title: Fabrication of single-crystal sapphire fibres for sensor applications
Creators: name: Xing, Mengchuan
Publisher: name: University of Glasgow
Year: 2020
Subjects: subject: Single crystal fibre, sapphire fibre, laser-heated pedestal growth, optical fibre sensor.

description: For this thesis, single crystal sapphire fibres, have been successfully fabricated as suitable sensor materials through the laser heated pedestal growth technique. Lab-produced aluminium oxide rods, commercial aluminium oxide ceramic rods, and sapphire wafer slices were used as source materials. Pure sapphire fibres with diameters in the range of 300 – 350 μm, both c-axis and a-axis, of high optical quality were fabricated. Some growth issues were encountered including diameter variation and an asymmetric molten zone. These were analysed and solved in the multiple growth trails. Seed orientation dependent longitudinal facets were observed in the sapphire fibres; by using accurately aligned c-axis sapphire crystals as a seed, these facets were eliminated. Three new types of sapphire fibre sensor structures were designed and fabricated via the laser heated pedestal growth system: a monolithic sapphire capillary blackbody radiation sensor, a monolithic sapphire capillary Fabry-Perot interferometer sensor and a long-period sapphire fibre grating sensor. For the blackbody radiation sensor, a robust blackbody cavity was fabricated by combining silicon carbide and sapphire at high temperature. The use of the sapphire capillary provides a strong protection of the inner transmission fibre, and the controllable size and shape of the cavity can help to manipulate the radiation signal generated. In the case of the Fabry-Perot interferometer sensor, the monolithic closed sapphire endcap design was developed, tested and evaluated. It was found that the internal surface of the end cap was not to be as good optical quality as was expected from the high surface tension of molten sapphire closing the capillary end under heating. The successful development of the crystal insertion method significantly improved the production success rate of the inner reflective surface of the sensor and would improve its stability as a potential high- temperature sensor. As for the long-period grating sensor, by employing the mechanical speed variant technique in the sapphire fibre growth procedure, innovative sapphire fibre gratings were successfully fabricated with the period in a range of 40 – 1000 μm. To select the suitable sensor for further investigation and development, initial tests were conducted to build the sensing systems for the three sensor candidates fabricated. Challenges were found during this procedure, which provided significant guidance for building sapphire-based sensing systems. Due to its innovative results and outcomes, the long-period sapphire fibre grating sensor was selected as the most suitable sensor for further development. In the investigation, a clear grating effect was observed in the form of the appearance of a resonant wavelength region, which exhibited a clear shift dependent on cladding refractive index. A back-coupling effect was seen in the gratings with a large period (700 – 900 μm) and fractional amplitude modulation values (0.02). The back-coupling effect was seen in the form of a rise in the signal strength as a resonant peak (RP), which could be clearly located in the spectrum. In the refractive index sensing experiment, the RP of the back-coupling sensor showed clearer wavelength shifts than the resonant wavelength region in the original sapphire fibre long-period grating sensor. In temperature sensing experiments, a strong linear relationship between the wavelength shift of the RP and the ambient temperature was obtained with sensitivity of 2.5 nm/ °C. Furthermore, a unique collimated white light output beam of the long-period sapphire fibre gratings was found and characterised with a significantly decreased divergence angle (6°) and various pseudo-coherent properties such as focusing, collimation, and directionality.