Definition
Pumping is the process to achieve population inversion .
For Laser action, the pumping mechanism maintains a higher population of atoms in the upper energy level relative to that in the lower level.
Active system is a system in which population inversion is achieved. Pumping is the method of raising the particles from a lower energy state to a higher energy state.
However, there are several ways to do this.
The most commonly used pumping methods are
- Optical
- Electrical Discharge
- Chemical
- Thermal
- Injection Current
- Inelastic Atom-Atom Collisions
1. Optical Pumping
As the name suggests light is used to achieve population inversion . Solid state lasers use this method. Generally for optical pumping Xenon flash lamps are used. Since these materials have very broadband absorption, a sufficient amount of energy is absorbed from the emission band of the flash lamp and population inversion is achieved.
Examples of optically pumped lasers are ruby, Nd: YAG Laser (Neodymium: Yttrium Aluminum Garnet).
2. Electrical Discharge Pumping
Gas lasers use this method. Since gas lasers have a very narrow absorption band, thus here flash lamps are of no use.
Electric discharge is electric current or flow of electrons. Thus electric discharge acts as the pump source. A high voltage electric discharge is passed through the laser medium or gas. After that the high electric field speeds up the electrons to high speeds. Resulting in collision with neutral atoms in the gas. Thus electrons gain energy and get excited.
Examples of Electrical discharge pumped lasers are He-Ne laser, argon-ion laser, CO2 laser, etc.
3. Chemical Pumping
Chemical reaction may also result in excitation and hence the creation of population inversion in a few systems.
An atom or a molecule produced through some chemical reaction. If they remain excited at the time of production, then it is the source of pumping.
For example, H2 + F2 → 2HF, in this chemical reaction, hydrogen (H2) and fluorine (F2) molecules are chemically react. These molecules combined to produce hydrogen fluoride molecule (2HF) in an excited state.
4. Thermal Pumping
In Some cases we can achieve population inversion by heating the laser medium. Heat acts as the pump source in thermal pumping.
5. Injection Current Pumping
In case of semiconductors, injection of current through the junction also results in population inversion among the minority charge carriers.
Examples of these systems are GaAs and InP.
6. Inelastic Atom-Atom Collisions
In addition to electric discharge method Here also a high voltage electric discharge acts as a pump source. However in this method, a combination of two types of gases X and Y are used. Both X and Y gases are having the same excited states (X+ and Y+).
During electric discharge, electrons in gas X from lower energy state or get excited to X+ due to continuous collision with electrons. Thus, the excited state electrons in gas X+ collide with the lower energy state electrons in gas Y. As a result, the lower energy state electrons in gas Y gains sufficient energy and jump into the exciting state Y+. Helium-Neon (He-Ne) laser uses this method.
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