| Diode-pumped solid-state short pulse lasers have been one of the research focuses in the solid-state laser field due to the advantages they possess such as:small volume, compactness, reliability, long lifetime, all-solid-state, high efficiency, high beam quality and low cost. They are of great interests in the field of controlled nuclear fusion, plasma physics, high speed imaging, spectroscopy, holography etc. Passive Q-switching technology is an efficient way to obtain short pulse lasers.In the passive Q-switching regime, beside the laser gain crystal, a saturable absorption unit is needed as a key part. For now the most widely used saturable absorption unit is the Cr4+:YAG crystal, using the saturable absorption property of Cr4+. But in the Cr4+:YAG materials, due to the substitution of Cr4+ ions for a fraction of Al3+ ions at Al-ion sites, Mg2+ or Ca2+ ions should also be co-doped to keep the balance of the charge, which brings some complexities in the crystal growth and problems in the applications.Recently S.A.Zolotovskaya et al. obtained Cr:YVO4 crystal with the substitution of Cr5+ ions for a fraction of V5+ ions in the YVO4 crystal and demonstrated its saturable absorption property. In the meanwhile, YVO4 is also an excellent laser gain material, Nd:YVO4 has been one of the most outstanding laser materials due to its broad absorption bandwidth, large absorption and emission cross-sections, polarized emission etc. If laser gain ions such as Nd3+ or Yb3+ and saturable absorption ion Cr5+ are co-doped in the laser host crystal YVO4, then the laser gain unit and the saturable absorber are combined together to make a new kind of self-Q-switched laser matrix.In this thesis, based on the above thoughts, a new kind of self-Q-switched laser crystal Cr:Nd:YVO4 is systematically studied. The main contents are as follows:1. Growth of Cr:Nd:YVO4 crystalCrack free Cr:Nd:YVO4 single crystals with high optical quality and comparatively large dimensions were successfully grown along a-axis direction by using Czochralski method. Since crystal growth is a complex process of phase transition from melt to solid state, the factors which affect the process and the crystal quality were analyzed including the choice of crystal seed, the design of suitable temperature field, the optimal growth parameters and the atmosphere.2. Characterization of the quality of Cr:Nd:YVO4 crystalThe rocking curves were measured along a-axis and c-axis by using the high resolution X-ray diffraction method to test the lattice integrity of the as-grown crystals, the results indicated that the rocking curves were without broadening and splitting, the crystal is of perfect lattice integrity.The optical uniformity of the crystal was tested by the conoscopic interference method, the patterns of the interference were complete and smooth, indicating that the as-grown crystal is of good optical uniformity.By using X-ray fluorescence method the compositions of the crystal, except for oxygen, were analyzed to calculate the effective segregation coefficients. The effective segregation coefficients calculated for Nd and Cr are 0.7899 and 1.3982, respectively, for the other two elements Y and V are approximately the stoichiometric ratio.The structure of the as-grown Cr:Nd:YVO4 crystal was determined by the X-ray powder diffraction method, which indicated the crystal belongs to the tetragonal system, space group I41/amd, and the lattice parameters of crystal are a=b=0.71121nm, c=0.62904nm.3. Characterization of the properties of Cr:Nd:YVO4 crystalThe density, thermal properties, spectral properties and self-Q-switched laser performance of the as-grown crystal were systematically characterized.The density of the Cr:Nd:YVO4 crystal was measured to be 4.258 g/cm3 by using the buoyancy method at room temperature, the experimental result is consistent to the theory value.By using the thermal-mechanical analyzer the thermal expansion coefficients along a-axis and c-axis were measured to be 0.7747×10-6/K and 6.5355x10-6/K, respectively. The coefficient along c-axis is much larger than the value along a-axis, which is similar to that of Nd:YVO4 crystal. The specific heat of the crystal was measured to be 0.501 Jg-1K-1 at room temperature, which is larger than that of glass. And the thermal diffusion coefficients measured at room temperature wereλ11=2.194 mm2/s andλ33=2.946 mm2/s, respectively. Using the above measured density, specific heat and thermal diffusion coefficients, the thermal conductivities of Cr:Nd:YVO4 crystal were calculated to be k11=4.78 W/mK and k33=5.75 W/mK, respectively.The absorption spectra of the crystal were measured before and after annealing, which showed polarized absorption character, the wide absorption band centered at 1100 nm enabled the crystal to be a self-Q-switched laser matrix. The strong absorption peak centered at 808 nm with FWHM of 11 nm is beneficial to the choice and efficiency of the pumping source. After annealing, the absorption intensity of the wide band centered at 1100nm increased much, which showed great affect on the quality of the self-Q-switched laser pulse. The measured fluorescent spectra excited by 808nm and 596nm sources showed similar curves but with different intensities.The self-Q-switched laser performance of the Cr:Nd:YVO4 crystal with different length, under different transmissions for the output mirrors, was demonstrated by using the flat-flat configuration. When the transmission for the output mirror T=10%, the optimal output pulse width was 85ns and the maximum average output power was 103mW. |
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