The construction and use of the laser
Quantum generators emitting in the range of visible and infrared radiation are called lasers. The word "laser" is an abbreviation of the expression: Light Amplification by Stimulated Emission of Radiation, which means the amplification of light as a result of induced or, as sometimes called, stimulated emission of quants.
The generalized laser consists of a laser active medium, a "pump" system - a voltage source and an optical resonator. The pumping system transfers energy to the atoms or molecules of the laser medium, giving them the opportunity to go into an excited "metastable state" creating a population inversion.
- Optical pumping uses photons from a source such as a xenon gas flash lamp or other laser to transfer energy to a laser substance. The optical source must provide photons that correspond to the permissible transition levels in the laser material.
- Collisional pumping is based on the transfer of energy to a laser substance as a result of collisions with atoms (or molecules) of the laser substance. In this case, the energy must also be provided corresponding to the permissible transitions. This is usually done with an electric discharge in a pure gas or in a mixture of gases in a tube.
- Chemical pumping systems use the binding energy released as a result of chemical reactions to transform the laser substance into a metastable state.
An optical resonator is required to provide the required force in the laser and to select photons that travel in the desired direction. When the first atom or molecule in a metastable state of inverse population is discharged due to stimulated radiation, it initiates the discharge of other atoms or molecules in a metastable state. If the photons move in the direction of the walls of the laser substance, usually a rod or tube, they are lost and the amplification process is interrupted. Although they may bounce off the walls of the rod or pipe, sooner or later they will be lost from the system and will not contribute to the creation of the beam.
On the other hand, if one of the destroyed atoms or molecules releases a photon parallel to the axis of the laser substance, it can initiate the release of another photon, and both of them will be reflected by a mirror at the end of the generating rod or tube. Then, the reflected photons pass back through the substance, initiating further radiation in exactly the same way, which will again be reflected by the mirrors at the ends of the laser substance. As long as this amplification process continues, some of the amplification will always exit through the partially reflecting mirror. As the gain or gain of this process exceeds the losses from the cavity, lasing begins. Thus, a narrow concentrated beam of coherent light is formed. The mirrors in the laser optical cavity must be fine-tuned so that the light beams are parallel to the axis. The optical resonator itself, i.e. the substance of the medium should not strongly absorb light energy.