In this thesis, the propagation of light in novel photonic devices has been studied theoretically, numerically and experimentally. In particular, self-similar solutions to the nonlinear Schrödinger equation have been investigated as a means of avoiding distortions associated with high power pulse propagation in optical fibres. The results show that it is the interplay between the nonlinear and dispersive effects that leads to stable formation of the self-similar solutions. By considering generalised nonlinear Schrödinger equations we have extended the previous investigations of linearly chirped parabolic pulse solutions, which exist in the normal dispersion regime, and have found a new broader class of self-similar solutions, which exist when the fibre parameters are allowed to vary longitudinally. Numerical simulations of these systems confirm the analytic predictions. Experimental confirmation of parabolic pulse generation in high gain cascaded amplifier systems and in highly nonlinear microstructured fibres is also reported. In addition, the propagation of light in modulated crystal structures has been investigated. By modifying the linear and nonlinear properties of the crystals it has been shown that it is possible to manipulate the speed and the wavelength of the propagating light. In particular, negative refractive index materials have been shown to support fast and/or slow propagating light, whilst two dimensional nonlinear photonic crystals have been used to demonstrate multiple harmonic generation over a wide range of phase matching angles. The influence of waveguiding geometries has also been considered to determine the optimum design for the efficiency of the devices

This thesis details work relating to the characterisation of microstructured fibres using SPM techniques. More specifically the optical properties of the fibres have been investigated by the use of a scanning near-field optical microscope and atomic force microscopy. The SNOM was constructed and fully characterised as part of this work. The current state of research into microstructured fibre fabrication, theory and applications is currently benefitting from a great deal of interest from academia and commercial investors alike. New fibre structures are being produced at a rate previously impossible. With this increase comes a need to be able to characterise more effectively the fibres that are produced. SNOM provides a number of significant features that address this issue. In this work four recently fabricated microstructured fibres have been investigated at a number of wavelengths. In each case accurate mode pro- files have been measured and compared with resolution that would be extremely difficult to obtain with traditional mode profiling techniques. A theoretical model has also been used to predict the mode profiles. Measurements of the mode profiles after propagation in free space are presented and are compared to a theoretical beam propagation technique. An interferometric technique at 1550nm was used to image electric field amplitude and phase of the fibre modes, including results on the phase evolution of the mode as it propagates in free space.

This thesis presents experimental and computational work on a variety of advanced fibre Bragg grating structures covering long dispersion compensating chirped Bragg gratings, superstructured Bragg gratings for identical multiple channel operation, Bragg gratings for pulse-shaping applications and Bragg gratings for add-drop applications in high bit-rate systems. Development of the fabrication-technique developed and analysed as a part of this work has led to a number of experimentai 'firsts', including the meter-long Bragg gratings with dispersion-characteristics designed to compensate simultaneous linear and higher order dispersion. Upon transfer of this technology to our industrial partners, a number of field-trial experiments utilising gratings written using this fabrication technique have been successfully performed. Some of the requirements identified from customers led to the discovery of the importance and understanding of high-quality reflection and time-delay profiles. Another product ofthe high flexibiiity provided by the developed fabrication technique have led to demonstrations of superstructured Bragg gratings for a number of exciting applications such as multiple-channel filters obtained through a periodic sinc modulation of the refractive index-profile in fibre Bragg gratings and pulse-reshaping from a soliton to square-pulse with applications in high-speed demultiplexing. Additionally, it is discussed how uniform apodised Bragg gratings filters for application in dense WDM networks, despite their near ideal spectral performance, suffer from non-linear phase attributes in the stop-band, that could limit their use in high bit-rate systems (10Gbit/s and above). Linear phase-filters for dispersion-free filtering are proposed and demonstrated as a solution to this problem for bit-rates up to 40Gbit/s and channel spacings as narrow as 25GHz.

