Growth And Properties Of Nd:LGS Laser Crystal

The laser technique develops very rapidly recently, and has been widely used in many fields. Actually, the laser equipment is just a cavity which makes the light oscillating and coherent by stimulated emission of radiation. It is highly monochromatic, directional, and coherent. Combining the merits of semiconductor and solid-state lasers Diode-pumped Solid-state Lasers (DPSSL) have been widely used in the fields of military, medical treatment, optical communication, material processing, laser measurement, etc, due to its advantages such as compactness, long lifetime, all-solid state efficiency and high stability, Diode pumped solid state lasers (DPSSL) have become the central focus in the field of lasers.Laser crystal is an important part of laser system, its physical and spectral properties play a significant role on characteristics of laser output. Searching for more efficient laser crystals has become a hot point since such materials play important roles both in laser designing and laser application. Crystal materials have been in dominant position of laser application owing to the excellent optical, thermal, mechanical and physicochemical properties that they possess. The Czochraski method is a preferred one among all the crystal growth methods for its short growth period, convenient observation and controllability. In the year of 1983, A.A.Kaminskii et al. reported the growth of (La(1-x)Ndx)3GasSiO14 (x=0.0001-1) crystals with different doped concentrations for Nd and La ions. Nd3+:LGS is a kind of disordered laser crystal possessing low pumping threshold and wide absorption,emission bands, which is suitable for laser application. Later on Kaminskii et al. achieved cw laser output using the Nd3+:LGS crystal pumped by Laser diode,but the maximum average output power is only 30mW. LGS can also be used as a piezoelectric and electro-optical crystal because of its remarkable piezoelectric and electro-optical capability. From 1980s when LGS crystal was first reported to now, it has been paid much attention to. Recently laser output of Nd:LGS crystal at 1.06 um has been made a breakthrough, the maximum average cw output power was enhanced to be 2.25W. In the past researchers focused on Nd:LGS crystals with low Nd-doping concentrations, while high Nd-doping concentrations in Nd:LGS crystal would distort the lattice and bring the optical quality down,beside which the high Nd-doping concentrations would also increase the thermal effect during the laser experiment to limit the laser application of Nd:LGS in high output power range. Owing to the severe self-absorption effect, it's difficult for Nd:LGS crystal to obtain quasi-three level laser output. Generally Nd:LGS crystal was cut along Z-axis in laser performance, but it belongs to trigonal system, thus the laser output carried out with X-cut and Y-cut Nd:LGS polished samples would be polarized, which should be of great using value in electro-optical and nonlinear field. However up to now, there is no report covering this point. In this work, crystal growth, crystal structure, crystal quality, thermal and spectral properties together with laser performance of Nd:LGS have been systematically investigated, the outline is shown as follows:1. Crystal growth.Polycrystalline material for Nd:LGS single crystal growth was synthesized by solid-phase reaction method and the starting reagents were La2O3, Nd2O3 and SiO2 with purity of 99.99%, and Ga2O3 with purity of 99.999%. Different doping concentration of Nd:LGS crystal were grown by the Czochraski method, and the influence on crystal growth and quality by such factors as growth parameters, purity and thermal environment were discussed. The equipment and technology for growth of Nd:LGS for growth are introduced.2. Characterization of Nd:LGS crystal.The concentrations of La, Nd, Ga and Si elements in the different Nd concentration of the as-grown Nd:LGS single crystal were measured by the X-ray fluorescence analysis method. The measured results indicate that as-grown Nd:LGS single crystal is closed to stoichimetric LGS and has high homogeneousness in components. Integrity property was measured by high resolution X-ray diffraction method. Conoscopic observation patterns of Nd:LGS show that the as-grown Nd:LGS crystal possesses high optical qualities.3. Basic physical properties of Nd:LGS crystal.Important properties of Nd:LGS crystal such as the specific heat, thermal expansion and thermal conductivity have been studied. The specific heat of crystals with different Nd3+ doping concentration has little difference. Thermal expansion measured by thermal-mechanical analyzer reveals little difference among them. Thermal conductivity of Nd:LGS crystal was calculated according to the density, the specific heat and the thermal diffusion coefficients measured by a laser pulse method. The calculated results show that the value of thermal conductivity increases according to the sequence of Nd3+ ion doping concentration increases. The absorption and emission spectra were analyzed at room temperature.4. Laser properties of Nd:LGS crystal.For the first time to our knowledge, a sample of 0.15at% Nd:LGS crystal was end-pumped by a Ti:Al2O3 laser, and a maximum cw output power of 18mW was obtained at 906nm. With a laser-diode (LD) as the pump source, continuous-wave (CW) laser performance at 1.06μm of 0.3at% and 0.15at% Nd:LGS crystals with Z-cut was demonstrated. The respective maximum powers of 2.23W and 0.47W were obtained with corresponding optical conversion efficiencies of 13% and 8.5%, and slope efficiencies were 13.9% and 4.4%. For the first time to our knowledge, CW laser at 1.06μm of 0.3at% Nd:LGS crystals with X-cut was demonstrated, and the maximum powers of 1.12W was obtained with corresponding optical conversion efficiencies of 8.5%, and slope efficiencies was 9.7%。...