Ionising radiation damage of optical fibre data link components in the CMS Inner Tracker

Results are presented from cobalt-60 photon irradiations of multi-quantum well (MQW) modulators (to ~20Mrad) and optical fibres (to ~30Mrad). These components form part of the optical data-link for transfer of signals from the silicon microstrip and MSGC elements in CMS. Measurements of the reflectivity of the modulators at different bias voltages were made before and after irradiation showing no significant change. The modulators were irradiated under bias with the dark current in the devices continuously monitored during and for several days after the irradiation. Increases in the dark current were observed due to radiation damage though the devices were fully annealed after several days. The results suggest that the modulators are sufficiently resistant to ionising radiation for use in CMS, though further tests are required. The transmission of the fibres was monitored throughout the irradiation and for part the recovery period after irradiation. The radiation induced attenuation losses in the fibres was fitted with a model describing the time and dose dependence of different defect states. The data obtained here for the fibres could therefore be projected over a ten year lifetime at LHC, resulting in only small transmission losses in fibres used in CMS.