Fibre optics tutorial
- an overview or tutorial covering fibre optics and its applications in
telecommunications and data networking.
Fibre optic communication has revolutionised the
telecommunications industry. It ahs also made its presence widely felt within
the data networking community as well. Using fibre optic cable, optical
communications have enabled telecommunications links to be made over much
greater distances and with much lower levels of loss than was previously
possible. In addition to this, fibre optics are being used increasingly for
Ethernet links. Here the fibre optic cable enables high speed data transmission
over considerable distances.
Development of fibre optics
Since the earliest days of telecommunications there has been
an ever increasing need to transmit more data even faster. Initially single line
wires were used. These gave way to coaxial cables that enabled several channels
to transmitted over the same cable. However these systems were limited in
bandwidth and optical systems were investigated.
Optical communications became a possibility after the first
lasers were developed in the 1960s. The next piece of the jigsaw fell into place
when the first optical fibers with a sufficiently low loss for communications
purposes were developed in the 1970s. Then, during the late 1970s a considerable
amount of research was undertaken. This resulted in the installation of the
first optical fibre telecommunications system. It ran over a distance of 45 km
and used a wavelength of 0.5 mm and had a data rate of just 45
Mbps - a fraction of what is possible today.
Since then, considerable improvements have been made in the
technology. Data rates have improved and in addition to this the performance of
the optical fibre has been improved to enable much greater distances to be
achieved between repeaters. As an indication of this the speeds that can now be
achieved along through a fibre optic system exceed 10 Tbps.
When the first fibre optic transmission systems were being
developed, it was thought that the fibre optic cabling and technology would be
prohibitively expensive. However, this has not been the case and costs have
fallen to the extent that fibre optics now provides the only viable option for
many telecommunications applications. In addition to this it is also used in
many local area networks where speed is a major requirement.
Advantages of fibre optics
There are a number of compelling reasons that lead to the
widespread adoption of fibre optic cabling for telecommunications applications:
- Much lower levels of signal attenuation
- Fibre optic cabling provides a much higher bandwidth allowing more data
to be delivered
- Fibre optic cables are much lighter than the coaxial cables that might
otherwise be used.
- Fibre optics do not suffer from stray interference pickup that occurs
with coaxial cabling
Fibre optic transmission system
Any fibre optic data transmission system will comprise a
number of different elements. There are three major elements (marked in bold),
and a further one that is vital for practical systems:
- Transmitter (light source)
- Fibre optic cable
- Optical repeater
- Receiver (Detector)
The different elements of the system will vary according to
the application. Systems used for lower capacity links, possibly for local area
networks will employ somewhat different techniques and components to those used
by network providers that provide extremely high data rates over long distances.
Nevertheless the basic principles are the same whatever the system.
In the system the transmitter of light source generates a
light stream modulated to enable it to carry the data. Conventionally a pulse of
light indicates a "1" and the absence of light indicates "0". This light is
transmitted down a very thin fibre of glass or other suitable material to be
presented at the receiver or detector. The detector converts the pulses of light
into equivalent electrical pulses. In this way the data can be transmitted as
light over great distances.
Fibre optic transmitter
Although the original telecommunications fibre optic systems
would have used large lasers, today a variety of semiconductor devices can be
used. The most commonly used devices are light emitting diodes, LEDs, and
semiconductor laser diodes.
The simplest transmitter device is the LED. Its main
advantage is that it is cheap, and this makes it ideal for low cost applications
where only short runs are needed. However they have a number of drawbacks. The
first is that they offer a very low level of efficiency. Only about 1% of the
input power enters the optical fibre, and this means that high power drivers
would be needed to provide sufficient light to enable long distance
transmissions to be made. The other disadvantage of LEDs is that they produce
what is termed incoherent light that covers a relatively wide spectrum.
Typically the spectral width is between 30 and 60 nm. This means that any
chromatic dispersion in the fibre will limit the bandwidth of the system.
