  (Abridged version) The CMS experiment at the LHC will begin operation in
2007. The CMS Tracker sub-detector, 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 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{\deg}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 100{\Omega} to 62{\Omega}. 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).
