|Title:||Responses of photonic crystal fibres to pressure, axial strain and temperature|
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Electrical Engineering|
|Pages:||xv, 123 leaves : ill. ; 30 cm.|
|Abstract:||Photonic crystal fibres (PCFs) show different elastic and optical properties from conventional silica fibres because they contain periodic transverse microstructures in their profiles. The responses of PCFs and conventional silica fibres to external measurands are also different, because of the same reason. With these special properties/responses, PCFs have the potential to be widely used in fibre-optic sensors and communication systems. However, to our knowledge, there is no explicit model so far that can simulate the responses of PCFs to axial strain, pressure and temperature. In this dissertation, the microstructure cladding of PCF is regarded as a honeycomb structure which has inhomogeneous elastic properties. Both Young's modulus and Poisson's ratio of this region are anisotropic, and are the functions of the air-filling ratio of the microstructure cladding. Based on this assumption, three theoretical models for three types of PCFs (solid-core PCF, hollow-core photonic bandgap fibre and hybrid PCF) are constructed. These theoretical models can not only be used to simulate the optical properties/responses of existing PCFs to different measurands, but also be used to predict the performance of PCFs with various fibre designs. Thus when PCFs are used in fibre-optic sensors or devices, these theoretical models are very useful to guide the designs of such sensors or devices. Using the theoretical models, the responses of PCFs to axial strain, acoustic pressure, temperature and lateral pressure are investigated respectively. The simulation results show that compared with conventional silica fibres, PCFs are predicted to have several novel or improved responses to external measurands, which can be used to enhance the performance of the fibre sensors or construct new PCF-based devices. The responses of both solid-core PCF and hollow-core photonic bandgap fibre (PBF) to axial strain are investigated theoretically and experimentally. For the solid-core PCF, the length term of its phase sensitivity to axial strain can be normalized to unit, and the index term is mainly determined by the strain-optic effect of the silica core. The experimental results show that the NL-3.3 fibre (one type of solid-core PCF) has the phase sensitivity of 0.7813±0.006 (ε⁻¹), which agree well with the theoretical prediction. For the hollow-core PBF in which most of light is confined in air, the index term of its phase sensitivity to axial strain is much smaller than the solid-core PCF, which is verified by the experimental results. In experiment, the phase sensitivity of HC-1550-02 fibre (one type of hollow-core PBF) is measured to be 0.9815±0.004 (ε⁻¹), which shows a good agreement with the theoretical prediction of ~0.9797 (ε⁻¹).|
The normalized responsivities (NR) of PCFs to acoustic pressure are studied theoretically and experimentally. The simulation results show that: 1) NR of PCF to acoustic pressure is mainly determined by the air-filling ratio of PCF's profile. PCF with a higher percentage of air and lower percentage of silica in its profile is more flexible to acoustic pressure and thus has larger NR; 2) hollow-core PBFs tend to have higher NR than both conventional silica fibres and solid-core PCFs, because the index term of silica-core fibres has opposite sign with the length term, and this negative index term is greatly reduced in a hollow-core PBF in which most of the light is confined in air. In experiment, NR of HC-1550-02 fibre to acoustic pressure is measured to be ~-334.4 (dB re μPa⁻¹), which is about 15 dB higher than conventional fibres. The simulation results show that NR of the hollow-core PBF can be improved further by both decreasing the thickness of its silica outer cladding and increasing the air-filling ratio of its microstructure inner cladding. Using proper fibre parameters, NR of the hollow-core PBF can reach as high as ~310 (dB re μPa⁻¹), which is about 35 dB higher than conventional fibres. The great improvement of NR is anticipated to have important practical benefits to simplify the sensor design of the fibre hydrophone, and increase the number of sensors that can be interrogated per optical source or the number of sensor channels that can be multiplexed onto a signal fibre. The simulation shows that the lateral pressure can result in deformation of the hollow-core PBF’s air core as well as its air-silica cladding, both of which induce linear birefringence of the hollow-core PBF. Applying pressures laterally to three segments of a HC-1550-02 fibre, a novel hollow-core PBF polarization controller (PC) can be constructed. By varying the magnitudes of the applied pressures in these three segments, the output state of polarization from the hollow-core PBF PC shows a good coverage of all the possible polarization states on the surface of the Poincare sphere, indicating a universal control of the polarization state can be achieved. Compared with former scheme for hollow-core PBF PC which makes use of the inherent birefringence of hollow-core PBF, the new scheme may be applicable to hollow-core PBFs with little or no inherent birefringence. Thus, a hollow-core PBF PC with broader bandwidth may be obtained by using this new scheme. Hybrid PCF guides light by a novel guiding mechanism, which is a combination of index-guiding and bandgap-guiding. Because the guiding mechanisms of the hybrid PCF are different in two orthogonal directions, high birefringence property is expected. To our knowledge, there is no theoretical model that can simulate the birefringence properties of hybrid PCFs. In this dissertation, a theoretical model for hybrid PCFs is constructed to simulate the birefringence property of hybrid PCFs and the responses of hybrid fibres' birefringence to axial strain and temperature. Using this theoretical model, the birefringence/responses of hybrid PCFs are predicted, as the functions of their design parameters. In experiment, the birefringence of one type of hybrid PCF and its responses to axial strain and temperature is measured. The experimental results agree well with the simulation results, which give us the confidence to use this theoretical model to guide the design of hybrid PCFs for many special applications.
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