Electrical Regeneration for Long-Haul Fiber-Optic Time and Frequency Distribution Systems

In recent years fiber-optic-based long-haul installations for time and frequency (T&F) distribution have become operational at various sites. The common practice to cope with large attenuation of long fiber path is to use bidirectional optical amplifiers. This, however, becomes insufficient in case of very long links and/or long spans between amplifiers, because of unavoidable deterioration of the signal-to-noise ratio (SNR). In this article, we present a solution where the optical signal amplification is combined with optical-electrical-optical (OEO) regeneration, performed in a few points along the link. We analyze the impact of replacing some optical amplifiers with OEOs and demonstrate the resulting improvement in terms of SNR of the received optical signal and the phase noise at the output of the T&F distribution system. Laboratory experiments performed with both spooled and metropolitan-area fibers (total length up to 900 km) confirmed the theoretical predictions and showed that placing the OEO regenerators in appropriate points along the link allows reaching the required SNR.