| Laser, an important invention in21century, has been widely used in national defense, scientific research, medical treatment and dialy life, after a development of fifty years. Due to its small volume, high stablity and compactness, solid-state laser has become the research emphasis and lead the development of laser. The working manners of the laser can be devided into continous-wave laser output and pulse laser output. Because of its high peak power and narror pulse width, pulse laser has many important amplications. With the pulsed laser we observe a series of nonlinear optical effect such as self-foucing, stimulated Brillouin scattering and stimulated Raman scattering. Meahwhile it has many important practical amplications in precision machinng and meanment of ultrafast processes. Q-switched and mode-locking are two main methods for obtaining pulse laser. The Q-switched technique is a key breakthrough in the history of laser, and the demonds on higher peak power and narrorer pulse width also promote the development of Q-switched technique. The type of Q-switches evolves from the meachinal Q-switches to the electro-optical Q-switches, acousto-optical Q-switches and passive Q-switches. Due to its advantages such like low-cost, simple design and compactness, passive Q-switches are widely used nowdays. Origianlly, the saturable absorber based on different organic dyes, Later the researcher found that crystals doped with absorbing ions (Cr4+:YAG) or containing color centers (F-2:LiF) also possess excellent saturated absorption property. In recent years, some new materiales were also used as Q-switches such as graphene, topological isolator and MoS2. With Q-switched technique, the maximum pulse energy and peak power reach to millijoule (mJ) and megawatt (MW), respectively. Mode-locking is another technique to generate ultrashort pulse from lasers, we distinguish mode-locking between active and passive mode-locking. For passive mode locking, originally the organic dyes were used as the saturable absorber to obtain laser pulse. Kerr-lens mode-locking represented a real breakthrough in providing a relatively simple means for mode locking in solid-state laser. Today the semiconductor saturable absorbor mirror (SESAM) is most commercial product as the saturable absorber for mode-locking, and some new material like carbon nanotube and graphene were also discovered as the saturable absorber.The researchers are also exploring the new materials for mode-locking actively while developing the pulse laser technique. They found that the active ions play an important rule in generating pulse laser. For example, the Tm3+ion, which has relative long upper level lifetime and big energy storage property, is suitable for Q-switched laser. For Ti3+, Cr3+and Yb3+ions, they have wide fluorescence spectra and are suitable for generating ultrashort pulse. And Ti:sapphire is an important material for femosecond laser. What’s more, host materials also play important rules for generating pulse laser. In case of Nd3+, when we use crystal as the host material, the gain medium usually has excellent thermal properties but sharp fluorescence line, the narror linewidth goes against obtaining ultroshort pulse. When we use glass as host material, the gain medium usually has wide linewidth which is favor of generating ultrashort pulse, but the thermal properties are poor. The disorder crystal possesses the excellent thermal properties such as crystal and the spectral character such as glass at the same time, which has been became a hot research topic. Though mixed crystal has the character of long-range order in structure such as crystal, when we use mixed crystal as host material, the active ions will occupy different site and the lattice field around them are different. Diffenrnt active ions will have different fluorescence line, they overlap with each other and the total fluorescence line will be broaden, which is called inhomogeneous broadening. This kind of gaim material has wide fluorescenc line and excellent thermal properties, and is also suitable for generating pulse laser.Vanadates crystals are kinds of excellent laser material, and YVO4is the earliest vanadate ever found, and has been commercialized producted and used in people’s life. Nd:YVO4was called "three foundation laser material" with Nd:YAG and Ti:Al2O3. LuV04and GdVO4are the homolog of YVO4, and also have excellent laser properties. They are all belong to tetragonal system, could be grown by Czochralski method and the melting point are all about1800℃. Because of these points mentioned above, high quality LuGdVO4, YGdVO4and LuYV04mixed crystal were grown by the Czochralski method, and lots of research had been done about these materials. Their spectra possess obvious inhomogeneous broadening and they have been domenstrated to be excellent pulse laser materials. LaV04is also a member of vanadate, and beleongs to monoclinic system. It is hard to obtain high quality LaVO4with Czochralski method because of its spiral growth. For the mixed vanadate crystal with La3+, they have specific advantages as pulse laser material. However there is little report on the crystal growth and thermal properties, especially the systematic research on pulse laser performance