Optical fibre gratings and their applications as acousto-optic devices
1.1 Background: Fibre optics has gained prominence in the fields of optical fibre communication, instrumentation, cable television and optical data networks. The major application however is in the area of the optical network where the availability of a wide variety of all-optical components is essential to the development of an all-optical network. Components based on optical fibres are extremely attractive due to their intrinsically low loss and compatibility with the fibre system. Recently fibre Bragg gratings (FBG) [1-12] as fibre components have been introduced and proved to have almost all the required qualities for rapid integration into commercial applications. They are currently the area of significant interest and have been used in numerous applications such as fibre lasers, fibre sensors, dispersion compensation, filtering components and wavelength division multiplexing. A grating is a device which periodically - modifies the phase and intensity of an incident wave incident on or transmitted through it. Fibre gratings can be regarded as the fibre equivalent of bulk dielectric mirror or diffraction gratings and be used to reflect, diffract or filter light within the fibre. Advantages of fibre gratings over competing technologies include all-fibre geometry, low insertioii loss, high extinction and potentially low cost. The most distinguishing feature of fibre gratings however is the design flexibility that they offer for achieving desired spectral characteristics. Numerous physical parameters can be varied, including: refractive index change, length, apodisation, chirp, blaze, and whether the grating supports counterpropagating or copropagating coupling at a desired wavelength. By varying these parameters, gratings can be fabricated for use in many different applications. Two basic methods are use to fabricate fibre gratings: internal and external writing. Internal writing is a holographic process where writing beams are launched into a fibre as counter-propagating coherent bound modes of the waveguide. The beams are normally from a blue laser and modify the core index by a two-photon absorption process . The external writing technique is the most flexible and popular method which can be achieved by a step-by-step technique [14,15] for mode converter grating, or a holographic interference of two coherent UV beams using either an interferometer or a phase mask for FBGs . The grating growth is dependent on the fibre photosensitivity. The mechanisms of the photosensitive effect are often linked to a germanium-oxygen defect centre absorption band near 243 nm. Indeed, a widely used interpretation of photosensitivity consists of the photogeneration of carriers by bleaching of the 243 nm band, with subsequent changes in the electronic state of the glass . An enhancement in the photosensitivity through high pressure, low-temperature H, loading also occurs in various silicate glasses [18-20]. Optical fibre acousto-optic devices have the potential to act as frequency shifters, switches, modulators or tunable filters. An all-fibre acousto-optic frequency shifter was first reported by B.Y. Kim  in which efficient coupling between LP01 and LP11 modes in an optical fibre was obtained by periodic microbending produced by an acoustic flexural wave travelling along the fibre, the acoustic wavelength matching the beat length between the LP01 and LP11 modes. The beat length is defined as LB = 2(pi)/(Delta)ß where (Delta)ß = ß01 - ß11 where ß01 and ß11 are the propagation constants of the LP01 and LP11 modes . This phenomenon has also been demonstrated using torsional acoustic waves, but a frequency shifter in this case was reported to have an increased electrical drive power of about 0.78W . A similar low power acousto-optic device based on a tapered single-mode fibre has also been described . In that case the maximum coupling between the LP01, and LP11 modes was obtained for an electrical drive power of only 0.5 mW, but was not suitable as a frequency shifter as the shifted light was coupled to the second mode and lost as a cladding mode at the exit of the device. This is not the case for the four-port acousto-optic device incorporating a null taper coupler . Here the coupler is manufactured from two identical strands of single-mode fibre, one of which is pre-tapered along a 4 cm length (typically from a diameter of 125 µm to 90 µm) prior to fusing the final device. The two fibres are then held in parallel and fused while the fibres are elongated such that the final waist diameter is about 10 pm. The asymmetry of the device means that the component has a broad band zero splitting, but in the presence of an acoustic flexural wave anything from 0% to 100% coupling of the light is possible via the coupled output port . The basic principle for an acousto-optic superlattice modulator was first proposed in 1986 [27-29]. In those papers it was predicted that the optical Bloch waves of a fine periodic stratified structure could be scattered by a weak periodic superlattice providing a certain Bragg-like resonant condition was satisfied. The general solutions of the coupled Bloch wave equations, which can be applied to a variety of different geometries, show that the addition of a coarse grating can enhance or impair the reflection efficiency of a normal fine period Bragg grating. This thesis reports the first ever successful observation of this effect. The experimental set up involved a photo-induced FBG and a longitudinal acoustic wave. 1.2 Objective The principal objective of this research project is to examine a technique which transforms FBGs into Bragg cells operating in reflection mode: "acousto-optic superlattice modulators" (AOSLM). The work includes understanding the optical properties of FBGs, designing, fabricating and characterising the required types of grating, and testing the acousto-optic performance of the superlattice modulators. This mainly includes: - understanding the basic principle of FBGs; - investigating different writing techniques of FBGs; - developing an energy level model of the grating formation process; - studying the growth and thermal decay of FBGs; - writing long period gratings for fibre polarisers; - demonstrating 100% efficiency of AOSLM. 1.3 Summary of contents This thesis discusses the basic principles, fabrication, characteristics, growth and decay of FBGs and their application in AOSLMs. Chapter 2 presents a basic theoretical analysis of their optical properties based on coupled-mode theory. In chapter 3 we review grating writing techniques and describe our set-up for fabricating fibre gratings and monitoring grating growth and decay. In general, there exist different variants of transverse holographic methods for grating fabrication such as the phase mask, step-by-step and interferometric techniques. In chapter 4 we propose a three energy level (EL) model for the process of Type IIa grating formation in boron-codoped germanosilicate fibres to account for our experimental observations which include: i) a slow negative index change following an initial rapid positive index change, ii) the growth rate of the index change being proportional to the intensity of the writing beam, and iii) the saturated index change being independent of the intensity of the writing beam. Thermal decays of fibre gratings are studied to confirm the prediction from the TEL mode 1. In chapter 5 we present the experimental results of uniform long-period optical fibre gratings, chirped long-period gratings and long-period gratings in birefringent optical fibres. For the fibre polariser, greater than 30-dB extinction for one polarisation mode with a splitting of 10.5 nm and an insertion loss < 0.5 dB was achieved. In chapter 6 the first-reported demonstration of an AOSLM is presented. The performance was greatly improved by reducing the fibre diameter by etching in HF. Its operating principle is based on an acoustic wave vector matching the wave-vector difference between the forward and backward Bloch waves. The theoretically derivative procedure based on Bloch waves and the calculated results are described. An acousto-optically induced reflectivity for the first side-band approaching 100% is achieved by an etched cladding from 125 µm to a final diameter ~ 35 µm. The conclusion is given in chapter 7.