Introduction of Laser: Applications of Laser Crystal Laser is an amazing substance. It’s a great discovery since the discovery of atomic power, computer and semiconductor.
It’s known that the luminosity of laser is quite high, which can reach up to billion times of that of the sun or even higher. It’s pure and with excellent mono-chromaticity. It has incomparable collimation (travel in straight lines). Laser has great power; the burst of energy can penetrate and melt even the hardest substance. So it has wide application in manufacturing, life as well as scientific research. It’s also a strong weapon for people to explore the nature. Equipment that can generate laser is called laser device. According to working substances, laser devices can be divided into gas laser, solid laser, liquid laser and semiconductor laser. According to working modes, there are continuous laser, pulse laser and ultra pulse laser. The most common lasers are He-Ne laser, CO2 laser, ruby laser, neodymium glass laser and so on Crystal material is widely used in lasers. Crystal used in laser is laser crystal. It’s a kind of crystal material can transfer power from the outside world into crystal material with high parallelism and monochromatic laser in space and time through optical resonator. For example, material used in the first laser device invented in the 1960s was ruby crystal (Cr: Al2O3).
In the 1970s, the advent of neodymium doped yttrium aluminum garnet crystal (Nd:YAG) pushed the development of solid laser; the advent of titanium doped sapphire in 1980s made the possibility of ultra short, ultra fast and ultra powerful laser and the popularity of femtosecond laser; and the advent of Neodymium doped yttrium vanadate (Nd:YVO4) crystal in 1990s made a new development era for solid laser.
DPL is the most promising laser among the different kinds of lasers. And laser crystal is the core part of DPL. From its micro structure, laser crystal consists of luminescence center and host crystal. Most luminescence center of laser crystal is made up of active ion. While active ion becomes part of the host crystal, then it will become self-activated laser crystal. Active ions used in laser crystal are mainly transition metal ions and trivalent rare-earth ions. The optical electron of transition metal ion is the 3d electron of the outer shell; this optical electron can be directly affected by the surrounding crystal field, its spectral characteristic differs in different structure type crystals. The 4f electron of the trivalent rare-earth ion is shielded by 5s and 5p electrons of the outer shell, thus reduces the effect of its surrounding crystal field; while the perturbation of the crystal field may lead to transition of the originally forbidden 4f electron, and forms narrow bandwidth absorption and fluorescence spectrums. So the change of the trivalent rare-earth ions in different crystals is obviously different from transition metal ions.
Host crystals used in laser crystal are mainly oxides and fluorides. As a host crystal, it should have stable physical and chemical characteristics; it can easily generate large size crystal with good optical homogeneity and low price. At the same time, we should notice the adaptability between the host crystal and the active ions: for example, the radius, electro negativity and valence state between host positive ion and active ion should be close as possible. Besides, we should also take into account of the impact of host crystal field upon active ion spectrum. For some host crystal with special function, it can directly generate laser with certain special function after mixed with active ions; for example, in some nonlinear crystals, after generating laser, active ion can be directly transferred into harmonic wave output through host crystal. In the new century, laser and laser technology is driving the development of photo electronic technique and industry with its strong vitality. Laser crystal is also developed in the four aspects such as single crystal, glass, optical fiber as well as ceramic. Micro nano crystal boundary, good integrity polycrystalline ceramics with simple production process as well as laser fiber with compact size, excellent heat exchange and low cost together are challenging Nd:YAG who has been dominating the laser crystal industry for more than 40 years. So it has been a hot issue for the global researchers as how to improve the quality and productivity of laser crystal as well as how to optimize its growth process.