Advanced optical fibre gratings for nano-structural characterisation and biosensing applications

This thesis presents detailed investigation on the fabrication, spectral characterisation and applications of UV-inscribed optical fibre gratings devices. Of prominent significance is the characterisation of the optical fibre gratings devices with nanoparticles and biological recognition elements for novel developments in the field of optical biosensing. A major contribution detailed in this thesis is the systematic study on fabrication, spectral characterisation and applications of different UV-inscribed in-fibre gratings. Specifically, uniform and apodized Fibre Bragg gratings (FBGs), normal and dual-peak long period fibre gratings (LPFGs), small-angle tilted fibre gratings (S-TFGs) and excessively tilted fibre gratings (Ex-TFGs) are presented. The holographic, phase-mask scanning and point-by-point methods are employed to fabricate these advanced optical fibre gratings using 244nm frequency-doubled Ar+ laser. Particular emphasis is laid on fabrication of dual-peak LPFGs in SMF-28 and thin-cladding single mode fibres of grating periods 140μm and 300μm respectively. Also, Ex-TFGs of different tilt angles are inscribed in single mode fibres using amplitude masks of different periods: 5.0μm, 6.6μm and 25μm. Another important contribution from this study is the nano-structural characterisation of the in-fibre gratings with nanoparticles such as carbon nanotubes (CNT), zinc oxide (ZnO) and gold nanoparticles for power demodulation, sensitivity enhancement and polarisation dependent SPR excitation respectively. Refractive index (RI) sensors based on 81° Ex-TFGs with carbon nanotube (CNT) overlay deposition have been investigated. The CNT, a dark material, with high absorption of light and high RI is responsible for the power demodulation of the attenuation band while the 81°-TFG induces the wavelength shift as the surrounding medium RI changes. Results show high sensitivities of 557.29 nm/RIU and 95.54 dB/RIU for the wavelength shift and power demodulation respectively. Also, nano-deposition of zinc oxide (ZnO) on Ex-TFGs inscribed in two different fibre types has been investigated using dissimilar morphologies (direct ZnO overlay and PSZnO overlay) for enhanced RI sensing. Significant improvement in sensitivity of ~ 21% (~ 522 nm/RIU) is obtained. The polarisation dependence of Au-coated S-TFGs on excitation of surface plasmon resonance (SPR) has also been investigated. Finally, the in-fibre gratings are surface-functionalized with bioreceptor elements such as enzymes (glucose oxidase) and antibody/antigen (Trx, IL-6). Enzyme functionalized biosensor based on dual-peak LPFG has been investigated for sugar concentration level and specific glucose detection and high sensitivities of ~4.67 nm/% and 12.21 ± 0.19 nm/ (mg/ml) are obtained respectively. Also, fibre optic biosensors based on antibodyfunctionalized 81º-TFGs have been presented for label-free specific recognition of interleukin-6 (IL-6) and thioredoxin (Trx) proteins. High saturation values (∧λ max ) of 35.05nm and 33.19nm are obtained respectively. The specificity validation of the biosensors in the presence of other interfering proteins is investigated using human plasma and results show high specificity

Fibre Bragg gratings fabrication in four core fibres

Due to the limitation of the lens effect of the optical fibre and the inhomogeneity of the laser fluence on different cores, it is still challenging to controllably inscribe different fibre Bragg gratings (FBGs) in multicore fibres. In this article, we reported the FBG inscription in four core fibres (FCFs), whose cores are arranged in the corners of a square lattice. By investigating the influence of different inscription conditions during inscription, different results, such as simultaneous inscription of all cores, selectively inscription of individual or two cores, and even double scanning in perpendicular core couples by diagonal, are achieved. The phase mask scanning method, consisting of a 244nm Argon-ion frequencydoubled laser, air-bearing linear transfer stage and cylindrical lens and mirror setup, is used to precisely control the grating inscription in FCFs. The influence of three factors is systematically investigated to overcome the limitations, and they are the defocusing length between the cylindrical lens and the bare fibre, the rotation geometry of the fibre to the irritation beam, and the relative position of the fibre in the vertical direction of the laser beam

High-sensitivity refractive index sensor based on large-angle tilted fiber grating with carbon nanotube deposition

This paper presents a highly sensitive ambient refractive index (RI) sensor based on 81° tilted fiber grating (81°-TFG) structure UV-inscribed in standard telecom fiber (62.5μm cladding radius) with carbon nanotube (CNT) overlay deposition. The sensing mechanism is based on the ability of CNT to induce change in transmitted optical power and the high sensitivity of 81°-TFG to ambient refractive index. The thin CNT film with high refractive index enhances the cladding modes of the TFG, resulting in the significant interaction between the propagating light and the surrounding medium. Consequently, the surrounding RI change will induce not only the resonant wavelength shift but also the power intensity change of the attenuation band in the transmission spectrum. Result shows that the change in transmitted optical power produces a corresponding linear reduction in intensity with increment in RI values. The sample shows high sensitivities of ∼207.38nm/RIU, ∼241.79nm/RIU at RI range 1.344-1.374 and ∼113.09nm/RIU, ∼144.40nm/RIU at RI range 1.374-1.392 (for X-pol and Y-pol respectively). It also shows power intensity sensitivity of ∼ 65.728dBm/RIU and ∼ 45.898 (for X-pol and Y-pol respectively). The low thermal sensitivity property of the 81°-TFG offers reduction in thermal cross-sensitivity and enhances specificity of the sensor

Enzyme-functionalized thin-cladding long-period fiber grating in transition mode at dispersion turning point for sugar-level and glucose detection

Enzyme-functionalized dual-peak long-period fiber grating (LPFG) inscribed in 80-μm-cladding B/Ge codoped single-mode fiber is presented for sugar-level and specific glucose detection. Before enzyme functionalization, the dual-peak LPFG was employed for refractive index sensing and sugar-level detection and high sensitivities of ∼4298.20  nm/RIU and 4.6696  nm/% were obtained, respectively. Glucose detection probe was attained by surface functionalization of the dual-peak LPFG via covalent binding with aminopropyl triethoxysilane used as a binding site. Optical micrographs confirmed the presence of enzyme. The surface-functionalized dual-peak LPFG was tested with D-(+)-glucose solution of different concentrations. While the peak 2 at the longer wavelength was suitable only to measure lower glucose concentration (0.1 to 1.6  mg/ml) recording a high sensitivity of 12.21±0.19  nm/(mg/ml), the peak 1 at the shorter wavelength was able to measure a wider range of glucose concentrations (0.1 to 3.2  mg/ml) exhibiting a maximum resonance wavelength shift of 7.12±0.12  nm/mg/ml. The enzyme-functionalized dual-peak LPFG has the advantage of direct inscription of highly sensitive grating structures in thin-cladding fibre without etching, and most significantly, its sensitivity improvement of approximately one order of magnitude higher than previously reported LPFG and excessively tilted fibre grating (Ex-TFG) for glucose detection