Plastic optical fiber (POF) has been used for 30 years in specialized applications that require low data rates and cabling runs less than 100 meters long. But now, because of new developments, POF is on the verge of gaining wide acceptance in high-bandwidth communications markets, such as industrial control, telecommunications, and home networking.
Most POF is called step index fiber (SI-MMF), which has a fiber diameter of 1 millimeter, or 1,000 micrometers, and a core diameter of 980 micrometers. Generally, polymethacrylate (PMMA) and acrylic resins are used as the core material, and fluorinated polymer is used as the clad material. One manufacturer offers a premium POF with a core of high-purity polymethyl methacrylate and a cladding made of special fluorinated polymer.
POF can be cut with a razor blade, and the cut end can be polished without much effort. Plastic fiber can be terminated by a variety of methods. One popular method involves heating the fiber end and pushing it against a mirrored surface.
The material can withstand rough treatment, and if the ends of the fiber are somewhat damaged or if the light axis is a bit off-center, it will still transmit light. The large diameter of the core makes it easy to insert into the fiber a large amount of light from a relatively inexpensive light source. POF optical transceiver modules typically use a 650-nanometer red LED as the light source. In addition, typical POF connectors cost substantially less than those for glass fiber.
SI-MMF has a loss of 0.15-0.2 decibels per meter with a 650 nanometer light source. Bandwidth is limited by the large numerical aperture of the core and the step-index profile. All of these parameters restrict SI-MMF fiber to relatively short links at low data rates. But the recent development of low-loss, high-bandwidth graded-index POF (GI-POF), which can offer 2 GHz of bandwidth at extended distances, should allow plastic to challenge glass optical fiber. Not only that, GI-POF could become a contender in applications like fiber-to-the-desk, fiber-to-the-home, and other short-haul links.
The advancements in GI-POF and low-cost transceivers that use red and green LEDs and economical laser diodes are driving renewed interest in many types of POF data links. Several companies in Japan and one U.S. firm have introduced improved SI-POF and the new family of GI-POF. A number of semiconductor companies have also introduced suitable transmitter diodes.
Home networks could become an important market for POF. The demand for high-speed Internet access and the wide use of digital devices, such as DVD players and video surveillance systems, requires a residential network that can interconnect computers, printers, digital cameras, music recording equipment, and all of the appliances of the future.
POF cabling continues to be a popular choice for low-cost, point-to-point links in noisy EMI/RFI instrumentation and factory environments. Additionally, the industrial networking segment, which has been a strong market for low-speed, step-index, POF links, is adopting the Industrial Ethernet standard. The industrial control networking market is moving from the 12-Megabits- to 16-Megabits-per-second Profibus and SERCOS standards to the 100-Megabits-per-second Industrial Ethernet standard. The next step is the integration of the Ethernet standard with field bus standards. And looking to the future, the industry is studying Gigabit Ethernet to further lower manufacturing costs in plants. Higher performance GI-POF should find application here.
In the telecommunications interconnect industry, GI-POF can also provide lower cost network components at a time when carriers are looking for cost savings in their network infrastructures. While maintaining the performance associated with glass optical fiber, GI-POF allows the use of less expensive transceivers and connectors. Most of this increased tolerance is in the transceiver, where important cost reductions can be achieved.
As a result, the overall cost of new interconnection equipment in data/telecom networks can be reduced as much as 75% with GI-POF, and it will be possible to meet the demand for 10-Gigabit-per-second speed over distances less than 30 meters. An example of a high-capacity, ultra-short distance connection is found in a server farm environment.
Additionally, in the next few years, wide use of 40-Gigabit-per-second optical interconnect systems is expected. The individual component alignment tolerance of plus or minus one-tenth the diameter of the glass fiber, or five microns, requires expensive components in connectors and transceivers. The 180-micrometer diameter of POF, which is three times the diameter of multimode GOF, affords a total alignment tolerance of plus or minus 85 micrometers, compared to a plus or minus 20-micrometer alignment tolerance for glass in a typical link. This makes it possible to use less expensive designs for all components.
In addition, placing optical fibers in parallel within a single ribbon cable offers a connection design that is less expensive, has more relaxed tolerances, and makes field installation easier. This type of GI-POF cable can meet the needs of existing OC-192 and emerging OC-768 optical transmission speeds for carriers.
Four connector styles are available for terminating plastic optical fiber: simplex, simplex latching, duplex, and duplex latching. The latching connector is designed for rugged applications that require greater retention force than nonlatching connectors. Duplex connectors are keyed to ensure proper orientation. For extreme temperature environments, the cable/connector attachment can be strengthened with an RTV adhesive.
Similar to glass fiber, POF test equipment includes optical power meters, stabilized 660-nanometer LED light sources, and a family of interface adapters.
Several worldwide industry organizations are boosting the awareness of POF applications. The most important is the POF Trade Organization, which has a Web site that offers information on suppliers of POF equipment and services to end-users and designers. Other organizations include the POF Consortium in Japan, the POF Applications Center at the University of Applied Sciences in Germany, and POF organizations in the United Kingdom, France, and Brazil.