The so-called Q-switching refers to the technology of adjusting the Q value of the laser. In the initial stage of laser pumping, the Q value of the resonator is adjusted to a very low level, so that the laser does not meet the oscillation conditions temporarily. When a high particle number density is obtained under the excitation of the pump pulse, the Q of the resonator is quickly adjusted. At this time, the inversion population density is far greater than the threshold inversion population density, the green laser pointer oscillation is quickly established and reaches a high peak power, at the same time the inversion population is quickly exhausted, and the pulse ends quickly. Laser pulse with narrow pulse width and high peak power. The use of Q-switching technology can establish the output of nanosecond pulses.
Mode locking is an important technology for laser pointers that generate ultrashort pulses. There are multiple modes of laser pulses in the laser cavity, and the laser ultrashort pulses or mode-locked pulses are output when the phases of these modes achieve constructive interference with each other. Mode locking is generally divided into two categories: one is active mode locking, and the other is passive mode locking. The former is to periodically modulate the gain or loss of the laser from the outside to the laser to achieve mode locking; the latter uses a saturable absorber (such as a thin semiconductor film), which uses its nonlinear absorption to lock the relative phase and produce super Short pulse output.
Pulse compression technology is a measure to overcome the dispersion effect caused by the change of the refractive index of the material with the wavelength. If the chirp is linear, the dispersion is easy to correct. However, most optical amplifier materials produce high-order effects, which are difficult to control when the pulse width is increased, and need to be solved by pulse compression technology. Pulse compression technology has four basic methods: the first is a parallel grating pair compressor.
It allows the long-wavelength part of the beam to pass through a longer optical path than the short-wavelength part, which reverses the dispersion effect of the material and becomes the grating extender of the pulse amplifier chain. This kind of compressor introduces negative dispersion at appropriate intervals, its structure is compact, but the light loss is large (close to 50%), and it will introduce high-order dispersion. The second is the prism pair compressor. The basic principle is similar to that of a grating pair, but the induced negative dispersion is smaller than the aforementioned grating type.
If the distance between two prisms is large enough, the laser pointer positive dispersion of the material can be balanced by moving a prism in and out of the optical path. The apex angle of the prism is cut so that the deviation of the center wavelength is the smallest, and the incident angle is the Brewster angle, so that the linearly polarized Fresnel reflection loss is the smallest, and the entire optical cavity system has almost no loss. It is worth pointing out that the grating pair compressor and the prism pair compressor introduce third-order dispersion components with opposite signs. If the two are used together, the higher-order components of dispersion can be offset. The third type is the more modern dual-chirped mirror (DcM) compressor.