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OPTICAL FIBER TYPES
Optical fibers are characterized by their structure and by their properties of
transmission. Basically, optical fibers are classified into two types. The first type is
single mode fibers. The second type is multimode fibers. As each name implies, optical
fibers are classified by the number of modes that propagate along the fiber. As previously
explained, the structure of the fiber can permit or restrict modes from propagating in a
fiber. The basic structural difference is the core size. Single mode fibers are
manufactured with the same materials as multimode fibers. Single mode fibers are also
manufactured by following the same fabrication process as multimode fibers.
Single Mode Fibers
The core size of single mode fibers is small. The core size (diameter) is typically
around 8 to 10 micrometers (μm). A fiber core of this size allows only the
fundamental or lowest order mode to propagate around a 1300 nanometer (nm) wavelength.
Single mode fibers propagate only one mode, because the core size approaches the
operational wavelength (λ). The value of the normalized frequency parameter (V)
relates core size with mode propagation.
In single mode fibers, V is less than or equal to 2.405. When V ≤ 2.405, single
mode fibers propagate the fundamental mode down the fiber core, while high-order modes are
lost in the cladding. For low V values (≤1.0), most of the power is propagated in
the cladding material. Power transmitted by the cladding is easily lost at fiber bends.
The value of V should remain near the 2.405 level.
Single mode fibers have a lower signal loss and a higher information capacity
(bandwidth) than multimode fibers. Single mode fibers are capable of transferring higher
amounts of data due to low fiber dispersion. Basically, dispersion is the spreading of
light as light propagates along a fiber. Dispersion mechanisms in single mode fibers are
discussed in more detail later in this chapter. Signal loss depends on the operational
wavelength (λ). In single mode fibers, the wavelength can increase or decrease
the losses caused by fiber bending. Single mode fibers operating at wavelengths larger
than the cutoff wavelength lose more power at fiber bends. They lose power because light
radiates into the cladding, which is lost at fiber bends. In general, single mode fibers
are considered to be low-loss fibers, which increase system bandwidth and length.
Q.34 The value of the normalized frequency parameter (V) relates the core size with
mode propagation. When single mode fibers propagate only the fundamental mode, what is the
value of V?
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