Pulse Spreading

" title="Atom feeThe data which is carried in an optical fibre consists of pulses of light energy following each other rapidly. There is a limit to the highest frequency, i.e. how many pulses per second which can be sent into a fibre and be expected to emerge intact at the other end. This is because of a phenomenon known as pulse spreading which limits the "Bandwidth" of the fibre.

Figure 11 Pulse Spreading in an Optical Fibre
The pulse sets off down the fibre with an nice square wave shape. As it travels along the fibre it gradually gets wider and the peak intensity decreases.
Cause of Pulse Spreading
The cause of cause spreading is dispersion. This means that some components of the pulse of light travel at different rates along the fibre. there are two forms of dispersion.
1. Chromatic dispersion
2. Modal dispersion
Chromatic Dispersion
Chromatic dispersion is the variation of refractive index with the wavelength (or the frequency) of the light. Another way of saying this is that each wavelength of light travels through the same material at its own particular speed which is different from that of other wavelengths.
For example, when white light passes through a prism some wavelengths of light bend more because their refractive index is higher, i.e. they travel slower This is what gives us the "Spectrum" of white light. The "red' and "orange" light travel slowest and so are bent most while the "violet" and "blue" travel fastest and so are bent less. All the other colours lie in between.
This means that different wavelengths travelling through an optical fibre also travel at different speeds. This phenomenon is called "Chromatic Dispersion".
Figure 10 Dispersion of Light through a Prism
Modal Dispersion
In an optical fibre there is another type of dispersion called "Multimode Dispersion".
More oblique rays (lower order modes) travel a shorter distance. These correspond to rays travelling almost parallel to the centre line of the fibre and reach the end of fibre sooner. The more zig-zag rays (higher order modes) take a longer route as they pass along the fibre and so reach the end of the fibre later.
Now:-
Total dispersion = chromatic dispersion + multimode dispersion
Or put simply: for various reasons some components of a pulse of light travelling along an optical fibre move faster and other components move slower. So, a pulse which starts off as a narrow burst of light gets wider because some components race ahead while other components lag behind, rather like the runners in a marathon race.
Consequences of pulse spreading
Frequency Limit (Bandwidth)
The further the pulse travels in the fibre the worse the spreading gets

Figure 12 - Merging of Pulses in a Fibre.
Pulse spreading limits the maximum frequency of signal which can be sent along a fibre. If signal pulses follow each other too fast then by the time they reach the end fibre they will have merged together and become indistinguishable. This is unaceptable for digital systems which depend on the precise sequence of pulses as a code for information. The Bandwidth is the highest number of pulses per second, that can be carried by the fibre without loss of information due to pulse spreading.
Distance Limit
A given length of fibre, as explained above has a maximum frequency (bandwidth) which can be sent along it. If we want to increase the bandwidth for the same type of fibre we can achieve this by decreasing the length of the fibre. Another way of saying this is that for a given data rate there is a maximum distance which the data can be sent.
Bandwidth Distance Product (BDP)
We can combine the two ideas above into a single term called the bandwidth distance product (BDP). It is the bandwidth of a fibre multiplied by the length of the fibre. The BDP is the bandwidth of a kilometre of fibre and is a constant for any particular type of fibre. For example, suppose a particular type of multimode fibre has a BDP of 20 MHz.km, then:-
1 km of the fibre would have a bandwidth of 20 MHz
2 km of the fibre would have a bandwidth of 10 MHz
5 km of the fibre would have a bandwidth of 4 MHz
4 km of the fibre would have a bandwidth of 5 MHz
10 km of the fibre would have a bandwidth of 2 MHz
20 km of the fibre would have a bandwidth of 1 MHz
The typical B.D.P. of the three types of fibres are as follows:-
Multimode 6 - 25 MHz.km
Single Mode 500 - 1500 MHz.km
Graded Index 100 - 1000 MHZ.km
NB: The units of BDP are MHz.km (read as megahertz kilometres). They are not MHz/km (read as megahertz per kilometres). This is because the quantity is a product (of bandwidth and distance) and not a ratio.
Choice of Fibre
Multimode Fibre
Muitimode fibre is suitable for local area networks (LAN's) because it can carry enough energy to support all the subscribers to the network. In a LAN the distances involved, however, are small. Little pulse spreading can take place and so the effects of dispersion are unimportant.
Single Mode Fibre.
Multimode Dispersion is eliminated by using Single Mode fibre. The core is so narrow that only one mode can travel. So the amount of pulse spreading in a single mode fibre is greatly reduced from that of a multimode fibre. Chromatic dispersion however remains even in a single mode fibre. Thus even in single mode fibre pulse spreading can occur. But chromatic dispersion can be reduced by careful design of the chemical composition of the glass.
The energy carried by a single mode fibre, however, is much less than that carried by a multimode fibre. For this reason single mode fibre is made from extremely low loss, very pure, glass.
Single mode low absorption fibre is ideal for telecommunications because pulse spreading is small.
Graded Index Fibre
In graded index fibre rays of light follow sinusoidal paths. This means that low order modes, i.e. oblique rays, stay close to the centre of the fibre, high order modes spend more time near the edge of core. Low order modes travel in the high index part of the core and so travel slowly, whereas high order modes spend
predominantly more time in the low index part of the core and so travel faster. This way, although the paths are different lengths, all the modes travel the length of the fibre in tandem, i.e., they all reach the end of the fibre at the same time. This eliminates multimode dispersion and reduces pulse spreading.
Graded Index fibre has the advantage that it can carry the same amount of energy as multimode fibre. The disadvantage is that this effect takes place at only one wavelength, so the light source has to be a laser diode which has a narrow linewidth.
Figure 13 - Ray Paths in Graded Index Fibred">Site Feed