| Laser display, on behalf of the development direction of display technology, has the advantages of high brightness, wide color gamut, high contrast ratio and long lifetime, etc. Recently, laser display with three primary colors (red, green and blue) solid-state lasers as light source has been the hot spot in the new generation of display technique. Normally, semiconductor lasers were used as the light sources of red and blue lasers. As the high-power green laser diode has not been successfully developed, a nonlinear frequency conversion technology based on solid-state lasers was adopted. Based on quasi-phase matching (QPM) theory, the periodically poled LiNbO3/LiTaO3 (PPLN/PPLT) crystals are very promising non-linear materials because of its very high nonlinear efficiency and abilities of mass production. LiNbO3/LiTaO3 crystals, belonging to trigonal system and 3m point group, have the main crystal structure of Nb/Ta-O octahedrons and Li-O tetrahedrons. Below Curie temperature, the center of cations likes Nb5+/Ta5+, Li+and anions will shift along c-axis, forming spontaneous polarization. Congruent LiNbO3/LiTaO3 crystals contain a large density of intrinsic defects, which affect some important physical properties seriously. Growth of magnesium doped LiNbO3/LiTaO3 crystals (MgO:LN/LT) and near-stoichiometric LiNbO3/LiTaO3 crystals (SLN/SLT) are two very effective ways to improve the properties of the crystals.In theory, The Mg-doped near stoichiometric LiNbO3/LiTaO3 crystals (MgO:SLN/MgO:SLT) combine the two improvement methods effectively and hence become the most superior type of LiNbO3/LiTaO3 crystals.It is attracting more and more international attention to expand laser wavelength by nonlinear optical techniques. Continuously tunable mid-infrared laser in the 3-5 μm band can be achieved by optical parametric oscillation (OPO) process using periodically poled LiTaO3 crystals (PPLT) through QPM theory. Owing to the low coercive field and high optical damage threshold, thicker SLT/MgO:SLT crystal wafers along Z direction (3 mm) can be periodically poled. So the increasing laser aperture can make the PPLT applied in higher-power laser in extensively fields. However, the lack of SLT and MgO:SLT crystals restricted its practical application severely.With the development of multi-functional composite crystal materials, the study of self-doubling frequency and self-pumping OPO with the rare earth and magnesium co-doped PPLN/PPLT has become an important research focus. For this, growth of high-quality rare earth and magnesium co-doped LiNbO3/LiTaO3 crystals is prerequisite.Based on the need of matrix materials in laser fields and the research foundation of our group, the main tasks of this thesis are listed as follows:1) Prepared MgO:SLT polycrystalline powder through a new crystalline powder synthesis process, wet-chemical method. The corresponding elements, Ta, Li and Mg, were mixed atomic-grade homogeneously in a transparent solution using hydrothermal method and organic complexation. Therefore, the problem of distribution of Mg element in the crystalline powder synthesized by the solid state reaction was resolved, which will promote the quality of MgO:SLT crystals. A comparative study was carried out between the polycrystalline materials by the new technology and that via conventional solid-state reaction method. XRD phase analysis demonstrated the advantages of wet-chemical method on the calcination temperature. Scanning electron microscopy, transmission electron microscopy and element analysis mapping showed the inhomogeneous distribution of Mg element in the polycrystalline powder synthesized by solid state reaction method. XPS spectra indicated Ta-Li antisite defects in the MgO:SLT polycrystalline powder were eliminated by the wet-chemical synthesis method.2) Growth of SLT and MgO:SLT crystals by Czochralski method. The evaporation of lithium from the non-sufficiently sintered Li-rich polycrystalline powder was proved. MgO:SLT crystals were successfully grown via Czochralski method from the polycrystalline powder prepared by wet-chemical method. The basic physical properties of CLT, MgO:CLT, SLT and MgO:SLT crystals were studied comparatively to discuss the influence of Li concentration and magnesium to optical, electrical and thermal properties of LiTaO3 crystals.3) Growth of Nd:MgO:CLT crystals with high Mg concentration of 5 mol%, which has reached the threshold concentration. A suitable thermal field for growth of LiTaO3 crystals was determined using the CGSim numerical simulation software. The concentration of Mg and Nd ions in the as-grown Nd:MgO:CLT crystal was measured by X-ray fluorescence elemental analysis, and the segregation coefficient was calculated. The temperature dependence of Nd:MgO:CLT crystal structure was studied by temperature-dependent XRD, and some distortions of a few particular planes, which is very close to the doping ions, were detected. The series of temperature-dependent XRD patterns were refined using GSAS software to obtain the temperature dependence of LT lattice constants. Thermal conductivity of Nd:MgO:CLT crystal was measured, ka=4.429 W/(m·K), kc= 5.155 W/(m-K). Both polarization absorption peaks and emission peaks of Nd:MgO:CLT crystal split obviously, and the full width at half-maximum (FWHM) of σ polarized absorption peak is wider than that of π polarized. Continuous-wave (CW) laser performance of Nd:MgO:CLT crystals was studied on c-and a-cut of crystal samples, and a maximum output power of 3.58 W was achieved using the c-cut crystal, with the conversion efficiency of 22.78%, which is superior to a-cut crystal.4) Growth of 3 inches 0.8 mol% Nd doped LiNbO3 (Nd:CLN) and 0.5 mol% Nd and 5 mol% Mg co-doped LiNbO3 (Nd:MgO:CLN) crystals using JPG auto-control systems. The study of XRD of Nd:CLN and Nd:MgO:CLN with the refinement using FULLPROF software showed the influence of the locatioin of doping ions to lattice constants of lithium niobate. The refractive index at different wavelengths of Nd:MgO:CLN crystal was measured by a minimum deflection angle method. Furthermore, the Sellmeier equation was obtained by fitting the wavelength-dependent refractive index using the least squares method. CW laser performance of a-and c-cut Nd:MgO:CLN crystals was studied, and the effect of different resonat cavity om the CW laser performance of a-cut sample was also studied using two output mirror with different transmission. The as-grown Nd:MgO:CLN crystal was periodically poled with regulated electric field, and self-frequency doubling green laser with the maximum power of 80 mW was obtained using the PPNd:MgO:CLN wafers for the first time, with the center wavelength of 542 nm. |
PDF Full Text Request