Advanced UV inscribed fibre grating structures and applications in optical sensing and laser systems

This thesis presents detailed investigation of UV inscribed fibre grating based devices and novel developments in the applications of such devices in optical sensing and fibre laser systems. The major contribution of this PhD programme includes the systematic study on fabrication, spectral characteristics and applications of different types of UV written in-fibre gratings such as Type I and IA Fibre Bragg Gratings (FBGs), Chirped Fibre Bragg Gratings (CFBGs) and Tilted Fibre Gratings (TFGs) with small, large and 45º tilted structures inscribed in normal silica fibre. Three fabrication techniques including holographic, phase-mask and blank beam exposure scanning, which were employed to fabricate a range of gratings in standard single mode fibre, are fully discussed. The thesis reports the creation of smart structures with self-sensing capability by embedding FBG-array sensors in Al matrix composite. In another part of this study, we have demonstrated the particular significant improvements made in sensitising standard FBGs to the chemical surrounding medium by inducing microstructure to the grating by femtosecond (fs) patterning assisted chemical etching technique. Also, a major work is presented for the investigation on the structures, inscription methods and spectral Polarisation Dependent Loss (PDL) and thermal characteristics of different angle TFGs. Finally, a very novel application in realising stable single polarisation and multiwavelength switchable Erbium Doped Fibre Lasers (EDFLs) using intracavity polarisation selective filters based on TFG devices with tilted structures at small, large and exact 45° angles forms another important contribution of this thesis

Near- and mid-IR fibre grating devices and applications

This thesis presents a detailed research work on the fabrication, characterisation and applications of optical fibre grating devices with operation wavelengths cover from near- to mid- infrared (IR) range. One of the major contributions described in this thesis is the systematic investigation on the structures, fabrication methods and spectral, thermal, strain and surrounding refractive index (SRI) characteristics of near-IR fibre gratings including fibre Bragg gratings (FBGs), chirped fibre Bragg grating (CFBGs) and long period grating (LPGs). For some applications, such as special engineering sensors and high power fibre laser, the fibre gratings have been fabricated on different novel fibres (metal coated fibre and large mode field fibre), respectively. Another important contribution from the studies is experimental investigation on 45º tilted fibre gratings (45°-TFGs) and excessively tilted fibre gratings (ex-TFGs), and their applications. 45º-TFGs with high polarisation dependent loss (PDL) in single mode and polarisation maintaining (PM) fibres have been fabricated. The 45°-TFG has been employed as in fibre polariser to obtain the single polarised laser, which has been further developed as transverse loading sensor achieving high sensitivities. Furthermore, all fibre Lyot filter with narrow bandwidth (26 pm), constructed by two 45º-TFGs with 100m long cavity in PM fibre has been demonstrated. For ex-TFGs, SRI sensor based on a surface modified 81°-TFG, showing capability to detect glucose concentration with relatively high RI sensitivity (~168nm/RIU). Finally, an all-fibre loading sensor based on a hybrid 45° and 81° TFG structure has been demonstrated. Finally, I have fabricated fibre gratings into mid-IR 2μm range. The mid-IR FBGs have been evaluated for thermal and strain response, revealing higher temperature sensitivities than that in near-IR range. The mid-IR LPGs have been investigated for the thermal and refractive index sensitivities, also showing significant enhancement. The 45°-TFGs in mid-IR have been investigated for their PDL characteristics. The mid-IR FBGs and 45°-TFGs have been employed in Tm-doped fibre laser cavity to realize multi-wavelength continued wave (CW) and single polarisation operation

Advanced UV inscribed fibre grating structures and applications in optical sensing and laser systems

