Page 60 - Optical Switching And Networking Handbook

03_200023_CH02/Batesx 1/17/01 8:17 AM Page 45

Basic Fiberoptics Technologies 45

Bending Cables

As light travels along the fiber, it is reflected from the interface
between the core and cladding whenever it strays from the path
straight down the center. If the fiber is bent during the installation
(by design or accidentally), the light only stays in the fiber because
the pouter cladding reflects it back into the cable. However, total
internal reflection only works under certain conditions. The angle of
incidence must be kept low. If you bend the fiber too much, the light
2
can escape. The bending radius is crucial in the installation of fiber.
Despite all its strong points, fiber does have some drawbacks, albeit
minor. The amount of allowable bending is specific to particular
cables because it depends on the difference in refractive index
between the cladding and the core. The greater the difference in
refractive index, the more stringent is the allowable bending radius.
Figure 2-12 illustrates this bending difference.

Figure 2-12
Light will bounce
out of the fiber if
Cladding
the bend is too
tight.

Cable

Light will bounce
out of the fiber

2 The angle of incidence is the angle in which the light hits the cladding.