Optical amplifiers amplify incident light through stimulated emission, the same mechanism as used with lasers. These amplifiers are the same as lasers without feedback. Optical gain is achieved when the amplifier is pumped either electrically or optically to realize population inversion.
There are semiconductor laser amplifiers, Raman amplifiers, Brillouin amplifiers, and erbium-doped fiber amplifiers (EDFAs). Certainly the EDFAs show the widest acceptance. One reason is that they operate near the 1.55-^m wavelength region, where fiber loss is at a minimum. Reference 11 states that it is possible to achieve high amplifier gains in the range of 30 dB to 40 dB with only a few milliwatts of pump power when EDFAs are pumped by using 0.980-^m or 1.480-^m semiconductor lasers. Figure 9.31 is a block diagram of a low-noise EDFA.
In Figure 9.31, optical pumping is provided by fiber pigtailed19 semiconductor lasers with typically 100 mW of power. Low-loss wavelength division multiplexers efficiently combine pump and signal powers and can also be used to provide a pump bypass around the internal isolator. The EDFA has an input stage that is codirectionally pumped and an output stage that is counterdirectionally pumped. Such multistage EDFA designs have simultaneously achieved a low-noise figure of 3.1 dB and a high gain of 54 dB (Refs. 10, 11).
Er3* -doped fiber
Er3* -doped fiber
980-nm or 1480-nm pump
High gain 20-50 dB
Figure 9.31 An EDFA block diagram. (From Ref. 9. Courtesy of Hewlett-Packard.)
19A pigtail is a length of fiber factory connected to an active component.
The loops of fiber should be noted in Figure 9.32. These are lengths of fiber with a dopant. The length of erbium-doped fiber required for a particular amplifier application depends on the available pump power, the doping concentration, the design topology, and gain and noise requirements.
EDFAs are often installed directly after a semiconductor laser source (transmitter) and/or directly before the PIN or APD receiver at the distant end. Figure 9.32 illustrates this concept. With the implementation of EDFAs, the length of a fiber-optic link can be extended without repeaters an additional 100-250 km, or it can extend the distance between repeaters a similar amount.
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