Step index fibers

We mentioned above three types of optical fibers step index single mode, step index multimode, and gradient index multimode. The last two types can carry more light that the step index single mode fiber. Lightness is a very important feature. Among the important features for such a system are the following (Oare, 1995) ... 

Fig. 3. Types of optical fiber (a) multimode stepped index, (b) multimode graded index, and (c) single-mode stepped index.

Step-Index Multimode. The step-index multimode fiber has a sharp difference between the index of refraction of the core and that of the cladding (Fig. 16.8a). Light propagation in such a fiber is... 

Figure 16.9. Light propagation in a mnltimode step-index fiber.

Single Mode. The single-mode fiber has a step-index profile (Fig. 16.6c) and a small core diameter (typically 10 im) such that only one mode can travel through it. This is now the preferred system, particularly for long-distance transmission. The normal light source for multimode fibers is the light-emitting diode (LED). But... 

Multimode optical fibers, 11 131, 132 intermodal dispersion in, 11 134 Multimode stepped index optical fiber, 11 131, 132... 

Figure 6.102 Schematic illustration of (a) step-index, (b) graded-index, and (c) single-mode optical fibers. Reprinted, by permission, from J. F. Shackelford, Introduction to Materials Science for Engineers, 5th ed., p. 611. Copyright 2000 by Prentice-Hall, Inc.

Step Index Fiber—A fiber having a uniform refractive index within the core and a sharp decrease in refractive index at the core/cladding interface. 

Both step index and graded index fibers have been prepared by this process, using Ge02 und B203 as dopant materials. Deposition efficiencies of about 45% have been obtained. The deposition rate is typically between 0.4—1.6 g/min and blanks yielding 10 km fibers have been made. 

Figure 7.8 Schematic of optical guidance in a multimode step-index fiber and the corresponding refraction index profile.

Figure 7.9 Refractive index profiles of three common types of optical glass fiber multimode step index, multimode graded index, and single mode.

Such fibers should have the same properties as good IRE, e.g. sufficient transmission for IR radiation, insolubility in solvents, and chemical inertness. Further, they should have good mechanical properties (e.g. flexibility), they must be step index fibers, and it must be possible to remove the original cladding and to apply the sensitive one without any damage to the fiber. 

Figure 8.9 Scattering of evanescent waves by a gold nanosphere wdth radius r. The refractive indices of the core of a step-index multimode fiber ( )...

Step-index and graded-index fibers differ in their (radial) refractive index profiles. 

Step-index fibers are manufactured using the rod/tube or the double crucible process, in which the core and cladding glasses are melted separately from ultrapure powders and transferred into two crucibles with concentric orifices at the bottom and drawn into a fiber. To obtain a semi graded refractive index profile, a diffusion process can be applied in a curing furnace. Step index fibers with a transmission loss of < 10 dB/km at a wavelength of 0.8. pm can be obtained. 

The bandwidth of graded index fiber is much higher (> 1000 MHz km) than of step index fibers (50 - 100 MHz km). 

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