CHAPTER 13
Fiber-Optics Communications
1. The information-carrying capacity of a cable or radio channel is directly proportional to its bandwidth.
2. The RF spectrum is heavily used and occupied. Only in the microwave region is there room for expansion.
3. Light is an electromagnetic signal like a radio wave but is much higher in frequency. It can be used as a carrier for information signals.
4. Because of the very high frequency of light compared to typical information signals, tremendous bandwidth is easily available.
5. Light waves carrying data signals can be transmitted in free space but are greatly attenuated by atmospheric effects and require pinpoint alignment.
6. Most light-wave communication is by way of a glass or plastic fiber cable that acts as a "light pipe" to carry light modulated by information signals.
7. The main components of a light-wave communications system are an AID converter, a light source transmitter, a fiber optic cable, a photo- or light detector with amplifier and shaper, and a DIA converter.
8. Because of the great attenuation of light in a fiber-optic cable, repeater units are used to amplify and regenerate the signals over long distances.
9. The primary application of fiber-optic communications is in long-distance telephone systems.
10. The primary advantages of fiber-optic cables over conventional cables and radio are wider bandwidth, lower loss, lightweight, small size, strength, security, interference immunity, and safety.
11. The main disadvantage of fiber-optic cable is that its small size and brittleness make it more difficult to work with.
12. Light waves, like radio waves, are a kind of electromagnetic radiation.
13. Light waves occur at very high frequencies in the range of 3 x 1011 to 3 X 1616 Hz.
14. Wavelength rather than frequency is used to express the place of light in the spectrum.
15. 15...The wavelength of light is expressed in terms of nanometers (1 nm = 10-9 m) or micrometers (1 mm = 10-6 m). Micrometers are also called microns.
16. The visible light spectrum is from 700 nm (red) to 400 nm (violet).
17. The optical spectrum is made up of visible light, infrared at lower frequencies and ultraviolet at higher frequencies.
18. Infrared rays cannot be seen, but they act like light waves and can be manipulated in similar ways as with a lens or mirrors.
19. Light waves, like microwaves, travel in a straight line.
20. The angle at which light strikes a surface is called the angle of incidence. The angle at which light is reflected from a surface is called the angle of reflection. The angle of incidence is equal to the angle of reflection.
21. When a light ray passes from one medium to another, it is bent. This is called refraction.
22. The amount of refraction is called the index of refraction n and is the ratio of the speed of light in air to the speed of light in another medium, such as water, glass, or plastic (n = 1 in air, n = 1.3 in water, n = 1.5 in glass).
23. The angle of the incident light ray determines whether the ray will be reflected or refracted.
24. The critical angle is the angle of incidence that causes the refracted light to travel along the interface between two different media.
25. If the angle of incidence is made greater than the critical angle, reflection occurs instead of refraction.
26. Light entering a fiber-optic cable has an angle of incidence such that the light is reflected or bounced off the boundary between the fiber and the external media. This is called total internal reflection.
27. Fiber-optic cables are made from glass and plastic. Glass has the lowest loss but is brittle. Plastic is cheaper and more flexible, but has high attenuation.
28. A popular fiber-optic cable with a glass core and plastic cladding is called plastic clad silica (PCS).
29. The cladding surrounding the core protects the core and provides an interface with a controlled index of refraction.
30. Step index means there is a sharp difference in the index of refraction between the core and cladding.
31. Graded index means that the index of refraction of the core varies over its cross section, highest in the center and lowest at the edges.
32. A single-mode cable is very small in diameter and essentially provides only a single path for light.
33. Multimode cores are large and provide multiple paths for the light.
34. Multiple light paths through a step-index core cause a light pulse to be stretched and attenuated. This is called modal dispersion and it limits the upper pulse repetition rate and thus the information bandwidth.
35. Multiple light paths in a graded-index core are controlled so that they converge at multiple points along the cable. Modal dispersion does occur, but it is not as severe as that caused by a step-index core.
36. Modal dispersion does not occur in single mode cores.
37. The three most widely used types of fiber optic cables are multimode step-index, single-mode step-index, and multimode graded-index.
38. The primary specification of a fiber-optic cable is attenuation which is usually expressed as the loss in decibels per kilometer.
39. Light loss in a fiber-optic cable is caused by absorption, scattering, and dispersion.
40. Cable attenuation is directly proportional to its length.
41. Cable losses range from 1 dB/km in glass single-mode step-index cable to 100 dB/km for plastic multimode step-index cable.
42. Fiber-optic cables can be spliced by gluing.
43. Special connectors are used to connect cables to one another and to the equipment.
44. Fiber-optic systems use light-emitting diodes (LEDs) and semiconductor lasers as the main light sources.
45. Light-emitting diodes are used in short distance low-speed systems. Injection laser diodes (ILDs) are used in long distance, high-speed systems.
46. Most LEDs and ILDs emit light in the in. visible near-infrared range (0.82 to 1.55mm).
47. A popular operating frequency is 1.3 mm because fiber-optic cable has an attenuation null at this wavelength.
48. Laser diodes emit monochromatic or single-frequency light. The light waves are coherent, so they reinforce one another to create an intense and finely focused beam.
49. Intense laser light is produced by an ILD because reflecting surfaces in the structure form a cavity resonator for the light waves.
50. The most commonly used light sensor is a photodiode.
51. A photodiode is a PN junction that is reverse-biased and exposed to light. Light increases the leakage current across the junction. This current is converted into a voltage pulse.
52. PIN junction diodes are faster and more sensitive than conventional photodiodes.
53. The fastest and most sensitive light detector is the avalanche photodiode (APD).
54. The APD operates with a high reverse bias so that when light is applied. Breakdown occurs and produces a fast, high-current pulse.
55. The receiver portion of a fiber-optic system is made up of a photodiode, amplifier, and shaper.
56. Fiber-optic systems are rated by the speed and the product of the bit rate and the distance.
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