An optical fiber amplifier is a key component for enabling efficient transmission of wavelength-division multiplexed (WDM) signals over long distances. Even though many alternative technologies were available, erbium-doped fiber amplifiers won the race during the early 1990s and became a standard component for long-haul optical telecommunications systems. However, owing to the recent success in producing low-cost, high-power, semiconductor lasers operating near 1450 nm, the Raman amplifier technology has also gained prominence in the deployment of modern light-wave systems. Moreover, because of the push for integrated optoelectronic circuits, semiconductor optical amplifiers, rare-earth-doped planar waveguide amplifiers, and silicon optical amplifiers are also gaining much interest these days.

Interestingly, even though completely unrelated to the conventional optical communications technology, optical amplifiers are also finding applications in biomedical technology either as power boosters or signal-processing elements. Light is increasingly used as a tool for stretching, rotating, moving, or imaging cells in biological media. The so-called lab-on-chip devices are likely to integrate elements that are both acoustically and optically active, or use optical excitation for sensing and calibrating tasks. Most importantly, these new chips will have optical elements that can be broadly used for processing different forms of signals.

There are many excellent books that cover selective aspects of active optical devices including optical amplifiers. This book is not intended to replace these books but to complement them.