How to Generate Laser?

Laser is a major invention of the 20th century, known as "the fastest knife", "the most accurate ruler" and "the brightest light". The word "laser" is an acronym for "light amplification by stimulated emission of radiation", which describes the principles of how lasers are produced.

Process of Generating Laser

There are three radiation processes in optical radiation. One is spontaneous emission, in which the excited state atom spontaneously transitions from a high energy level to a low energy level without any external action, and radiates a photon at the same time. The second is stimulated radiation, in which the luminescent atoms in the excited state radiate photons when they transition to a low-energy state or a ground state under the action of an external radiation field. The third is stimulated absorption, in which particles in a low-energy state absorb the energy of external light and transition to a high-energy state.

For spontaneous emission, even if two particles simultaneously transition from a high-energy state to a low-energy state, the phase, polarization state, and emission direction of the light they emit may be different, but this is not the case for stimulated emission. When a particle in a high-energy state transitions to a low-energy state under the excitation of foreign photons, it will emit light that is exactly the same as the foreign photon in terms of frequency, phase, and polarization state.

In 1917, Einstein first proposed the concept of stimulated radiation when he derived Planck's formula for black body radiation. He theoretically predicted the possibility of stimulated emission of atoms, which is the basis of lasers.

In a laser machine, the radiation that occurs is stimulated radiation, and it emits laser light that is identical in frequency, phase, polarization state, and so on. Any stimulated light system has both stimulated radiation and stimulated absorption. Only when stimulated radiation is dominant can the external light be amplified to emit laser light. In general light sources, stimulated absorption dominates. Only when the equilibrium state of the particles is broken, so that the number of particles in the high-energy state is greater than the number of particles in the low-energy state (this situation is called ion number inversion), the laser can be emitted.

Conditions for Generating Laser

• Population inversion: This is the primary condition for the stimulated emission of light. In semiconductors, electrons in the valence band are pumped to the conduction band. In order to obtain ion number inversion, heavily doped P-type and N-type materials are used to form the PN junction. In this way, under the action of an applied voltage, ion number inversion occurs near the junction region, which stores electrons in the conduction band below the high Fermi level EFC and stores holes in the valence band above the low Fermi level EFV.
• Satisfy the threshold condition: The realization of particle number inversion is a necessary condition for laser generation, but it is not a sufficient condition. To generate laser light, a resonator with minimal loss is required. The main part of the resonator is two parallel mirrors, and the stimulated radiation emitted by the activated material is reflected back and forth between the two mirrors, constantly causing new stimulated radiation, which is continuously amplified. Only when the gain of stimulated radiation amplification is greater than the various losses in the laser, that is, if a certain threshold condition is met, can a stable laser be output.
• Satisfy the resonance conditions: the laser is reflected back and forth in the resonator cavity, and only when the phase difference between these beams at the output end is Aф=2qmq=1, 2.3.4, can there be enhanced interference at the output end and stable laser output can be generated. Assuming that the length of the resonant cavity is L, and the refractive index of the active medium is N, the formula f=qc/2NL is called the resonance condition. It shows that after the resonator length L and the refractive index N are determined, only certain frequencies of light can form optical oscillation and output stable laser light.