1.5 terabit/s submarine 4000 km system validation over a deployed line with industrial margins using 25 GHz channel spacing and NRZ format over NZDSF

We have transmitted 1.5 Tbit/s over a deployed line of 4000 km of NZDSF using only C-band EDFAs with 25 GHz spacing and NRZ modulation format. The average performance obtained correspond to a 4.8 dB margin for a system designed with Enhanced 7% redundancy FEC scheme based on concatenated Reed-Solomon code. This experiment demonstrates for the first time the industrial feasibility of a 1.5 Tbit/s regional network transmission which is compatible with 0.4 (bit/s)/Hz. The use of NRZ format has two advantages. The first one is a low cost and compact terminal equipment due to less modulation stages compared to RZ or CRZ. The second one is a better compatibility with high channel density allowing simpler and lower cost repeaters for a given capacity. Furthermore, the demonstration of the excellent robustness of NRZ format over NZDSF allow direct field deployment or upgrades.

1. Introduction

Currently installed submarine systems are based on NZDSF and CRZ format. Increasing the capacity above the one it has been designed for raises a key issue on the spectral efficiency. The transmission loop experiments reported with spectral efficiency of 0.4(bit/s)/Hz or better^{[1] – [5]} exhibited however a minimum Q factor which is not sufficient to account for system margin required in an industrial submarine system. Indeed, time varying system performance impairment (1.2 dB), manufacturing impairment (0.5 dB), allowance for repair and aging (0.7 dB), represent at least 2.4 dB impairment which should be taken into account in the transmission feasibility. In this experiment, we demonstrate for the first time that the NRZ format at 10 Gbit/s over a deployed line of NZDSF combined with a true 25 GHz spacing can be employed in a 1.5 Tbit/s 4000 km industrial transmission system since all the 150 transmitted channels exhibit a 3.1 dB margin above the Q factor required to get BER with a concatenated Reed-Solomon FEC with 7% redundancy.6,7 The average system margin is 4.8 dB.