Fibre optic transmitters
- an overview or tutorial covering fibre optic transmitters that are used to
launch modulated light streams carrying data into fibre optic cables.
In order that data can be carried along a fibre optic cable,
it is necessary to have a light source or optical transmitter. This fibre optic
transmitter is one of the key elements of any fibre optic communications system
and the choice of the correct one will depend upon the particular application
that is envisaged.
Fibre optic transmitter choices
There is a variety of different aspects to any fibre optic
transmitter. For any application, the different specifications need to be
examined to ensure that the particular fibre optic transmitter will meet the
requirements.
One of the major aspects to any fibre optic transmitter, is
its power level. It is obvious that the fibre optic transmitter should have a
sufficiently high level of light output for the light to be transmitted along
the fibre optic cable to the far end. Some fibre optic cable lengths many only
be a few metres or tens of metres long, whereas others may extend for many
kilometres. In the case of the long lengths, the power of the fibre optic
transmitter is of great importance.
The type of light produced is also important. Light can be
split into two categories, namely coherent and incoherent light. Essentially,
coherent light has a single frequency, whereas incoherent light contains a wide
variety of light packets all containing different frequencies, i.e. there is no
single frequency present. While some emitters may appear to emit a single colour,
they can still be incoherent because the light output is centred around a given
frequency or wavelength.
The frequency or wavelength of the light can also be
important. Often fibre optic systems will operate around a given wavelength.
Typically the wavelength of operation is given.
It is also necessary to consider the rate at which the
transmitter can be modulated as this affects the data rate for the overall
transmission. In some instances low rate systems may only need to carry data at
a rate of a few Mbps, whereas main telecommunications links need to transmit
data at many Gbps.
Types of fibre optic transmitter
There are two main types of fibre optic transmitter that are
in use today. Both of them are based around semiconductor technology:
- Light emitting diodes (LEDs)
- Laser diodes
Semiconductor optical transmitters have many advantages. They
are small, convenient, and reliable. However, the two different types of fibre
optic transmitter have very different properties and they tend to be used in
widely different applications.
LED transmitters These fibre optic
transmitters are cheap and reliable. They emit only incoherent light with a
relatively wide spectrum as a result of the fact that the light is generated by
a method known as spontaneous emission. A typical LED used for optical
communications may have its light output in the range 30 - 60 nm. In view of
this the signal will be subject to chromatic dispersion, and this will limit the
distances over which data can be transmitted
It is also found that the light emitted for a LED is not
particularly directional and this means that it is only possible to couple them
to multimode fibre, and even then the overall efficiency is low because not allt
he light can be coupled into the fibre optic cable.
LEDs have significant advantages as fibre optic transmitters
in terms of cost, lifetime, and availability. They are widely produced and the
technology to manufacture them is straightforward and as a result costs are low.
Laser diode transmitters These fibre optic
transmitters are more expensive and tend to be used for telecommunications links
where the cost sensitivity is nowhere near as great.
The output from a laser diode is generally higher than that
available from a LED, although the power of LEDs is increasing. Often the light
output from a laser diode can be in the region of 100 mW. The light generation
arises from what is termed stimulated emission and this generates coherent
light. In addition to this the output is more directional than that of a LED and
this enables much greater levels of coupling efficiency into the fibre optic
cable. This also allows the use of single mode fibre which enables much greater
transmission distances to be achieved. A further advantage of using a laser is
that they have a coherent light output and this means that the light is
nominally on a single frequency and modal dispersion is considerably less.
A further advantage of lasers is that they can be directly
modulated with high data rates. Although LEDS can be modulated directly, there
is a lower limit to the modulation rate. One of the disadvantages of a laser
diode fibre optic
Nevertheless laser diode fibre optic transmitters have some
drawbacks. They are much more expensive than LEDs. Furthermore they are quite
sensitive to temperature and to obtain the optimum performance they need to be
in a stable environment. They also do not offer the same life as LEDs, although
as much research has been undertaken into laser diode technology, this is much
less of an issue than previously.
Fibre optic transmitter summary
In view of the different characteristics that LEDs and laser
diode fibre optic transmitters posses they are used in different applications.
The table below summarises some of the chief characteristics of the two devices.
| Characteristic |
LED |
Laser
Diode |
| Cost |
Low |
High |
| Data rate |
Low |
High |
| Distance |
Short |
Long |
| Fibre type |
Multimode fibre |
Multimode and single mode fibre |
| Lifetime |
High |
Low |
| Temperature sensitivity |
Minor |
Significant |
LEDs tend to be used for the more cost sensitive applications
and ones where lower data rates and shorter distances are required. Local area
networks with speeds up to a maximum of 100 Mbps and distances up to a kilometre
or so represent the upper limits. Long distance telecommunications fibre optic
links with Gbps data rates require the use of the more expensive laser diode
fibre optic transmitters.
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