Block 1 - prt 1 - 3.5.1, 2.5.2, 3 PDF

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Summary

in depth notes on the blocks used on the TM355 module...

Description

3.5.1 Optical transmitters and detectors The purpose of an optical transmitter is to convert input data in the form of an electrical signal into a light signal that is sent along the fibre. There are two main types, both semiconductor devices: the light-emitting diode (LED) and the laser diode. Both are versions of technology that is also found elsewhere: . LEDs for optical transmitters are similar in principle to the LEDs seen in displays and as indication lights in consumer goods. However, rather than emitting visible light, they emit in the infrared region of the spectrum, where optical fibres are most transparent. . Laser diodes are also found in CD, DVD and Blu-ray drives, where they read and write data from the disc. LEDs are inexpensive compared to laser diodes and so are used in some multimode fibre systems. However, they have a number of disadvantages: they are lower in power and emit over a range of wavelengths, leading to dispersion. Importantly, LEDs emit a relatively broad cone of radiation, whereas a laser diode emits a strongly aligned beam. Because of this, the laser diode is much more efficient at transferring its energy to the fibre. The beam of light from the LED or laser diode is modulated to convey a useful signal: either varied in intensity, or simply switched on and off. The data rate that can be obtained from the transmitter depends on how fast the beam can be modulated. With both LEDs and laser diodes, the beam can be modulated directly by varying the electrical power supplied to them. Again, the laser diode has an advantage over the LED in the speed at which it can switch. However, this method of modulation can result in transient changes in frequency (known as ‘chirp’) when switching. For the highest data rates, a technique is used in which the task of modulation is separated from supplying the light. In this technique a laser runs continuously, and the beam is stopped or allowed through by an external modulator, a device that changes transparency in response to an electrical signal. At the other end of the fibre, a detector converts the light signal back into an electrical signal. The type of detector commonly used is called a photodiode. It provides a current output that varies with the intensity of the light it receives.

3.5.2 Optical amplifiers You have already seen that even though optical fibres are very transparent, they attenuate markedly over long distances. For example, the best-case attenuation in Table 1.1 is 0.35 dB km−1, which over a 300 km link would amount to 105 dB, a huge drop in power.

One way of increasing the range of an optical-fibre link is to use a repeater or regenerator. These are devices that counteract the effects of attenuation by restoring an optical signal to its original form. The optical signal is converted back to an electrical signal,

which is then processed electronically and retransmitted optically. Although definitions vary, repeaters and regenerators are often distinguished by the extent of the processing that is carried out, the term ‘repeater’ tending to include simpler devices than ‘regenerator’. A repeater amplifies the signal to bring it back to its original amplitude, but at the same time it may also amplify any noise that is mixed with the signal. A regenerator does further processing, so that the degraded received pulse you saw in Figure 1.21 would be reshaped and retimed as well as being restored to its original amplitude. Thus the regenerated pulse is a copy of the original transmitted pulse with any noise removed, provided the signal has not deteriorated too far. Recall from Section 2.1 that an essential benefit of digital transmission as compared to analogue is that, as long as it is possible to read a signal correctly as a digital sequence, the signal can be regenerated exactly. If the signal is degraded too far – for example, if in a binary signal a 1 is read as a 0 – then regeneration can fail. However, it will be successful if the signal is not too degraded by attenuation and spreading, or contaminated with noise. The relationship between noise in the signal and the possibility of errors will be explored in Part 3 of this block. In principle, any distance can be covered using a chain of regenerators, but there are some practical disadvantages: these devices have to be powered and maintained, and many of them may be needed to cover long distances. This is a particular problem for international cables, which often run under the ocean, making power provision and maintenance very difficult. Optical amplifiers have been developed as a better solution for long-haul links. They amplify the optical signal directly, without converting it back to an electrical signal. A number of techniques are used, but all share the idea of a laser that allows energy to be ‘pumped’ into the atoms of a material and then released later when ‘stimulated’ by radiation, thus creating more radiation of the same wavelength. In an optical amplifier, a weak pulse triggers this stimulated emission process and becomes a stronger pulse. Figure 1.23 shows one type of optical amplifier, the erbium-doped fibre amplifier (EDFA). It consists of a section of fibre that contains a small proportion of erbium atoms. Energy from a pump, itself a laser, is combined with the incoming signal in a device known as a coupler. Stimulated emission takes place within the doped fibre and amplifies the signal. Note that the amplified signal has the same wavelength as the incoming signal, but the pump uses a shorter wavelength, so the signal can be distinguished from the energy supplied by the pump.

Another type of optical amplifier is the Raman amplifier, which differs in the physical mechanism involved, but like the EDFA has a pump laser with a different wavelength from the signal. While some Raman amplifiers use a relatively short length of fibre (a kilometre or so), a distributed Raman amplifier has the advantage of amplifying a signal along the whole of its transmission path, which can be many kilometres. Pumping can be done at either end

of the fibre, or both, but is usually done backwards from the receiver end. One advantage of this is that amplification is then greatest at the receiver end, where it is most needed.

Activity 1.10

Optical amplifiers mitigate the effects of attenuation by boosting the signal, but do they solve the problem of dispersion? Discussion No. Dispersion results from light of different wavelengths travelling at different speeds, spreading out a pulse as it travels along the fibre. Simply increasing the size of the signal will not remedy this – unlike a repeater, which completely regenerates the signal.

A semiconductor optical amplifier (SOA) works by similar principles to a semiconductor laser, so can be manufactured using the same types of techniques. SOAs have the advantages of compactness and low cost, but they do have some disadvantages compared to other types of optical amplifier, such as limited power (at least at the time of writing).

3.6

Optical fibre in a network

You have now considered the various components of a single optical-fibre link. However, combining optical fibres into a network requires further components, and it is some of these that I will look at next....