Laser refers to a large number of photons with exactly the same characteristics generated by the transition of metastable electrons in atoms from high energy levels to low energy levels under the excitation of incident photons. The process of laser generation is the process of stimulated radiation light amplification, that is, the laser working material absorbs external energy, causing more and more particles at the high energy level of the working material to transition to the low energy level, and at the same time, photons are released, and the photons pass through the resonance. The continuous oscillation and amplification in the cavity forms a laser. Therefore, to generate laser, three conditions must be met: laser working material, external excitation source, and optical resonant cavity.
In the thermal equilibrium state, particles such as atoms in general media all satisfy the Boltzmann distribution, that is, the number density of particles at low energy levels is greater than that at high energy levels. To generate laser light, the distribution of particles must first be changed so that the number density of particles at high energy levels is greater than that at low energy levels. This distribution state is called "particle number inversion." Particle number reversal can only be achieved in special media, and only a few hundred such media are currently found in nature. Only these special media can serve as the working substance of the laser, also known as the activation medium. They are the necessary conditions for the generation of laser.
In addition, the particles in the excited state must also have a long enough life. In the working material of the laser, the average lifetime of a certain excited state particle is particularly long, up to 10^-3 seconds or even 1 second, which is called "metastable state". Only in the metastable state can particle number reversal be achieved, thus providing the necessary conditions for the generation of laser light.
To achieve the inversion of the particle number between the upper and lower energy levels of the working material, energy must be provided from the outside to excite the particles at the low energy level to the high energy level. This process is called "pumping" or "pumping" . Substances that provide energy to perform this function are sources of stimulation. The spectral lines emitted by the excitation source should match the absorption spectral lines of the working material as much as possible, so as to achieve the maximum conversion of energy. Common excitation methods generally include optical excitation, electrical excitation, chemical excitation, nuclear excitation, thermal excitation, etc.
The excitation source can cause the working material to achieve particle number inversion, but to produce high-purity laser, the stimulated radiation must be much greater than the spontaneous emission of the material (the background noise of the laser) to ensure that the number density of photons in a specific spectrum is high enough. This is An optical resonator is required.
The optical resonant cavity can not only provide necessary optical feedback for the oscillation of laser photons, but also limit the frequency and direction of the laser, improving the monochromaticity and directionality of the laser. Coaxial mirrors are installed at both ends of the resonant cavity. The excitation source excites atoms or molecules in the working material through the pumping process to generate stimulated radiation. The photons radiated along the axis of the resonant cavity are reflected back along the axis by the mirrors, and further The generation of excitation photons forms an avalanche effect-type oscillation amplification process. One of the two mirrors is a partial mirror. The oscillated and amplified light beam along the axis direction can be released through the partial mirror to generate a laser beam; while the photons that encounter the other total reflection mirror are reflected back and continue to oscillate. and zoom in again.
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