| As one of the greatest inventions in 21 century, laser plays an important role in industry, scientific research, medical treatment, military and daily life. Diode-pumped all-solid-state laser has become the central focus of the area of lasers due to its properties of compactness, stable operation, good beam quality and high efficiency. The operating manners of the lasers can be divided into continuous-wave operation and pulse operation. The pulsed laser possesses the advantages of short pulse width, large pulse energy and high peak power, and then has important and wide applications. Q-switching, which can be divided into active and passive Q-switching, is a main method for obtaining pulsed laser. Q-switching laser can emit energy in a narrow pulse by adjusting the Q-value of the cavity, and then output pulse with nanosecond-level pulse width and megawatt-level peak power. Mode-locking, which mainly includes active and passive mode-locking, is another method for obtaining pulsed laser. Mode-locking laser can force the oscillating longitudinal modes to maintain a fixed-phase relationship to each other, lead to coherent superposition, and then output ultrashort pulse. Pulse with picosecond-level even femtosecond-level pulse width can be obtained by mode-locking. Passive Q-switching and mode-locking are widely used due to their simple structure, low cost and flexible adjusting, which can meet the requirements for large pulse energy and ultrashort pulse, respectively. To obtain larger pulse energy for a passive Q-switching laser, laser material with smaller emission cross sections and longer upper level lifetime is needed. To obtain narrower pulse for a mode-locking laser, laser material with wider emission spectra is needed. So researchers are exploring new pulsed laser materials actively with the development of the laser techniques.The laser material of solid-state laser is obtained by host material doped with active ion. Among the active ions, Nd3+ ion is the most widely researched. The advantages of Nd3+ active ion are as following:its absorption peak near 808 nm matches well with the emission wavelength of GaAlAs; Nd3+-doped crystal possesses large absorption emission cross sections; it has several fluorescence lines and can give out lasers of 0.9, 1.06 and 1.3 μm.. Among the host materials, mixed crystals have attracted much attention. The same sites of a mixed crystal are randomly occupied by different ions and lead to the fluctuations of lattice fields, which cause the variations of the lattice fields surrounding the active ions, and then the absorption and emission spectra are inhomogeneously broadened and the emission cross sections are reduced. Therefore, the mixed crystal possesses high pulse energy storage capacity and is suitable for generating short pulses. The Nd3+ doped mixed vanadate crystals such as Nd:LuxGd1-xVO4, Nd:YxGd1-xVO4 and Nd:LuxY1-xVO4 (0<x<1) have been widely researched, which possess inhomogeneously broadened spectra and significantly enhanced Q-switching pulse energies. The mixed garnet crystals have also aroused researchers’interests.The mixed garnet crystals were developed on the basis of the garnet crystals. The garnet crystals contain Y3Al5O12 (YAG), Lu3AI5O12 (LuAG), Y3Ga5O12 (YGG), Gd3GasO12 (GGG), Lu3GasO12 (LuGG), and so on, which are good host materials due to the advantages of high thermal conductivity, high mechanical strength and good chemical stability. As a representative of garnet crystal, Nd:YAG is the most successful laser crystal for commercial use, and its laser output power has reached tens of kilowatts. However, the applications of Nd3+ doped garnet crystals in pulsed lasers are restricted by their narrow spectra bands and large emission cross sections. In order to solve these problems, researchers developed mixed garnet crystals. Mixed garnet crystals such as Nd:Gd3AlxGa5-xO12 (0<x<5, Nd:GAGG), Nd:LuxY3-xAl5O12 (0<x<3, Nd:LuYAG), Nd:LuxGd3-xGa5O12(0<x<3, Nd:LGGG), Nd:Lu3ScxGa5-xO12 (0<x<2, Nd:LuSGG) have been proved to possess good pulsed laser properties.In the Nd:LuSGG mixed crystal,inhomogeneous broadening was induced by the fluctuation of the lattice fields caused by the random distributions of Sc3+ and Ga3+ in the anionic group. To further broaden the spectra, the Y3+ ions were introduced into the Nd:LuSGG crystal by partly substituting Lu3+ ions, which leaded to fluctuation of the lattice fields by cationic group, and then Nd:LuYSGG mixed crystal with broader inhomogeneously broadened spectra was formed. As the chemical formula of the congruent melting Y3ScxGa5-xO12 (0<x<2, YSGG) crystal was Y3Sc1.43Ga3.57O12 and Nd:Lu3Sc1.5Ga3.5O12 crystal had been proved to possess good pulsed laser performances, we grew Nd:LuYSGG crystal with the chemical formula of Nd:Lu3-xYxSc1.5Ga3.5O12 (0<x<3).For garnet crystals involving Ga components, to avoid the oxidation of the crucible, crystal growth by conventional Czochralski method should be at N2 atmosphere, which induced oxygen defects and valence alternation of Ga elements. The problems could be solved by optical floating zone method, which