Definition of the gain medium
In laser physics, the laser gain medium is the medium (generally in the form of a beam of light) that magnifies the power of light. In a laser, the medium requires to make up for the loss of the resonator and also is typically described as the active laser medium. The gain medium can also apply it to optical fiber amplifiers. Gain describes the degree of amplification.
Since the gain medium enhances the energy of the magnified light beam, the medium itself requires to obtain the power, that is, with a pumping procedure, usually created to either present (electrical pumping) or input light wave (optical pumping), and the pump wavelength is smaller than that of the signal light.
Types of laser gain media
There are several kinds of gain media. The typical ones are the following:
- Some direct bandgap semiconductors, such as GaAs, AlGaAs, as well as InGaAs, are typically pumped by an electrical present in the form of quantum Wells (see semiconductor lasers).
- Laser crystals or glasses, such as Nd: YAG crystal (neodymium-doped yttrium aluminum garnet, see yttrium aluminum garnet laser), Yb: YAG (Ytterbium aluminum garnet laser), Er glass, Er: YAG (Erbium doped YAG), or titanium sapphire, in solid sheet form (see volume laser) or optical glass fiber (fiber laser, fiber amplifier). These crystals or glasses are doped with laser-active ions (mostly trivalent rare-earth ions, often transition steel ions) and pumped with light waves. Lasers utilizing these media are typically referred to as doped insulator lasers.
- Ceramic gain media are usually also doped with rare earth aspect ions.
- A laser dye, generally a fluid service, is made use of in color lasers.
- Gas lasers make use of a number of gases or a blend of gases, typically pumped by a discharge gadget (such as CO2 as well as excimer lasers).
- Unique gain mediators consist of chemical gain mediators (which transform chemical power right into light), nuclear pumping arbitrators, as well as oscillators in complimentary electron lasers (which transfer energy from a rapid electron beam right into a beam).
Crucial physical impacts
Most of the times, the physical basis of the amplification process is promoted radiation, in which the incident photon causes even more photon radiation and the thrilled laser-active ion very first changes to a somewhat lower power thrilled state. There is a distinction in between the four-level gain medium as well as the three-level gain medium.
An amplification procedure that occurs much less often is boosted Raman spreading, which involves changing some of the greater energy pumped photons into reduced energy photons and also phonons (pertaining to latticework vibrations). If the occurrence light power is really high, the gain will certainly reduce after the gain medium gets to gain saturation. The amplifier can not include an arbitrarily huge amount of power to the occurrence beam of light at a limited pump power. In laser amplifiers, the number of ions in the upper degree reduces at saturation as a result of stimulated radiation.
The gain medium has a thermal impact due to the fact that part of the pump light power is converted into warm. The resulting temperature gradient as well as mechanical stress and anxiety will cause the prism effect and also distort the enhanced light beam. These impacts can destroy the light beam quality of the laser, decrease its effectiveness, as well as even damage the gain medium (thermal breaking).
Associated physical homes of laser gain medium
In laser applications, the physical residential or commercial properties of lots of gain media are essential. It generally includes:
- In the laser shift process needing wavelength area, the best optimal gain takes place in this region.
- The substratum has a high degree of openness in the functioning wavelength area.
- Great pump source of light, efficient pump absorption.
- Suitable high-ranking life time: enough time for Q-switched applications and also short enough for promptly regulated power.
- High quantum effectiveness is acquired from common quenching effects, excited state absorption, and comparable procedures or helpful effects such as multiphoton shifts or power transfers.
- Suitable four-level habits due to the fact that quasi-three-level actions presents some other additional constraints.
- High toughness and long life, chemical security.
- For solid-state gain media: Base media need to be of great optical top quality, can be cut or brightened of extremely premium quality (suitable solidity), enable high concentration of laser-active ions to be doped without forming collections, have good chemical security, have a good thermal conductivity and also reduced thermo-optical coefficient (weak thermal prism effect at high power procedure), resistance to mechanical stress and anxiety, optical isotropy is generally called for, Yet sometimes birefringence (reducing the effect of thermal depolarization) as well as gain connected with polarization is needed (see the polarization of laser radiation).
- Low pump power limit at a high gain: The product of radiation cross-section and high-ranking life time is larger.
- The light beam quality of the pump light is low: high pump absorption is called for.
- Wavelength adjusting: Requires huge gain transmission capacity
- Ultrashort pulse generation: gain range is broad and also level; Ideal diffusion and nonlinearity.
- Passive mode-locked lasers without Q-switching stability: adequately big laser cross-sections.
- High power pulse boosting (positive comments amplifier): Result of high optical damage threshold as well as not too high saturation on gain.
Keep in mind that there are situations where contradictory needs are required. For example, extremely reduced quantum defects are inappropriate with a four-level system. A big gain data transfer corresponds to a smaller sized laser cross-section than the ideal situation, and the quantum problem is not so little. The condition in the solid-state gain medium enhances the gain bandwidth as well as decreases thermal conductivity.
