In laser physics, the laser gain medium is the medium (usually in the form of a beam of light) that enhances the power of light. In lasers, this medium is required to compensate for the loss of the resonant cavity and is often referred to as the high energy laser medium. Gain media can also be applied to optical fiber amplifiers. Gain refers to the level of amplification. Since the gain medium increases the power of the amplified beam, the medium itself needs to receive the energy, i.e., through the pumping process, which generally progresses to either existing (electric pumping) or input light wave (optical pumping), and the pump wavelength is smaller than that of the signal light. Type of laser gain medium There are many types of gain media. The usual is: 1. Some direct bandgap semiconductors, such as GaAs, AlGaAs, and InGaAs, are usually pumped by electric currents in the form of quantum Wells (see semiconductor lasers). 2. Laser crystal or glass, such as Nd: YAG(neodymium-doped Yttrium aluminum garnet, see Yttrium aluminum garnet laser), Yb: YAG(Ytterbium-doped garnet laser), Yb: glass, Er: YAG(Erbium-doped YAG) or titanium sapphire, in solid sheet form (see doped lasers) or optical glass fiber (fiber lasers, fiber amplifiers). These crystals or glasses are doped with laser-active ions (mainly trivalent rare earth ions, which sometimes transfer metal ions) and pumped with light waves. Lasers using these media are often described as doped insulator lasers 3. Rare earth ions are usually added to ceramic gain media Laser dyes, usually liquid solutions, are used in color lasers Gas lasers utilize multiple gases or mixtures of gases, usually pumped by discharge tools (such as CO2 and excimer lasers) 4. The unique gain retarder consists of a chemical gain retarder (which converts chemical energy into light), a nuclear pump retarder, and an oscillator in a completely free electron laser (which transfers energy from a fast electron beam to a beam).