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This thesis describes a detailed study of advanced optical fibre sensors based on fibre Bragg grating (FBG), tilted fibre Bragg grating (TFBG) and long-period grating (LPG) and their applications in optical communications and sensing. The major contributions presented in this thesis are summarised below. The most important contribution from the research work presented in this thesis is the implementation of in-fibre grating based refractive index (RI) sensors, which could be the good candidates for optical biochemical sensing. Several fibre grating based RI sensors have been proposed and demonstrated by exploring novel grating structures and different fibre types, and employing efficient hydrofluoric acid etching technique to enhance the RI sensitivity. All the RI devices discussed in this thesis have been used to measure the concentration of sugar solution to simulate the chemical sensing. Efforts have also been made to overcome the RI-temperature cross-sensitivity for practical application. The demonstrated in-fibre grating based RI sensors could be further implemented as potential optical biosensors by applying bioactive coatings to realise high bio-sensitivity and bio-selectivity. Another major contribution of this thesis is the application of TFBGs. A prototype interrogation system by the use of TFBG with CCD-array was implemented to perform wavelength division multiplexing (WDM) interrogation around 800nm wavelength region with the advantages of compact size, fast detection speed and low-cost. As a high light, a novel in-fibre twist sensors utilising strong polarisation dependant coupling behaviour of an 81°-TFBG was presented to demonstrate the high torsion sensitivity and capability of direction recognition.

This thesis describes work on fibre transitions made in photonic crystal fibres (PCF) and conventional standard fibres. Three post-processing techniques were used to make the transitions: fibre tapering, ferrule drawing and a new technique – PCF hole inflation. All these methods change the fibre dimensions on a centimeter scale while maintaining very low loss. In the hole inflation technique, cladding holes are pressurized and can be enlarged while heat-treating, unlike other techniques where the holes can only be reduced in size. Controlled hole expansion was used to produce devices for applications such as supercontinuum generation. Furthermore, differential pressurization of holes could create a diversity of core shapes in a PCF section. For example they were investigated to improve interfacing of laser diodes to fibres. Differential pressurization was also used to introduce new cores into PCFs. Introducing a larger core asymmetrically by the original core resulted in a fundamental to second-order mode conversion with a high extinction ratio. Alternatively, similar mode conversion was demonstrated by fusing two unequal standard fibres. Also with standard fibres, low-loss multimode to single-mode fibre transitions were made using a modified fibre fabrication technique. These fibre transitions and optical devices have a wide range of potential applications, for example in supercontinuum generation and low-loss interfacing of fibres to other optical systems.

Optical fibres have been used as a key medium for telecommunication and networking for more than two decades because in principle they offer sufficient transmission capacity, reaching total rates as high as Tbits/s per fibre. Critical fibre properties such as mode field profiles, single-mode propagation conditions and dispersion characteristics can all be related to the optical fibre refractive index profiles. For this reason, it is of fundamental importance to be able to determine the optical fibre refractive index profiles. In this thesis, a novel Transmission-Line technique has been studied and extended for both the forward and inverse solutions. In the forward solution of the Transmission-Line technique, it is shown that the technique is not only capable of determining exactly the propagation constants in optical fibres with real refractive index profiles, but also evaluating accurately the complex propagation constants in single-mode fibres with arbitrary complex refractive index profiles. To illustrate the effectiveness of this technique, it is applied to the evaluation and manipulation of the gain in a typical 980 nm pumped Erbium-Doped fibre as well as to the calculation of the attenuation of optical fibres when radial loss factors are presented. Moreover, based on the Transmission-Line equivalent circuit model, the exact analytical formulas are derived for a recursive algorithm which allows direct and efficient calculation of dispersion of arbitrary refractive index profile optical fibres. The proposed algorithm computes dispersion directly from the propagation constants without the need for curve fitting and successive subsequent numerical differentiation. The algorithm results in savings for both storage memory and computation time. In the inverse solution using the Transmission-Line technique, the optical fibre refractive index profile synthesis from the given mode electric field distribution is developed and demonstrated. The application of the Transmission-Line principles in the study of optical fibre properties was developed for the first time in the early 80's. However, until now the potential of using Transmission-Line technique for the design of optical fibres based on the given electric field pattern had not been examined. From Maxwell's equations, the Transmission-Line equivalent circuits are derived for a homogeneous symmetric optical fibre. This work demonstrates how to use the Transmission-Line model to reconstruct the exact refractive index profile from the electric field data. The accuracy of the reconstructed optical fibre refractive index profile is examined numerically.

