Plastic Optical Fiber
The large core area and easy-to-cut and terminate properties of plastic optical fiber have long held the promise of a low-cost, easy-to-install communications medium that offers all the benefits of optical fiber with the ease of termination of copper. It was once presumed that plastic fiber could also be manufactured at an extremely low price compared to glass or even copper cables. Unfortunately plastic fiber is not yet proven to be cost competitive or to exhibit sufficiently high bandwidth or low enough attenuation to make it a serious rival to either glass fiber or copper cable. Plastic fiber continues to be developed, however, and has found some applications in the automotive field and may yet offer a viable product for short-distance, lower-speed data communications, perhaps in the small
office, home office, or SoHo arena.
Plastic fiber available today is step index, which by its very nature limits the bandwidth available. Current designs are based on a material called PMMA, poly methyl methacrylate. Step index plastic optical fiber, or SIPOF, today has a best bandwidth of 12.5 MHz.km and an attenuation of 180 dB/km. Compare this to the 500 MHz.km bandwidth and 1 dB/km attenuation available from 50/125 glass optical fiber.
The manufacturing costs of PMMA fiber are thought to be about the same as for conventional glass optical fiber, but SI-POF currently sells at a premium compared to glass or all-silica fiber. The thermal stability of PMMA is also questionable. High temperatures combined with high humidity can raise the attenuation of the fiber significantly.
SI-POF fibers are available in sizes of 500, 750 and 1000 micron total diameter. Most of this is a PMMA core with a thin layer of fluorinated PMMA for the cladding.
Deuterated PMMA has been proposed as an advancement. It can reduce attenuation to 20 dB/km in theory but this has not been achieved in practice. Deuterated PMMA is also very expensive to produce.
To really improve plastic fiber a graded index version has to be produced to overcome the poor bandwidth properties of SI-POF. Graded index plastic optical fiber, or GI-POF, offers the potential of 3 Gb/s transmission over 100 m and 16 dB/km attenuation at 650 nm. Even 1300 nm operation may be possible with next generation materials.
GI-POF experiments have been undertaken based on a material called perfluorinated plastic, PF. PF fibers could have an attenuation as low as 1 dB/km at 1300 nm with a fiber of about 750 micron diameter and a 400 micron core. Perfluorinated graded index plastic optical fiber, PFGIPOF is available today offering a minimum of 50 dB/km around 1300 nm and 200 dB/km at 650 nm. For example, Lucina® from Asahi Glass, which has a 120 micron core and 500 micron cladding with a numerical aperture of 0.18 and a claimed bandwidth of 200 MHz.km. Manufacturers are aiming for 10 dB/km across the useable spectrum in the near future.
Today plastic fibers are mostly used for illumination or very shortdistance communication systems, such as in a car. The main advantage of plastic fiber is ease of connectorization but it has yet to prove itself in terms of cost, bandwidth, attenuation and long-term thermal stability.