How exactly is a laser formed and why does a cospiter be needed?
The principle of laser formation is stimulated radiation. When an atom's electron absorbs the energy of a foreign photon, it transitions from the ground state to the excited state, a process called stimulated absorption. However, the electrons are not stable in the excited state, and after a period of time they fall back to the ground state and emit a photon at the same time, a process called spontaneous emission (as is the case with ordinary light sources).
If an electron in the excited state encounters an external photon, the electron in the excited state will quickly fall back to the ground state and emit a photon with the same frequency in its direction, which is stimulated radiation.
If we use a strong light or other pumping method, most of the electrons of the laser crystal are pumped to the excited state to achieve a particle number reversal. If an electron spontaneously emits a photon first, this photon induces other electrons in the excited state to fall back to the ground state, emitting another photon, which in turn continues to affect other electrons, which is like a chain reaction.
However, because the direction of the photons emitted spontaneously is random, the direction of the stimulated light is also random, which requires the use of a resonator. A total mirror and a half mirror are perpendicular to the crystal rod, and it does not matter if the light is reflected in all directions, as long as it is not coaxial with the crystal rod, the light will be reflected by the mirror, and the final light left is the same direction as the crystal rod.
It is reflected back and forth inside, and a part of it is emitted through a semi-reflector. The direction of the light produced by stimulated radiation is related to the photon that causes the stimulated radiation. We want the direction of the light to be as same as possible, which is why lasers have better directionality, but not all lasers need resonators.
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