Clock and Data Recovery-Free Data Communications Enabled by Multi-Core Fiber With Low Thermal Sensitivity of Skew

Abstract

Optical switching has the potential to scale the capacity of data center networks (DCN) with a simultaneously reduction in latency and power consumption. One of the main challenges of optically-switched DCNs is the need for fast clock and data recovery (CDR). Because the DCN traffic is dominated by small packets, the CDR locking time is required to be less than one nanosecond for achieving high network throughput. This need for sub-nanosecond CDR locking time has motivated research on optical clock synchronization techniques, which deliver synchronized clock signals through optical fibers such that the CDR modules in each transceiver only need to track the slow change of clock phase, due to change of the time of flight as temperature varies. It is desired to remove the need for clock phase tracking (and thereby the CDR modules) if the temperatureinduced clock phase drift can be significantly reduced, which would reduce the power consumption and the cost of transceivers. Previous studies have shown that the temperature-induced skew change between multi-core fiber (MCF) cores can be forty times lower than that of standard single mode fibers. Thus, clock-synchronized transmission maybe possible by using two different MCF cores for clock and data transmission, respectively, enabling the sharing of an optical clock with stable clock phase. To investigate the potential of MCF for CDR-free short-reach communications, we first improve the measurement method of the temperature dependent inter-core skew change by using a modified delay interferometer, achieving a resolution of 3.8 femtoseconds for accurate inter-core skew measurements. Building on the MCF measurement results, we carried out an MCF-based clock-synchronized transmission experiment, demonstrating the feasibility of CDR-free data communications over a temperature range of 43 • C that meets DCN requirements. Index Terms-Data center networks, clock synchronization, Multi-core fiber, Thermal coefficient of delay.

Modeling the Performance of the Clock Phase Caching Approach to Clock and Data Recovery

“…This worst-case overhead can be lowered by reducing the optical fiber TCD, τ , and therefore reducing the required rate of clock phase updates, f φ , by the same factor, by using low TCD fibers such as hollow core fiber (20 times smaller TCD and therefore 20 times smaller required f φ and worstcase overhead than SMF-28 [9]). This overhead can also be potentially reduced by using homogeneous multicore fibre (MCF), which features a low thermal coefficient of skew of about 40 fs/km/ • C between different cores [21]. Further modeling of system performance and scalability in a clock phase cached optical switch using SMF-28 and HCF may be found in Chapters 6 and 7 respectively of [20].…”

Clock-Synchronized Clock and Data Recovery to Enable Sub-Nanosecond Optically-Switched Networks

“…The network overhead can be reduced by reducing the TCD, 𝜏, by using low TCD fibers such as hollow core fiber (20 times lower than SMF28) [8]. It can also be potentially reduced by using homogenous multicore fibers (MCF), which features a low thermal coefficient of skew of about 40 fs/km/°C [9]. A comprehensive analytical model of system performance and scalability in a clock phase cached optical switch may be found in [10].…”

Clock Synchronized Transmission of 51.2 GBd Optical Packets for Optically Switched Data Center Interconnects

“…For a given transmitter to receiver pair, the total timing jitter arises from two sources: a) random temperature variation that changes the light propagation delay in optical fibers [3], and b) jitter of the clock sources that the transmitter and receiver are both synchronized to. The temperature induced timing jitter is relatively slow and, therefore, can be compensated by using clock phase caching [2] or significantly reduced by using low thermal sensitivity fibers [4,5]. The clock source induced jitter, however, is primarily high frequency and hard to compensate.…”