Front-end Optical Links for the Liquid Argon Calorimeter

System Overview

We are currently evaluating a system based around the Hewlett-Packard 'G-Link' (HDMP-1022/4) chipset. The chip allows 32 bits of data to be serialised at 40MHz (operating in the 'double-frame' mode), which is the LArg specification. Since the chip takes care of clock encoding / extraction, DC balance and framing it is very easy to use and can be thought of a virtual ribbon cable. Since the G-link protocol (CIMT) adds an extra 4 control bits per 16 bit word, the total baud-rate of the link is 1.6 Gb/s. This single chip solution would require one fibre optic link per front-end board.

At present, the G-Link chip drives commerical VCSEL-based Gb Ethernet transceiver units. These allow a low cost, reliable interface to the fibre without the need to work with laser drivers. Since only half of a transceiver is used at either end of the link, they may not be optimal for use in ATLAS.

A standard 50/125 GRaded INdex Multimode fibre is used to connect the transmitter to the receiver. Reliable operation over distances in excess of 200m is simple to achieve. A demonstrator link (see below) has been operated for over six months in the lab. with no errors recorded.

The bulk of our work at present centres around characterising the radiation tolerance of the demonstrator link. We have irradiated G-Link transmitter chips, packaged VCSEL emitters, transceivers and optical fibres.

Click here for an on-line presentation of the current status of the project.
Click here to look at papers.
Click here to look at talks.

M.P.