| According to different materials, lasers can be divided into four main categories: solid state lasers, liquid lasers, gas lasers, and semiconductor lasers. Generally, LD (laser diode) pumped solid state lasers (all solid state lasers) combine the advantages of solid state lasers and semiconductor lasers, and possess the properties of compactness, high efficiency, high beam quality, stable output and reliability. Nowadays, all solid state lasers have been widely used in the field of industry, medical treatment, national defence, information technology, scientific research, etc.The laser materials play a central role in laser equipments. Up to now, hundreds of solid state materials have realized lasing, and give out thousands of laser lines. From familiar blue laser pointer to laser fusion, Nd3+doped materials have found a wide variety of practical applications, and have attracted many attentions. Among various Nd3+doped crystals, Nd:YAG and Nd:YV04are the most commonly used in all solid state lasers. But every crystal has its defects, e. g., low doping level and high cost of Nd:YAG, the serious thermal effect and low damage threshold of Nd:YVO4. Therefore, it is always necessary to search for new laser materials, and research their properties.Generally, oxyorthosilicates allow large activator concentration and possess good mechanical, chemical, and thermal durability. Thereinto, both YaSiO5(YSO) and Lu2SiO5(LSO) have low symmetry crystal structure of positive monoclinic C2/c space group and two non-equivalent crystallographic sites which can be substituted by rare-earth dopants. Nd:YSO has attracted much interest due to its favorable growth properties, strong natural birefringence, wide absorption and emission lines, large absorption cross section, etc. By contrast, LSO provide better thermal properties. Replacing Y with Lu will not change the structure intensively because the radii difference between Lu3+and Y3+was only5%. Thus,(LuxY1-x)2SiO5(LYSO) has the same crystal structure with YSO or LSO and combines the advantages of them. More importantly, the structural disorder is further enhanced. This induces more broadened inhomogeneous broadening of spectrum and leads to some novel laser performance, such as enhancement of pulse energy in Q-switching, much shorter pulse width in mode-locking and adapted for LD pumping no demanding of strict spectrum matching.Garnet crystals occupy an important place in laser materials. Among them LU3Al5O12(LuAG) has high thermal conductivity, excellent physical and chemical properties. Substituting large Gd3+ions for Lu3+ions is favorable for doping Nd3+ions, and increasing pumping efficiency.The research of this thesis was focused on the two new laser crystals, Nd:(LuxY1-x)2SiO5(Nd:LYSO) and Nd:(LuxGd1-x)3Al5O12(Nd:LuGdAG), including optical properties, LD pumped CW and passively Q-switching operation performance, nonlinear optical frequency conversion, and so on. The main contents are as following:The history and the advantages of all solid state lasers, laser transitions of Nd3+ions, characteristics of passively Q-switching technique and saturable absorption properties of Cr4+:YAG crystal were introduced. Progress of the research on Nd:LYSO and Nd:LuGdAG solid state lasers were also summarized.The optical orientation and refractive index of Nd:LYSO crystal were determined utilizing a polarized microscope and a prism coupler:(b, X)=180.0°,(a, Y)=23.3°,(c, Z)=-10.5°, nX=1.7915, nY=1.7933, nZ=1.8144(λ=632.8nm). The room temperature polarized absorption spectra in the wavelength range of300-1000nm were measured by a spectrophotometer. Based on Judd-Ofelt, the optical spectral parameters of Nd:LYSO were calculated. The results show that the radiative lifetime is240μs,σa (E//X)(the absorption cross section around811nm when the vibration direction of the electric field is parallel to the X-axis of the optical indicatrix)>σa (E//Z)>σa (E//Y). In the wavelength range of800-1400nm, the room temperature polarized emission spectra, together with the room temperature and low temperature (78K) non-polarized emission spectra were recorded by a spectrofluorometer. According to the Fuchtbauer-Ladenburg formula, the stimulated emission cross section was calculated. It was found that Nd:LYSO crystal has potential to lase at about912nm, 1076nm and1359nm, and the fluorescence spectrum is mainly inhomogeneously broadened.The model of LD end-pumped solid state lasers was studied, and the formulas of threshold and slope efficiency of four-level system were obtained. In addition, rate equations of Cr4+:YAG passively Q-switching were discussed, the expressions of pulse energy, peak power and pulse width were given, too. And then LD pumped Nd:LYSO (b-cut,0.5at.%)1075and1079nm dual-wavelength laser was studied. When the absorbed pump power was3.87W, the continuous-wave (CW) laser output was1.1W, corresponding to an optical conversion efficiency of28.4%and a slope efficiency of32.4%. Using a Cr+:YAG crystal as the saturable absorber, passively Q-switched operation of Nd:LYSO crystal produced the maximum average output power of294mW, the shortest pulse width of27.5ns, the largest pulse energy of34.3μJ, and the highest peak power of1.18kW. Furthermore, the1075and1079nm dual-wavelength operation performance of Nd:LSO (b-cut,0.5at.