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  • Passive optical networks, higher power lasers keys to broadband service in rural areas

Photonics Spectra
May 2009
Caren B. Les,

MELBOURNE, Australia – The benefits of high-speed Internet – videoconferencing, high-definition television, interactive online gaming – often are not available to rural dwellers, those who live well beyond the limits of urban and suburban areas. Technologies such as digital subscriber lines and cable are inherently impractical for covering areas where there are great distances between customers, and satellite and fixed wireless services, although suitable for broad-area coverage, are expensive.

Researchers at the University of Melbourne and at NEC Australia, a Mulgrave-based supplier of information and communications technology, both in Victoria, believe that passive optical networks are a good choice for providing broadband services in rural areas.

Researchers demonstrated an approximately 37-mile extended reach Gigabit passive optical network system for broadband access in rural areas using a dual-wavelength distributed Raman amplifier. Shown is the experimental setup. Courtesy of Ka Lun “Alan” Lee.

According to lead investigator Dr. Ka Lun “Alan” Lee, a research fellow at the university, the group found that, in Victoria, for access rates of 50 Mb/s or more, optical fiber solutions are the cheapest. Passive optical networks are advantageous in that they have no active electronics in the street, critical in rural areas where the cost of maintenance is very high, Lee said. Compared with wireless and asymmetric digital subscriber lines, the networks provide much higher capacities for rural customers, he added.

Lee said that the technology, which would enable rural customers to enjoy the same quality of service as urban dwellers, has the advantage of being “future proof” because any improvements in technology or increase in demand would be met without having to rebuild the entire access network.

Loss of signal strength

Gigabit passive optical networks are considered cost-effective in providing high bandwidth over long distances and typically have a power budget of 28 dB, enabling 32 optical network units to be served from a single optical line terminal for a maximum reach of approximately 18 miles. The networks carry data long distances over optical fibers to passive optical splitters, which split the signal to individual households. The problem is that there is a loss of signal strength along the optical fiber and that 30 km is not a long enough reach for rural areas.

The researchers demonstrated an approximately 37-mile-reach Gigabit passive optical network system to serve 32 subscribers without using any active element in the field. They employed a dual-wavelength distributed Raman amplifier to increase upstream and downstream data at 1350 and 1490 nm, respectively, and reported that symmetric bidirectional transmissions at 2.5 Gb/s exhibited good bit-error-rate performance over the entire reach.

Lee said that the long-range technology involves modifying existing commercial passive optical network systems. The amplifiers can be installed in existing telephone exchanges, and standard optical fiber can be used. One main challenge would be the expense involved in rolling out optical fibers to rural households, Lee noted.

The technology is comparable in cost to alternative rural broadband technologies, is cheaper than alternatives if the household demand is 50 Mb/s or higher, and could be implemented around the world because it is similar to existing standard passive optical networks, Lee said.

Another drawback to the researchers’ proposed system, however, is that, to achieve the extended range, they must amplify the laser light to a higher-than-normal power level. Lee said that “this could pose a health hazard if people were to look directly into the fiber during laser operation. In practice, this is possible only if the fiber breaks, and then automatic safety systems will cut off the laser light before it can damage the eyes.” The main difficulty, he noted, is that a new procedure will be needed for restarting the laser.

The researchers are continuing to explore modifications to the technology to ensure that they have the most economic approach and are working with NEC Australia to explore commercialization of the system.

The group’s economic modeling has shown that providing broadband services in rural areas is significantly more expensive than in urban areas. Each technology has its advantages and disadvantages, but in the end, the right choice depends on the capacity required per household and the desired coverage. Passive optical networks are likely to play a larger role as demand increases for services such as high-definition video on demand, Lee said.

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