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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 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.
Large aperture, high-performance Nd: YAG crystals are key materials for high average power solid-state lasers, and have shown many unique applications and promising application prospects in industrial, scientific, and military fields. In the military field, the development of high-power laser technology has brought about a revolution in military weapons, and high-power solid-state lasers with an output power of 100kW have begun to be practical.
The research and production of Nd: YAG crystals are mainly concentrated in the United States and China.
In the United States, II-VI and Northrop Grumman Synaptic companies focus on the preparation of Nd: YAG laser crystals, with their crystal quality and processing level leading in the world; Specifically, the new convex interface growth technology has been used for crystal growth, with a crystal diameter of 150mm and optical uniformity of 0.1 per inch λ (with a wavelength of 250-300nm), and achieved doping concentration of different components less than 10%.
In addition to neodymium doped crystals, ytterbium doped yttrium aluminum garnet (Yb: YAG) has shown advantages in the field of high-efficiency high-power lasers due to its high quantum efficiency, and is considered an important way to develop high-power lasers.
Since the early 1990s, multiple internationally renowned research institutions have conducted research on Yb: YAG crystals and their laser applications and devices.
In 1991, the Lincoln Laboratory at Massachusetts Institute of Technology (MIT) successfully developed the first Yb: YAG laser with an indium gallium arsenic laser diode as the pump source at room temperature, with an output power of 12mW;
In 2004, the Hughes Research Laboratory in the United States achieved a laser output power of 4.4kW using Yb: YAG discs, and recently achieved laser output power exceeding 5kW;
In 2004, Trump, a German company, also achieved a 4kW Yb: YAG disc laser and demonstrated that a single disc can achieve laser output power exceeding 10kW.
In 2013, the Singapore National Laboratory achieved a 1.1kW near-diffraction limit single chip Yb: YAG laser, which is currently the highest output power of a single chip laser. In China, Tsinghua University has also achieved Yb: YAG laser output with a power of 1kW. In terms of mode-locked lasers, the international shortest Yb: YAG mode-locked laser has a pulse width of 136fs and an output power of 3W. In 2014, the Max Planck Institute of Quantum Optics achieved a self mode-locked Yb: YAG crystal 270W mode-locked laser output, with a peak power of 28MW, which is the highest peak power achieved in self mode-locked lasers to date.
