Two-photon laser scanning fluorescence microscopy using photonic crystal fibre

We report the application of a simple yet powerful modular pulse compression system, based on photonic crystal fibres which improves upon incumbent twophoton laser scanning fluorescence microscopy techniques. This system provided more than a 7-fold increase in fluorescence yield when compared with a commercial two-photon microscopy system. From this, we infer pulses of infrared radiaton of less than 35 fs duration reaching the sample

Investigations towards the development of a novel multimodal fibre optic sensor for oil and gas applications.

Oil and gas (O&G) explorations are moving into deeper zones of earth, causing serious safety concerns. Hence, sensing of critical multiple parameters like high pressure, high temperature (HPHT), chemicals, etc., are required at longer distances. Traditional electrical sensors operate less effectively under these extreme environmental conditions and are susceptible to electromagnetic interference (EMI). Compared to electrical sensors, fibre optic sensors offer several advantages like immunity to EMI, electrical isolation, ability to operate in harsh environmental conditions and freedom from corrosion. Existing fibre optical sensors in the O&G industry, based on step index single mode fibres (SMF), offer limited performance, as they operate within a narrow wavelength window. A novel multimodal sensor configuration, based on photonic crystal fibre (PCF) and utilising a multiwavelength approach, is proposed for the first time for O&G applications. This thesis reports computational and experimental investigations into the new multimodal sensing methodology, integrating both optical phase-change and spectral-change based approaches, needed for multi-parameter sensing. It includes investigations to improve the signal-to-noise ratio (SNR) by enhancing the signal intensity attained through structural, material and positional optimisations of the sensors. Waveguide related, computational investigations on PCF were carried out on different fibre optic core-cladding structures, material infiltrations and material doping to improve the signal intensity from the multimodal sensors for better SNR. COMSOL Multiphysics simulations indicated that structural and material modifications of the PCF have significant effects on light propagation characteristics. The propagation characteristics of the PCF were improved by modifying the geometrical parameters, and microstructuring the fibre core and cladding. Studies carried out on liquid crystal PCF (LCPCF) identified its enhanced mode confinement characteristics and wavelength tenability features (from visible to near infrared), which can be utilised for multi-wavelength applications. Enhancing core refractive index of the PCF improved the electric field confinements and thereby the signal intensity. Doping rare earth elements into the PCF core increases its refractive index and also provides additional spectroscopic features (photoluminescence and Raman), leading to a scope for multi-point, multimodal sensors. Investigations were carried out on PCF-FBG (Fibre Bragg grating) hybrid configuration, analysing their capabilities for optical phase-change based, multipoint, multi-parameter sensing. Computational investigations were carried out using MATLAB software, to study the effect of various fibre grating parameters. These studies helped in improving understanding of the FBG reflectivity-bandwidth characteristics, for tuning the number of sensors that can be accommodated within the same sensing fibre and enhancing the reflected signal for improved SNR. A new approach of FBG sensor positioning has been experimentally evaluated, to improve its strain sensitivity for structural health monitoring (SHM) of O&G structures. Further, experimental investigations were carried out on FBGs for sensing multiple parameters like temperature, strain (both tensile and compressive) and acoustic signals. Various spectroscopic investigations were carried out to identify the scope of rare earth doping within the PCF for photoluminescence and Raman spectroscopy based multimodal sensors. Rare earth doped glasses (Tb, Dy, Yb, Er, Ce and Ho) were developed using melt-quench approach and excitation- photoluminescence emission studies were carried out. The studies identified that photoluminescence signal intensity increases with rare earth concentration up to an optimum value and it can be further improved by tuning the excitation source characteristics. Photoluminescence based temperature studies were carried out using the rare earth doped glasses to identify their suitability for O&G high temperature conditions. Raman spectroscopic investigations were carried out on rare earth (Tb) doped glasses, developed using both melt-quench and sol-gel based approaches. Effect of 785 nm laser excitation on Raman signatures and suitability of rare earth doped materials for fibre-based Raman distributed temperature sensing (DTS) were also studied. Finally, a novel multimodal fibre optic sensor configuration is proposed for the O&G applications, consisting of rare earth doped photonic crystal fibre integrating Bragg gratings and operating in multiple wavelength regimes in a multiplexed fashion. The integrated sensor combination is expected to overcome the limitations of existing sensors with regards to SNR, sensing range and multimodal sensing capability

Silica hollow core fibres for mid-infrared medical applications

In this thesis two types of silica hollow core microstructured fibres - the Negative Curvature Fibre and the Photonic Bandgap Fibre - are presented as a novel solution for the flexible delivery of Er:YAG laser radiation. The Negative Curvature Fibre and Photonic Bandgap Fibre had attenuations of 0.06 dB/m and 1.1 dB/m at 2.94 μm wavelength, respectively. This is an important wavelength regime for medical applications, specifically surgery, due to the existence of a strong absorption peak for water around 3 μm. The guidance of high energy pulses of the order of 195 mJ and 14.4 mJ respectively is demonstrated. These energies are sufficient to ablate soft and hard biological tissue. As verification, porcine bone was ablated in air and submerged in water to simulate practical application of a surgical device. The presented fibres are compared to alternative state-of-the-art solid and hollow core fibres, in respect of the fabrication, attenuation, pulse energy delivery capability, bend sensitivity and the output beam profile. The fabrication and characterisation of a novel sapphire endtip is also presented, which seals the hollow cores of the fibres from contamination and therefore increases the usability significantly. The endtip was shown to be mechanically robust, provide a hermetic seal and able to survive practical tissue ablation in air and water. These encapsulated fibres provide a new fully flexible delivery system for high energy Er:YAG laser radiation and hence will open up the possibility of new minimally invasive surgical procedures

