High Capacity Backhaul of Submarine Networks

High Capacity Needs for Submarine Backhaul

Submarine systems terminate in landing stations which are usually as close a possible to the beach where the submarine cable comes ashore (figure 1a). There are many reasons for this including reducing the total distance traversed on the submarine cable and limiting the land exposure to high DC voltage that the submarine power-feed equipment uses. However once the wavelengths are terminated at the landing station, they still have to be transported to the network operators POP which is normally in a city quite far from the landing station.

Since a submarine cable typically has 4-6 fibers each carrying 64-96 10G waves, the terrestrial backhaul can require very high capacity. This can be accomplished using multiple terrestrial fiber pairs but each pair must be purchased (or leased) and then terrestrial amplifiers installed on every fiber pair along the length of the cable. This can be extremely expensive especially when diverse routes (for route protection) are required.

High-capacity terrestrial transport with Nu-Wave CXR/ES

Xtera’s Nu-Wave CXR/ES transmission systems can transmit 240 (150) wavelengths up to 3000 (4000) kilometers. As these wavelengths can be either 10Gb/s or 40Gb/s (or any mix) , a single fiber pair with Nu-Wave CXR can carry up to 9.6 (6.0) Terabits/s of capacity. This makes it possible to place the capacity of an entire submarine system on a single fiber pair (figure 1b).

The Nu-Wave CXR/ES use All-Raman technology to provide high capacity over such long distances. Raman amplification provides distributed amplification along the installed fibers by using pump lasers to feed additional light energy into line fibers. The bandwidth achieved and amplification generated can be controlled by varying the number of pump lasers and the pump power launched into the line fiber, respectively.from each laser. For Nu-Wave CXR high-capacity systems, these Raman pumps extend the usable fiber bandwidth to 100nm, significantly expanding the capacity beyond typical EDFA amplifiers which can only use the C- (or L-) band (figure 2).

Raman amplification also results in an improved Optical Signal to Noise Ratio (OSNR) compared to EDFA amplifiers and this makes it possible to transmit the full 240 channels for up to 3000 km if required. Where distances are less than 3000 km it is often possible to use the improved Raman performance to reduce the number of ILAs required, further reducing backhaul cost.

Deployment

In a typical application (figure 1b) the Nu-Wave CXR/ES would be deployed in the landing station and connect directly to the operators city POP. All of the capacity of a submarine cable can be terminated on a single fiber so only one fiber pair per route is required. Normally, a protection route following a separate path is also specified. Again, only one fiber pair is required.

The capacity available in the Nu-Wave CXR/ES can be divided into separate bands. The granularity of these bands is either 30 or 60 channels. These bands can be installed one at a time, as required, to reduce the start-up costs in the early years when the submarine cable is less full. To further reduce start-up costs and reduce planning complexity, 10Gb/s and 40Gb/s wavelengths can be deployed at any point in the cable lifetime.

Figure 1: Submarine backhaul

Figure 2: Nu-Wave CXR optical bandwidth