%), an isostructural analog of Nd:LYSO, was also researched. The maximum CW dual-wavelength output power was1.09W, the optical conversion efficiency and the slope efficiency were28.2%and30.9%, respectively. For passively Q-switching operation, the maximum average output power of630mW, the shortest pulse width of42.5ns, the largest pulse energy of54.8μJ, and the highest peak power of1.16kW were achieved, respectively. For both Nd:LYSO and Nd:LSO, higher efficiencies were obtained with smaller transmittance of output couplers due to the influence of inhomogeneous broadening of spectra on the laser performance.Laser performance of three Nd:LYSO crystals cutting along different principal axis of the optical indicatrix at4F3/2→4I11/2transition were researched, respectively. Both the lowest threshold pump power and the highest efficiency were obtained with Y-cut crystal since it has the strongest absorption at the pump wavelength, and the emission cross sections of the three crystals at about1.08μm are similar. Using X-cut crystal the maximum CW output power of7.56W was obtained, corresponding to an optical conversion efficiency of26.5%and a slope efficiency of33.1%calculated by incident pump power. Calculated by absorbed pump power, they were52.5%and 55.8%, respectively. For Y-cut crystal, the maximum output power was10.3W, the highest optical conversion efficiency and slope efficiency were36.4%and45.9%(50.2%and54.8%, calculated by absorbed pump power). As for Z-cut crystal, they were7.61W,26.7%and32.3%(49.1%and51.5%), respectively. All of them produced1076and1079nm lines under high pump level.1079nm laser line was very weak for X-cut crystal, while it's the stronger one for Y-cut and Z-cut crystals. As far as the polarization is concerned, except the vibration of1076nm line produced in X-cut crystal along Y-axis was obviously stronger than that along Z-axis, all the other lines vibrated approximately along a certain principal axis of the optical indicatrix.Passively Q-switched laser experiments at4F3/2→4I11/2transition with three Nd:LYSO crystals cutting along different principal axis of the optical indicatrix were developed with Cr4+:YAG crystal, respectively. For X-cut crystal, the maximum average output power of2.64W (corresponding to an optical conversion efficiency of9.3%and a slope efficiency of14.9%), the shortest pulse width of10.9ns, the biggest pulse energy of120.9μJ and the highest peak power of7.8kW was obtained. For Y-cut crystal, they were4.36W (16.8%and24.8%),8.3ns,150.8μJ and14.7kW. In the case of Z-cut crystal, they were3.17W (12.2%and18.4%),10.1ns,52.1μJ and4.7kW, respectively. The output spectra were complex probably due to the thermal and Fabry-Perot etalon effects of Cr4+:YAG crystal. While under the biggest pump power in our experiments the stronger output laser line coincided with CW operation.Laser performance of three Nd:LYSO crystals cutting along different principal axis of the optical indicatrix at F3/2→4I13/2transition were also studied, respectively. For X-cut crystal, the maximum output power of2.22W was obtained with an optical conversion efficiency of9.0%and a slope efficiency of12.5%(18.3%and24.8%, calculated by absorbed pump power). For Y-cut crystal, they were2.61W,13.2%and17.1%(19.6%and24.7%). As regards Z-cut crystal, they were3.05W,11.7%and16.1%(22.1%and27.2%). It was evident that, as far as operation efficiencies were concerned,Y-cut crystal was still the best. Yet Z-cut crystal possessed the highest output power. We attributed this to the larger stimulated emission section of Z-cut crystal. The room temperature fluorescence spectrum of Nd:LuGdAG crystal was recorded in the range of800-1400nm. The central wavelengths of the three emission bands corresponding to4F3/2→4I9/2,4F3/2→4I11/2and4F3/2→4I13/2transitions were discovered to locate near946.5nm,1063.5nm and1338nm. And then laser operations at these three transitions were carried out, respectively.LD pumped1064nm Nd:LuGdAG laser was demonstrated. In CW operation, the maximum output power of6.88W was obtained with an optical conversion efficiency of21.9%and a slope efficiency of26.3%(75.6%, calculated by absorbed pump power) at an incident pump power of31.4W.In passively Q-switched operation with Cr4+:YAG crystal as the saturable absorber, the maximum average output power of3.94W was achieved, corresponding to an optical conversion efficiency of12.5%and a slope efficiency of15.1%(43.4%, calculated by absorbed pump power). The shortest pulse width, largest pulse energy, and highest peak power were5.8ns,36.4μJ, and6.1kW, respectively.In the experiments of LD pumped1.3μm Nd:LuGdAG lasers, the maximum output power of3.31W was obtained, giving an optical conversion efficiency of10.5%and a slope efficiency of13.0%(37.4%, calculated by absorbed pump power).The model of quasi-three-level system was analyzed. Then LD pumped948nm Nd:LuGdAG lasers were studied experimentally. Under an incident pump power of26.3W, a maximum CW output power of3.03W with an optical conversion efficiency of11.5%and a slope efficiency of15.6%(44.8%, calculated by absorbed pump power) was obtained. With Cr4+:YAG crystal as the passively Q-switched component, the maximum average output power of1.01W, the minimum pulse width of20.7ns, the largest pulse energy of124.0μJ, and the highest peak power of5.8kW were achieved at the maximum incident pump power of26.3W.The basic theory of frequency doubling was introduced. Utilizing intracavity second harmonic generation technique, LD pumped474nm Nd:LuGdAG/LBO blue lasers were realized successfully with a type-Ⅱ cut LBO crystal. A highest CW output power of802mW was obtained, corresponding to an optical conversion efficiency of3.0%(8.6%, calculated by absorbed pump power). On Cr4+:YAG passively Q-switched operation, the maximum average output power, optical conversion efficiency, shortest pulse width, largest pulse energy, and highest peak power were measured to be331mW,1.3%(3.7%, calculated by absorbed pump power),38.0ns,6.4μJ, and146W, respectively.
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