Development of ultrastable fiber-optic time and frequency reference networks in Africa

Abstract

The unparalleled accuracy of modern-day atomic clocks has stimulated the development of time and frequency comparison techniques, with optical frequency transfer over fiber networks emerging as the preferred method. It has been demonstrated that frequency transfer over optical fibers has an order-of-magnitude better stability and accuracy than traditional satellite-based techniques. Precise time has become an essential service for most critical infrastructure and applications. New progress in LTE and 5G will demand more access to precise time with accuracies of under 10 ns. Although this technology exists in Africa, continuous improvements are required. With the arrival of the Square Kilometre Array (SKA) in Africa, the National Metrology Institute of South Africa (NMISA) upgraded its time and frequency infrastructure in order to support the stringent time accuracy requirements of the MeerKAT and SKA telescopes. Over the past five years, the Centre for Broadband Communication at the Nelson Mandela University has been conducting exciting and cutting-edge research looking at new and innovative ways for coherently disseminating high-speed data and clock over optical fiber networks. This paper reports on recent research progress made in developing synchronous optical networks in South Africa and across the African continent. This paper begins by presenting a pioneering all-optical approach for measuring the round-trip latency time along a spooled G.652 single-mode fiber. This has been realized by optically injecting a pulse-per-second (PPS) signal from a distributed feedback laser into the slave mode of a 1550 nm vertical cavity surface emitting laser (VCSEL) located at the receiver end. A round-trip fiber time delay of 113.2 µs was experimentally measured over 22 km. Furthermore, the jitter instability of an optically modulated PPS was measured as a function of temperature. A jitter of 434.82 ps was measured during the night-to-day temperature cycle (5°C–25°C). The impact of polarization fluctuations on jitter stability is presented. A maximum jitter of 417.88 ps was measured for the transmitted PPS along the aerial fiber. Lastly, a novel technique for distributing a stable microwave reference frequency, using an intensity modulated VCSEL, is presented. The novel frequency dissemination and synchronization system proposes the use of a VCSEL-based phase correction actuator together with the inherent chromatic dispersion properties of the fiber. Frequency instabilities of 4.18939 × 1 0 − 12 at 1 0 4 s without active noise cancellation and 4.86 × 1 0 − 14 at 1 0 4 s with active noise cancellation were successfully measured across the 26 km G.655 fiber link.