ABSTRACT
An erbium-doped fiber amplifier (EDFA) has five main optical components: an erbium-doped fiber, a laser diode (LD) to pump the fiber, a wavelength-dependent fiber coupler, a polarization-independent optical isolator, and an optical bandpass filter. The LDs operate at wavelengths which include 1.48, 0.98, and 0.82 μm. Fabry–Perot multimode LDs, namely InGaAsP and InGaAs type utilizing a strained superlattice, and GaAlAs type, operate these wavelengths, respectively. The polarization-insensitive optical isolator plays a very important role in suppressing laser oscillation and amplified spontaneous emission. The advantage of using a polarization-insensitive isolator is that uniform gain can be obtained for an arbitrary input polarization. An EDFA operates as a three-level optical amplifier and, therefore, has different gain saturation characteristics from those of a four-level amplifier. The chapter describes a simple rate equation analysis to characterize the EDFA, and several experimental results related to gain saturation power, signal power, and their dependence on the pump wavelengths, for comparison with the analysis.
