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The rapid growth of data transferred within data centres, combined with the slowdown in Moore's Law, creates challenges for the future scalability of electronically-switched data-centre networks. Optical switches could offer a future-proof alternative and photonic integration platforms have been recently demonstrated with nanosecond-scale optical switching times. End-to-end switching time, however, is currently limited by the clock and data recovery time, which typically takes microseconds, removing the benefits of nanosecond optical switching. Here we show a clock phase caching technique that can provide clock and data recovery times of under 625 ps (16 symbols at 25.6 Gb/s). Our approach uses the measurement and storage of clock phase values in a synchronised network to simplify clock and data recovery versus conventional asynchronous approaches. We demonstrate our technique using a real-time prototype with commercial transceivers and validate its resilience against temperature variation and clock jitter, based on measurements from a production cloud data centre. Main T he rate of data transmitted between servers within data centres has rapidly increased over the last few years [1], driven by cloud adoption and data-intensive cloud workloads such as data analytics and machine learning. Cloud providers have been able to accommodate this fast growth by relying on Moore's Law for networking: every two years the electronic switch integrated circuits (ICs) double their bandwidth at same cost and power. The long-term sustainability of this trend, however, is being questioned by two upcoming challenges: Firstly, similar to processor ICs, scaling transistor density on electronic switch ICs is fundamentally limited by power dissipation as few-nm transistor sizes are approached [2]. Secondly, electronic high-speed serial transceiver data rates are predicted to be hard to scale beyond 112 Gb/s due to the steep increase in dielectric loss when operating at high frequencies [3, 4]. Consequently, increasing the aggregate switch capacity will require a proportional increase in the number of serial transceivers surrounding the chip, resulting in greater power density and packaging complexity. Although continued bandwidth scaling in the near future could be supported by architectural optimisations such as co-packaged optics [5], preserving cost neutrality in the medium-to-long term appears very challenging. This uncertainty has motivated research in optical switches as a viable alternative to electronic switches [6]. Optical switches simply redirect the incoming signals onto output ports without any optical/electronic conversion or digital processing and, hence, they do not suffer from the limitations of transistor or transceiver technology. They could, therefore, provide a future-proof solution for bandwidth scaling within the data centre [7].

We demonstrate a clock and data recovery technique that achieves <625ps locking time for 25.6Gb/s-OOK and show its robustness under worst-case data centre temperature variation. The locking time was improved by 12×, making nanosecond optical switching viable in data centres.

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