Fiber Optic Communications for the Premises Environment

THE FIBER OPTIC DATA COMMUNICATIONS LINK FOR THE PREMISES ENVIRONMENT
2.1 The Fiber Optic Data Communications Link, End-to-EndIn this chapter we consider the simple fiber optic data link for the premises environment. This is the basic building block for a fiber optic based network. A model of this simple link is shown in Figure 2-1.

The illustration indicates the Source-User pair, Transmitter and Receiver. It also clearly shows the fiber optic cable constituting the Transmission Medium as well as the connectors that provide the interface of the Transmitter to the Transmission Medium and the Transmission Medium to the Receiver.All of these are components of the simple fiber optic data link. Each will be discussed. Consideration will be in the following order: fiber optic cable, Transmitter, Receiver and connectors. We will conclude by taking up the question of how to analyze the performance of the simple fiber optic data link.2.2 Fiber Optic CableWe begin by asking Just what is a fiber optic cable? A fiber optic cable is a cylindrical pipe. It may be made out of glass or plastic or a combination of glass and plastic. It is fabricated in such a way that this pipe can guide light from one end of it to the other.The idea of having light guided through bent glass is not new or high tech. The author was once informed that Leonardo DaVinci actually mentioned such a means for guiding light in one of his notebooks. However, he has not been able to verify this assertion. What is known for certain is that total internal reflection of light in a beam of water - essentially guided light - was demonstrated by the physicist John Tyndall [1820-1893] in either 1854 or 1870 - depending upon which reference you consult. Tyndall showed that light could be bent around a corner while it traveled through a jet of pouring water.Using light for communications came after this. Alexander Graham Bell [1847-1922] invented the photo-phone around 1880. Bell demonstrated that a membrane in response to sound could modulate an optical signal, light. But, this was a free space transmission system. The light was not guided.Guided optical communications had to wait for the 20th century. The first patent on guided optical communications over glass was obtained by AT &T in 1934. However, at that time there were really no materials to fabricate a glass (or other type of transparent material) fiber optic cable with sufficiently low attenuation to make guided optical communications practical. This had to wait for about thirty years.During the 1960's researchers working at a number of different academic, industrial and government laboratories obtained a much better understanding of the loss mechanisms in glass fiber optic cable. Between 1968 and 1970 the attenuation of glass fiber optic cable dropped from over 1000 dB/km to less than 20 dB/km. Corning patented its fabrication process for the cable. The continued decrease in attenuation through the 1970's allowed practical guided light communications using glass fiber optic cable to take off. In the late 1980's and 1990's this momentum increased with the even lower cost plastic fiber optic cable and Plastic Clad Silica (PCS).Basically, a fiber optic cable is composed of two concentric layers termed the core and the cladding. These are shown on the right side of Figure 2-2. The core and cladding have different indices of refraction with the core having n1 and the cladding n2. Light is piped through the core. A fiber optic cable has an additional coating around the cladding called the jacket. Core, cladding and jacket are all shown in the three dimensional view on the left side of Figure 2-2. The jacket usually consists of one or more layers of polymer. Its role is to protect the core and cladding from shocks that might affect their optical or physical properties. It acts as a shock absorber. The jacket also provides protection from abrasions, solvents and other contaminants. The jacket does not have any optical properties that might affect the propagation of light within the fiber optic cable.The illustration on the left side of Figure 2-2 is somewhat simplistic. In actuality, there may be a strength member added to the fiber optic cable so that it can be pulled during installation

This would be added just inside the jacket. There may be a buffer between the strength member and the cladding. This protects the core and cladding from damage and allows the fiber optic cable to be bundled with other fiber optic cables. Neither of these is shown.How is light guided down the fiber optic cable in the core? This occurs because the core and cladding have different indices of refraction with the index of the core, n1, always being greater than the index of the cladding, n2. Figure 2-3 shows how this is employed to effect the propagation of light down the fiber optic cable and confine it to the core.

As illustrated a light ray is injected into the fiber optic cable on the right. If the light ray is injected and strikes the core-to-cladding interface at an angle greater than an entity called the critical angle then it is reflected back into the core. Since the angle of incidence is always equal to the angle of reflection the reflected light will again be reflected. The light ray will then continue this bouncing path down the length of the fiber optic cable. If the light ray strikes the core-to-cladding interface at an angle less than the critical angle then it passes into the cladding where it is attenuated very rapidly with propagation distance.Light can be guided down the fiber optic cable if it enters at less than the critical angle. This angle is fixed by the indices of refraction of the core and cladding and is given by the formula: