The Large Hadron Collider (LHC), currently under construction at CERN, Geneva, will collide proton beams of energy 7 TeV. The high luminosity of the machine will lead to a severe radiation environment for detectors such as ATLAS. The ATLAS Semiconductor Tracker (SCT) must be able to tolerate a radiation field equivalent to an ionising dose of 10 Mrad (Si) and a neutron fluence of 2x10<sup>14</sup>cm<sup>-2</sup> (1MeV,Si) over the 10 year lifetime of the experiment.
The SCT is instrumented by silicon microstrip detectors and their front-end chips (ABCDs). Data is transferred from, and control signals to, the ABCDs using multimode optical links carrying light at 840 nm. The incoming timing, trigger and control (TTC) link uses biphase mark encoding to send 40 Mbit/s control signals along with a 40 MHz clock down a single fibre. Optical signals are received by a p-i-n diode and decoded by DORIC chips. Data in electrical form from the ABCDs is used to moderate two VCSELs by means of a VCSEL driver chip (VDC). Each detector module carries 12 ABCDs and is served by two optical fibres for data readout and one for TTC signals. There are 4088 such modules within the SCT.
The system performance specifications and architecture are described, followed by test results on individual components and complete links. The optical fibre, active optical components, chips, packaging and interconnects have all been qualified to the necessary radiation levels. This has involved studies of total dose effects, single event upset and ageing at elevated temperatures and details of these studies are presented.