Meaning of the gain medium
In laser physics, the laser gain medium is the medium (typically in the form of a beam) that enhances the power of light. In a laser, the medium needs to make up for the loss of the resonator as well as is typically referred to as the active laser medium. The gain medium can additionally use it to fiber optics amplifiers. Gain describes the level of amplification.
Considering that the gain medium enhances the power of the magnified light beam, the medium itself needs to obtain the energy, that is, with a pumping process, usually designed to either existing (electric pumping) or input light wave (optical pumping), and the pump wavelength is smaller sized than that of the signal light.
Kinds of laser gain medium
There are lots of type of gain media. The usual ones are the following:
- Some direct bandgap semiconductors, such as GaAs, AlGaAs, and also InGaAs, are generally pumped by an electric present in the form of quantum Wells (see semiconductor lasers).
- Laser crystals or glasses, such as Nd: YAG ( neodymium-doped yttrium aluminum garnet), Yb: YAG( Ytterbium aluminum garnet laser), Yb: glass, Er: YAG (Erbium doped YAG), or titanium sapphire, in strong sheet form (see volume laser) or optical glass fiber (fiber laser, fiber amplifier). These crystals or glasses are doped with laser-active ions (primarily trivalent rare-earth ions, often transition steel ions) and pumped with light waves. Lasers making use of these media are often referred to as doped insulator lasers.
- Ceramic gain media are generally likewise doped with rare earth aspect ions.
- A laser dye, normally a fluid option, is utilized in dye lasers.
- Gas lasers make use of a number of gases or a combination of gases, generally pumped by a discharge gadget (such as carbon dioxide and also excimer lasers).
- Unique gain arbitrators consist of chemical gain arbitrators (which convert chemical energy into light), nuclear pumping moderators, and oscillators in cost-free electron lasers (which transfer power from a rapid electron beam into a beam).
Crucial physical results
In most cases, the physical basis of the boosting procedure is boosted radiation, in which the event photon causes more photon radiation and the fired up laser-active ion first transitions to a somewhat lower energy ecstatic state. There is a difference in between the four-level gain medium as well as the three-level gain medium
A boosting procedure that takes place less often is stimulated Raman scattering, which entails changing several of the greater power pumped photons right into lower energy photons and phonons (pertaining to lattice vibrations). If the occurrence light power is extremely high, the gain will certainly lower after the gain medium gets to gain saturation. The amplifier can not include an arbitrarily large amount of power to the incident beam of light at a limited pump power. In laser amplifiers, the number of ions in the top degree reduces at saturation due to promoted radiation.
The gain medium has a thermal effect since part of the pump light power is converted into warm. The resulting temperature slope and also mechanical stress will cause the prism effect as well as misshape the enhanced light beam. These results can ruin the light beam high quality of the laser, decrease its effectiveness, and also damage the gain medium (thermal fracturing).
Related physical buildings of laser gain medium
In laser applications, the physical buildings of lots of gain media are very important. It mostly includes:
- In the laser shift procedure requiring wavelength region, the very best peak gain occurs in this region.
- The substratum has a high level of transparency in the functioning wavelength region.
- Excellent pump light, efficient pump absorption.
- Appropriate upper-level life time: enough time for Q-switched applications and short enough for rapidly regulated power.
- High quantum efficiency is obtained from common quenching effects, fired up state absorption, and also similar procedures or advantageous results such as multiphoton shifts or energy transfers.
- Perfect four-level behavior because quasi-three-level habits introduces some other extra constraints.
- High toughness and lengthy life, chemical security.
- For solid-state gain media: Base media need to be of excellent optical high quality, can be cut or polished of really top quality, allow high focus of laser-active ions to be doped without forming collections, have great chemical stability, have an excellent thermal conductivity and low thermo-optical coefficient, resistance to mechanical anxiety, optical isotropy is usually called for, However often birefringence and also gain associated with polarization is required.
- Reduced pump power limit at a high gain: The product of radiation cross-section and also upper-level lifetime is larger.
- The beam high quality of the pump light source is low: high pump absorption is required.
- Wavelength adjusting: Needs large gain bandwidth
- Ultrashort pulse generation: gain spectrum is large and also level; Suitable dispersion and nonlinearity.
- Passive mode-locked lasers without Q-switching security: completely large laser cross-sections.
- High power pulse amplification (favorable responses amplifier): Result of high optical damages limit and not too high saturation on gain.
Note
Keep in mind that there are scenarios where contradictory needs are called for. For example, very low quantum defects are inappropriate with a four-level system. A huge gain bandwidth corresponds to a smaller laser cross-section than the perfect instance, as well as the quantum defect is not so tiny. The disorder in the solid-state gain medium enhances the gain transmission capacity as well as reduces thermal conductivity.
A brief pump absorption length is helpful but intensifies the thermal result.
The needs for the gain medium differ from case to instance. Consequently, many gain media are very essential for applications, as well as it is essential to select the right gain media when maximizing the layout of the laser.
Read more: Q Switch Crystals
