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Optoelectronics & Communications

Tighter tolerances demand attention to linear optical properties of waveguides

From OE Reports Number 198 - June 2000
31 June 2000, SPIE Newsroom. DOI: 10.1117/2.6200006.0003

As the optical networking industry grows rapidly, so does research on linear optical properties of waveguides and fibers. Robert Norwood of Photon-X said, "Optical networking companies like Nortel Networks, Cisco, Lucent, and others are driving to provide ever-increasing bandwidth to their customers. In the process of providing this bandwidth, they are constantly pushing the limits of existing optical fiber and optical waveguide technology."

As systems are pushed to provide more bandwidth, acceptable tolerances shrink. Relatively small effects, which could be ignored in the past, can now keep the system from working as intended. Effects such as chromatic dispersion and polarization mode dispersion (PMD) in standard materials must be measured with a high degree of accuracy.

Those using new waveguiding materials also need to know optical properties such as waveguide loss, polarization-dependent loss, chromatic dispersion, and PMD. Norwood said, "Many subtle effects, which are of little interest at the research level, are critically important to understand if one is to compete effectively with existing materials such as planar silica.

"In the case of active materials," continued Norwood, "such as electro-optic materials, it is often the linear optical properties that are the most difficult to control and reproduce."

New plastic fiber

Among the new materials is a plastic fiber with good transparency in the near IR. Whitney White of Lucent's Bell Labs (Murray Hill, NJ) is working with Asahi Glass in Japan to understand the new material's properties. "It has good transparency compared to other plastic fibers because it doesn't contain hydrogen," White said. While the plastic fiber's properties aren't as desirable as glass, it could be used with conventional transmitters for near-IR transmission, at 830 nm and 1300 nm, over distances as long as 300 m. In addition to cost advantages, plastic fiber is easier to connect and install than standard silica fiber. Such a fiber might be useful for increasing the bandwidth of networks in offices, in homes, or in computers that can use an optical backplane.

The fiber appears to exploit the fact that many different electromagnetic modes are strongly coupled. The fact that the power is spread among different modes coupled together keeps the pulse from dispersing linearly; a pulse transmitted through the plastic fiber has a sharp rise and fall at the average propagation velocity.

Mass-producing waveguides

Paul Ferm at Corning is working on another approach to polymer waveguides. His group is investigating processes to make single-or multimode waveguides using methods of mass production, which resemble printing more than they resemble semiconductor manufacturing. Corning is not alone in trying to develop waveguide printing methods. Wafer-based methods, such as spin coating, limit the economies of scale that could be applied if the waveguides could be made the same way that embossed plastic holograms are made.

The production steps for that system require first making a master and tooling. The waveguide is fabricated using printing, filling, and overcoating. Then the waveguides are die-cut and otherwise prepared for mounting and packaging. Some sort of passive alignment system will be needed. Although these methods exist for other sorts of polymer manufacturing, applying them to waveguides requires much higher precision at many of these steps.

As part of developing polymer waveguides, however, the material's properties need to be investigated. "The linear optical properties of waveguides and fibers often relate as much to the processes used to produce the waveguides as to the materials comprising the waveguides and fibers," Norwood said.

Finally, developers need to understand the optical properties of waveguides under operating conditions. "It is also critically important," Norwood said, "that the behavior of the linear optical properties be understood over a large range of temperature and humidity regimes, reflecting the conditions experienced in actual use."

Yvonne Carts-Powell

Yvonne Carts-Powell, based in Boston, writes about optoelectronics and the Internet.