Integrated Optical Components for Optical Fibre Sensors

Bristow, Julian Paul Gregory (1985) Integrated Optical Components for Optical Fibre Sensors. PhD thesis, University of Glasgow.

Abstract

The signal processing requirements of optical fibre sensors have been examined and a range of integrated optical processing systems on lithium niobate substrates considered. Requirements on the constituent components of the system have been analysed. It was found that mode filters with extinction ratios of 60dB were required for Mach-Zender interferometers and for short-term gyroscopes, with the figure increasing to 90dB for inertial single-pass systems. Couplers were required to be stable and have as low an insertion loss as possible. A need for a new processing scheme for single fibre polarimetric sensors was identified. The phenomenon of surface plasma wave TM attenuation resonance was examined, and the properties of the various surface waves which could propagate along the surface of a thin metal film examined. These considerations were used to design metal clad stripe waveguide polarisers using silicon monoxide/aluminium claddings. Extinction ratios of up to 80+/-13dB with excess losses of 2+/-3dB were measured for an individual device, and methods for the measurement of such high ratios examined. These results were shown to be better than any reported in the literature. Extrapolation of such results to yield extinction ratios normalised with respect to length has been shown to be invalid. Attempts at excitation of the short-range surface plasmon using waveguide geometries have been unsuccessful to date, but are shown to offer greatly enhanced extinction ratios or shorter devices. Other applications of a variety of surface plasma waves have been considered. The design of waveguide polar isers using proton exchanged sections was considered and a novel geometry proposed. Due to fundamental limits of the devices and experimental considerations it was not possible to produce polarisers with sufficiently high extinction ratio and low excess loss. A device to enable efficient coupling between single-mode optical fibres and integrated stripe guides was designed and developed. The device used ion-milled grooves in lithium niobate to locate the fibres. A loss of 2.6dB was determined for the device with a projected loss of 1.8dB for the same device using index matching fluid. The device would be suitable for mass production, but further work is necessary to reduce the losses to acceptable limits. The use of field overlap calculations to evaluate coupling losses due to waveguide dissimilarties has been investigated. The method has been shown to be suitable for the optimisation of the waveguide parameters but is not generally accurate. An experimental system to evaluate the integrals has been demonstrated and the results obtained validated. The method has been shown to be a useful tool for design of integrated optical/fibre optical systems. Other applications of the recorded data have been considered, but further work is necessary before refractive index distributions may succesfully be reconstructed from the field profiles. The use of the data in optimising the design of electro-optic guided wave modulators is proposed. A novel detection and signal processing scheme has been proposed for the single fibre polarimetric sensor. The system operates in a closed loop configuration and uses different optical frequencies in the two eigenmodes. The system requires two orthogonal polarisers and a frequency shifter operating on one mode. The requirements on integrated optical components for an implementation of such a system have been analysed. Frequency shifters have been reported in the literature with sufficiently high performance, and TE-polarisers have been demonstrated in this thesis with the requisite performance. Edge polishing has been shown to be an unsuitable technique for the fabrication of TM-like mode filters, but a novel geometry using ion milling to produce a vertical wall in the crystal has been demonstrated. The extiction ratio was measured to be 17+/-3dB while the associated excess loss was 7+/-3dB. Although these values preclude the inclusion of the device in the proposed system, they are important as they enable the TM-like modes to be selected. It is shown that the devices would be useful for other systems incorporating phase modulators on Z-cut LiNbO3. Other implementations, including all-fibre versions, of the proposed system have been considered, but are unlikely to offer comparable dynamic range. A number of suggestions have been offered for work based on the contents of this thesis.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Electrical engineering
• Date of Award: 1985
• Depositing User: Enlighten Team
• Unique ID: glathesis:1985-77348
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 14 Jan 2020 11:53
• Last Modified: 14 Jan 2020 11:53
• URI: https://theses.gla.ac.uk/id/eprint/77348