Optical Networks - _5_5 Optical Amplifiers_60

Optical Networks - _5_5 Optical Amplifiers_60 - 5.5 Optical...

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5.5 Optical Amplifiers 295 Table 5.2 Typical sensitivities of different types of receivers in the 1.55 μ m wavelength band. These receivers also operate in the 1.3 μ m band, but the sensitivity may not be as good at 1.3 μ m. Bit Rate Type Sensitivity Overload Parameter 155 Mb/s pinFET 36 dBm 7dBm 622 Mb/s pinFET 32 dBm 2 . 5 Gb/s pinFET 23 dBm 3dBm 2 . 5 Gb/s APD 34 dBm 8dBm 10 Gb/s pinFET 18 dBm 1dBm 10 Gb/s APD 24 dBm 6dBm 40 Gb/s pinFET 7 dBm 3 dBm and above 2.5 Gb/s. However, a pinFET receiver with an optical preamplifier has a sensitivity that is comparable to an APD receiver. The overload parameter defines the dynamic range of the receiver and can be as high as 0 dBm for 2.5 Gb/s receivers, regardless of the specific receiver type. 5.5 Optical Amplifiers Optical amplifiers have become an essential component in transmission systems and networks to compensate for system losses. The most common optical amplifier today is the erbium-doped fiber amplifier (EDFA) operating in the C-band. L-band EDFAs and Raman amplifiers are also used. EDFAs are used in almost all amplified WDM systems, whereas Raman amplifiers are used in addition to EDFAs in many ultra- long-haul systems. These amplifiers are described in Section 3.4. In this section, we will focus mainly on EDFAs. The EDFA has a gain bandwidth of about 35 nm in the 1.55 μ m wavelength region. The great advantage of EDFAs is that they are capable of simultaneously amplifying many WDM channels. EDFAs spawned a new generation of transmission systems, and almost all optical fiber transmission systems installed over the last few years use EDFAs instead of repeaters. The newer L-band EDFAs are being installed today to increase the available bandwidth, and hence the number of wavelengths, in a single fiber. Amplifiers are used in three different configurations, as shown in Figure 5.2. An optical preamplifier is used just in front of a receiver to improve its sensitivity. A power amplifier is used after a transmitter to increase the output power. A line amplifier is typically used in the middle of the link to compensate for link losses. The
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296 Transmission System Engineering design of the amplifier depends on the configuration. A power amplifier is designed to provide the maximum possible output power. A preamplifier is designed to provide high gain and the highest possible sensitivity, that is, the least amount of additional noise. A line amplifier is designed to provide a combination of all of these. Unfortunately, the amplifier is not a perfect device. There are several major imperfections that system designers need to worry about when using amplifiers in a system. First, an amplifier introduces noise, in addition to providing gain. Second, the gain of the amplifier depends on the total input power. For high input powers, the EDFA tends to saturate and the gain drops. This can cause undesirable power transients in networks. Finally, although EDFAs are a particularly attractive choice for WDM systems, their gain is not flat over the entire passband. Thus some channels
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This note was uploaded on 01/15/2011 for the course ECE 6543 taught by Professor Boussert during the Spring '09 term at Georgia Tech.

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Optical Networks - _5_5 Optical Amplifiers_60 - 5.5 Optical...

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