Advanced Bragg grating based integrated optical devices for wavelength division multiplexing systems
This thesis presents experimental and theoretical work towards the development of advanced integrated planar optical devices for wavelength division multiplexing (WDM) applications in optical communication systems. The presented work lies within a broad range of research areas, namely: design and simulation of novel Bragg grating based WDM passive devices, their performance characterization in simulated optical communication systems, fabrication and characterization of photosensitive thin films and finally device fabrication using ultraviolet induced refractive index changes. A complete model for the analysis of Bragg grating assisted devices in waveguide structures has been developed. Bragg grating based optical Add/Drop multiplexers (OADM) have been simulated and studied by using this modelling tool. A fully optimised design for an OADM based on null coupler and tilted Bragg grating has been proposed. This device can exhibit optimised Add and Drop actions with suppressed backreflections and crosstalk to a level lower than -40 dB. A novel interferometric OADM configuration based on a full cycle full (100%) coupler is also proposed. This design exhibits fully optimised and symmetrical Add/Drop actions in contrast to the compromised performance of traditional interferometric configurations. A software simulation tool has also been developed and employed for the theoretical characterization of optical filters, linking this way the proposed devices to real communication system issues related to high bit rate WDM networks. The effect of group delay ripple in WDM filters is studied for different modulation formats and a simplified figure of merit is proposed for the characterization of the effect. For the fabrication of photosensitive thin film structures, a prototype in-house Flame Hydrolysis Deposition facility was built and it was initially characterized. Further experimental work involved detailed analysis and characterization of highly photosensitive Lead Germanate glasses grown by Pulsed Laser Deposition. A developed direct UV-writing facility is presented and major issues around the functionality of the technique are discussed, by demonstrating solutions for the control of the optical system. A rapid heat treatment technique is also proposed for locking the enhanced photosensitivity in deuterium loaded germanosilicate glasses and successful preliminary results are demonstrated. This method should allow finally the fabrication of complex all-UV-written devices, which has been limited to date by the degrading photosensitivity during UV writing, due to rapid deuterium outdiffusion.