The trapping of microparticles by optical methods using a focussed laser beam, known as optical tweezers, has evolved rapidly in the last thirty years. However this process is limited to the trapping of a small number of particles. Evanescent wave trapping allows simultaneous trapping of many particles due to the long length over which a strong intensity gradient is present. A channel waveguide used to produce such an evanescent wave can be photolithographically defined on a flat substrate and thus can be integrated with other micron scale processes. This therefore has potential applications in the lab-on-a-chip field that is currently proving so successful, particularly in biochemical areas where evanescent wave manipulation is ideal for its properties of being cheap, robust, contaminant-free and designed for use with aqueous solutions. This thesis describes both a theoretical and experimental study into the optical trapping and propulsion of gold nanoparticles and latex microparticles above caesium ion-exchanged waveguides. Gold particles of radii varying from 50nm to 250nm and latex particles varying from 1.5μm to 7.5μm were propelled above a waveguide in an aqueous medium. The evanescent field of the channel waveguide was used to both optically trap and propel these particles and speeds of up to 500μm/s were achieved, a full order of magnitude faster than has previously been reported. The optical forces on the particles were derived and used to predict the trapping ability and the speed of particles and physical, electric, and thermophoretic forces, that also affect the particles were described. In addition, modelling allowed the theoretical optimisation of the waveguides for this process. Using a counter-propagating wave it is demonstrated that it is possible to both render particles stationary and position them at any point along the waveguide. In addition, devices were fabricated that allow particles to be automatically sorted down either branch of a Y-junction waveguide. These results demonstrate, that evanescent wave based, integrated optical devices for trapping are feasible and it is anticipated that this will lead to devices for real-life applications being realised.

This thesis presents the results of investigations into the nano-structuring of lithium niobate using the combined or sequential action of laser light and a chemical etch (a mixture of hydrofluoric and nitric acids). Novel methods to characterise ferroelectric domains using Raman spectroscopy and surface second harmonic generation are also described. The use of Raman spectroscopy as a diagnostic technique for recently poled lithium niobate was investigated. The position of several Raman peaks were observed to be shifted following electric-field domain inversion; some these peaks returned to their original positions with a characteristic time constant that is related to the recovery of the internal field of lithium niobate. Surface second harmonic generation was also shown to be able to distinguish between opposite z faces of virgin and poled lithium niobate.

There are many applications for small scale, solid state lasers in the near infrared, where conversely there are very few such devices. A lasing device in a rare earth doped gallium-lanthanum-sulphide thin film is attractive due to emission at wavelengths in the 2 to 5 µm region where many gasses and liquids have fundamental vibrations and overtones and so are detectable. This region also covers the 3 to 5 µm atmospheric 'windows'. Some examples of such detection is presented in this thesis. Using Pulsed Laser Deposition, a relatively new deposition technique, we are able to grow thin films of the chalcogenide glass; gallium-lanthanum-sulphide, gallium-sodium-sulphide and variations of oxysulphides, on a variety of substrates. EXAFS measurements have shown that some of the elements in the glass structure change their bonding arrangement when grown at different energy density producing 'wrong bonds'. This points to the origin of the increased absorption and shift of the optical bandgap which is seen in the materials. It is this tail absorption which ultimately prevented the production of an actual solid state, rare earth laser device. These amorphous semiconductors have a transmission range from the visible through to the mid infrared part of the spectrum. Chalcogenides can be photomodified. i.e. they have an ability to change refractive index when illuminated with photons whose energies lie in the optical bandgap of the material. This process can be reversible or irreversible depending on post deposition treatment and so gives them potential applications such as optical memory, holographic recording media, lithographically written waveguide structures and potentially laser mediums. For such uses a detailed knowledge of the chalcogenide materials optical parameters is needed. A novel technique for the optical characterisation of the thin films has been developed which has is able to measure differences in refractive index to an accuracy of 8.5 x 105. We are able to map refractive index changes across an entire surface and more uniquely whilst they are occurring during, and after, photomodification or heating.

This thesis describes the development of novel optical fibres, microstructured optical fibres (MOFs), and demonstrates device applications based on these structures. A particular emphasis is made on incorporating rare-earth ions within these fibres in order to realise novel active devices. Together with the development of the fabrication technique, characterisation and applications of these radically different fibre types are presented. First, the fabrication techniques of MOFs, which heavily rely upon fibre drawing, are studied. A mathematical model developed for the capillary drawing process is experimentally examined. Good agreement is obtained whilst it is also found that the model provides useful physical insights for determining the fibre draw parameters even for MOFs with complex geometries. Details of the fabrication techniques developed to optimise fibre structures are also presented. Transmission properties of highly nonlinear MOFs are then studied experimentally. It is found that the transmission losses are strongly influenced by the core dimensions due to the high Rayleigh scattering coefficient that originates from the holey cladding. A simple model is used to explain the observations. In addition, a continuous effort towards reducing OH-induced losses of this fibres type is outlined. Rare-earth doped highly nonlinear MOFs are fabricated and characterised. Then, three device demonstrations are carried out for the first time. These include a mode-locked ytterbium doped MOF laser, a nonlinear amplifier based on an ytterbium doped MOF, and a continuous wave erbium doped MOF laser with a very low threshold and high efficiency. Using the ytterbium doped MOF, wide tunability of ultrashort pulses from 1µm to 1.58µm is demonstrated using the soliton self frequency shift effect. For the erbium doped MOF, a pump power threshold of 0.5mW and a slope efficiency of 57% are demonstrated. Novel cladding pumped fibres, air clad MOFs, which use a conventional inner cladding and a holey outer cladding, are developed aiming at improved performance of cladding pumped fibre lasers. Wide tunability over 110nm and pure three level operation at 980nm of ytterbium doped cladding pumped fibre lasers are demonstrated. Finally, the fabrication and characterisation of large mode area microstructured fibres (LMA-MOFs) are described, and a comparison with conventional counterparts is made in terms of bend losses and corresponding effective mode areas. The results show that a slight refractive index difference introduced in the core region of this fibre type strongly modifies its waveguide characteristics. By applying this knowledge, a novel ytterbium doped cladding pumped fibre, which uses different sizes of air holes to define the inner and outer cladding, is developed. A continuous wave output power in excess of 1W is obtained. Results concerning various forms of pulsed laser operation using this fibre are presented and future possibilities are discussed.

