6 Tb/s Unrepeatered Transmission of 60 x 100Gb/s PDM-RZ QPSK channels with 40 GHz Spacing over 437 km.

We compare 100 Gb/s transmission with 40 GHz instead of standard 50 GHz spacing, and describe a 60 x 100 Gb/s unrepeatered transmission experiment over a record distance of 437 km of ultra-low loss fibre with a high power booster and third-order Raman pumping. Introduction Unrepeatered systems target long distance transmission without any in-line active elements, thus reducing the line complexity and the overall system cost. They connect islands to each other or with the mainland, provide festoons along the coast, or bridge long spans in terrestrial systems. Typical transmission lengths are a few hundred kilometres. Even if most of today's systems operate at 10 Gb/s, the demand for increased capacity is rapidly moving interest toward bit rates of 100 Gb/s and above. At 100 Gb/s, Polarisation Division Multiplexed Quadrature Phase Shift Keying (PDM-QPSK) combined with a coherent receiver is seen as the solution of choice because digital signal processing can compensate chromatic dispersion and polarisation mode dispersion, thus enabling upgrade of already installed systems. Several unrepeatered laboratory experiments have highlighted the potential of this format, either in few-channel experiments, with the transmission of 4 x 100Gb/s over 465 km [1] and 8 x 100Gb/s over 444 km [2], or in high-capacity demonstrations, with the transmission of 26 x 100Gb/s over 401 km [3] and 40 x 100Gb/s over 365 km [4] at 2 b/s/Hz spectral efficiency. A 100 Gb/s system opens a capacity in excess of 8 Tb/s in the C-band.