Fiber Optic Communications: Fundamentals and Applications

Chapter 6Optical Amplifiers

6.1 Introduction

The optical amplifier may be considered as a laser without feedback, or one in which the feedback is suppressed. In the 1980s, optical amplifiers were not commercially available and long-haul fiber-optic communication systems used electrical amplifiers to compensate for the fiber loss. The optical signal was first converted to the electrical signal (O/E conversion) using a photodetector and then converted back to the optical domain (E/O conversion) after amplification in the electrical domain. However, this type of optoelectronic regenerator is expensive for multi-channel optical communication systems. With the advent of optical amplifiers, the optical signal can be amplified directly without having to do O/E and E/O conversion.

There are different physical mechanisms that can be used to amplify the optical signal. In semiconductor optical amplifiers (SOAs), an electrical pump (power supply) is used to achieve population inversion. In the presence of signal photons that have energy close to the band gap, electrons are stimulated to recombine with holes and, thereby, emit photons due to stimulated emission. Thus, the input signal photons are amplified. In EDFAs, an optical pump is used to achieve population inversion. In the presence of signal photons, the erbium ions in the excited state emit light by stimulated emission and make transitions to the ground state. In Raman amplifiers, an optical pump gives up its energy to create a ...