need no crucible and could grow crystals at O2 atmosphere,and then oxygen defects could be reduced and the valence alternation of Ga elements could be avoided.We grew the Nd:LuYSGG mixed crystal by optical floating zone method for the first time and studied crystal structure and components. The optical properties were studied to validate the inhomogeneous broadening effect and the laser performances were studied to validate the pulse energy enhancement effect. The main works are as following:1. Crystal growthThe equipments and growth procedures of the optical floating zone method were introduced. The important factors that affected crystal quality were discussed, including feed rod, seed crystal, stability of the molten zone, shape of the growth interface, growth rate, rotation rate and growth atmosphere. High quality feed rod and seed crystal were the basises of growing high quality crystal; stable molten zone and convex growth interface could restrain the generation of defects; proper growth rate and rotation rate were the most important factors of growing high quality crystal; the O2 atmosphere could reduce the oxygen defects and improve the crystal quality. The proper growth parameters were determined, and high quality Nd:Lu3-xYxSc1.5Ga3.5O12 (x=1,1.5,2) series crystals were obtained.2. Physical propertiesThe crystal structures and components of Nd:Lu3-xYxSc1.5Ga35O12 (1,1.5,2) series crystals were studied. The X-ray diffraction spectra revealed that the as-grown crystals belonged to cubic system with Ia3d space group, and the lattice parameters were calculated. The lattice parameters of Nd:Lu3-xYxSc1.5Ga3.5O12 crystals increased with the increase of x value, which agreed with the larger radies of Y3+. The segregation coefficients of the elements in Nd:Lu3-xYxSc1.5Ga3.5O12 series crystals were determined by X-ray fluorescence analysis. The segregation coefficients of Nd in Nd:Lu3-xYxSc1.5Ga35O12 series crystals were about 0.8, which indicated that Nd3+ had been doped into the crystals well.The absorption and emission spectra of Nd:Lu3-xYxSc1.5Ga3.5O12 series crystals were measured at room temperature.In addition, the spectra parameters were calculated by J-O theory, such as absorption and emission cross section, fluorescence lifetime, and so on. For Nd:Lu2YSc1.5Ga3.5O12, Nd:Lu1.5Y1.5SC1.5Ga35O12 and Nd:LuY2Sc1.5Ga35O12 crystals, the absorption cross sections at 807 nm were 3.075×10-20,3.151×10-20 and 3.189×10-20 cm2, the emission cross sections at 1.06μm were 7.2×10-20,8.3×10-20 and 9.1×10-20 cm2,and the fluorescence lifetimes were 259.98,262.20 and 264.00μs, respectively. Compared to Nd:Lu3Sc1.5Ga3.5O12 crystal, the Nd:Lu3-xYxSc1.5Ga3.5O12 series crystals possessed broader absorption and emission spectra lines, larger absorption cross sections, smaller emission cross sections and longer lifetimes, so they were more suitable for being used in pulsed lasers.3. Laser performancesThe continuous-wave Nd:LuYSGG lasers of 1.06 and 1.33μm were demonstrated. For 1.06μm laser, the slope efficiency was 48.0%, and the output power reached 4.39 W under the absorbed pump power of 10.34 W, corresponding to an optical conversion efficiency of 42.5%. Two wavelengths of 1058.6 and 1061.8 nm were found from the laser spectra. For 1.33 μm laser, the slope efficiency was 15.9%, and the maximum output power was 1.24W under the absorbed pump power of 8.62 W with corresponding optical conversion efficiency of 14.4%.By using Cr4+:YAG crystal as the saturable absorber, the 1.06μm Q-switching laser performances of Nd:LuYSGG crystal were studied. The largest repetition rate of 9.1 kHz, shortest pulse width of 4.1 ns and largest pulse energy of 157.1 μJ were obtained. Compared to Nd:LuSGG crystal, the pulse width decreased, and the pulse energy increased more than one time, which proved that Nd:LuYSGG crystal possessed better energy storage capacity. The theoretical pulse energies were calculated, which agreed well with the experimental results.By using topological insulator Bi2Se3 as the saturable absorber, the 1.33μm Q-switching laser of Nd:LuYSGG crystal was realized. The maximum average output power of 0.36 W was recorded, corresponding to an optical conversion efficiency of 4.7%. The minimum pulse width, maximum pulse repetition rate and maximum pulse energy were 146 ns、349.5 kHz and 1.03 μJ, respectively.By using V3+:YAG as the saturable absorber, the 1.33μm Q-switching laser of Nd:LuYSGG crystal was realized. The maximum average output power of 0.39 W was recorded, corresponding to an optical conversion efficiency of 6.2%. The minimum pulse width, maximum pulse repetition rate and maximum pulse energy were 20.8 ns、 41.6 kHz and 9.38μJ, respectively.By using saturable output coupler (SOC) as the saturable absorber and output coupler, the 1.06 μm passive mode-locking laser of Nd:LuYSGG crystal was realized. Stable dual-wavelength mode-locking was obtained. The synchronous dual-wavelength pulse could be used for generation of THz-wave pulses, measurement of pump-probe ultrafast spectroscopy and plasma beat-wave accelerator. |
PDF Full Text Request