Unrepeatered Submarine Solution Upgrade

The High Cost of Submarine Repeaters

Submarine systems normally use underwater repeaters every 60-100 kms to amplify the optical signals on long distance systems. Repeaters significantly increase the initial cost of a submarine system because they are expensive to install and must be remotely powered using complex high voltage systems. Once under the water, they add multiple potential single points of failure and when failures occur, a ship must be dispatched to repair them at high cost. In addition, since repair time is often more than a week, repeater failures can have a major affect on customer service.

The Xtera Solution: Nu-Wave XLS – Ultra Long-Haul submarine system with no repeaters

Xtera’s Nu-Wave XLS (eXtra Long Span) is an industry-leading unrepeatered transmission system that provides a cost-effective alternative to repeatered submarine technology. It enables an operator to achieve spans of up to 500 km on standard submarine cable without deploying any repeaters (figure1). This delivers a much simpler and more cost effective solution for higher capacity submarine systems. It also lowers the start-up costs enough to make many smaller capacity systems economically feasible.

The Nu-Wave XLS uses All-Raman technology to provide high capacity over such long spans. Raman 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 power launched into the line fiber, respectively. This makes it possible to have a modular approach with shorter links requiring less equipment.

Raman technology provides a number of advantages over standard EDFA amplifiers including improved Optical Signal to Noise Ratio (OSNR), wider bandwidth windows (up to 100nm) and modular deployment. Typically 60 10G wavelengths are deployed up to 65dB of span loss and 16 10G wavelengths can be delivered over spans of up to 85 dB (figure 2).

Deployment

The Nu-Wave XLS can be deployed in new submarine cables as well as used to upgrade existing submarine cables.

New installations

For new installations, Xtera has the capability to provide either a full turnkey solution or simply install the appropriate terminal equipment onto existing installed fiber pairs. The design of Nu-Wave XLS makes it easy to specify the most cost effective solution for any combination of distance and capacity.

Upgrade of existing fiber pairs

Many operators have existing unrepeatered systems using older technology that are limited either in the bandwidth per wave or the total number of waves per fiber (or both). Older cables are often a challenge to upgrade. These challenges include:

• Non-WDM optimized cables (DSF, LS…)
• High-loss aged cables
• Presence of ROPAs

Fortunately, the Raman technology used by Nu-Wave XLS can resolve these issues enabling an operator to significantly increase the capacity available on older generation systems.

For example, Dispersion Shifted Fiber (DSF) is not well suited for DWDM transmission and nonlinear distortions limit the number of wavelengths that can be transmitted in the standard C band. For a customer with this fiber type, the Xtera design team used the wide bandwidth ability of the Nu-Wave XLS  to transmit outside of the C band (figure 3). 600Gb/s of capacity (60 x 10G) was made possible on a system on which the closest competition could only offer 160Gb/s (16 x 10G).

Figure 1: Unrepeatered submarine system with Nu-Wave XLS

Figure 2: Nu-Wave XLS transport capability Vs loss

Figure 3: Upgrade of non DWDM optimized cable system