Low Thermal Sensitivity Hollow Core Fiber for Optically-Switched Data Centers

Optical switches offer several benefits over electronic switches, including better scalability, lower power consumption, and lower latency. The end-to-end switching latency in optical switching systems is the sum of the optical switching time and the clock and data recovery (CDR) locking time. In short packet dominated data center networks, the CDR locking time limits the end-to-end switching latency. It has been shown that sub-nanosecond CDR locking time is required to achieve high network throughput. Recent research shows that scalable and sub-nanosecond CDR can be achieved by synchronizing the clock frequency and phase of all end-points connected to an optical switch. In such a system, the clock phases must be tracked because of the thermally-induced change of propagation time through standard single mode fiber (SMF-28). Hollow core fiber has been shown to have a 20 times smaller thermal coefficient of delay than SMF-28, offering the potential to simplify clock phase tracking. In this article, we investigate the benefits of the low thermal coefficient of delay of hollow core fiber for clock-synchronized data center networks, showing under 625 ps CDR locking time in both a point-to-point and a 2-to-1 optically switched system, using real-time 60-ns packets operating at 25.6 Gb/s. Based on our results, we estimate that sub-nanosecond CDR locking time can be achieved for a 100 m size data center cluster interconnected by an optical switch using hollow core fiber, without active tracking of clock phase.