Each laser is has three basic elements, it needs to generate laser light:
On this page we will discuss the various gain or lasing media of lasers:
Within the context of laser physics, the gain medium (aka optical medium or active laser medium) is the material and part of a laser in which the optical gain (light amplification) occurs. As already described, this gain is usually generated by a process of stimulated emission. The gain medium is a required element of a laser to compensate for the losses occurring in the resonator cavity. The gain medium utilizes the energy from the pumping source to add energy to the amplified light. There is a variety of different gain media available for lasers. The type of gain medium also defines the type of laser. Examples of such gain media include:
- Crystals, such as ruby or sapphire
- Crystals which are “doped” with rare earth ions, such as Nd:YAG (neodymium-doped yttrium aluminum garnet) lasers or Yb:YAG (ytterbium-doped yttrium aluminum garnet) lasers
- Glasses, such as Yb:glass (silicate glasses doped with laser-active ions, in this case ytterbium)
- Ceramics which are usually also doped with rare-earth ions
- Laser dyes, which are usually suspended in a liquid form and usually used in dye lasers
- Gases or gas mixtures, which are usually pumped with electrical discharges. Examples are CO2 lasers or Excimer lasers.
- Semiconductors such as GaAs, GaAlAs, or InGaAs, which are typically pumped with electrical currents
The optical amplification in the gain medium of a laser is the product of stimulated emission, where the re-circulating light within the optical cavity induces transitions of laser-active ions from an “excited” state to a lower energy state, during which a photon is emitted. Gain media can be roughly divided into three-level systems and four-level systems.
In a three-level system, the laser transition ends in the ground state for the individual electrons. The laser transition in the un-pumped gain medium exhibits strong absorption of energy, so that no net laser gain results can be achieved. Only by pumping more than half of the atoms into the upper laser level (population inversion), net laser gain results can be achieved. The pump energy (threshold pump energy) required to achieve this population inversion in a three-level gain medium is quite high, however. Additionally, population inversion can only be achieved by pumping the atoms into an energy level higher than the upper laser level, where the electrons will then rapidly transfer back to the upper laser level. This avoids stimulated emission caused by the pumping action. Theodore Maiman’s ruby laser is a good example of a three-level laser medium, as are certain dye lasers.
A
four-level laser medium will exhibit a lower threshold pump energy than a three-level medium will. In this system the lower laser level is well above the ground state and is usually quickly depopulated after the laser transition. This avoids population build-up on this level and thus reduces the chances of re-absorption of laser radiation. Another consequence of this system is that there is no absorption in the un-pumped state and that the gain usually increases proportionally to the absorbed pump energy. Popular examples of lasers with a four-level gain medium is the neodymium-doped gain media in a Nd:YAG laser and the CO2 laser.
