Speaker
Description
Summary
Approximately 40 000 analogue readout optical links are being installed in the CMS
Tracker sub-detector for operation in the LHC. Each readout link transmits data
using analogue Pulse Amplitude Modulation (PAM) and has been specified to be
equivalent to a baseband digital PAM system conveying 8 bits of information at 40
MSamples/s. Hence the equivalent digital data rate for the analogue optical links
is 320Mbit/s. 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 foreseen tenfold increase in luminosity. The cost of the
optoelectronic components represents a large fraction of the CMS Tracker
electronics budget. Hence, a digital system reusing the existing components while
delivering sufficient performance for SLHC operation could potentially be a cost-
effective alternative to a full replacement of the installed links. The feasibility
of such a conversion must therefore be explored in terms of performance that can be
achieved and implementation complexity.
The theoretically achievable data rate over the existing analogue link has
previously been investigated using established communication theory. This
theoretical estimate has now been augmented with experimental results from
characterization tests performed in the laboratory. An Agilent 4438C Vector Signal
Generator was used to transmit randomly generated data through an analogue optical
link, using Quadrature Amplitude Modulated (QAM) RF carriers. At the output of the
optical link, an Agilent 4440A Spectrum Analyzer demodulated the signals obtained,
allowing a detailed analysis of the link’s bandwidth and noise characteristics. The
experimental setup and method for determining the capacity of the optical link will
be described.
RF digital modulation over fiber has been experimentally proven to be a candidate
for a future readout link system operating in the SLHC environment and future
experiments. The first estimate of the achievable data rate based upon detailed
experimental measurements will be presented, along with potential hardware
implementation options for a prototype of the proposed system. Also, the
difficulties associated with the proposed upgrade scheme will be reviewed and
compared to the expected benefits.
The performance of future detectors will impose even greater requirements on the
readout systems, with large amounts of data having to be collected and transmitted
over optical fiber. Achieving higher data rates will undoubtedly require employing
novel techniques derived from communication engineering and information theory. The
current work represents a stepping stone to understanding the benefits – as well as
limitations – of the application of such a novel data transmission concept in the
context of HEP instrumentation.
