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Optical Networks - _3_6 Detectors_40

Optical Networks - _3_6 Detectors_40 - 198 Components...

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198 Components Figure 3.61 Block diagram of a receiver in a digital communication system. 3.6 Detectors A receiver converts an optical signal into a usable electrical signal. Figure 3.61 shows the different components within a receiver. The photodetector generates an electrical current proportional to the incident optical power. The front-end amplifier increases the power of the generated electrical signal to a usable level. In digital communication systems, the front-end amplifier is followed by a decision circuit that estimates the data from the output of the front-end amplifier. The design of this decision circuit depends on the modulation scheme used to transmit the data and will be discussed in Section 4.4. An optical amplifier may be optionally placed before the photodetector to act as a preamplifier. The performance of optically preamplified receivers will be discussed in Chapter 4. This section covers photodetectors and front-end amplifiers. 3.6.1 Photodetectors The basic principle of photodetection is illustrated in Figure 3.62. Photodetectors are made of semiconductor materials. Photons incident on a semiconductor are absorbed by electrons in the valence band. As a result, these electrons acquire higher energy and are excited into the conduction band, leaving behind a hole in the valence band. When an external voltage is applied to the semiconductor, these electron-hole pairs give rise to an electrical current, termed the photocurrent. It is a principle of quantum mechanics that each electron can absorb only one photon to transit between energy levels. Thus the energy of the incident photon must be at least equal to the bandgap energy in order for a photocurrent to be generated. This is also illustrated in Figure 3.62. This gives us the following constraint on the frequency f c or the wavelength λ at which a semiconductor material with bandgap E g can be used as a photodetector: hf c = hc λ eE g . (3.19) Here, c is the velocity of light, and e is the electronic charge. The largest value of λ for which (3.19) is satisfied is called the cutoff wavelength and is denoted by λ cutoff . Table 3.2 lists the bandgap energies and the corresponding
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3.6 Detectors 199 Valence band Conduction band Electron Hole Photon E g Electron energy (eV) h e n/ Figure 3.62 The basic principle of photodetection using a semiconductor. Incident pho- tons are absorbed by electrons in the valence band, creating a free or mobile electron-hole pair. This electron-hole pair gives rise to a photocurrent when an external voltage is applied. cutoff wavelengths for a number of semiconductor materials. We see from this table that the well-known semiconductors silicon (Si) and gallium arsenide (GaAs) cannot be used as photodetectors in the 1.3 and 1.55 μ m bands. Although germanium (Ge) can be used to make photodetectors in both these bands, it has some disadvantages that reduce its effectiveness for this purpose. The new compounds indium gallium arsenide (InGaAs) and indium gallium arsenide phosphide (InGaAsP) are commonly used to make photodetectors in the 1.3 and 1.55
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