Optical amplifiers are devices that amplifies light directly in the optical domain without the need to convert to electrical singal. Optical amplifiers are at the heart of long haul fiber optics communication. Often, people recongize optical amplifiers as Lasers without cavity/resonator or feedback.
EDFA is the most popular fiber based optical amplifier. The gain medium is erbium and it is a backbone of the modern OFC. EDFA is now a matured technology and based on the gain requirement, one can use different pumping schemes. We have done several works in the EDFA aiming at high gain and improved noise figure.
These are all optical amplifiers made out of a combination of either different fiber types, or different active materials. The purpose of these amplifiers are to add more functionalities and improved performance.