Second harmonic generation in novel optical waveguides

Second Harmonic Generation has traditionally been restricted to crystals; however, due to the development of highly sophisticated fabrication technology, poling and phase matching techniques, it has now become feasible in glass fibres and waveguides which are widely used in nonlinear optics. For instance, silica glass based Photonic Crystal Fibres (PCF) exhibit long coherence lengths and controllable optical properties such as chromatic dispersion despite low second order nonlinearity. Given such benefits, the numerical modelling and analysis of optical waveguides accurately and efficiently has become vital for the advancement of nonlinear optics. Hence, this thesis focuses on enhancing the Second Harmonic Generation in optical waveguides through the use of different structures and different materials, and demonstrating the same by numerical simulations. In this thesis, the accurate and numerically efficient Finite Element based Beam Propagation Method has been employed to investigate the evolution of Second Harmonic Generation in highly nonlinear SF57 soft glass Equiangular Spiral PCFs. Further, the H-field based Finite Element Method has been employed for the stationary analysis of Photonic Crystal Fibres. It is shown here that the second harmonic output power in highly nonlinea

Boundary effects in finite size plasmonic crystals: Focusing and routing of plasmonic beams for optical communications

Plasmonic crystals, which consist of periodic arrangements of surface features at a metal-dielectric interface, allow the manipulation of optical information in the form of surface plasmon polaritons. Here we investigate the excitation and propagation of plasmonic beams in and around finite size plasmonic crystals at telecom wavelengths, highlighting the effects of the crystal boundary shape and illumination conditions. Significant differences in broad plasmonic beam generation by crystals of different shapes are demonstrated, while for narrow beams, the propagation onto the smooth metal film is less sensitive to the crystal boundary shape. We show that by controlling the boundary shape, the size and the excitation beam parameters, directional control of propagating plasmonic modes and associated beam parameters such as angular beam splitting, focusing power and beam width can be efficiently achieved. This provides a promising route for robust and alignment-independent integration of plasmonic crystals with optical communication components

Design and optimisation of micro-structured waveguides in nonlinear crystals

Direct femtosecond laser inscription has emerged as one of the most efficient methods for direct three dimensional micro-fabrication of integrated optical circuits in dielectric crystals.Lithium niobate is one of the most widely used dielectric crystal for a wide range of optical functions. Using the direct femtosecond inscription technology, it is possible to produce almost circular tracks of 1-2:5μm diameters with negative refractive index changes up to -0:012 in lithium niobate crystals. Those tracks can be used as a cladding region to confine the propagating light inside a core region of a micro-structured waveguide. This dissertation is focused on the numerical investigation of the propagation properties of depressed-cladding,buried micro-structured waveguides in z-cut lithium niobate crystals which can be fabricatedby direct fs laser inscription method. First of all, we discuss how experimentally achievable parameters of cladding tracks such as their position, total number, refractive index contrasts between the low index cladding structure and the core region can be used to design buried micro-structured waveguides with good confinement properties and to achieve any control over the propagation properties of different polarisation modes specific to a wide range of applications of lithium niobate. Numerical analysis of micro-structured waveguides are implemented by using finite element method. The high nonlinear coefficient and wide transparency region of lithium niobate enable its use for frequency conversion applications towards mid-infrared wavelength ranges. In this thesis, optimisation of the guiding properties, specifically the confinement losses, of microstructured waveguides in lithium niobate is realised for both around telecom and mid-infrared wavelength regions. Optimisation is based on a practical approach which takes into account the variation of experimentally achieved track parameters over cladding region. It is shown that the spectral region where confinement losses are below 1 dB/cm can be extended up to a wavelength of 3:5μm. In recent years, a variety of design geometries for micro-structured waveguides has been a focus of research interest as a means of manipulating and controlling the properties of propagating light. The flexibility of writing tracks at various depths inside lithium niobatecrystals allows direct fabrication of micro-structured waveguides with advanced design geometries.The ability to write tracks at varying sizes by femtosecond laser inscription method enables the fabrication of micro-structured waveguides with highly complex spiral geometries. Here, we explore design issues of equiangular, Fermat and Archimedes spiral geometries in accordance with experimentally available track parameters. Optimisation of each geometry is separately implemented for telecom and mid-infrared wavelength ranges. The primary advantage of designing waveguides with spiral geometries is a much finer control and better manipulation of propagating light stemming from a higher number of parameters available for design. Also, it is found that the spectral region where confinement losses are below 1dB/cm can be further extended up to a wavelength of 3:66 μm