Optical Fiber

Up until this point, we have discussed conductive (metal) cables that transmit signals in the form of current. Optical fiber, on the other hand, is made of glass or plastic and transmits signals in the form of light. To understand optical fiber, we first need to explore several aspects of the nature of light.

The Nature of Light Light is a form of electromagnetic energy. It travels at its fastest speed in a vacuum: 300,000 kilometers/second (approximately l86,000 miles/second). The speed of light depends on the density of the medium through which it is traveling (the higher the density, the slower the speed).

Refraction Light travels in a straight line as long as it is moving through a single uniform substance. If a ray of light traveling through one substance suddenly enters another (more or less dense) substance, its speed changes abruptly, causing the ray to change direction. This change is called refraction. A straw sticking out of a glass of water appears bent, or even broken, because the light by which we see it changes direction as it moves from the air to the water.

The direction in which a light ray is refracted depends on the change in density encountered. A beam of light moving from a less dense into a more dense medium is bent toward the vertical axis (examine Figure 5.1-11). The two angles made by the beam of light in relation to the vertical axis are called I, for incident, and R, for refracted. In Figure 5.1-11a, the beam travels from a less dense medium into a denser medium. In this case, angle R is smaller than angle I.

In Figure 5.1-11b, however, the beam travels from a denser medium into a less dense medium. In this case, the value of I is smaller than the value of R. In other words, when light travels into a more dense medium, the angle of incidence is greater than the angle of refraction; and when light travels into a less dense medium, the angle of incidence is less than the angle of refraction.

Fiber-optic technology takes advantage of the properties shown in Figure 5.1-11b to control the propagation of light through the fiber channel.

Fig. 5.1-11 Refraction

Critical Angle Now examine Figure 5.1-13. Once again we have a beam of light moving from a denser into a less dense medium. In this example, however, we gradually increase the angle of incidence measured from the vertical. As the angle of incidence increases, so does the angle of refraction. It, too, moves away from the vertical and closer and closer to the horizontal.

Fig. 5.1-12 Critical angle

At some point in this process, the change in the incident angle results in a refracted angle of 90 degrees, with the refracted beam now lying along the horizontal. The incident angle at this point is known as the critical angle.

Reflection When the angle of incidence becomes greater than the critical angle, a new phenomenon occurs called reflection (or, more accurately, complete reflection, because some aspects of reflection always coexist with refraction). Light no longer passes into the less dense medium at all. In this case, the angle of incidence is always equal to the angle reflection (see

Fig. 5.1-13 Reflection

Optical fibers use refection to guide light through a channel. A glass or plastic core is surrounded by a cladding of less dense glass or plastic. The difference in density of the two materials must be such that a beam of light moving through the core is reflected off the cladding instead of being refracted into it. Information is encoded onto a beam of light as a series of on-off flashes that represent 1 and 0 bits.

Propagation Modes

Current technology supports two modes for propagating light along optical channels, each requiring fiber with different physical characteristics: multimode and single-mode. Multimode, in turn, can be implemented in two forms: step-index or graded-index (see Figure 5.1-14).

Fiber Optical Cable Propgation Mode
Fig. 5.1-14 Propagation modes

Multimode Multimode is so named because multiple beams from a light source move through the core in different paths. How these beams move within the cable depends on the structure of the core.

In step-index multimode, the density of the core remains constant from the center to the edges. A beam of light moves through this constant density in a straight line until it reaches the interface of the core and the cladding. At the interface there is an abrupt change to a lower density that alters the angle of the beam's motion. The term step-index refers to the suddenness of this change.

Figure 5.1-15 shows various beams (or rays) traveling through a step-index fiber. Some beams in the middle travel in straight lines through the core and reach the destination without reflecting or refracting. Some beams strike the interface of the core and cladding at an angle smaller than the critical angle; these beams penetrate the cladding and are lost. Still others hit the edge of the core at angles greater than the critical angle and reflect back into the core and off the other side, bouncing back and forth down the channel until they reach the destination.

Fig. 5.1-15 Multimode step-index

Every beam reflects off the interface at an angle equal to its angle of incidence. The greater the angle of incidence, the wider the angle of reflection. A beam with a smaller angle of incidence will require more bounces to travel the same distance than a beam with a larger angle of incidence. Consequently, the beam with the smaller incident angle must travel farther to reach the destination. This difference in path length means that different beams arrive at the destination at different times. As these different beams are recombined at the receiver, they result in a signal that is no longer an exact replica of the signal that was transmitted. Such a signal has been distorted by propagation delays. This distortion limits the available data rate and makes multimode step-index cable inadequate for certain precise applications.

A second type of fiber, called graded-index, decreases this distortion of the signal through the cable. The word index here refers to the index of refraction. As we saw above, index of refraction is related to density. A graded-index fiber, therefore, is one with varying densities. Density is highest at the center of the core and decreases gradually to its lowest at the edge. Figure 5.1-16 shows the impact of this variable density on the propagation of light beams.

Fig. 5.1-16 Multimode graded-index

The signal is introduced at the center of the core. From this point, only the horizontal beam moves in a straight line through the constant density at the center. Beams at other angles move through a series of constantly changing densities. Each density difference causes each beam to refract into a curve. In addition, varying the refraction varies the distance each beam travels in a given period of time, resulting in different beams intersecting at regular intervals. Careful placement of the receiver at one of these intersections allows the signal to be reconstructed with far greater precision.

