Optical Link systems for the cms strip tracker

The CMS Tracker’s ~10 million individual detector channels are read out by ~40000 uni-directional analog (40MS/s) optical links that transmit the data between the detector and the ~65m distant counting room. The corresponding control system consists of ~2500 bidirectional digital (40Mb/s) optical links based upon the same components as far as possible. The on-detector elements (lasers and photodiodes) of both readout and control links will be distributed throughout the detector volume in close proximity to the silicon detector elements. For this reason, strict requirements are placed on minimal package size, mass, power dissipation, immunity to magnetic field and radiation hardness. It has been possible to meet the requirements with the extensive use of commercially available components with a minimum of customization.

Imperial College has collaborated with the CERN group responsible for developing the optical links (http://cms-tk-opto.web.cern.ch) in the characterization, performance assessment, and optimization of the analog readout links. The optical links built must match the performance requirements of the overall readout system in terms of dynamic-range and resolution. They are required to transmit 3.2 MIPs with an equivalent digital resolution of 8 bits. A simulation has been developed to predict the gain spread throughout the entire Tracker, at the nominal front-end operating temperature of -10°. The results have been used to adjust the links’ gain such that optimum performance is ensured in the final system.

The transmitter: The Analog OptoHybrid (AOH)

With a year left before CMS goes into operation, we have already begun researching potential upgrades to the readout optical links. The next iteration of the CMS Tracker will be operated in the Super LHC (SLHC) environment, and will have to cope with significantly increased data rates due to the tenfold increase in luminosity that is foreseen. In contrast to the telecoms industry where the optical fiber and its installation drive the cost of a transmission system, it is the cost of the optoelectronic components that represents a large fraction of the CMS Tracker electronics budget. Hence, a digital system based on the existing components that can deliver sufficient performance for SLHC operation could potentially be a cost-effective solution. The feasibility of such a conversion must therefore be explored in terms of performance that can be achieved and implementation complexity. We have shown that multi-Gbit/s data-rates are possible over the current optical links by employing techniques similar to those used in ADSL. The concept involves using digitally-modulated radio frequency (RF) sinusoidal carriers in order to make efficient use of the available bandwidth. The work represents a stepping stone to understanding the benefits - as well as limitations - of the application of such a novel concept in the context of HEP instrumentation.

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