of these mixed crystals.The mixed vanadate crytals with La3+ion mainly refer to LaYVO4, LaLuVO4and LuGdVO4. Compare with LuGdVO4, YGdVO4and LuYVO4mention above, this series of mixed vanadate crystals have two advantages. Firstly, LaVO4belongs to monoclinic system, while YVO4, LuVCh and GdVO4belong to tetragonal system. The difference on crystal system of the two mixed elements will bring great change on lattice field and the fluorescence line will has bigger inhomogeneous broadening. Secondly, compare with Y3+, Lu3+and Gd3+, La3+has biggest ionic radius, the big difference on ionic radius also will bring great change on lattice field and the fluorescence line will has bigger inhomogenous broadening. Based on the two advantages mentioned above, we could dope relative less La3+to obtain the same amount of inhomogeneous broadening and meanwhile maintain the thermal properties to the hilt. For the crystal growth, In order to overcome the spiral growth, through controlling the amount of La3+doped in mixed crystal to make the mixed crystal maintain tetragonal structure. In this thesis high quality LaYVO4, LaLuVO4and LuGdVO4mixed crystal were successfully grown and characterizations on crystal component, crystal structure, thermal properties, spectral properties and laser performances were carried out. The outline is shown as follows:1. Crystal growth, component and structureHigh quality LaYVO4, LaLuVO4and LuGdVO4mixed crystal were grown by Czochralski method, and a brief introduction about the growth equipment and growth technics was given. Some problems in growth process were also discussed. Using X-ray fluorescence analysis, the component of the mixed crystal was determined and the segregation coefficients of Nd3+ions in LaYVO4, LaLuVO4and LuGdVO4mixed crystal were also calculated. The X-ray powder diffraction was used for the structure measurements, and the space group of the mixed crystals is I41/amd. Based on the X-ray powder diffraction data, the unit cell parameters were calculated.2. Thermal propertiesFor the Nd:La0.11Yo:89V04, Nd:La0.05Lu0.95VO4, Nd:Lao.i2Gd0.88VO4and Nd:Lao.25Gd0.75VO4, the systematic thermal properties were studied including thermal expansion coefficient, specific heat, thermal diffusion coefficient and thermal conductivity. The variations of the thermal properties with temperature were also determined. The thermal properties of the mixed crystal are not as good as Nd:YVO4, Nd:LuVO4and Nd:GdVO4, but they still could satisfy the demands of moderate power laser.3. Spectral propertiesThe refractive indics of Nd:La0.11Y0.89VO4, Nd:La0.12Gdo.88VO4and Nd:La0.25Gd0.75VO4were measured by the minimum-deviation method and the Sellmeier’s equations were fitted by the least-squares method. For the Nd:Lao.iiYo.89V04, Nd:La0.05Lu0.95VO4, Nd:Lao.i2Gd0.88VO4and Nd:La0.25Gd0.75VO4, the polarized absorption and emission spectra were measured, and the spectral parameters such as polarized absorption cross-sections, polarized emission cross-sections and life of stimulated radiation were calculated based on the J-O theory. The results show that the mixed vanadate with La3+has wider absorption and emission spectra, smaller emission cross-sections and longer upper lever lifetime than Nd:YVO4, Nd:LuVO4and Nd:GdVO4, respectively, which indicated that they were suitable for pulsed laser. The variations of emission spectra and fluorescence lifetime with temperature were also measured. With the increase of temperature, the width of emission spectra became wider and the fluorescence lifetimes became longer.4. Laser experimentThe continuous-wave laser output properties of the mixed crystal were studied with LD pumped. The laser output properties of a-cut crystals were better than those of c-cut crystals. For Nd:La0.11Y0.89VO4, the slope efficiency for a-cut crystal and c-cut crystal are46.5%and43.9%, respectively; For Nd:La0.05Lu0.95V04, they are43.7%and16.8%, and for Nd:Lao.i2Gdo.88V04, they are38.8%and26.9%. What’s more, we realized dual-wavelength laser output with c-cut Nd:La0.11Y0.89VO4and Nd:Lao.o5Luo.95V04crystals.With a Cr4+:YAG, the Q-switched laser experiment of a-cut Nd:Lao.nYo.89V04, Nd:Lao.o5Luo.95V04and Nd:La0.12Gd0.88VO4were carried out and the maximum pulse energy obtained were97.3μJ,114.9μJ and124.7μJ, respectively. Compare with the results of Nd:YVO4, Nd:LuVO4and Nd:GdVO4, the pulse energy of the mixed crystal were enhanced. The Q-switched laser properties of a-cut Nd:Lao.i2Gdo.88V04, Nd:Lao.25Gdo.75V04and Nd:Lao.3Gdo.7V04were also studied on the same conditions. We obtained the maximum pulse energy and peak power with Nd:Lao.3Gdo.7V04, and they were131.5μJ and15.3KW, respectively.With a SESAM, the passive mode-locking performances of a-cut Nd:La0.11Y0.89VO4, Nd:La0.05Lu0.95V04and Nd:Lao.i2Gdo.88V04were demonstrated and the6.8ps,7.6ps and3.4ps mode-locking pulse were ontained, respectively. Utilizing c-cut Nd:Lao.iiYo.89V04, stable dual-wavelength mode-locking was obtained. Based on the polarized output characteristics of vanadate, with a-cut Nd:La0.11Yo.89V04and Nd:Lao.25Gd0.75VO4, we realized π polarization mode-locking, σ polarization mode-locking and the mode-locking of π and σ polarizations coexisted, respectively. |
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