This thesis presents detailed investigation of UV inscribed fibre grating based devices and novel developments in the applications of such devices in optical sensing and fibre laser systems. The major contribution of this PhD programme includes the systematic study on fabrication, spectral characteristics and applications of different types of UV written in-fibre gratings such as Type I and IA Fibre Bragg Gratings (FBGs), Chirped Fibre Bragg Gratings (CFBGs) and Tilted Fibre Gratings (TFGs) with small, large and 45º tilted structures inscribed in normal silica fibre. Three fabrication techniques including holographic, phase-mask and blank beam exposure scanning, which were employed to fabricate a range of gratings in standard single mode fibre, are fully discussed. The thesis reports the creation of smart structures with self-sensing capability by embedding FBG-array sensors in Al matrix composite. In another part of this study, we have demonstrated the particular significant improvements made in sensitising standard FBGs to the chemical surrounding medium by inducing microstructure to the grating by femtosecond (fs) patterning assisted chemical etching technique. Also, a major work is presented for the investigation on the structures, inscription methods and spectral Polarisation Dependent Loss (PDL) and thermal characteristics of different angle TFGs. Finally, a very novel application in realising stable single polarisation and multiwavelength switchable Erbium Doped Fibre Lasers (EDFLs) using intracavity polarisation selective filters based on TFG devices with tilted structures at small, large and exact 45° angles forms another important contribution of this thesis.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Growth and characterisation of single-crystal fibres for sensing applications

The laser heated pedestal growth technique has been successfully employed to grow pure and doped sapphire crystal fibres for characterisation as suitable sensor materials. Source materials used were polycrystalline and crystalline sapphire rods while fibres with typical diameters in the range 80 - 170 mm were grown. Pure sapphire fibres, both a- and c-axis, were found to grow easily with no complications such as melt instability. C-axis fibre growth was readily initiated while a-axis fibres required an appropriate a-axis oriented seed crystal. Dip-coating has been used to prepare suitably coated sapphire source rods for growth into doped fibres. Doped fibres grown included Cr3+:, Er3+:, Er3+:Yb3+: and Yb3+:Er3+:Al2O3. Er3+:Yb3+:Al2O3 fibres have been prepared with approximately equal concentration of both dopants while a 10:1 Yb3+ to Er3+ concentration ratio was used for preparing Yb3+:Er3+:Al2O3 fibres. Ruby fibres were also found to grow easily although brownish-green deposits have been observed on some of these fibres. Large transmission losses have been found in fibres with these deposits. Acid cleaning was not effective in removing these deposits, suggesting that they have diffused beneath the surface of the fibres. This was attributed to the condensation of chromium oxide on the fibre surface during growth. Growth of rare earth-doped fibres was initially problematic due to the constant breaking-off of the crystallising fibres from the melt. This was thought to be due to the flexibility of the small diameter source fibres used as well as the high concentration levels of doping. Replacing these small fibres with larger source rods thus permitted RE-doped fibres with relatively good optical quality to be grown. Fibres were grown with typical growth rates of 0.5 - 1 mm/min

The Gut-Kidney Axis in Kidney Transplantation

For the majority of patients with end-stage kidney disease, kidney transplantation remains the optimal treatment modality, offering superior survival and quality of life at a fraction of the cost of dialysis. However, the requirement for long-term immunosuppression increases the susceptibility of transplant recipients to metabolic complications, infection, cancer, and cardiovascular disease, meaning that a “normal” lifespan is rarely, if ever achieved. Innovative strategies are needed to reduce the burden of current immunosuppressive regimens and to maintain the balance between immunosuppression and the maintenance of protective immunity. The gut microbiome is a critical determinant of human health and modulates host immunity and metabolism through a number of recognised mechanisms, and likely others as yet unrecognised. In transplantation, the gut microbiome offers a novel target to alleviate the deleterious immune and metabolic responses that result from exposure to alloantigen and long-term immunosuppression. Herein, we show for the first time, that dietary modification of the gut microbiome can prevent allograft rejection in a murine model of kidney transplantation through signalling via GPR43 and mediated by the action of T regulatory cells. The aims of this thesis were to investigate the role of the gut microbiome in mediating immune responses in kidney transplantation, and to assess strategies to measure glycaemic risk (pre-transplant) and status (post-transplant). This thesis comprises a pre-clinical model of kidney transplantation and translational studies in the clinic, and is presented as a collection of published and submitted works