40 Gb/s over existing 10 Gb/s system
A key to successful and seamless migration to 40 Gb/s line rates is the ability to utilize the existing transmission infrastructure. For example, it is very important that 40 Gb/s systems can use the same type of transmission fibers, optical amplifiers and dispersion compensating fibers as currently deployed 10 Gb/s systems. However, to overlay 40 Gb/s on an existing 10 Gb/s line system as shown in Fig. 1, requires thorough interoperability assessments between the 10 Gb/s line system and the added 40 Gb/s channels. For example, adding 40 Gb/s channels to an existing l0G line system requires the 40 Gb/s receivers to have an OSNR sensitivity, a dispersion tolerance and a power dynamic range similar to the 10 Gb/s line cards. In the same way, the 40 Gb/s transmitter must provide an output power similar to the 10 Gb/s signals and the 40 Gb/s modulation format must be transmittable through typical 10 Gb/s optical mux/demux without significant filtering penalty. To meet the above requirements, the 40 Gb/s line cards must for example include powerful forward error correction to improve the OSNR sensitivity and adaptive dispersion compensators (ADC's) to widen the otherwise tight dispersion tolerance of 40 Gb/s. Commercially available ADC's typically provide a tuning range of >400 ps/nm, enabling 40 Gb/s receivers to have practically the same dispersion tolerance after transmission as 10 Gb/s receivers.
The 40 Gb/s signals might need dispersion pre-compensation to convert a deployed 10 Gb/s dispersion map into an optimum dispersion map for the 40 Gb/s signals. This is illustrated in Fig. 2, which shows the calculated transmission penalty after 8x100 km of SSMF as function of pre-compensation and the inline dispersion compensation ratio. As seen, to convert a typical deployed 10 Gb/s dispersion map with a 95% inline compensation ratio and no pre-compensation into an optimum 40 Gb/s dispersion map, -500 ps/nm of pre-compensation (30% of 100 km SSMF) is needed for the 40 Gb/s signals.
For seamless interoperability, any already installed optical performance monitoring (OPM) systems must also be able to handle the 40 Gb/s signals. Moreover, deployment of 40 Gb/s systems should not require extra training of carrier personal or any advanced pre-characterization of the fiber plant, and it should use simple and common turn-up and installation procedures.
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