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ABSTRACT
Communication is an important part of our daily life. The communication process involves information generation, transmission, reception and interpretation. As needs for various types of communication such as voice, images, video and data communications increase demands for large transmission capacity also increase. This need for large capacity has driven the rapid development light wave technology; a worldwide industry has developed. An optical or light wave communication system is a system that uses light waves as the carrier for transmission. An optical communication system mainly involves three parts. Transmitter, receiver and channel. In optical communication transmitters are light sources, receivers are light detectors and the channels are optical fibers. In optical communication the channel i.e, optical fibers plays an important role because it carries the data from transmitter to the receiver. Hence, here we shall discuss mainly about optical fibers.
Optical Fibers in Communications
1. Introduction
Optical fibers are arguably one of the world’s most influential scientific developments from the latter half of the 20th century. Normally we are unaware that we are using them, although many of us do frequently. The majority of telephone calls and internet traffic at some stage in their journey will be transmitted along an optical fiber. More indirectly, many of the systems that we either rely on or enjoy in everyday life such as banks, television and newspapers as (to name only a very limited selection) are themselves dependent on communication systems that are dependent on optical fibers.
There are various other uses of optical fibers which are irrelevant to this essay, although it is interesting developed to detect chemicals along pipelines (by using unprotected chemically sensitive fiber), detect plutonium smuggling, monitor strain in yacht masts, allow communication with CAT scan patients, construct gyroscopes with no moving parts, transmit images from telescopes, and possibly guide atoms (although this is very early in the stages of development).
In this essay I shall attempt to cover the many areas of importance in optical fiber design. Only some crucial areas such as fiber design will be covered in detail; others such as signal sources and detectors will be discussed more briefly.
I shall also give some indication of the systems currently in use commercially and some of the systems currently being developed.
2. Why Optical Fiber?
Why has the development of fibers been given so much attention by the scientific community when we have alternatives? The main reason is bandwidth – fibers can carry an extremely large amount of information. I shall discuss the advantages and disadvantages of fiber compared to the four other commonly used media.
Twisted Pair Cable is used for, and is still suitable for, simple telephone links (known as the local loop) from the consumer to the nearest telephone exchange. The bandwidth is low, but is adequate for carrying low quality analogue voice signals. Attenuation of the signal is not significant over the short distances such signals are carried. The main advantage of twisted pair cable is the very low cost.
Coaxial Cable can carry a much larger amount of data – especially by multiplexing (the process of transmitting several signals of different wavelengths along the same cable) analogue signals. Multiplexing however is also possible with fiber, and fiber provides significantly higher bandwidth. Digital signals can be transmitted, but the bandwidth is limited if signal quality is to be maintained. Again, fiber is more expensive for many applications where coaxial cable is still used.
3. Fundamentals Of Fibers
The fundamental principle that makes optical fibers possible is total internal reflection. This is described using the ray model of light
From Snell’s Law we find that refraction (as shown by the dashed line) can only occur when the angle theta1 (between the incident ray and the material boundary) is large enough. This implies that as the angle is reduced, there must be a point when the light ray is reflected, where theta1 = theta2 (note that this is only true when the refractive index of the initial medium is greater than that of the adjacent medium, as shown by the value of n on the diagram). The angle where this happens is known as the critical angle and is:
Cladding. The core is (according to the ray model) where the light rays travel and the cladding is a similar material of slightly lower refractive index to cause total internal reflection. Usually both sections are fabricated from silica (glass). The light within the fiber is then continuously totally internally reflected along the waveguide.
When light enters the fiber we must also consider refraction at the interface of the air and the fiber core. The difference in refractive index causes refraction of the ray as it enters the fiber, allowing rays to enter the fiber at an angle greater than the angle allowed within the fiber
This acceptance angle, theta, is a crucial parameter for fiber and system designers. More widely recognized is the parameter NA (Numerical Aperture) that is given by the following equation:
Also crucial to understanding fibers is the principle of modes. A more in-depth analysis of the propagation of light along an optical fiber requires the light to be treated as an electromagnetic wave (rather that as a ray). Unfortunately there is not room for such a mathematical treatment in this essay, but we should note that it leads to a quantisation of the ‘angles’ at which ‘rays’ can travel through the fiber.
Figure 3 – Modes
The solid line is the lowest order mode shown on figure 3. It is clear that according to the ray model the lowest order mode will travel down a given length of fiber quicker than the others. The electromagnetic field model predicts the opposite – that the highest order mode will travel quicker. However, the overall effect is still the same – if a signal is sent down the fiber as several modes then as it travels along the fibre the pulse will spread out (this process is known as modal dispersion); this can lead to the pulses merging and becoming indistinguishable.
One further classification of rays can be made; meridional rays pass through the fiber axis; skew rays (hybrid rays) constantly rotate without passing through the fiber axis.
One other significant point should be noted from the electromagnetic field model – the evanescent field. The model predicts that the EM field does not suddenly drop to zero at the core-cladding boundary – it instead decays as a negative exponential within the cladding (see figure 4). This is crucial for various technologies relating to fibers.
This method of signal transmission has benefits in terms of security – for the signal to be ‘tapped’ the fiber must be broken (since effectively no energy escapes from the fiber) and this can easily be detected (when no signal reaches the other end of the fiber!). This is one of the many advantages of the medium.
4.The Development of Fiber
I shall consider the development of fiber in several sections rather than giving a general discussion of fiber properties. Two types of material are used to manufacture fibers – glass and plastic. There are several properties of a material that dictate how useful it is as a fiber:
• Purity
• Refractive Index (wavelength-dependent)
• Attenuation (wavelength-dependent)
We must remember that there are many different types of fiber and applications for fiber. The different properties of fibers can be combined to provide a suitable fiber for a particular job – not all fibers will be applicable for every situation.
The purity of the fiber will be reflected in both its attenuation properties (consider the scattering effect of an impurity particle) and its refractive index. The original breakthrough in reducing fiber attenuation was achieved by purifying the glass used to make the fibers. There are other intrinsic and extrinsic factors which contribute to the attenuation, such as absorption by OH- ions, absorption of infra-red radiation leading to molecular vibrations, leakage from the core (can be caused by Rayleigh Scattering and fiber curvature) and leaky modes. Curvature is important in fiber specification; again a more detailed analysis of the propagation of light through fibers is required to fully explain this, but essentially a small amount of the light is radiated as the fiber bends. Leaky modes are modes slightly below the cut-off, but can be propagated for a short distance along the fiber; they can be initially avoided at the light source by restricting the angle at which light enters the fiber, but can be introduced along the fiber by microbending. Microbends are minute bends in the fiber which can be introduced during manufacture or cabling (see later for more detail on cabling); they can cause power to be transferred between modes, possibly to leaky modes and hence can result in power loss.