Performance of the CMS Tracker Optical Links and Future Upgrade Using Bandwidth Efficient Digital Modulation

The Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC) particle accelerator will begin operation in 2007. The innermost CMS subdetector, the Tracker, comprises ~10 million detector channels read out by ~40 000 analog optical links. The optoelectronic components have been designed to meet the stringent requirements of a high energy physics (HEP) experiment in terms of radiation hardness, low mass and low power. Extensive testing has been performed on the components and on complete optical links in test systems. Their functionality and performance in terms of gain, noise, linearity, bandwidth and radiation hardness is detailed. Particular emphasis is placed on the gain, which directly affects the dynamic range of the detector data. It has been possible to accurately predict the variation in gain that will be observed throughout the system. A simulation based on production test data showed that the average gain would be ~38% higher than the design target at the Tracker operating temperature of -10°C. Corrective action was taken to reduce the gains and recover the lost dynamic range by lowering the optical receiver's load resistor value from 100Ω to 62Ω. All links will have gains between 0.64 and 0.96V/V. The future iteration of CMS will be operated in an upgraded LHC requiring faster data readout. In order to preserve the large investments made for the current readout system, an upgrade path that involves reusing the existing optoelectronic components is considered. The applicability of Quadrature Amplitude Modulation (QAM) in a HEP readout system is examined. The method for calculating the data rate is presented, along with laboratory tests where QAM signals were transmitted over a Tracker optical link. The results show that 3-4Gbit/s would be possible if such a design can be implemented (over 10 times the equivalent data rate of the current analog links, 320Mbits/s).