Overview of the RICH Optical Data Link and Evaluation of its Optical Attenuation

The RICH optical link is required to transport the Gigabit/s data flow from the on-detector Level 0 front-end boards to the Level 1 receiver electronics located approximately 100 meters away in the counting room. To ensure the light intensity observed at the Level 1 end of the optical link is compatible with the optical receiver specifications, a knowledge of the optical attenuation across the proposed link is necessary. This report presents the results obtained from performing attenuation tests on the RICH optical link and its constituent interfaces. These results are then used to predict the absolute maximum and minimum light intensities that would be expected at the Level 1 optical receiver when the data link is driven by the Level 0 optical transmitter, the VCSEL. To confirm these results explicitly, Bit Error Rate (BER) tests were performed using a variable optical attenuator inserted into the data link. The report concludes that by operating the VCSEL with a baseline driving current below 4.2 mA the RICH front-end data will be efficiently transferred across the proposed optical data link to the Level 1 receiver.