In view of their performance, LEDs are used mainly in
local-area-network applications where the data rates are typically in the range
10-100 Mb/s and transmission distances are a few kilometres.
Where higher levels of performance are required, i.e. it is
necessary that the fibre optic link can operate over greater distances and with
higher data rates, then lasers are used. Although more costly, they offer some
significant advantages. In the first instance they are able to provide a higher
output level, and in addition to this the light output is directional and this
enables a much higher level of efficiency in the transfer of the light into the
fibre optic cable. Typically the coupling efficiency into a single mode fibre
may be as high as 50%. A further advantage is that lasers have a very narrow
spectral bandwidth as a result of the fact that they produce coherent light.
This narrow spectral width enables the lasers to transmit data at much higher
rates because modal dispersion is less apparent. Another advantage is that
semiconductor lasers can be modulated directly at high frequencies because of
short recombination time for the carriers within the semiconductor material.
Laser diodes are often directly modulated. This provides a
very simple and effective method of transferring the data onto the optical
signal. This is achieved by controlling current applied directly to the device.
This in turn varies the light output from the laser. However for very high data
rates or very long distance links, it is more effective to run the laser at a
constant output level (continuous wave). The light is then modulated using an
external device. The advantage of using an external means of modulation is that
it increases the maximum link distance because an effect known as laser chirp is
eliminated. This chirp broadens the spectrum of the light signal and this
increases the chromatic dispersion in the fibre optic cable.
Fibre optic cable
The full details and description of fibre optic cabling are
found in a separate article / tutorial on this area of the website. In essence a
fibre optic cable consists of core, around which is another layer referred to as
the cladding. Outside of this there is a protective outer coating.
The fibre optic cables operate because their cladding has a
refractive index that is slightly lower than that of the core. This means that
light passing down the core undergoes total internal reflection when it reaches
the core / cladding boundary, and it is thereby contained within the core of the
optical fibre.
Repeaters and amplifiers
There is a maximum distance over which signals may be
transmitted over fibre optic cabling. This is limited not only by the
attenuation of the cable, but also the distortion of the light signal along the
cable. In order to overcome these effects and transmit the signals over longer
distances, repeaters and amplifiers are used.
Opto-electric repeaters may be used. These devices convert
the optical signal into an electrical format where it can be processed to ensure
that the signal is not distorted and then converted back into the optical
format. It may then be transmitted along the next state of the fibre optic
cable.
An alternative approach is to use an optical amplifier. These
amplifiers directly amplify the optical signal without the need to convert the
signal back into an electrical format. The amplifiers consist of a length of
fibre optic cable that is doped with a rare earth mineral named Erbium. The
treated fibre cable is then illuminated or pumped with light of a shorter
wavelength from another laser and this serves to amplify the signal that is
being carried.
In view of the much reduced cost of fibre optic amplifiers
over repeaters, amplifiers are far more widely used. Most repeaters have been
replaced, and amplifiers are used in virtually all new installations these days.
Receivers
Light travelling along a fibre optic cable needs to be
converted into an electrical signal so that it can be processed and the data
that is carried can be extracted. The component that is at the heart of the
receiver is a photo-detector. This is normally a semiconductor device and may be
a p-n junction, a p-i-n photo-diode or an avalanche photo-diode.
Photo-transistors are not used because they do not have sufficient speed.
Once the optical signal from the fibre optic cable ahs been
applied to the photo-detector and converted into an electrical format it can be
processed to recover the data which can then be passed to its final destination.
Summary
Fibre optic transmission of data is generally used for long
distance telecommunications network links and for high speed local area
networks. Currently fibre optics is not used for the delivery of services to
homes, although this is a long term aim for many telcos. By using optical fibre
cabling here, the available bandwidth for new services would be considerably
higher and the possibility of greater revenues would increase. Currently the
cost oft his is not viable, although it is likely to happen in the medium term.
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