An optical fiber is the core component of an optical fiber communication link. Popularly known as optical fiber cables, they are the most promising type of guided transmission medium for virtually all forms of digital and data communications applications. With optical fibers, electromagnetic light waves propagate through the media composed of a transparent material without using electrical current flow. Optical fibers are mostly made of glass or plastic material having properties such that the phenomena of total internal reflection takes place that enables light waves to propagate within it in a properly guided manner similar to that of electromagnetic waves through a metallic transmission medium. This chapter begins with an easy-to-understand ray model of the propagation of light through optical fibers. It is followed by a discussion on the concept of modes and the modal analysis of step-index as well as the graded-index type of fibers. Finally, the type of losses and dispersions are explained to assess the limitation of optical fibers.
Review of Optical Ray Theory
In essence, an optical fiber communications system is one that uses light (optical signal) as the carrier of analog or digital information signal. Propagating light waves, carrying information, through the earth's atmosphere is difficult and often impractical. The optical energy in a light wave follows narrow paths, called light rays or beams. For most practical applications, the light rays are used to describe a number of optical phenomena geometrically. In fact, ray theory is known as geometric optics. It is these rays (geometrical paths traversed by light) which actually carry the optical energy.
Velocity of Propagation
Electromagnetic energy, such as light waves, travels at a velocity of c = 3 × 108 m/sec approximately in free space (a vacuum). Moreover, the velocity of propagation is the same for all light frequencies in free space. However, it has been demonstrated that
• All light frequencies are not propagated with the same velocity.
• Since materials are denser (possess higher refractive index) than free space, electromagnetic waves travel slower in materials than in free space.
• When the velocity of an electromagnetic wave is reduced as it travels from one medium to another medium of denser material, the light ray refracts (i.e., bends or changes direction) toward the normal.
• Likewise, when an electromagnetic wave travels from a denser material into a lighter one, it gets refracted away from the normal.