Low-cost, high-precision propagation delay measurement of 12-fibre MPO cables for the CMS DT electronics upgrade

CMS DT electronics upgrade involves laying down 3500 optical links from the CMS experimental cavern to the service cavern, whose lengths must be matched to minimize skew, so that the present upstream electronics can be reused at an initial stage. In order to assess the cables' compliance, a high resolution and cost-effective system has been developed to measure the length uniformity of these fibres. Transit-time oscillation method has been implemented with matched MTP 12-channel fibre optic transmitter and receiver and a Spartan-6 FPGA. After proper corrections and averaging, millimetre-range accuracy has been achieved.