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.