A laser is a device that emits laser light using the principle of stimulated radiation.

The pump source is the source of energy supply for the laser; The gain medium is the core of the laser, which absorbs the energy provided by the pump source and amplifies the laser; A resonant cavity is a mirror with two parallel sides, which reflects light back and forth between the mirrors and passes through the gain medium multiple times. Therefore, while shortening the length of the working material, it can also achieve the goal of amplifying laser power.

By using light, electricity, or other methods to excite matter, a portion of the particles are excited to a higher energy state. When the number of particles in this state is greater than the number of particles in the lower energy state, the substance can amplify the radiation of a certain wavelength due to stimulated radiation. That is, when the radiation of this wavelength passes through the material, the emission intensity will be amplified, and the radiation will be consistent with the position, frequency, and direction of the incident light wave, This type is called a laser amplifier.

If the excited material is placed in the resonant cavity, the light radiation is reflected and propagated back and forth along the axis direction in the resonant cavity, and passes through the material multiple times, the light radiation is amplified many times, forming a beam of high intensity and concentrated laser, which is called a laser oscillator.