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4.1 Light in communications
Light is increasingly important in communications, particuarly in the medium of optical fibres. This importance derives from two important facts. First, light waves are of a very high frequency. Second, the frequency of a signal that can be carried on a particular medium is limited by the frequency of the waveform that it can support. The frequency of a light waveform is so high that, in principle, data can be carried on an optical fibre thousands of times faster than we can currently achieve. At present, it is largely the capabilities of the associated electronic equipment that limit the speed of optical communications.
In the future we may start to see computer equipment which uses light exclusively for its internal operation. Already some promising results have been achieved in this field. If this technique becomes prominent, then optical communication will be even more important.
4.2 Properties of light
An important property of light is its wavelength (frequency). Despite the name `light', not all light is visible to human beings; when light is visible its colour is dictated by its wavelength. Red light has the longest wavelength that we can see; longer wavelengths are called `infra-red', and may be important in some communications systems. Some animals can see light of wavelenghts that are invisible to people.
Another important property is the intensity of the light. We perceive variations in intensity as variations in brightness. Normally the intensity of light decreases as we get further from the source, even when the light is focussed into a narrow beam. This happens because, as it travels through the medium, some of the light is scattered (randomly reflected away) and some is absorbed. Thus, in the same way that long electrical cables attenuate (reduce) the strength of an electrical signal, long light beams are less intense at the receiver than the transmitter. The mathematical principles that govern the attenuation of light are slightly more complicated than those that apply to electricity (at least for direct current), and I won't describe them here.
The light that comes from the sun, or from a light bulb, contains a range of different wavelengths and is said to be incoherent. This means that the overall intensity of the light is reduced as different wavelengths cancel each other out. Coherent light, as obtained from a LASER, does not suffer this effect, so it remains intense over long distances.
If left uninterrupted, light beams tend to travel in straight lines. Until the advent of optical fibres, light was mostly only useful for line-of-site communication, although it can be guided by mirrors.
The speed of light in space is 3x108 m/sec, but may be slower in some other media.
4.3 Light sources and detectors
Ordinary light bulbs are not much use in communications sytems, as they generate light with a wide range of wavelengths. The most common light sources in communications are light-emitting diodes (LEDs). These generate light with a narrow band of frequencies, and which therefore appears coloured, usually red. LASERs (`light amplification by stimulated emmision of radiation') are capable of generating very intense light beams, which can be tightly focussed. These are normally used only for specialized applications, as they require high voltages and may be bright enough to burn. a LASER diode, on the other hand, generates a beam that is more intense than an LED, but requires only a low voltage to operate. LASER diodes are widely used in laser printers and CD players, as well as communications apparatus.
Remember that all computers currently available are electronic devices. Therefore we need some mechansism for detecting the light beam from an optical transmitter and producing a corresponding electricial signal. In communications, the most common device for this application is a photodiode. In essense this has an electrical resistance that varies with the intensity or wavelength of the light that shines on it.
4.4 Reflection and refraction
Everyone should be familiar with the phenomenon of reflection. When a light beam strikes a reflective surface it `bounces back' at the same angle. The most common reflective device is the mirror. High-purity mirrors are used to guide the beams for LASERs and LASER diodes. However, using mirrors as a means of contructing a communications channel is not effective in an urban or office environment, as it is too easy to obstruct the beam accidentally.
Refraction is the bending of a light beam when it crosses from one medium to another, e.g., from water to air. Refraction is what makes objects underwater appear closer to the surface than they really are.
These phenomena are illustrated in the figure below.
Figure 1: Reflection (above) and refraction (below)
4.5 Optical fibres
Optical fibres combine the high-speed communications available with light, with the flexibility of cables. Optical fibres can be bent and twisted without too much loss of light, so they can be used in most places that a cable can be used. Optical fibres do not bend light beams; they direct the beam by reflecting it from its internal surfaces. By means of a large number of reflections the light appears to travel in a curved path. Because it is difficult to make flexible reflective materials, optical fibres make use of a phenomenon that is a combination of reflection and refraction called `total internal reflection'. When light strikes a surface at a particular angle, even though the material would normally be transparent (i.e., the light would pass through it) it is instead reflected. This is what makes puddles in the road appear shiny: light is reflected off their surfaces rather than passing into the water. Optical fibres are constructed with two types of platic or glass, one inside the other. It is the interface between these surfaces that causes the reflections. Clever, innit?