The report describes the optical links developed for the data transmission of the
LHCb calorimeter system. They are used to establish high speed (1.6Gbps)
connections over long distances of about 100 m among the front-end electronics
cards of the calorimeter system, the L0 trigger system and the DAQ boards.
The optical transmitters are built as mezzanines boards, i.e. as cards to be
plugged to carrier-boards (to the CROC calorimeter boards, to the SPD control
boards and to the validation cards). They get power, control signals and reference
clock from the underlying carrier-boards. To plug the mezzanines boards to the
carrier-boards we plan to use the high-speed connectors by Samtec.
To transmit 32 bits patterns at 40.08MHz through the optical fibers we use the GOL
chip (Gigabit Optical Link), radiation hard, produced by the CERN Microelectronic
Group. The data transfer rate, running the transmitter at 40.08MHz, including
header and parity bits, using the 8B/10B encoding mode, is of about 1.6 GHz per
The reference clock will be generated by the carrier-board and distributed to the
GOL chips from a clock distributor. The jitter introduced by these devices is
guaranteed to be less than 2ps RMS.
As optical transducer in the single-channel transmitter boards we use the VCSEL
(Vertical Cavity Surface Emitting Laser) laser diode (1mW at 6mA) by ULM Photonics.
It operates on multimode optical fibers at a wavelength of 850nm and is equipped
with SMA type fibre connector.
In the multi-channel boards we use a parallel transmitter made by Agilent. The
optical transducer is the SNAP12 standard compliant, equipped with the MPO/MTP
ribbon fibre connector interface. It operates on multimode optical fibre at a
wavelength of 850 nm.
The start up of the GOL is managed by means of the CRT4T power switches.
Special care has been spent in the projecting the PCBs and in placing the bypass
capacitors, in order to minimize the noise level and the bit error rate.
The report will describe the test performed on the prototypes to fully qualify the
optical link. The link has been qualified using the eye diagram and the BERT. The
results show that the BER is better than 10-13 as expected.