Laser crystal can transform the energy provided by the outside world into crystal materials with highly parallel and monochromatic laser, which are coherent in space and time. It is the working material of crystal laser. The laser crystal is composed of two parts: the luminescence center and the matrix crystal. The luminescence centers of most laser crystals are composed of active ions, which partially replace the cations in the host crystal to form doped laser crystals. When the active ions become a part of the matrix crystal, they form a self activated laser crystal.

The active ions used in laser crystal are mainly transition metal ions and trivalent rare earth ions. The optical electrons of transition metal ions are 3D electrons in the outer layer. In the crystal, the optical electrons are easily affected by the surrounding crystal field, so the spectral characteristics are very different in different types of crystals. The 4f electrons of trivalent rare earth ions are shielded by the outer electrons of 5S and 5p, which weakens the effect of crystal field on them. However, the perturbation of crystal field makes the forbidden 4f electron transition possible and produces narrow band absorption and fluorescence lines. Therefore, the spectrum of trivalent rare earth ions in different crystals does not change as much as that of transition metal ions.

The main matrix crystals used in laser crystal are oxide and fluoride. As a matrix crystal, in addition to its stable physical and chemical properties, easy to grow large-size crystals with good optical uniformity and low price, it should consider its adaptability with the activated ions, such as the radius, electronegativity and valence of the matrix cations and the activated ions should be as close as possible. In addition, the influence of the crystal field of the matrix on the spectrum of the active ions should be considered. For some matrix crystals with special functions, the laser with certain characteristics can be directly generated by doping the active ions. For example, in some nonlinear crystals, the laser generated by the active ions can be directly converted into harmonic output through the matrix crystal.

The active ions used in laser crystal are mainly transition metal ions and trivalent rare earth ions. The optical electrons of transition metal ions are 3D electrons in the outer layer. In the crystal, the optical electrons are easily affected by the surrounding crystal field, so the spectral characteristics are very different in different types of crystals. The 4f electrons of trivalent rare earth ions are shielded by the outer electrons of 5S and 5p, which weakens the effect of crystal field on them. However, the perturbation of crystal field makes the forbidden 4f electron transition possible and produces narrow band absorption and fluorescence lines. Therefore, the spectrum of trivalent rare earth ions in different crystals does not change as much as that of transition metal ions.