Maximizing the capacity of ultra-long haul submarine systems

The capacity of ultra-long haul (ULH) submarine systems is ultimately limited by the electrical power that can be delivered to the submerged amplifiers within the repeaters. An excessive power feed voltage will cause damage to the cable and joints and so the system should be designed to maximize the power available per repeater using the available voltage. Furthermore, the optical system design must use the available power efficiently in order to maximise the system capacity. Here, both electrical and optical design methodologies are considered which achieve this aim. It is found that the ideal span loss is in the range 8-10dB and that the system should be operated in the linear regime of the fibre, obviating the need for high effective area fibres. Furthermore it is shown that higher order modulation formats are inefficient in ULH systems as sufficient bandwidth is available and standard PM-QPSK allows for greater capacities. In this respect, spectral efficiency is a poor metric to assess such systems. Through optimization, it is shown that transpacific systems of up to 250 Tb/s over 10 SDM channels are feasible using amplifier bandwidths of 70nm.