Plastic fiber heads for the mainstream

Low-loss gradient-index plastic optical fibers (POFs) based on perfluorinated materials are making plastic an increasingly attractive alternative to copper and glass for short-haul data transmission. Advances in POF for local-area communications were the focus of the POF Conference `97 (September 22-25; Kauai, HI), the eighth annual international technical meeting for POF. Some 200 attendees from around the world attended the meeting, sponsored by the Japan POF Consortium, the US-based POF Inter

Plastic fiber heads for the mainstream

Kristin Lewotsky

Low-loss gradient-index plastic optical fibers (POFs) based on perfluorinated materials are making plastic an increasingly attractive alternative to copper and glass for short-haul data transmission. Advances in POF for local-area communications were the focus of the POF Conference `97 (September 22-25; Kauai, HI), the eighth annual international technical meeting for POF. Some 200 attendees from around the world attended the meeting, sponsored by the Japan POF Consortium, the US-based POF Interest Group, and the US Office of Naval Research. According to Edward Berman of Boston Optical Fiber (Westborough, MA), the quality of presentations was very high, with speakers coming from industry players such as E. I. DuPont de Nemours, NTT, Asahi Glass, and Keio University.

In the keynote address, for example, David Kalish and Jane Clayton of Lucent Technologies (Norcross, GA) discussed the market position of POF with respect to glass fiber and copper wiring. Plastic fiber offers high bandwidth and noise insensitivity compared to copper wire, while remaining more economical than glass fiber.

Driven by the cost of optoelectronic components, however, plastic optical networks are significantly more expensive than copper networks--about one-third higher for a 100-Mbit/s local network. Eventually, this disadvantage may be outweighed by the demand for bandwidth, especially as copper networks become increasingly susceptible, as data rates rise, to crosstalk and errors. Kalish emphasized the need for full-network solutions that addressed the cost issue.

Perfluorinated fiber

Perfluorinated fiber was the big story of the conference. Although plastic fiber, in general, is more economical than glass fiber, with large core diameters that simplify alignment and installation, the high transmission losses induced by poly(methyl methacrylate) fibers have previously limited the use of POF in even short-range data applications. With demonstrated losses of 50 dB/km, which are expected to dro¥as low as 10 dB/km, perfluorinated fiber shows the potential to open u¥the premises-wiring market for data communications.

Asahi Glass Co. (Yokohama, Japan), in collaboration with Yasuhiro Koike of Keio University (Yokohama, Japan), has developed an amorphous perfluorinated polymer called Cytop. Noriyuki Yoshihara of Asahi reported that the material is designed to minimize absorption over the 650 to 1550-nm band, while offering low scattering loss. In addition, it has an index of refraction of about 1.34, making it suitable for doping with higher-index perfluorinated materials to produce gradient-index fiber.

In experiments, Cytop-based fiber demonstrated attenuation of less than 100 dB/km over the wavelength range from 650 to 1300 nm, dropping to 50 dB/km at 1300 nm. The fiber bandwidth was between 300 and 600 MHzkm, and simulations suggest that optimized fiber could achieve bandwidths as high as 10 GHzkm. Both attenuation and bandwidth characteristics were stable over the operating range of -30°C to 75°C.

Robert Smith and Bill Van Hoeven of DuPont (Wilmington, DE) discussed the suitability of perfluorinated materials, particularly the company`s Teflon AF, for local-area communications, noting that the new materials can be pulled into fiber with conventional techniques. To generate gradient indexes in the fiber, low molecular weight, high-refractive-index dopants are thermally diffused into the existing low-density polymer matrix (1.7 g/cc).

The pair agreed with the keynote speakers that cost was an issue that needed to be tackled before plastic fiber could really gain a foothold in the premises-wiring market. The current market cost of amorphous perfluoro polymer is approximately $10,000/kg, while the required price for competitiveness in at least some of the potential markets is $1000/kg. According to a duPont analysis on the costs of a large-scale fabrication plant, however, POF could approach that figure.

A somewhat surprising development at the plastic optical fiber conference involved reports of demonstrations of wavelength-division multiplexing (WDM) in plastic-fiber-based networks. Keihi Uehara and Jun-ichi Mizusawa of NTT Multimedia Networks Laboratories (Tokyo, Japan) reported an experimental eight-channel WDM system operating between 1.2 and 1.6 µm. The minimum channel spacing was 20 nm to reduce the effects of source instability. The grou¥successfully transmitted voice and video over a transmission distance of approximately 157 m, with a total signal loss from both components and fiber of about 9 dB.

Plastic optical fiber has undergone dramatic performance improvements in the past several years. If cost reduction keeps pace with the technical improvement, the fiber should begin to gain a foothold in the premises market as demand for bandwidth rises.

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