A laser generally emits light through a type of amplification. Atomic photons can be manipulated to concentrate the energy of light into a small area, or direct it over large distances. The method in which energy is directed into the light beam to amplify it is called pumping, and a laser pump typically stimulates particles into a higher energy state. Each state is often referred to as a level; a three-level laser, for example, can direct light in three different stages before it is emitted. Such a configuration is often used to ensure that the beam is at the desired power level.
The energy used in a laser typically excites the photon particles, but can also have the opposite reaction. In a three-level laser, the light is transferred from a resting state to a high energy level; at the second stage this energy decays, yet no particles are emitted as radiation. Generally, no more than half of the particles have enough energy in this process. Engineers have designed systems that can get them to be more energetic than necessary. The final stage is the activation of the laser beam, and a fast drop in particle energy typically occurs as the particles are emitted.
After the beam is fired, then the energy level normally decreases to its lowest state. The ground state of a three-level laser is therefore called the lower laser level, or E1, which is also usually the system state after emission occurs. Energy is often directed to the E3 level directly from E1, while the upper laser level, called E2, typically occurs just before the beam is activated.
The use of just half of the electrons generally requires that more power be added to the three-level laser. Much of the energy transitions to a lower state without emitting light or other radiation, which is generally enabled by the transport of energy by particles called lattice photons. This type of laser, therefore, is usually not as efficient as some other varieties.
Energy states at each level last for fractions of a second, despite the substantial gains and losses that can occur. Another phenomenon that often occurs in a three-level laser is a population inversion, in which the quantity of particles in a higher energy state is larger than those in lower states. Amplification of light is typically the result. These energy states, however, are reached in just a portion of the particles in this type of laser, so energy efficiency sometimes needs to be addressed.