Instrumental Delay Calibration of White Rabbit Switches for Sub-10 ns Timing in the REFIMEVE Network

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REFIMEVE is a French national research infrastructure, officially recognized in 2021, based on a fiber-optic network that disseminates ultra-stable time and frequency signals over long distances. Reference signals originate at LTE (former SYRTE) at Paris Observatory and are primarily transported over the optical fibre network backbone of RENATER [1]. It currently connects more than 20 research laboratories across metropolitan France, with a goal to reach over 40 in the next five years. The network also extends internationally through four cross-border links. Within the T-REFIMEVE project, we aim to establish a national-scale White Rabbit (WR) timing network, targeting time accuracy better than 10 nanoseconds across more than 80 nodes interconnected by over 5,000 km of optical fiber. Unlike the bidirectional optical carrier used in the REFIMEVE infrastructure, this deployment leverages the unidirectional optical amplifiers of the RENATER backbone. WR signals are transmitted as alien wavelengths using standard xWDM technology, which introduces the challenge of mitigating optical path asymmetries inherent in unidirectional links. A critical first step toward achieving this level of timing precision is the precise calibration of instrumental delays in the White Rabbit Switches (WRS). This work reports on the calibration of instrumental delays of dozens of WRS units. We performed the calibration according to the CERN procedure [2] and the EMPIR/VSL good practice guide [3], for unidirectional links, using SFP transceivers operating at a wavelength of 1560.61 nm. We defined port 1 of one of our WRS to be our golden calibrator, while choosing to leave its ingress and egress delays at their default firmware values, and then used it to calibrate port 1 of another WRS, thus constituting the usual pair of reference ports in a slightly unusual way. We then used these two ports to calibrate all other WRS ports. As expected the synchronisation was already good using the default…