Bending Losses of Optical Fiber Cable

   A variety of outside influences can change the physical characteristics of optical fiber cable, affecting how they guide light.    Typically these effects are modest and must be enhanced or accumulated over long distances to make the kind of sensors described in Chapter 29. However, significant losses can arise if the fiber is bent so sharply that light strikes the core cladding interface at a large enough angle that the light can leak out.
   Bending loss is easiest to explain using the ray model of light in a multimode fiber. When the fiber is straight, light falls within its confinement angle. Bending the fiber changes the angle at which light hits the core-cladding boundary, as shown in Figure 5.6. If the bend is sharp enough, it hits the boundary at an angle outside the confinement angle  c, and is refracted into the cladding where it can leak out.
   Bend losses fall into two broad categories. Macrobends are single bends obvious to the eye, such as a fiber bent sharply where a cable ends at a connector. Microbends are tiny kinks or ripples that can form along the length of fibers that become squeezed into too small a space. This can happen in a cable when the cabling material shrinks relative to the fiber, or the fiber stretches relative to the cable. Microbends are smaller, but they cause similar light leakage because they also affect the angle at which light hits the core-cladding boundary.