The Generation of Lasers

The Generation of Lasers
The generation of lasers was proposed by Einstein in 1916 with his theory of “spontaneous and stimulated emission”. This theory forms the physical basis of modern laser systems. The interaction between photons and atoms may lead to three transition processes: stimulated absorption, spontaneous emission, and stimulated emission. As long as stimulated emission can be sustained and stable, lasers can be obtained. Therefore, special devices – lasers – must be manufactured. The composition of a laser is generally composed of three main parts: the working substance, the excitation device, and the optical resonator.

1. Working substance

The substance in a laser that can generate laser light is called the working substance. Under normal circumstances, the distribution of atomic numbers in the substance at each energy level is a normal distribution. The number of atoms at the lower energy level is always greater than that at the higher energy level. Therefore, when light passes through the normal state of the luminescent substance, the absorption process is dominant, and the light always weakens. To make the light strengthened after passing through the luminescent substance and achieve light amplification, it is necessary to make stimulated emission dominant. To make the number of atoms at the higher energy level greater than that at the lower energy level, this distribution is opposite to the normal distribution and is called particle number inversion.
2. Excitation Device
The function of the excitation device is to excite atoms in a lower energy level to a higher energy level, enabling the working substance to achieve a particle number inversion. The energy levels of the substance include the ground state and the excited state, as well as a metastable state. The metastable state is less stable than the ground state, but much more stable than the excited state. Relatively speaking, atoms can remain in the metastable state for a longer period of time. For example, the chromium ions (Cr3+) in ruby have a metastable state with a lifetime of the order of 10-3 seconds. After the working substance is excited and achieves particle number inversion, initially, due to the different propagation directions of the photons emitted by spontaneous radiation, the stimulated radiation photons also have different propagation directions, and there are many losses in output and absorption; stable laser output cannot be generated. To enable the stimulated radiation to continue to exist in the limited volume of the working substance, an optical resonator is needed to achieve the selection and amplification of light.
3. Optical Resonator
It is a pair of mutually parallel reflecting mirrors installed at both ends of the working substance, perpendicular to the main axis. One end is a total reflection mirror (with a reflection rate of 100%), and the other end is a partially transparent and partially reflective mirror (with a reflection rate of 90% to 99%).
The functions of the resonator are: ① generating and maintaining optical amplification; ② selecting the direction of the output light; ③ selecting the wavelength of the output light. For a specific working substance, due to various factors, the actual emitted light wavelength is not unique, and the spectrum has a certain width. The resonator can play a frequency selection role, making the monochromaticity of the laser better.