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SUMMARY
Now that you have completed this chapter, let's review some of the new terms, concepts,
and ideas that you have learned. You should have a thorough understanding of these
principles before moving on to chapter 4.
OPTICAL FIBER CLASSIFICATION depends on more than the number of modes that a fiber
can propagate. The optical fiber's refractive index profile and core size further
distinguish different types of single mode and multimode fibers.
The REFRACTIVE INDEX PROFILE describes the value of the fiber's refractive index
as a function of axial distance at any fiber diameter.
In STEP-INDEX fibers, the refractive index of the core is uniform and undergoes
an abrupt change at the core-cladding boundary.
In GRADED-INDEX fibers, the refractive index of the core varies gradually as a
function of radial distance from the fiber center.
MULTIMODE STEP-INDEX FIBERS have a core of radius (a), and a constant refractive
index n1. A cladding of slightly lower refractive index n2 surrounds
the core.
The RELATIVE REFRACTIVE INDEX DIFFERENCE (Δ) is the difference in the
core and cladding refractive index. The ability of the fiber to accept optical energy from
a light source is related to Δ.
MULTIMODE STEP-INDEX FIBERS have relatively large core diameters and large
numerical apertures. Unfortunately, multimode step-index fibers have limited bandwidth
capabilities and poor bend performance.
Short-haul, limited bandwidth, low-cost applications use multimode step-index fibers.
MULTIMODE GRADED-INDEX FIBERS have a core of radius (a). Unlike step-index fibers,
the value of the refractive index of the core (n1) varies according to the
radial distance (r). The value of n1 decreases until it approaches the value of
the refractive index of the cladding (n2). Like the step-index fiber, the value
of n2 is constant and has a slightly lower refractive index than n1.
The PROFILE PARAMETER (α) determines the shape of the core's
refractive index profile. As the value of α increases, the shape of the core's
profile changes from a triangular shape to a step.
The gradual decrease in the core's refractive index from the center of the fiber causes
propagating modes to be refracted many times.
Multimode graded-index fibers have less MODAL DISPERSION than multimode
step-index fibers. Lower modal dispersion means that multimode graded-index fibers have
higher bandwidth capabilities than multimode step-index fibers.
SOURCE-TO-FIBER COUPLING EFFICIENCY and INSENSITIVITY TO MICROBENDING AND
MACROBENDING LOSSES are distinguishing characteristics of multimode graded-index
fibers. 62.5 μm fibers offer the best overall performance for multimode
graded-index fibers.
Coupled power increases with both core diameter and Δ, while bending losses
increase directly with core diameter and inversely with Δ. However, a smaller
Δ improves fiber bandwidth.
MATCHED-CLAD and DEPRESSED-CLAD are two types of single mode step-index
fibers. Matched cladding means that the fiber cladding is a single homogeneous layer of
dielectric material. Depressed cladding means that the fiber cladding consists of two
regions: an inner and outer cladding region.
SINGLE MODE FIBER CUTOFF WAVELENGTH is the smallest operating wavelength where
single mode fibers propagate only the fundamental mode. At this wavelength, the 2nd-order
mode becomes lossy and radiates out of the fiber core.
SINGLE MODE FIBERS have low attenuation and high-bandwidth properties. Present
applications for single mode fibers include long-haul, high-speed telecommunication
systems.
VAPOR PHASE OXIDATION and DIRECT-MELT PROCESS are two methods of fabricating
multimode and single mode optical fibers.
CABLE STRUCTURES include buffers, strength members, and the jacket, or sheath.
TIGHT-BUFFERED, LOOSE-TUBE, and GEL-FILLED LOOSE-TUBE are types of
fiber optic buffering techniques.
FIBER OPTIC CABLES use strength members to increase the cable's strength and
protect the optical fibers from strain.
JACKET MATERIAL should have low smoke generation, low toxicity, low-halogen
content, flame retardance, fluid resistance, high abrasion resistance, and stable
performance over temperature.
Navy systems require that fiber optic cables meet stringent environmental conditions.
The types of cable designs considered by the Navy include the OPTICAL FIBER CABLE
COMPONENT (OFCC), STRANDED, and RIBBON cable designs.
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