PCS/HCS (plastic or hard clad silica

PCS/HCS (plastic or hard clad silica, plastic cladding on a glass core) has a smaller glass core (around 200 microns) and a thin plastic cladding.

Step index multimode was the first fiber design. It has higher attenuation and is too slow for many uses, due to the dispersion caused by the different path lengths of the various modes travelling in the core. Step index fiber is not widely used - only POF and PCS/HCS (plastic or hard clad silica, plastic cladding on a glass core) use a step index design today.

Graded index multimode fiber uses variations in the composition of the glass in the core to compensate for the different path lengths of the modes. It offers hundreds of times more bandwidth than step index fiber - up to about 2 gigahertz. Two types are in use, 50/125 and 62.5/125, where the numbers represent the core/cladding diameter in microns.

Singlemode fiber shrinks the core down so small that the light can only travel in one ray. This increases the bandwidth to almost infinity - but it's practically limited to about 100,000 gigahertz - that's still a lot! Singlemode fiber has a core diameter of 8-10 microns, specified as "mode field diameter," the effective size of the core, and a cladding diameter of 125 microns.

Specialty Fibers have been developed for applications that require unique fiber performance specifications. Erbium-doped singlemode fibers are used in fiber amplifiers, devices used in extremely long distance networks to regenerate signals. Fibers are optimized for bandwidth at wavelengths appropriate for DWDM systems or to reverse chromatic dispersion. This is an active area of fiber development.

October 25, 2011

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