Optical Fibre Communications Between Radio Telescopes in the European VLBI Network

The increase in availability of installed optical fibre within Europe is presenting a unique opportunity to upgrade the European Very Long Baseline Interferometry Network (EVN). This network of European radio observatories regularly take part in joint sessions to observe celestial radio sources at high resolution, using sophisticated magnetic tape equipment to record data at rates up to 1 Gbit/sec. There are clear advantages to replacing the current recorder technology with an optical fibre network. These include earlier availability of scientific results and perhaps more importantly, increased efficiency during the observing sessions. This report investigates the feasibility of interconnecting the EVN with optical fibres. The work involves the construction of optical device and system models in MATLAB script for both analogue and digitally modulated optical links. These models are used to predict the performance of optical links over distances in excess of 3000km, taking account of chromatic dispersion, non-linear phenomena such as SBS and polarisation mode dispersion if appropriate. In addition to the theoretical modelling, experimental work is performed to verify the models and better characterise phase instabilities for the analogue case. Furthermore, contact has been made with the optical fibre industry to investigate the availability of dark fibre across the EVN span and close to the VLBI station sites leading to cost estimates for both a single experimental link and full network implementation. The work predicts that digital modulation should be the preferred option over analogue for optical fibre links over distances up to 3000km. Commercial equipment is already available which can provide multi-wavelength long haul optical fibre links at speeds up to 10Gbps per wavelength and fibre is, and will become available within continental Europe to provide a backbone for this communication link. In conclusion we find that the connection of VLBI observatories in EVN to a central correlator at 1 Gbit/sec or higher data rates is certainly feasible with fibre optic technology. Current costs using installed fibre are probably prohibitive. However plans for a very high data rate system to connect European large scale facilities and academic institutions make could make such a system affordable in future. The report consists of the main part (sections 1 to 11) and three appendices.