Ultra-long-haul coherent transmission for submarine applications

A review of recent experimental demonstrations supports the conclusion that all transatlantic cables can be immediately upgraded to 40G per channel, thus doubling their capacity, and that the technology is mature for 100G submarine transmissions. Introduction Historically, optical coherent detection has been proposed as a method for increasing the sensitivity of the receivers due to the lack of optical amplification. Soon after, the advent of Erbium doped fibre amplifiers (EDFA), chromatic dispersion management, and of the wavelength division multiplexing (WDM) has changed the horizons of the optical transport industry, making coherent detection an uninteresting technique. Recent experimental demonstrations showed that a polarization diversity coherent receiver with ultra-fast digital signal processing can enable for a new generation of commercial systems working at 40G and 100G over ultralong distances1,2. In the optical transport network domain, the requirements of the submarine market are well known; it is characterized by long distance pointto-point transmissions where the total transported capacity of a cable is the crucial parameter. The majority of the actual submarine cables are based on non-zero dispersion shifter fibre (NZ-DSF) with negative dispersion which is compensated by standard single-mode fibre (SSMF). These systems operate with 10G return-to-zero on/off keying (RZ-OOK) transponders or with 10G return-to-zero differential phase shift keying (RZ-DPSK) transponders.