Meaning of the gain medium
In laser physics, the laser gain medium is the medium (usually in the form of a beam) that magnifies the power of light. In a laser, the medium requires to compensate for the loss of the resonator and also is frequently referred to as the active laser medium. The gain medium can also use it to optical fiber amplifiers. Gain refers to the degree of amplification.
Considering that the gain medium boosts the energy of the enhanced light beam, the medium itself needs to get the power, that is, via a pumping process, normally developed to either current (electrical pumping) or input light wave (optical pumping), and also the pump wavelength is smaller than that of the signal light.
Types of laser gain media
There are many sort of gain media. The usual ones are the following:
- Some straight bandgap semiconductors, such as GaAs, AlGaAs, and also InGaAs, are generally pumped by an electrical existing in the form of quantum Wells (see semiconductor lasers).
- Laser crystals or glasses, such as Nd: YAG( neodymium-doped yttrium aluminum garnet, see yttrium aluminum garnet laser), 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 (mainly trivalent rare-earth ions, sometimes transition metal ions) and pumped with light waves. Lasers using these media are usually referred to as drugged insulator lasers.
- Ceramic gain media are generally also doped with rare earth aspect ions.
- A laser dye, usually a fluid solution, is used in dye lasers.
- Gas lasers utilize several gases or a blend of gases, generally pumped by a discharge tool (such as carbon dioxide and excimer lasers).
- Unique gain arbitrators consist of chemical gain mediators (which transform chemical energy into light), nuclear pumping mediators, and oscillators in complimentary electron lasers (which transfer power from a rapid electron light beam into a beam).
Erbium-doped glass (mainly silicate and phosphate) exists in large quantities in solid-state lasers, fiber lasers and fiber amplifiers. The most common laser transition is from the 4I13/2 state to the ground state 4I15/2 (as shown in Figure 1). Depending on the composition of the glass, the wavelength of the laser produced by the transition is between 1.53um and 1.6um. Because the transition is a quasi-three-level system, Erbium-doped lasers and amplifiers need to provide a relatively significant excitation to erbium ions, so ERbium-doped lasers usually have a higher pumping threshold power.
Essential physical effects
In most cases, the physical basis of the amplification process is stimulated radiation, in which the incident photon triggers more photon radiation and also the ecstatic laser-active ion first shifts to a somewhat lower energy fired up state. There is a distinction in between the four-level gain medium and the three-level gain medium
A boosting process that happens much less frequently is stimulated Raman spreading, which includes changing a few of the higher power pumped photons into lower power photons and phonons (related to latticework resonances). If the incident light power is extremely high, the gain will certainly decrease after the gain medium reaches gain saturation. The amplifier can not add an arbitrarily huge amount of power to the case light beam at a minimal pump power. In laser amplifiers, the number of ions in the top degree reduces at saturation because of stimulated radiation.
The gain medium has a thermal effect due to the fact that part of the pump light power is exchanged heat. The resulting temperature gradient and also mechanical stress and anxiety will create the prism impact and misshape the magnified beam of light. These results can destroy the light beam top quality of the laser, decrease its performance, and also ruin the gain medium (thermal cracking).
Relevant physical properties of laser gain medium
In laser applications, the physical homes of numerous gain media are essential. It generally consists of:
- In the laser transition procedure requiring wavelength area, the most effective peak gain occurs in this area.
- The substratum has a high level of openness in the working wavelength area.
- Good pump light source, effective pump absorption.
- Suitable upper-level life time: enough time for Q-switched applications and short sufficient for promptly modulated power.
- High quantum performance is acquired from typical quenching impacts, fired up state absorption, and comparable procedures or helpful impacts such as multiphoton changes or energy transfers.
- Ideal four-level actions because quasi-three-level habits presents a few other additional restrictions.
- High stamina and long life, chemical security.
- For solid-state gain media: Base media require to be of good optical quality, can be cut or polished of very premium quality (suitable firmness), allow high focus of laser-active ions to be doped without forming clusters, have great chemical stability, have an excellent thermal conductivity and also low thermo-optical coefficient (weak thermal prism effect at high power operation), resistance to mechanical anxiety, optical isotropy is typically needed, But sometimes birefringence (reducing the impact of thermal depolarization) and gain connected with polarization is needed (see the polarization of laser radiation).
- Low pump power threshold at a high gain: The item of radiation cross-section and high-ranking lifetime is bigger.
- The beam of light top quality of the pump source of light is low: high pump absorption is required.
- Wavelength tuning: Requires huge gain transmission capacity
- Ultrashort pulse generation: gain spectrum is broad and level; Proper diffusion as well as nonlinearity.
- Passive mode-locked lasers without Q-switching security: sufficiently big laser cross-sections.
- High power pulse amplification (favorable comments amplifier): Effect of high optical damages limit and also not expensive saturation on gain.
Note that there are scenarios where conflicting requirements are called for. For instance, extremely reduced quantum flaws are inappropriate with a four-level system. A big gain transmission capacity corresponds to a smaller sized laser cross-section than the optimal situation. But the quantum defect is not so little. The disorder in the solid-state gain medium enhances the gain data transfer as well as lowers thermal conductivity.
A short pump absorption length is helpful yet worsens the thermal result.
The demands for the gain medium vary from situation to case. For that reason, numerous gain media are extremely important for applications. And it is essential to choose the right gain media when maximizing the style of the laser.
Read more: laser frequency doubling