Plastic optical fiber flexes its muscle

High bandwidth and low loss make plastic optical fiber a viable solution for many short-haul networking problems.

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Plastic optical fiber flexes its muscle

High bandwidth and low loss make plastic optical fiber a viable solution for many short-haul networking problems.

I. Edward Berman

Network bandwidth is the most important issue facing vendors of resource-intensive data, voice, and video applications. It is the primary obstacle blocking the expeditious transfer of large amounts of information. Plastic optical fiber (POF) can transfer data at rates ranging from 300 Mbit/s to 3 Gbit/s, which is higher than the transmission rate of copper wire. Also, with fiber diameters on the order of 1000 µm, POF is easier to install and align than glass fiber.

Plastic optical fiber offers a viable solution to short-haul (100 m or less) data-transfer applications such as local area networks (LANs), in which inadequate bandwidth is a fundamental weakness. Currently, almost all LANs are based on copper, a material that cannot support the bandwidth requirements of multimedia and Internet technologies. In addition, copper is vulnerable to electromagnetic interference and can be easily tapped, making it a poor choice for secure environments. Despite some improvements, antiquated wire-based technology is accepted because there has been no practical alternative. To meet the stringent performance requirements of asynchronous transfer mode (ATM) communications, for example, copper networks require expensive electronics to preserve signal strength and integrity.

While high-bandwidth glass optical fiber seems to be the logical alternative to copper, it has several disadvantages. The small core diameter of glass fiber (from 100 µm to as low as 8 µm) makes the critical alignment of fiber and related devices extremely difficult. Glass fiber is also very fragile and brittle, leaving it vulnerable to breakage at the connection. When cost and availability considerations are added to the mix, for short-haul applications, glass fiber offers no practical advantages over POF. In contrast, POF is ideally suited to ATM operation, conveying information more rapidly than copper and offering almost unlimited capacity to meet the high-speed transport requirements (see Fig. 1).

Fluorinated materials

The use of fluorinated materials represents an advance in POF technology. Most POF is based on polymethylmethacrylate (PMMA), which limits transmission wavelength to 650 nm and results in attenuations of about 150 dB/km. Although POF transceivers are available at 650 nm, the material does not function at telecommunications wavelengths (850, 1300, and 1550 nm), so system designers cannot take advantage of high-performance, economical components developed for the high-volume telecom market. Fluorinated POF, on the other hand, operates over a broad range of wavelengths that includes 850 and 1330 nm. Attenuation levels remain on the order of a few decibels per kilometer.

Fully fluorinated monomers contain a ring structure in the structural backbone similar to that of amorphous Teflon AF or Asahi Cytop. The properties of this type of ring-structure polymer are well documented and exhibit theoretical attenuation of 25 to 50 dB/km over the wavelength range from 600 to 1300 nm. Operating temperature can be increased by varying the amount of comonomers or the additions of side groups. Boston Optical Fiber is developing a fluorinated POF capable of performing like glass fiber over a wide spectral range and at temperatures in excess of 125°C. The material will yield a robust, large-diameter fiber that can transmit data at rates above 3 Gbit/s.

These materials are available in monomer form for final in-house purification prior to polymerization. They are readily polymerized with free radical initiators. The actual fiber is drawn from a preform using techniques similar to those for producing glass fiber. Core diameters are about two-thirds of the total fiber diameter, typically either 750 or 1000 µm.

Plastic-fiber standards

Indicating the growing acceptance of POF, the ATM Forum and other standards groups have decided to develop POF standards for 155-Mbit/s office LANs and 50-Mbit/s residential LANs. The Defense Advanced Research Projects Agency-backed High Speed Plastic Network (HSPN) consortium established by Boston Optical Fiber (Westborough, MA), Boeing (Seattle, WA), Lucent Technologies (Murray Hill, NJ), Packard Hughes Interconnect (Irvine, CA), and Honeywell (Minneapolis, MN) is taking the lead in the development of commercially available products incorporating plastic optical fiber and vertical-cavity surface-emitting lasers (VCSELs). Applications for home and ATM LANs, as well as for aerospace, automotive, multimedia, and the Internet, will emerge from the HSPN consortium research and development activities.

The evolution of applications such as those under development by the HSPN consortium can be linked directly to breakthroughs in the design of graded-index plastic optical fibers. Interfacial gel polymerization technology produces a graded-index fiber with a highly parabolic refractive index profile, resulting in well-behaved dispersion compensation (see Fig. 2). Earlier designs based on co-polymerization techniques were not successful, while stepped-index designs were bandwidth-limited as a result of the high-loss step configuration. Today, stepped-index POF with a numerical aperture (NA) of 0.33 raises the bandwidth to about 300 Mbit/s, compared to only about 15 Mbit/s achieved by stepped-index fiber with an NA of 0.50. In a recent demonstration, graded-index POF carried data at 650 nm across a 100-m link at more than 3 Gbit/s (see Fig. 3).

Various industries are considering integrating POF into research and development areas including automobile engineering, airline entertainment, medical instrumentation, and defense. The versatility and ruggedness of POF make it suitable for applications such as high-speed, efficient image transfer over local networks. In addition to having nearly unlimited capacity for high-speed transport of information, plastic optical fiber costs less than glass fiber and exceeds the data-transfer rate of copper. Thus, it offers a practical solution to the current bandwidth dilemma. o

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FIGURE 1. New generations of graded-index plastic optical fiber, like the 10-m length shown here, are capable of low-attenuation, high-performance, and short-haul data transmission.

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FIGURE 2. Interfacial gel polymerization technology for preform production of plastic optical fiber results in a fiber with a highly parabolic refractive index profile.

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FIGURE 3. In a recent demonstration, graded-index plastic optical fiber carried data at 650 nm across a 100-m link at a rate of 3 Gbit/s. The 45-ps input pulse (left curve) is broadened to 154 ps by transmission through the fiber (right curve), but the intensity remains high.

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