Experimental and theoretical analysis of polarization-maintaining fibre for sensing applications
Fibre optic sensors have been developed extensively over the past several decades, showing their potential and suitability for real-time measurement and thus meeting various industrial challenges. This has been achieved through the continuous innovation both in the fibre structural design and in the exploration of a variety of sensing mechanisms employed. Optical fibre sensors have shown advantages over conventional electrical counterparts by being of small size, immunity to electromagnetic interference and resistance to chemical attack, thus revealing their potential for a range of industrial applications. High-birefringence (Hi-Bi) optical fibres represent an important type of specialist fibres that have been explored widely by researchers. The focus of this thesis is to explore their potential for different sensing applications as the birefringence of these polarization maintaining (PM) fibres varies in response to the change in the surrounding environment, such as force and pressure.\ud Extensive evaluation of high-birefringence optical fibres, such as those with an elliptical core or elliptical inner cladding, Bow-Tie, Panda and Hi-Bi Photonic Crystal Fibres (PCFs), has been made both experimentally and theoretically, using numerical or analytical techniques.\ud \ud In this thesis, the material birefringence of Polarization-Maintaining fibres, e.g. PM side hole fibre(s) with one or two hole(s) located in its cladding, is calculated using a thermo-elastic displacement potential method through the superposition of sectional displacement potentials. These PM fibres have shown the potential for pressure/force measurement based on their birefringence characteristics when exposed to a transverse force. The methodology used is generic and thus applicable to any one-hole fibre structures, should the hole diameter or position vary in the fibre cladding, or the fibre hole be empty or filled in with any material. This enables the analysis to be applied more widely in a range of optical fibre sensor applications.\ud \ud Direct measurement of transverse force through the interrogation of induced birefringence variation has also been investigated in this thesis by using two specialist single mode Polarisation-Maintaining (PM) side-hole(s) fibres and four different types of Hi-Bi and low-Bi PCFs. The variation in the pressure sensitivity of these PM fibres has been investigated both theoretically and experimentally and it was confirmed that they are dependent upon several key parameters of the system, including the fibre structure, the magnitude and the direction of the applied external force, the fibre length used and the birefringence of the fibre. The theoretical data obtained have shown a good agreement with those from experiments, confirming the suitability of the use of such PM fibres for the measurement of pressure, force and mass of an object, applied in different directions, over a wide range and in real time.\ud \ud It is concluded based on the theoretical and experimental data obtained that PCFs of low birefringence are more sensitive than those of high birefringence (Hi-Bi) although the former require a longer length to achieve a similar level of birefringence. Compared to conventional Hi-Bi fibres (e.g. Panda and Bow-Tie fibres), PCFs have demonstrated much lower temperature sensitivity and this suggests that they are well suited to measure pressure, force, and mass in real time when temperature varies by using a fibre loop mirror (FLM) configuration. To solve the length problem in the low birefringence PCFs, a joint PCF approach can be used by fusion-splicing a short length of Low-Bi with a short section of a Hi-Bi PCF and their sensitivity to lateral pressure has been investigated and reported in detail in this thesis.