 to flow: step-index and graded-index multimode optical fiber. Returning to Figure 3.6, in a third light flow scenario, the angle of the light ray is greater than the critical angle. This is shown by the numeral 3 in Figure 3.6. When this situation occurs, a portion of the ray will be par- Team-Fly® Understanding Optical Fiber Figure 3.6 The flow of light rays cladding into and through an optical fiber. Light Source cladding n cladding The ability to transmit light through an optical fiber depends upon the focusing capability of the light source and the indices of refraction of the core and cladding. In (1) a laser is used with single mode fiber to direct a ray through the fiber without any refraction. In (2) the angle of a ray is less than the critical angle, resulting in the ray bouncing along the core/cladding boundary. In (3) the angle exceeds the critical angle, resulting in a portion of the ray refracted into the cladding and a portion refracted into the core. tially refracted into the cladding, where it will eventually be lost while a portion will be reflected back into the core. Now that we understand the basic principles of fabrication and general flow of light within an optical fiber, let's try to define the characteristics of this light conductor or waveguide. First, let's discuss cable metrics and terms that define different optical fiber characteristics. Optical Fiber Metrics and Terms If you study the specification sheet (specs) produced by a manufacturer of fiber-optic cable, you will note a series of terms that specify various aspects of the cable. In this section we discuss those terms and attempt to understand their meaning. Cable Size In one common method of specifying a fiber-optic cable, size of the cable's core and cladding areas is a key factor. If you use this method, you will usually encounter a pair of numbers separated by a slash, such Chapter Three as 50/125. These numbers refer to the diameter of the core and cladding area of the cable in micrometers, where a micrometer represents 10-