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Learn more about Fiber Optic Cable:
The second factor in fiber bandwidth, chromatic dispersion, affects both multimode and singlemode fiber. Remember a prism spreads out the spectrum of incident light since the light travels at different speeds according to its color and is therefore refracted at different angles. The usual way of stating this is the index of refraction of the glass is wavelength dependent. Thus a carefully manufactured graded index profile can only be optimized for a single wavelength, usually near 1300 nm, and light of other colors will suffer from chromatic dispersion. Even light in the same mode will be dispersed if it is of different wavelengths.
Chromatic dispersion is a big problem with LED sources in MM fiber, which have broad spectral outputs, unlike lasers which concentrate most of their light in a narrow spectral range. Systems like FDDI, based on broad spectral output surface emitter LEDs, suffered such intense chromatic dispersion that transmission was limited to only two km of 62.5/125 fiber.
Chromatic dispersion (CD) also affects long links in singlemode systems, even with lasers, so fiber and sources are optimized to minimize chromatic dispersion in the long distance links. Fiber defects and stress on the fibers can cause polarization mode dispersion (PMD) on long links. Both CD and PMD are tested on the cable plants for long high-speed fiber optic networks, a process called fiber characterization. More on CD and PMD.
Bend-Insensitive (BI) Fibers
When installing small fiber count cables indoors and routing patchcords around patch panels, fiber optic cables may be subjected to tight bends. This stress can cause bending losses in the fibers and even long term failure. Fiber manufacturers now offer bend-insensitive fibers, both singlemode and multimode, that are more tolerant of tight bending. One manufacturer even demonstrates the fiber by attaching it to wooden studs with a staple gun, a practice we strongly suggest you do not try – it’s just for demonstrations! Bend insensitive fibers are a big advantage for patchcords or whenever fibers are subjected to stress, but manufacturers should be consulted to see if these fibers require special techniques for splicing, termination or testing. More on BI fibers.