Single Mode Single mode uses step-index fiber and a highly focused source of light that limits beams to a small range of angles, all close to the horizontal. The fiber itself is manufactured with a much smaller diameter than that of multimode fibers and with substantially lower density (index of refraction). The decrease in density results in a critical angle that is close enough to 90 degrees to make the propagation of beams almost horizontal. In this case, propagation of different beams is almost identical and delays are negligible. All of the beams arrive at the destination "together" and can be recombined without distortion to the signal (see Figure 5.1-17).

Fig. 5.1-17 Single mode

Fiber Sizes Optical fiber are defined by the ratio of the diameter of their core to the diameter of their cladding, both expressed in microns (micrometers). The common sizes are shown in Table 5.1-1. The last size listed is used only for single mode.

Table 5.1-1 Fiber types

Fiber type

Core (microns)

Cladding (microns)

62.5/125

62.5

125

50/125

50

125

100/140

100

140

8.3/125

8.3

Figure 5.1-18 shows the composition of a typical fiber-optic cable. A core is surrounded by cladding, forming the fiber. In most cases, the fiber is covered by a buffer layer that protects it from moisture. Finally, the entire cable is encased in an outer jacket.

Figure 5.1-18 shows the composition of a typical fiber-optic cable. A core is surrounded by cladding, forming the fiber. In most cases, the fiber is covered by a buffer layer that protects it from moisture. Finally, the entire cable is encased in an outer jacket.

Fig. 5.1-18 Fiber construction

Both core and cladding can be made of either glass or plastic but must be of different densities. In addition the inner core must be ultrapure and completely regular in size and shape. Chemical differences in material, and even small variations in the size or shape of the channel, alter the angle of reflection and distort the signal. Some applications can handle a certain amount of distortion and their cables can be made more cheaply, but others depend on complete uniformity.

The outer jacket (or sheath) can be made of several materials, including Teflon coating, plastic coating, fibrous plastic, metal tubing, and metal mesh. Each of these jacketing materials has its own purpose. Plastics are lightweight and inexpensive but do not provide structural strength and can emit fumes when burned. Metal tubing provides strength but raises cost. Teflon is lightweight and can be used in open air, but it is expensive and does not increase cable strength. The choice of the material depends on where the cable is to be installed.

Light Sources for Optical Cable

As we have seen, the purpose of fiber-optic cable is to contain and direct a beam of light from source to target. For transmission to occur, the sending device must be equipped with a light source and the receiving device with a photosensitive cell (called a photodiode) capable of translating the received light into current usable by a computer. The light source can be either a light-emitting diode (LED) or an injection laser diode (ILD). LEDs are the cheaper source, but they provide unfocused light that strikes the boundaries of the channel at uncontrollable angles and diffuses over distance. For this reason, LEDs are limited to short-distance use.

Lasers, on the other hand, can be focused to a very narrow range allowing control over the angle of incidence. Laser signals preserve the character of the signal over considerable distances.

Fiber-Optic Connectors

Connectors for fiber-optic cable must be as precise as the cable itself. With metallic media, connections are not required to be exact as long as both conductors are in physical contact. With optical fiber, on the other hand, any misalignmet of one segment of core either with another segment or with a photodiode results in the signal reflecting back toward the sender, and any difference in the size of two connected channels results in a change in the angle of the signal. In addition, the connection must be complete yet not overly tight. A gap between two cores results in a dissipated signal; an overly tight connection can compress the two cores and alter the angle of reflection.

Given these constraints, manufacturers have developed several connectors that are both precise and easy to use. All of the popular connectors are barrel shaped and come in male and female versions. The cable is equipped with a male connector that locks or threads into a female connector attached to the device to be connected.

Advantages of Optical Fiber

The major advantages offered by fiber-optic cable over twisted pair and coaxial cable are noise resistance, less signal attenuation, and higher bandwidth.

• Noise resistance. Because fiber-optic transmission uses light rather than electricity noise is not a factor. External light, the only possible interference, is blocked from the channel by the outer jacket.

• Less signal attenuation. Fiber-optic transmission distance is significantly greater than that of other guided media. A signal can run for miles without requiring regeneration.

• Higher bandwidth. Fiber-optic cable can support dramatically higher bandwidths (and hence data rates) than either twisted-pair or coaxial cable. Currently, data rates and bandwidth utilization over fiber-optic cable are limited not by the medium but by the signal generation and reception technology available.

Disadvantages of Optical Fiber

The main disadvantages of fiber optics are cost, installation/maintenance, and fragility.

• Cost. Fiber-optic cable is expensive. Because any impurities or imperfections in the core can throw off the signal, manufacturing must be painstakingly precise. Also, a laser light source can cost thousands of dollars, compared to hundreds of dollars for electrical signal generators.

• Installation/maintenance. Any roughness or cracking in the core of an optical cable diffuses light and alters the signal. All splices must be polished and precisely fused. All connections must be perfectly aligned and matched for core size, and must provide a completely light-tight seal. Metallic media connections, on the other hand, can be made by cutting and crimping using relatively unsophisticated tools.

• Fragility. Glass fiber is more easily broken than wire, making it less useful for applications where hardware portability is required.

As manufacturing techniques have improved and costs come down, high data rates and immunity to noise have made fiber optics increasingly popular.

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