  The optical fibres that transmit the beam permit loop signals at the CERN
accelerator complex are deployed along radiation areas. This may result in
increased attenuation of the fibres, which reduces the power margin of the
links. In addition, other events may cause the links to not function properly
and result in false dumps, reducing the availability of the accelerator chain
and affecting physics data taking. In order to evaluate the state of the
fibres, an out-of-band fibre monitoring system is proposed, working in parallel
to the actual beam permit loops. The future beam interlock system to be
deployed during LHC long shutdown 2 will implement online, real-time monitoring
of the fibres, a feature the current system lacks. Commercial off-the-shelf
components to implement the optical transceivers are proposed whenever possible
instead of ad-hoc designs.
