THE PAST AND PRESENT LIFE OF ND: YAG LASER CRYSTALS

Laser crystals are functional crystal materials that can achieve laser output through electric or optical pumping, and are the core of all solid-state lasers. Laser crystals are usually composed of laser matrix crystals and activated ions. Typically, activated ions include rare earth ions, transition metal ions, and color centers.


Since the implementation of laser output in ruby (Cr: Al2O3) crystals in 1960, approximately 350 matrix materials and over 20 active ions have been discovered and developed, achieving effective laser output of over 70 wavelengths.


According to the matrix materials, laser crystals can be generally divided into three categories: oxide crystals (such as Al2O3, Y3Al5O12, YAlO3, Y2O3, Sc2O3), fluoride crystals (such as CaF2, BaF2, SrF2, LaF3, MgF2, LiYF4, LiCAF, LiSAF), and metal oxide salt crystals (such as Ca5 (PO4) 3F, Y2SiO5, YVO4, YAl3 (BO3) 4, CaWO4), etc.

The most widely used laser crystals currently are Nd: YAG, Nd: YVO4, and Ti: Al2O3, which are known as the three fundamental laser crystals. Among them, Nd: YAG is mainly used in medium and high-power lasers; Nd: YVO4 dominates in low-power and high-efficiency lasers; Titanium sapphire is applied in the fields of wide tuning and ultrafast pulse lasers. In recent years, many new laser crystals have been developed to meet specific needs or performance improvements, meeting the growing demand for all solid-state lasers and related high-tech industries.


Garnet is a natural mineral and one of the earliest studied crystalline materials. Structurally speaking, garnet crystals belong to the cubic crystal system, with the general formula A3B2C3O12. Among them, A represents atoms such as Y, Gd, Lu, La, etc., occupying the dodecahedral lattice; B represents atoms such as Sc, Al, Ga, Fe, etc., occupying octahedral lattice sites; C is an atom of Al, Ga, Fe, etc., occupying tetrahedral sites; Among them, yttrium aluminum garnet (YAG), yttrium gallium garnet (YGG), and gadolinium gallium garnet (GGG) are representatives of garnet laser crystals, and yttrium aluminum garnet is the most widely used laser crystal.


THE PAST AND PRESENT LIFE OF ND: YAG LASER CRYSTALS


The Y-O bond length of yttrium aluminum garnet crystal is 0.245nm. In this crystal, Y3+and other rare earth ions have similar radii.


Y3+located on the dodecahedron can be replaced by laser activated ions (trivalent rare earth cations) such as Nd3+, Er3+, Tm3+, Ho3+, and Yb3+, becoming laser crystals. Meanwhile, ions on octahedral lattice sites can also be replaced by trivalent metal sensitized ions such as Cr3+, V3+, Mn3+, and Fe3+.


At present, Nd3+: YAG, (Nd3+, Ce3+): YAG, (Nd3+, Ce3+): Tb3+: YAG, and (Nd3+, Ce3+): Cr3+: YAG have been commercialized and widely used. Generally speaking, garnet structured crystals have excellent thermal mechanical properties and laser characteristics, making them suitable for high-power laser applications.


However, neodymium doped crystals have low doping concentration and narrow absorption peaks, which have certain limitations in laser diode pumped laser applications. Table 1 lists the physical, chemical, and thermal properties of yttrium aluminum garnet crystals.


Geusic et al. first reported on the laser output of Nd: YAG crystals in 1964. Since then, this series of crystals and their lasers have attracted widespread research interest, promoting the research and application of this series of crystals.


Currently, kilowatt level Nd: YAG lasers have been commercialized and have shown advantages in industrial processing applications. In the past decade, with the research on high-power thermal capacity lasers, Nd: YAG has once again become a research focus. Compared with high-power thermal capacity laser crystal neodymium doped gadolinium gallium garnet (Nd: GGG) crystal, the theoretical laser output of Nd: YAG crystal is one-third higher, and the thermal lens effect is only half of that of Nd: GGG crystal.


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