Basically, an optical amplifier solves the traditional distance problem as in any long-distance telecommunication system such as a trans-Atlantic link. Due to optical signals travelling through the fiber, the signals become weaker in power. The farther you go, the signals become weaker till it gets too weak to be reliably detected.

With the help of fiber amplifiers fiber optic communication systems solve this problem along the way.

An optical fiber amplifier is an optical device purely. It never converts the incoming optical signal to an electronic signal at all. You can even call it an in-line laser. Dozens of optical channels can be simultaneously amplified by an optical fiber amplifier as they separately do not convert each channel into electronic signals. The optical switch is also useful.

An optical fiber amplifier is a section of optical fiber that with a rare-earth element such as erbium or praseodymium is doped.

By high power light (pump laser) the atoms of erbium or praseodymium can be pumped into an excited state. From the high power level excited state into low power level stable state the erbium atoms will jump, and at the same time, they release their energy in the form of emitted light photons. Same as the input optical signal, the emitted photons have the same phase and wavelength, thus amplify the optical signal.

For an optical fiber communication system, this is a very convenient form of the amplifier as it is an in-line amplifier and it eliminates the requirement to do the optical-electrical and electrical-optical conversion process.

For the operation of fiber amplifiers, the pump laser wavelengths and the corresponding optical signal wavelengths are key parameters. These wavelengths totally rely on the type of rare-earth element doped in the fiber and also on the composition of the glass in the fiber.

Gain is an important term in understanding fiber amplifiers and fiber adapter. The amplification per unit length of fiber is measured by gain. Gain usually depends on both the materials and the operating conditions, and for all materials, it varies with wavelength.