Standard POF
The abbreviation SI (Step Index) defines a fiber with
a homogeneous core. The core made of PMMA (polymethylmetacrylate)
is covered by a thin (10 μm) optical cladding made of a fluorinated polymer, having
a refractive index, which is lower by a few percent.
The light is guided by total internal reflection in the core. The lowest losses are obtained at 520 nm(green), 560 nm (yellow) and 650 nm (red).
Currently, SI-PMMA-POF are by far the majority of practi-cally used polymer fibers.
The different transmission times of different light rays in the core leads to a broadening of the launched light pulse. The differences in the propagation times grow with increasing refractive index difference between core and optical cladding (described by the Numerical Aperture mainly). The so called standard POF has a refractive index difference of nearly 5%. This enables a maximum angle of 20° relative to the fiber axis. Accordingly, the rays with the highest angle are slower by 5%.
For a 50 m long SI-POF a theoretical difference of 12 ns in the propagation times can be expected. Mode mixing and a high attenuation for the high angle rays lead to propagation time differences, which are almost 1/3 of the theoretical value. Thus, the band-width of a 50 m long SI-POF is approx. 100 MHz. Bit rates of about 200 Mbit/s over 50 m can be obtained without any additional measures (correspondingly higher for shorter distances),
Bit rates of »1000 Mbit/s over 50 m have been realized with special transmission schemes (equalizing, OFDM) over the last years.
Modified refractive index profiles allow even higher bit rates. Depending on the application the maximum operation temperature of the PMMA-POF is +70°C to +85°C (158°F to 185°F).