A number of developmental stages are still required to advance and mature optical waveguide technology in non-silica glasses. The primary stage includes raw material purification and improving quality and thermal stability of an optical glass for waveguide fabrication processes. Further stages can include design, application and integration of these waveguides with other photonic devices. Gallium lanthanum sulphide (Ga:La:S) chalcogenide glass (ChG), first discovered in 1976, is a material proposed as an optical waveguide for use in the infrared (IR). Interest in this glass system has been maintained, over the years, primarily due to its exceptional and unusual optical properties. The aim of this project is to advance the current state of art for Ga:La:S glass by demonstrating working solutions for fibre and planar waveguides. Chapter 1 of this thesis provides a general overview of current glass technology and the motivations of this project. The optical glass system under study has yet to attain acceptable stability for fibre production and as such investigation into fundamental manufacturing steps is still required. Raw material purity is an important aspect, of fabricating practical optical glasses, and directly affects performance. Chapter 2 of this thesis describes the purification and synthesis processes performed to produce raw materials with purity far superior to similar products available commercially. Each powdered precursor synthesised in our labs and used in fabrication of Ga:La:S based optical glasses has a transition metal impurity content of less than 1 parts-per-million (ppm wt%). The water content, OH-, of these fabricated glasses has been reduced to < 2 ppm. The primary concern when fabricating Ga:La:S based optical fibre is crystallisation. Optimising the composition to obtain a glass suitable for fibre fabrication is significant in providing thermal stability for fibre drawing. Chapter 3 describes some of the steps taken towards the fabrication and improvement of Ga:La:S based glasses for waveguide technology. The invention of a new variant in the Ga:La:S family of glasses provides key enhancements over existing Ga:La:S and Ga:La:S:O glasses. The hybrid oxy-chalcohalide glass, Ga:La:S:O:F, contains compounds of sulphide, oxide and fluoride as constituents. This new glass type provides significant thermal stability, in the context of fibre drawing. Fibre drawn from a single piece of polished Ga:La:S:O:F glass had attenuation at 1.5 and 4.0 µm of 3.3 and 2.1 dB m-1 respectively. The reduction of the OH- absorption at 2.9 µm to < 1 ppm in Ga:La:S:O:F glass, can potentially allow development of planar waveguide devices for the mid-IR. A range of extremely stable compositions for Ga:La:S, Ga:La:S:O and Ga:La:S:O:F glasses was also identified. These glasses were amorphous upon slow cooling in the furnace (8 oC min-1) indicating danced thermal stability against crystallisation. In Chapter 4 and 5, the fabrication and characterisation of channel waveguides is discussed. Photoinduced changes were introduced by directly writing waveguides into Ga:La:S glass through exposure to short wavelength light (l = 244 nm). Focused fluence of 1.5 - 150 J/cm2 from a continuous wave laser operating at 244 mn was applied, inducing photocompaction and photochemical changes. These passive channel waveguides were spatially single-mode and bad Dn ~ +10-3. The first chalcogenide channel waveguide laser in Nd3+-Ga:La:S glass was also demonstrated. Maximum laser output (l = 1075 nm) of 8.6 mW for an absorbed laser pump power of 89 mW and slope efficiency of 17% was achieved with measured device attenuation of < 0.5 dB cm-1. Discussed in Chapter 6 is the first demonstration of the hotdip spin coating process used to fabricate thin films of a ChG (Ga:La:S). This promising technique is presented as an enhancement to waveguide development. In addition, buried (50 µm) channel waveguides were directly written into the spun thin film using a pulsed laser source (l = 830 nm). These buried channel waveguides had a measured attenuation of < 1 dB cm-1.

Microstructured optical fibres, where an arrangement of air holes running longitudinally along the fibre guides light in either a solid or a hollow core, have created new opportunities in diverse areas of science and technology. Applications range from the generation of supercontinuum light to optical sensing, nonlinear telecom devices and the generation and delivery of extremely high optical powers. Photonic bandgap fibres, allowing light guidance in a hollow core, are also extensively studied. The main issues preventing accurate simulations of the properties of fabricated fibres are identified and addressed. An ideal, accurate representation of a realistic fibre is then proposed and employed to obtain fundamental scaling rules and to study the interactions between air guided and surface guided modes. Anticrossings between these modes in slightly asymmetric structures are identified as the cause for the unusual polarisation effects experimentally observed in these fibres. And finally, guidelines for fabricating fibres with the widest possible operational bandwidth possible are developed and presented.

This thesis reports on the progress that has been made improving the sensing performance of a distributed temperature and strain optical fibre sensor based on the coherent detection of spontaneous Brillouin scattering. To further improve the sensing range and the sensor's performance, the use of Raman amplification was investigated utilising different Raman pump configurations. Using frequency measurements only, a temperature resolution of 5 °C over a 150km sensing range with 50m spatial resolution was achieved using a counter-propagating Raman pump. Using co-propagating Raman pump the temperature resolution was 1.7 °C with 20m spatial resolution at 100km. Measuring both the power and frequency of the Brillouin signal, a simultaneous temperature and strain measurement was performed over 50km; temperature and strain resolutions of 3.5 °C and 85με with 5m spatial resolution were achieved, respectively, using co-propagating Raman pump which has the advantage of requiring access to just one end of the sensing fibre.

