Optical amplifiers are a a technology used to amplify light signals, also called optical signals. An optical amplifier amplifies an optical signal directly, without converting it into some other intermediate form. This distinguishes it from another type of device, called a regenerator or repeater, that boosts an incoming optical signal by converting it into electricity and then generating a new optical signal. Optical amplifier technology boosts optical signals in devices such as lasers and fiber-optic cables and is important for applications such as telecommunications.
The most common amplification method, which exploits the same phenomenon used in optically pumped lasers, is stimulated emission. Every optical amplifier using this method contains a physical medium that can amplify incoming light, called a gain medium. When the gain medium receives incoming optical energy, a process referred to as pumping the gain medium, the incoming photons of light are absorbed temporarily and excite some of the medium's electrons to higher energy levels. These electrons then quickly drop back to their original level. When they do, the energy they lose is released in the form of photons, amplifying the original signal. If very high amplification is needed, multiple amplifiers can be chained together so that the optical signal can pass through each of them in sequence.
Many different substances can be used as gain media in an optical amplifier, depending on the optical power, wavelength, and other characteristics desired for a particular device. The most common gain medium for optical amplifiers is silicon dioxide doped with small amounts of rare earth elements such as erbium and ytterbium. Other gain media, such as semiconductors or yttrium aluminum garnet crystals, can also be used.
A type of amplifier called a Raman amplifier boosts optical signals by exploiting a phenomenon called Raman scattering. When incoming light comes into contact with matter, photons that are not absorbed are scattered in different directions. The great majority of them retain the same wavelength and frequency as before. A very small percentage of them, which have exchanged energy with the particles of matter due to the vibrational energy of those particles, do not.
Exploiting this for optical amplification involves two light sources, a high-frequency pump laser and a lower-frequency, higher-wavelength light source from an optical fiber that actually carries the signal. The gain medium in a Raman amplifier is pumped with high-frequency light from a pump laser, exciting the particles of the medium and increasing their vibrational energy to stimulate Raman scattering. As the higher-wavelength photons of the optical signal pass through the medium, the signal gains energy at the expense of the pump beam and is amplified. This method is used primarily in telecommunications technology to boost long-distance signals sent through fiber-optic cables.