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.

By Ian Poole


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