Optical Amplifier Options for Wideband Submarine Systems

The main wideband amplifier candidates for use in submarine systems are hybrid DRA/EDFA and C+L band EDFAs. We explore the relative merits of the two technologies and demonstrate through simulations that the hybrid approach is advantageous for most systems. Introduction Submarine systems have enjoyed the tremendous advances in coherent transmission brought about by high speed DSP technology over the last few years. These techniques have brought us remarkably close to the Shannon limit. Increasing the transmission capacity further is achievable by selecting improved system parameters such as a smaller fibre attenuation coefficient and a lower nonlinear coefficient. However, these improvements only yield logarithmic increases in channel capacity. For linear increases in capacity the three main options are multi-core transmission, multi-mode transmission or increased bandwidth. In this paper we explore the latter, which is, perhaps, the most pragmatic option. Current repeater amplifier bandwidths are typically 35nm and here we explore the relative merits of the two most viable options to double this bandwidth to 70nm, namely hybrid Distributed Raman/EDFAs (DRA/EDFAs) and C+L band EDFAs. Amplifier topologies The basic structure of the two amplifier types considered are shown in Figs 1 and 2 for a single direction of transmission. Fig. 1: Hybrid DRA/EDFA structure Fig. 1 shows the typical hybrid structure in which the EDFA is pumped by a 980nm laser and the DRA is backward pumped by a 14xx laser. The Raman laser wavelength is chosen according to the required wave band, and in this work a wavelength of 1495nm has been chosen. The number of pumps is typically minimised for reliability reasons and so a single Raman wavelength is employed in this study. The typical structure of a C+L band EDFA is shown in Fig.2. The separate C and L band amplifiers firstly require that the waveband is split by a C/L band splitter at the front end. Also, a 2 stage L-band amplifier is required to enable both a low noise figure and a high output power to be achieved. There are many L-band amplifier configurations possible and here we have chosen simple forward pumped stages with a 1480nm pump for the second stage. Fig. 2: C+L Band EDFA structure In both cases, we need to provide 3dB more output power compared with C-band amplifiers if we are to achieve the same channel power over the increased bandwidth. Hybrid Amplifier Properties By using a single Raman pump, the on-off gain is necessarily sloped. The dB on-off gain increases in an approximately linear fashion with wavelength until it reaches a peak at 1600nm. This positive slope is complemented by the EDFA design which has a negative gain slope. One of the most unusual properties of the DRA is the variation of noise figure with wavelength. The noise figure decreases as the on-off gain increases. This is illustrated in Fig. 3. Since the DRA comprises the complete span, the noise figure includes the span loss. In the figure, the span loss has been subtracted from the noise figure to produce an effective noise figure, which eases comparison with the EDFA noise figure. The noise figure of the hybrid amplifier is dominated by the DRA and so, given a flat input spectrum, the system OSNR will also improve with increasing wavelength. One way to exploit this is to use a flex-rate transceiver. At higher wavelengths we can, for example, use higher order modulation formats or a low latency FEC. Alternatively, we can pre-emphasize the channel 980 nm 14xx nm 980 nm C/L Band splitter