Unleashing the light around the accelerators

Optical fibres play today a vital role in communications, machine controls, instrumentation and safety systems. CERN will count at LHC commissioning over 25'000 installed fibre kilometres and more than 40'000 optical terminations. This paper describes the technology that will be used to provide the LHC complex with a sound optical fibre infrastructure via the surface and the underground. The optical fibre network must be extremely reliable and redundant loops shall exist to avoid single points of failure. Laser power used at the transmitter rarely exceeds 1 mW and optical connections must therefore have a high quality standard in order to keep optical reflections and attenuation within acceptable limits. The long distance surface optical fibres (in ducts) may suffer from mechanical stress and will therefore be permanently monitored via an autonomous monitoring system using optical time domain reflectometry. The optical fibres in the LHC tunnel will be subject to irradiation and will therefore darken, thus increasing their attenuation. This process will be closely monitored as well, in order to set off a replacement of these fibres in time.