Coherent fiber links are becoming the gold standard for frequency transfer, with recent improvements allowing statistical uncertainty around 10-20 for optical links in the range of thousands of kilometers. The latest developments include digital implementation for the detection and compensation of fiber link-induced phase noise, as described by A. C. Cárdenas, S. Micalizio, M. Ortolano, C. E. Calosso, E. Rubiola, and J.-M. Friedt.
Frequency transfer and dissemination is increasingly performed via fiber links, allowing frequency stability for optical clocks in the 10-18 range. The main source of phase fluctuations are fiber length variations, caused by mechanical and temperature stresses on the fiber. Several techniques have been developed in the past to compensate for these fluctuations in order to avoid a degradation of the clock information, including the classical Doppler compensation and the more recent two-way cancellation. The classic method has demonstrated its reliability, but shows low flexibility and has problems with regard to efficient reconfiguration, monitoring and remote operation.
The effectiveness of digital implementation on coherent fiber links to overcome these drawbacks has already been demonstrated, using the Tracking Direct Digital Synthesizer (DDS) technique with direct fiber phase noise detection. This system was flexible enough to allow a 6 dB improvement of the unsuppressed noise limit of the two-way method with simple software reconfiguration, enabling its use for distances up to 47 km. Extending this range to the thousand-kilometer mark requires an increased tracking bandwidth from 20 kHz to the MHz region through a redesign of the board with parallel communication components. This was done by introducing an entirely digital implementation, all the way from phase detection to fiber noise compensation, using the fast ADCs, DACs and SoC of the Red Pitaya STEMlab 125-14. This system adds reduced communication delay between components on top of the previously demonstrated features of the tracking DDS. The critical aspects of this considerable step forward are the latency and resource usage of each functional block, implemented in the Red Pitaya FPGA, and the residual noise of the main components.
The difference in architecture between the initial Tracking DDS system and the new setup with the STEMlab 125 can be seen when comparing Figs. 1 and 2, where the latter clearly shows how all the main components, initially external devices, are basically included within the Red Pitaya unit: mixers and DDSs are replaced by digital blocks (Numerical Controlled Oscillators, or NCOs), implemented on the FPGA, therefore simplifying the hardware interconnection.
Implementation and characterization of the digital blocks was done directly on the STEMlab 125-14 using Linux, one of the different possible interfaces for the Red Pitaya configuration. Implementation of the complete system is currently underway and will be used on the 642-km long Italian link for time and frequency (LIFT), installed between Turin and Florence. We look forward to the successful outcome of Red Pitaya´s contribution in this breakthrough in long-distance optical communication.
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