acceptable results for the spectrum produced. Furthermore, the 2-D model of the phase-mask method was also able to produce tilted gratings by changing the incident angle of light on the phase mask. Therefore, this FEA approach provides insight into not only complex FBG structures but also tilted FBGs using a simple computational tool which will be useful in further research to understand the behaviour of a variety of FBG structures. For this study, material properties of standard single mode fibre (SMF-28) was considered. However, the model is able to simulate any optical fibre used in FBG fabrication by changing the material properties. The thesis also considers the understanding of FBG pressure sensors and observed non-linear behaviour. Therefore, a thorough literature review was carried out to find the influence of structural and material properties of optical fibres and FBGs which is believed to be the cause of non-linear behaviour. It investigates in depth the birefringence effect on fibres due to point load and distributed load on FBGs using the Structural Mechanics and Wave Optics module in COMSOL software. Many research studies have employed a plane strain assumption for structural mechanics problems; however, they do not clearly explain the true nature of FBGs under stress generalized strain. This study overcomes that problem by introducing proper mathematical equations to develop 3-D behaviour in a 2-D computational model. The behaviour of a distributed load on FBGs was discussed in detail with the help of the computational model. It provides new information about an asymmetric peak produced as a result of birefringence effects. The research proposes a new FBG uniform pressure sensor using a 2-D computational model. It was designed in simulation by adding a polymer material to the cladding of an SMF-28 by reducing the cladding diameter. In this study, polymers of PDMS and PTFE were chosen to further investigate the pressure enhancement in the suggested pressure range. The results show similar pressure sensitivity for both materials. Both materials are highly capable of enhancing pressure sensitivity in the range of 0 – 50 kPa. The suggested pressure range is most suitable for biomedical application. The positive results of the current study lend credibility for using the envisaged sensor for commercial use.

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