100 g raman-combination-xtera-technical-tutorial

Maximizing Network Capacity, Reach and Value Over land, under sea, worldwide www.xtera.com 1

100G + Raman Combination Raman optical amplification technology already offered very valuable benefits to N x 10G optical networks: high capacity of 2.4 Tbit/s (via the multiplexing of 240 waves at 10G each) and non-regenerated transport distances up to 4,000 km with simple 10G interface card technology. Similar capacity and reach benefits are found with the combination of 100G and Raman technologies, for both unrepeatered links and terrestrial networks with in-line amplifiers.

Benefits from Raman for 100G Transport

Combining Xtera Communications technology for flexible Raman amplification and 100G

long-haul interface cards, the line capacity can reach 15 Tbit/s in the line fiber thanks to the

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wide optical Raman bandwidth accommodating up to 150 channels with standard 50-GHz channel spacing.

From a transport reach perspective, Raman amplification allows a better control of the longitudinal per channel optical power along the optical path, minimizing the impact of nonlinear effects on data transport. Although digital coherent receiver can bring a small amount of nonlinear effect compensation, line equipment based on Raman amplifiers offers longer reach and smaller channel spacing compared to EDFA amplifiers.

A typical example is provided by link design for backhaul networks connecting cable landing stations to Points of Presence (PoPs). Up to 150 x 100G can be transported all-optically on 3,000 km, with no intermediate regeneration, with standard repeater spacing. This 15-Tbit/s line capacity represents a 10-fold increase compared to what would be achieved with 10G optical channels. The 15-Tbit/s line capacity increases by a factor of 70% the line capacity typically provided by conventional EDFA-based transport systems with 100G channels.

100G + Raman for Unrepeatered Links

100G and Raman amplification offer also unique capabilities in link designs for unrepeatered applications. Xtera Communications demonstrated in 2010 the transmission of 8 channels at 100G each on an unrepeatered distance of 444 km, with a Remote Optically Pumped Amplifier (ROPA) located about 110 km in front of the receive end. This 800-Gbit/s line capacity is twice the line capacity that is obtained with 10G channels on the same unrepeatered distance. With the current state of technology, it is not possible to transport more than 8 channels at 100 each because of the extremely high nonlinear effects that take place inside the line fiber when the optical channels need to span more than 400 km with no in-line amplifiers. Hence the smaller capacity benefit when moving from 10G to 100G channels compared to the terrestrial link with repeaters every 80 km.

Figure 1: 440-km unrepeatered link with 8 x 100G capacity.



Because of its highly nonlinear nature, this 444-km unrepeatered link was an excellent test bed to assess whether 10G, 40G and 100G channels could be mixed and matched on a 50-GHz grid with no need for guard band between them. Tests clearly shown that 10G, 40G and 100G channels can be mixed on a 50-GHz grid, with no inter-channel degradation, even for high optical powers and gains in the link.

Guaranteeing operation with 50-GHz channel spacing is obviously a crucial requirement to effectively raise total capacity of transmission systems, like unrepeatered submarine cable systems, when upgrading from 10G channels to 100G channels.

100G + Raman for Terrestrial Networks with In-Line Amplifiers

With optical repeaters periodically placed in intermediate sites to enable long all-optical path, Raman amplification is crucial to minimize the accumulation of optical noise and better control the strength of nonlinear effects. Both effects (noise and nonlinearity) accumulate in a string of optical amplifiers as found in long-distance optical networks: therefore optical amplifiers, with their excellent optical noise performance and moderate signal power launched into the line fiber, are highly beneficial to long-haul transmission infrastructures.

The figure below illustrates in a simple way the different behavior of line amplification equipment when it is based either on EDFA (top part of Figure 2) or Raman (bottom part of Figure 2) amplifiers.

Figure 2: Per channel optical power profile as a function of the transmission distance in a chain of EDFA or Raman amplifiers.



The quality of the common equipment is key to take the most of the interface cards at the ingress and egress points of the optical path. Lower noise accumulation and smaller peak-to-peak power excursion result in transmission distance of 3,000 km for 150 x 100G waves. Such a [Capacity x Reach] metric cannot be offered by traditional EDFA-designed transmission equipment.

In addition to the maximization of the [Capacity x Reach] metric in long-haul optical transmission infrastructures, Raman amplification makes it possible the transport of 100G channels over long spans in a multi-span optical path. To achieve this, Xtera takes advantage of the modular amplification subsystem proposed by Nu-Wave Optima platform. Depending on the actual span loss between two adjacent sites, the following amplification configurations can be implemented in the field:

Core amplifier;

Core amplifier with the help of Raman backward pumping;

Core amplifier with the help of both Raman backward and forward pumping;

Core amplifier combined with Raman backward and forward pumping, and Remote Optically Pumped Amplifiers (ROPAs).

Figure 3 provides an example of how Raman amplification technology can be used, in different flavors, to efficiently accommodating for various span lengths in backbone networks. The long-haul route of Figure 3 results from the concatenations of multiple spans, with different lengths. When the lengths of two adjacent spans are moderate, the two spans can be combined into a “superspan”: this avoids having to deploy intermediate equipment between the two original spans.

Figure 3: Typical long-haul route in a backbone network including various span lengths.



The optical path described in the Figure above includes a long span of 250 km, with an End-of-Life attenuation of 60 dB. With conventional EDFA-based transport system, such a span would impose the implementation on terminal equipment on either side of it to terminate the optical channels before they experience too severe degradations along the other spans. The terminal equipment would lead to back-to-back configuration at the interface card level for regeneration of the signal on a per wavelength basis. In addition to high Day-One CapEx, this implementation will result in a high incremental cost for new wavelengths.

With Raman-based integration, the 250-km span is seamlessly integrated in the end-to-end optical path which offers N x 100G capabilities and low incremental cost for new wavelengths.

The implementation depicted in Figure 3 not only takes advantage of Xtera industry-leading capacity but also its ability to skip intermediate amplifier sites in N x 100G networks. Site skipping provides space and power savings that continue beyond the initial implementation.



Maximizing Network Capacity, Reach and Value Over land, under sea, worldwide

Edition Date: September 2011

Version: 1.2


Technology

100 g raman-combination-xtera-technical-tutorial