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| cms-opto:tracker-home:Project Description | |
| Project Description |
| Both CMS readout and
control systems will rely heavily on optical links for data transfer in
order to minimise the power and material budget at the front end and allow
transfer of data at high rates, while remaining immune to electrical interference. The CMS tracker readout system extracts data from the 12 million detector channels over 50000 optical fibres. It is analogue and operates at a 40 MS/s transfer rate. The CMS tracker digital control system is responsible for broadcasting timing and trigger signals to the front-end, as well as sending and receiving control information to and from the detector. It is based on the same components as the analogue readout system and operates at 40Mb/s. |
| History |
The optical system
originally considered by CMS was developed by the RD-23
collaboration. It was based on a reflective electro-optic modulator situated
at the front-end and bi-directionally coupled to a transceiver located at
the link back-end. An alternative optical system based on directly modulated
laser transmitters was evaluated from 1995 onwards. In August 1996, CMS
decided to concentrate its optical link effort on one technology only: directly
modulated laser transmitters coupled by optical fibre to PIN photodiode
receivers. Additional information is available in Developments. |
| Characteristics |
| The CMS tracker optical
link is a multi-way unidirectional system based on edge-emitting laser transmitters
coupled to single-mode optical fibers, multi-way MT connectors and pin-photodiode
receivers. The wavelength of operation is 1310nm. The driving and receiving
electronics is designed according to the system application: analog for
the readout-, and digital for the control-system. The total length of the
link is approximately 100m, of which about 10m is within the high radiation
environment. The optoelectronic transmitter components are assembled using
Si-submount technology. The laser diodes are commercially available Multi-Quantum-Well
(MQW) InGaAsP edge-emitting devices selected for their good linearity, low
threshold current (~9mA) and proven reliability. Photodiodes are epitaxially
grown, planar InGaAs devices of small active volume. Optical Link specification
information is available in EDMS. |
| Environment |
| The front-end link
components will be subject to severe radiation levels: 30Mrad Gamma dose
and 5*1014 (1MeV) neutron equivalent/cm2 over 10 years
for the most exposed parts of the detector. Radiation hardness tests of
lasers, photodiodes, fibers and connectors have been performed with gammas,
neutrons, protons and pions. Increases of laser threshold and pin leakage
currents with hadron fluence have been observed together with decreases
in laser slope-efficiency and photodiode responsivity. Short lengths of
single-mode optical fiber and multi-way connectors have been found to be
little affected by radiation damage. Additional information is available
in Radiation hardness. |
| Performance |
| Complete analogue and digital data link prototypes have been successfully prototyped using the same optoelectronic components as tested for radiation hardness, and discrete off-the-shelf electronics. Test results with the analogue link operating at a 40MSample/s rate confirm that a dynamic range of 50dB and a linearity deviation of less than 2% are achievable with some safety margin. Digital links with bit error rates of less than 10-12 have been measured at 40Mb/s rates with -32dBm and -30dBm photodetector sensitivities. Additional information is available in Performance. |
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| Last modified by JT: Tue, 16-Dec-2008 11:55 |