Development and characterization of fibre devices for optical communications
This thesis is concerned with the investigation and development of two types of fibre devices for applications in optical communications: surface-plasmon fibre devices and chirped fibre grating devices. Firstly the suitability of long range surface plasmons (LRSP) for selective fibre mode excitation was assessed both theoretically and experimentally. The envisaged application was for mode tapping or coupling in modal multiplexed systems, where different regions of the fibre mode spectrum carry different communication channels. The idea was achieved by depositing a thin metal layer on a side-polished fibre and arranging that the LRSP mode is phase-matched to a fibre mode. Under this condition, resonant tunnelling of light from the core into the metal layer and from the metal layer into the core occur; this is then tapped off/in through a high index prism (or a second identical side-polished fibre) placed against the fibre face. the intervening space being filled with a layer whose refractive index may be varied. Different fibre modes are then selected by altering the index of this layer, i.e., by altering the phase index of the plasmon mode. An extensive theoretical modelling was conducted. the fabrication process mastered and experimental characterization techniques were implemented and used to fully understand the device behaviour and performance. Tests with dual-mode fibre devices proved that good modal selection can be achieved for higher order modes whereas lower order mode selection suffer from some inherent cross coupling. Additionally, the potential use of surface plasmon resonance in conjunction with optical fibres was studied and proposals for other fibre devices based on this effect were pointed. Next, linearly chirped fibre gratings were investigated. The objective was to develop and characterize devices for dispersion compensation in long fibre telecommunications links and thus allow to increase the information capacity of the installed fibre network. A high resolution measurement system was set up which provided unique and accurate information about the grating dispersion and spectrum. Several types of fibre gratings fabricated using different technologies were measured and the results used by the fabrication groups to refine their techniques. A variable-chirp fibre grating was implemented and investigated in detail, providing important information about the properties of linearly chirped gratings in general. An original chirped grating device which has a fixed dispersion and tunable central wavelength was demonstrated. This device can track the laser signal over satisfactorily broad wavelength ranges, thus allowing to partially overcome the grating narrow-band limitation.