TOKYO and KAWASAKI, JAPAN — By upgrading their fiber optic networks from 1 Gbps or 2.5 Gbps to 10 Gbps, service providers can support more network users with each fiber. However, though 10G technology is available for the access network today, it is still expensive and little used. Now Fujitsu Limited and Fujitsu Laboratories Limited have announced a new optical amplifier technology that will make 10G networks more economical. Using this technology allows operators to double the reach of their passive optical networks (PONs) and to increase the number of optical splits from 32 to 128, thus increasing the number of users.

Fujitsu and Fujitsu Laboratories developed a burst-mode optical amplifier technology with a semiconductor optical amplifier (SOA), an integrated SOA-array module fabrication technology and an SOA chip fabrication technology enabling uncooled operation. These three technologies, used together, make it possible to quadruple the splitting number in an optical access system and double the transmission distance between the central office equipment and the terminal equipment.

Figure 1. Increased splitting number and transmission distance in an optical access system.

Because it would allow many small routers to be consolidated into a relatively few large routers, this type of next-generation optical access system could operate at low cost and with low power consumption, paving the way for cloud infrastructure. In cloud computing, which requires a large number of terminal equipment and devices to be connected to the network, technologies that allow for low-cost, low-power, and long-distance connections are becoming indispensable.

Fujitsu’s New Technologies
1. Burst-mode optical amplification

One method that has been considered for increasing the number of optical network terminals (ONTs, or customer-premises equipment) per fiber is to install optical amplifiers in remote nodes between the optical line terminal (OLT) in the central office and the ONT. The amplifiers would compensate for fiber and splitting losses in the network.

Because conventional amplifiers are always on, they not only drain power continuously but also introduce a great deal of “noise” into the network. However, an amplifier based on the new Fujitsu technology switches on only when it detects an incoming upstream burst. Even with four amplifiers installed in the remote node, a 128-way split can now be supported with the same optical signal-to-noise ratio as a 32-way split (Figure 2).

2. Integrated SOA-array module fabrication

In an industry first, four integrated semiconductor optical amplifiers are fabricated as a single module (photograph in Fig. 2). The four SOAs are simultaneously and efficiently coupled with four single-mode fibers, which reduces cost and footprint per piece.

3. SOA chip fabrication enabling uncooled operation for small footprint and power-saving

An aluminum composite material is deployed in the active layer of the semiconductor optical amplifier to obtain high gain, even at high temperatures, eliminating the temperature control of SOA (photograph in Fig. 1). This dramatically contributes to the reduction of the module volume to one-fifth and one-sixth the power of a conventional cooled SOA module. It operates at temperatures of up to 85°C, so it can be installed outdoors on utility poles or in street gutters, allowing for remote nodes that need to operate under harsh conditions.

With this technology, a next-generation access system applied to optical aggregation networks can have 128 terminal-equipment connections, four times today’s splitting number, with distances of over 40 km, double today’s distance.

Figure 2. Optical signal-to-noise ratios (OSNRs) for upstream bursts. Burst-mode optical amplification before excessive reduction of optical signals is essential to keep high OSNR. Deployment of four SOAs in a remote node allows the OSNR of a 32-way split to be maintained even with a 128-way split.