Raman fibres and rare-earth doped silica fibres have been investigated for many years as gain media for the amplification of optical signals in telecommunications thanks to their broad gain bandwidth. They are now widely used for that application. It is only recently that power scaling of fibre laser sources has taken place with the development of double-clad fibres and highpower laser diode pump sources. Cladding-pumped fibre lasers are now a rapidly expanding and emerging technology with a wide range of applications, where high-power and high-brightness laser sources are required. Nevertheless, so far, most high-power lasers have been based on highly efficient ytterbium-doped fibre, while progress to power-scale other rare-earth doped fibres and fibre Raman laser has been much more modest. This thesis can be divided into two main themes. The first concerns the power-scaling and study of erbium-ytterbium doped fibre laser sources for optical amplification or as laser sources. The second theme concerns the development and study of high-power Raman fibre lasers and amplifiers based on the novel concept of a cladding-pumped Raman fibre. The themes are jointed in that the cladding-pumped Er:Yb doped lasers, developed in the first theme, are used as pump sources in this second part for the Raman devices. They are also jointed in that they both concern power-scaling of “eye-safe” sources at around 1.6 mm. Firstly, in collaboration with co-workers, high-power, large core, erbium-ytterbium doped fibre laser sources are developed. Output powers in excess of 70 W are obtained. Good beam quality output is achieved thanks to a tapered fibre section. The taper is compatible with standard single-mode fibre which enabled the realization of tuneable fibre lasers free from bulk external grating. The laser’s tuning characteristics are investigated in the C- and L-band range. Subsequently, a master-oscillator power-amplifier (MOPA) based on large core Er:Yb doped fibres is developed for the generation of high-energy pulses. The details of the MOPA are studied and presented. With careful design considerations, pulses free from non-linear effects, with energy up to 1 mJ and peak powers up to 6.6 kW, with narrow spectral linewidth, are obtained at 1535 nm. Secondly, using a double-clad fibre, consisting of raised index, germanium doped, core and inner cladding, with a pure silica outer cladding, a high-power CW single mode Raman fibre laser, pumped by a multi-mode erbium-ytterbium doped fibre laser, is demonstrated for the first time. The laser slope efficiency is 67% and the output power is in excess of 10 W. An experimental and theoretical study of the laser is performed. Then, the pulse amplification in a cladding-pumped Raman fibre is studied in a single pass amplifier configuration. The effects on the laser performance of the pump and Stokes seed powers, fibre length and four-wave mixing are presented. The Stokes’s small-signal gain can be as high as 50 dB. Using this configuration, 700 ns long pulses are amplified up to 10 mJ which shows that, potentially, optical pulses could be amplified to much higher energy. Finally, these results together, let predict that, soon, cladding-pumped Raman fibre could be used as direct brightness converter.

The aims of the study presented here are to develop methods of accurately predicting bend loss in arbitrary index fibres, to use these techniques to explore the potential offered by holey fibres in the large-mode-area, single-mode regime, and to place their performance in context against conventional step-index fibres. In the study presented here, numerical and experimental techniques have been developed that are capable of accurately evaluating the bending losses, the fundamental mode area and the modedness of holey fibres. Note that these techniques are also applicable to conventional solid fibres, which is essential in order to form accurate comparisons. These techniques are applied here to the problem of understanding the bending losses of large-mode-area holey fibres and are successfully used to assess the practical limits that bend loss imposes on large-mode-area holey fibres designed for single-mode operation. These properties are also evaluated for a range of equivalent conventional fibres, the results of which are used to benchmark the potential of holey fibre technology in this regime. The results of this study reveal that the performance of large-mode-area holey and conventional fibres at any given wavelength are similar, and that holey fibres offer advantages for broadband applications. Methods of improving bend loss in holey fibres are also investigated, and it is shown that more complex hole arrangements can be used to improve bend loss in a holey fibre.