7.6 OPTICAL AMPLIFIERS IN FIBER LINKS
Significant advances in fiber technology has occurred because of the development of on-line optical amplifiers. As stated in Section 1.8, optical amplifiers provide field intensity amplification that directly increases the power level of the fiber signal. This permits the amplifier to help overcome fiber-propagation loss, thereby increasing power levels at the detector input. At the same time, optical amplifiers produce spontaneous emission noise that adds to the detector input signal, and it is not immediately obvious how the overall link is affected.
Optical amplifiers can be used in fibers in two basic arrangements, as shown in Figure 7.10. In Figure 7.10a, the amplifier is placed at the fiber output, acting as a receiver preamplifier prior to photodetection. In Figure 7.10b, the amplifier is used at the fiber input to amplify the source power inserted into the fiber. If the amplifier was an ideal noiseless device, with gain values independent of input power levels, the systems would be identical in terms of source power delivered to the detector. The nonideal gain, and the addition of the amplifier spontaneous emission noise, means that each system must be separately examined.
Consider the link in Figure 7.10a. Let Pf be the fiber input power, Lf the fiber propagation loss, Pr the fiber output power, and assume the optical amplifier has gain G and spontaneous noise coefficient nsp. This means the amplifier inserts at its output the spontaneous ...
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