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Fiber Optic Technology Overview - 1/6

An Overview of Fiber Optic Technology - 59059 An Overview of Fiber Optic Technology

The use of fiber optics in telecommunications and wide area networking has been common for many years, but more recently fiber optics have become increasingly prevalent in industrial data communications systems as well. High data rate capabilities, noise rejection and electrical isolation are just a few of the important characteristics that make fiber optic technology ideal for use in industrial and commercial systems. Most often used for point-to-point connections, fiber optic links are being used to extend the distance limitations of RS-232, RS-422/485 and Ethernet systems while ensuring high data rates and minimizing electrical interference. Conventional electrical data signals are converted into a modulated light beam, introduced into the fiber and transported via a very small diameter glass or plastic fiber to a receiver that converts the light back into electrical signals. Fiber's ability to carry the light signal, with very low losses, is based on some fundamental physics associated with the refraction and reflection of light.

Fiber Optic Principles

Whenever a ray of light passes from one transparent medium to another, the light is affected by the interface between the two materials. This occurs because of the difference in speeds that the light can travel through different materials. Each material can be described in terms of its refractive index , which is the ratio of the speed of light in the material to its speed in free space. The relationship between these two refractive indices determines the critical angle of the interface between the two materials. There are three actions that can happen when a ray of light hits an interface. Each action depends on the angle of incidence of the ray of light with the interface. If the angle of incidence is less than the critical angle, the light ray will refract, bending toward the material with the higher refractive index. If the angle of incidence is exactly equal to the critical angle the ray of light will travel along the surface of the interface. If the angle of incidence is greater than the critical angle, the ray of light will reflect. The refractive index of vacuum is considered to be 1. Often, we consider the refractive index of air also to be 1 (although it is actually slightly higher). The refractive index of water is typically about 1.33. Glass has a refractive index in the range of 1.5, a value that can be manipulated by controlling the composition of the glass itself.

Fiber Optic Characteristics

Optical fibers allow data signals to propagate through them by ensuring that the light signal enters the fiber at an angle greater than the critical angle of the interface between two types of glass. As shown in Figure 1, optical fiber is actually made up of three parts. The center core is composed of very pure glass, with a refractive index of 1.5. Core dimensions are usually in the range of 50 to 125 um. The surrounding glass, called cladding , is a slightly less pure glass with a refractive index of 1.45. The diameter of the core and cladding together is in the range of 125 to 440 um. Surrounding the cladding is a protective layer of flexible silicone called the
WHITE PAPER sheath .
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