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Synchronous subnanosecond clock and data recovery for optically switched data centres using clock phase caching


The rapid growth in the amount of data being 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 demonstrated with nanosecond-scale optical switching times. End-to-end switching time is, however, currently limited by the clock and data recovery time, which typically takes microseconds, removing the benefits of nanosecond optical switching. Here we show that a clock phase caching technique can provide clock and data recovery times of under 625 ps (16 symbols at 25.6 Gb s−1). Our approach uses the measurement and storage of clock phase values in a synchronized network to simplify clock and data recovery versus conventional asynchronous approaches. We demonstrate the capabilities of our technique using a real-time prototype with commercial transceivers and validate its resilience against temperature variation and clock jitter.

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Fig. 1: Example clock phase cached data-centre architecture, operational principle and demonstration.
Fig. 2: Increase of data centre network utilization enabled by clock phase caching.
Fig. 3: Demonstration of the stability of clock phase caching.
Fig. 4: Impact of rate of change of temperature on clock phase caching.
Fig. 5: Impact of clock jitter on CDR locking time.
Fig. 6: Estimated worst-case optical switch overhead from the clock phase caching technique.

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We acknowledge financial support from Microsoft, Inphi Inc. and EPSRC grants EP/R041792/1 and EP/R035342/1 and Royal Society Paul Instrument Fund PIF/R1/180001. Eblana Photonics provided the lasers used in this work. We thank P. Watts for helpful discussion at the early stages of the work and E. Vonhof for assistance in the generation of the figures.

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Authors and Affiliations



K.A.C., P.C., H.B., I.H., K.J., B.T., H.W. and T.G. conceived the concept of clock phase caching, which was later refined with help from K.S., D.C. and Z.L. K.A.C. and Z.L. conceived and constructed the experimental set-up. K.A.C. implemented clock phase caching in the experiment. K.A.C. designed and implemented all FPGA hardware code. K.A.C. led the experiment and collected all experimental results, supervised by Z.L., with support provided by P.C., H.B., B.T., P.B. and G.Z. P.C. and H.B. collected data-centre traffic and performed the optical switch network utilization analysis. K.A.C., Z.L., P.B., P.C. and H.B. wrote and revised the manuscript. All authors discussed the results and commented on the manuscript.

Corresponding authors

Correspondence to Kari A. Clark, Paolo Costa or Zhixin Liu.

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Competing interests

A patent application, entitled ‘Phase Caching for Fast Data Recovery’, has been filed by Microsoft Technology Licensing, LLC with the US Patent and Trademark Office on 27 October 2017, on the technology described in this Article. This patent is currently pending (patent no. US20190132112A1).

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Supplementary information

Supplementary Information

Supplementary Figs. 1–3, Discussion 1–6 and Table 1.

Source data

Source Data Fig. 2

B, Data centre traffic pattern; C, Impact of traffic on throughput.

Source Data Fig. 3

A, 48 hour stability; B, Tolerance to rapid temperature change.

Source Data Fig. 4

A, Impact of rate-of-change of temperature on clock phase caching; B, Minimum required phase update rate as a function of rate-of-change of temperature.

Source Data Fig. 5

A, Impact of sinusoidal jitter on clock phase caching; B, Impact of Gaussian jitter on clock phase caching.

Source Data Fig. 6

Clock phase caching scalability.

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Clark, K.A., Cletheroe, D., Gerard, T. et al. Synchronous subnanosecond clock and data recovery for optically switched data centres using clock phase caching. Nat Electron 3, 426–433 (2020).

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