In classical wave optics, the medium parameters are considered to be independent of the intensity of light, and the optical process is usually expressed by linear differential equation. However, many new phenomena have been found when a strong laser passes through. If it is found that the refractive index is related to the field intensity of the laser, the refractive angle of the light at the interface between the two media changes with the change of the beam intensity; Self focusing and self defocusing of the beam; After passing through some media, the frequency of light wave changes, resulting in frequency doubling, sum frequency and difference frequency. All these phenomena belong to the study of nonlinear optics.

Lasers can now produce light sources with high directivity, high monochromaticity, polarization and frequency tunability and may obtain ultrashort pulses. High resolution spectroscopy, picosecond (10-12s) ultrashort pulses and tunable laser technology have made profound changes in classical spectroscopy and developed into laser spectroscopy. At the same time, it can also obtain high-power, femtosecond ultrashort pulse laser. The study of the interaction between this kind of laser and matter has developed into super ultrafast optics. These emerging disciplines have become important means to study the microstructure and micro dynamic process of materials, and provide unprecedented new technologies for the study of the structure and dynamic process of atomic physics, molecular physics, condensed matter physics, molecular biology and chemistry.

With the development of laser science and technology and the application of laser in many fields, new requirements are put forward for the performance of laser materials and corresponding laser devices. The basic problems involved in the development of new light sources and lasers have become an important content of modern optics. Its development trend is wavelength expansion, frequency modulation and optical pulse width compression, And the miniaturization and solidification of devices.

The development over the past decades shows that laser science and technology are also the vigorous development of medical treatment and biology, and laser crystals have become particularly important.