This thesis reports on theoretical and experimental studies of wavelength-selective waveguide structures for high-power Nd3+- and Yb3+-doped fibre lasers. Cladding-pumped high-power fibre lasers based on these novel waveguide designs and operating at desired unconventional wavelengths were investigated through numerical simulations and fibre laser experiments. Rare earth doped fibres have typically multiple emission bands of different effective strengths. Stimulate emission from strong bands dominates over, and via a reduced population inversion normally even suppresses, emission from weaker bands in conventional step-index waveguides. For efficient emission and laser operation on the weaker emission bands, it is necessary to suppress unwanted stimulated emission on the strong transitions by preventing power from building up at the unwanted wavelengths. Discrete "bulk" (non-waveguide) devices at a single or a few points are ineffective, if the gain at unwanted wavelength is sufficiently high to generate high-power amplified spontaneous emission even between filters. In such cases, waveguide structures which reduce the gain at unwanted wavelengths and prevent build-up of unwanted emission can be considered. The fibre itself acts as a distributed wavelength-selective filter, and a compact all-fibre laser can be made. For short-wavelength operation when the gain at longer wavelengths needs to be suppressed, a helical core fibre is proposed. This induces a large bending loss at unwanted longer wavelengths while the bending loss at desired shorter wavelength remains relatively low. The required bending loss properties, for efficient operation at the desired shorter wavelength, can be achieved by designing the helix pitch and offset along with fibre core diameter and NA (numerical aperture). A Nd3+-doped helical fibre laser operating at 0.92 μm was investigated through computer simulations. Alternatively, there are fibres in which the fundamental mode can be cut off at a certain wavelength. I have studied fibres with a W-type refractive index profile and fibres with a hollow (air-filled) central region surrounded by a core and then a region with depressed refractive index, known as depressed-clad hollow fibre. With these fibre designs, the doped core guides the desired shorter wavelength but not the unwanted longer wavelengths. Nd3+-doped W-type fibre lasers operating at 0.92 μm were simulated and experimentally demonstrated. Also Yb3+-doped depressed-clad hollow fibre lasers operating at 0.98 μm were simulated and experimentally demonstrated. For long wavelength operation, with a suppressed gain at shorter wavelengths, modified W-type designs are proposed. By designing the refractive index profile and using ring-shaped gain regions, the net gain on an intrinsically weak long-wavelength transition may become larger than that on an intrinsically stronger short-wavelength transition. Adopting this technique, Nd3+-doped fibre amplifiers and lasers operating at 1.38 μm were simulated. While fibre lasers that generate a nearly diffraction-limited single-mode beam are normally targeted, a multimode output is often obtained, e.g., in development stages with nonideal fibres. Then it is important to characterise the modal properties of the beam. For this, two different modal power decomposition methods based on intensity measurements are proposed. The first method is based on a tomography technique that uses a Wigner function followed by an inverse Radon transform. The second method adopts a wavelength-sweeping optical source which induces beat patterns after propagation through a certain length of fibre. The feasibilities of the two proposed ideas were verified through numerical simulations.

Frequency-Resolved Optical Gating (FROG) is a well-established and widely-employed technique for the intensity and phase characterisation of ultrashort optical pulses. Essentially, FROG involves mixing an ultrashort optical pulse with its time-delayed replica, or another pulse, in a nonlinear material or device to yield a two dimensional data set called a spectrogram, from which the electric field of the ultrashort pulse can be retrieved by an iterative algorithm. The most commonly used configuration is based on second-order nonlinear interactions in bulk materials, mainly because of its high efficiency compared to other schemes based on third-order nonlinear interactions. The research work in this thesis led to the first successful implementation of an integrated Lithium Niobate for the FROG device, based on sum-frequency generation. We demonstrated simultaneous complete characterisation of two ultrashort pulses of durations 4-25 ps in the 1.55 μm-band with a coupled energy of 430 fJ in a 26mm long PPLN waveguide device. The temporal walk-off between the interacting pulses in this interaction resulted in an acceptance bandwidth of 0.75 nm, limiting the measurable pulse duration to ~ 4.5 ps. In order to overcome this limitation, we proposed and demonstrated a novel FROG configuration based on cascaded second-harmonic and difference-frequency generations. Theoretical and numerical analyses of this configuration revealed its robustness against the temporal walk-off effect, resulting in improved temporal resolutions. This was experimentally verified by characterising a 2.1 ps pulse train with a coupled average power (energy) of 72 μW (29! fJ) i n the PPLN waveguide device previously mentioned.

Photonic crystal fibres (PCFs) are special fibres with air holes which run along the whole fibre length. These holes not only determine the fibres' unique properties, but also provide a new degree of freedom for fibre modications. In this thesis, we focus on hole control in PCFs from two perspectives: during their fabrication and after they have been made. We found for the first time that the direct information of viscosity was not necessary for description of the fibre drawing process. This conclusion matched our experimental results without recourse to any adjustable fitting parameters. By post-processing of PCFs, which modifies the cladding and core structure and shape, we have achieved a series of novel devices for both linear and nonlinear applications. We have demonstrated fibre devices with cores resembling Young's double slits that have good performance in terms of compatibility and intensity enhancement for a specific application in fibre optic spectrometers. The bulk of this thesis reports on higher-order modes and their nonlinear applications. We achieved all-fibre, low loss and broadband mode converters in highly nonlinear PCFs (HNPCFs) which converted the fundamental mode (LP01) to a higher-order mode (LP02), which can then be converted back if necessary. This higher-order mode has been used for supercontinuum (SC) generation and four wave mixing (FWM) at wavelengths unobtainable for the fundamental mode. This is achieved by utilising the profound dispersion properties of the higher-order mode. We also demonstrated another kind of mode conversion: from the fundamental mode to a Bessel-like beam or its Fourier transform version, an annular beam. Three different methods were implemented experimentally to achieve this non-diffractive, self-healing beam.