【作者】 于浩海；

【导师】 张怀金； Dingyuan Tang；

【作者基本信息】 山东大学 ， 材料学， 2008， 博士

【摘要】 激光--Laser（light amplification by stimulated emission of radiation），是利用受激辐射实现光的放大。实际上，激光器是一个振荡器，利用其受激辐射实现光的振荡，产生相干光。激光因具有单色性，方向性和相干性等特点，已经成为军事、医学、科研乃至人们日常生活不可或缺的一部分。近几年，随着激光二极管（laserdiode，简称LD）技术的发展，LD泵浦的固体激光器（Diode-pumped solid-state lasers，简称DPSSL），已经在激光器领域掀起了一场新的激光革命。如今，DPSSL已经在材料加工、医学、光通讯、激光显示、制导、雷达技术和激光核聚变等方面得到了重要的应用。作为激光器的一个重要部分，激光增益介质的性能最终决定了激光器的输出特性和应用范围。与Nd：YAG相比，钕离子（Nd）掺杂的钒酸盐晶体，如Nd：YVO₄，Nd：GdVO₄和Nd：LuVO₄，因其具有大的吸收和发射截面，偏振输出等特性，在众多的激光材料中脱颖而出，成为中小功率连续激光器的首选材料，其中Nd：YVO₄晶体已经实现产业化。随着脉冲激光器的发展，人们迫切需要高能量、短脉冲的小型激光器，因此寻求适合脉冲激光器中应用的激光材料成为研究的热点之一。鉴于这三种钒酸盐（Nd：YVO₄，Nd：GdVO₄和Nd：LuVO₄）都具有的锆石结构和I4₁／amd空间群，以及相接近的熔点（1800℃），若按照一定比例将其中的两两组合，可以得到三种新的钒酸盐混晶：Nd：Y_xGd_1-xVO₄，Nd：Lu_xGd_1-xVO₄和Nd：Y_xLu_1-xVO₄（0≤x≤1）。在这些新的混晶中，原来的一种离子占据的格位，被两种离子随机占据，使Nd离子周围的晶格场不同，造成其特征谱线，如吸收和发射谱线，会发生明显的非均匀增宽，并使得受激发射截面减小。小发射截面的激光材料适合于在调Q激光领域的应用，而宽的发射谱线的激光材料是产生锁模（尤其是飞秒脉冲）激光所梦寐以求的。除了激光晶体，在被动调Q甚至被动锁模激光器中，可饱和吸收体也是不可缺少的一部分，现在在被动调Q领域一直处于垄断地位的是Cr⁴⁺：YAG(Y₃Al₅O₁₂，简称YAG)，但是由于Cr⁴⁺取代Al³⁺离子，在取代的同时需要掺入Ca²⁺或Mg²⁺离子对电荷进行补偿，这给晶体的生长和应用都带来了很多的麻烦。本论文从材料制备、基本性质的测试和激光领域的应用三个方面对Nd：Y_xGd_1-xVO₄和Nd：Lu_xGd_1-xVO₄系列晶体进行了研究，发现：相对于单晶，混晶系列晶体具有明显的能量增强效应。还在钒酸盐生长的基础上，针对传统的可饱和吸收片Cr⁴⁺：YAG在制备和应用上固有的缺陷，对新型的可饱和吸收体Cr⁵⁺：GdVO₄和自调Q材料Nd：Cr：GdVO₄进行了初步的探索。其主要工作如下：（一）晶体生长1、采用提拉法（Czochralski），首次生长了Nd：Lu_xGd_1-xVO₄系列晶体并在此基础上对Nd：Y_xGd_1-xVO₄系列晶体进行了生长，得到了高质量的晶体，并对其生长过程进行了系统的研究。2、介绍了晶体生长的设备及工艺过程，讨论了影响晶体质量的因素。其中温场、纯度、参数的实时监控以及退火是最为重要的几个环节。（二）Nd：Lu_xGd_1-xVO₄和Nd：Y_xGd_1-xVO₄系列晶体物理性质的研究1、首次对Nd：Lu_xGd_1-xVO₄系列晶体的组分和结构的进行了研究，发现Nd：Lu_xGd_1-xVO₄系列晶体都具有锆石结构，属于四方晶系。2、首次对Nd：Lu_xGd_1-xVO₄系列晶体的热学性质和Nd：Y_xGd_1-xVO₄晶体的热导率进行了研究，发现Nd：Lu_xGd_1-xVO₄的热学性质随x趋向于0．5而逐渐变差，而Nd：Y_xGd_1-xVO₄随x趋向于0．37逐渐变差。通过对材料常数的计算，发现这两个系列的晶体都适合应用在较高功率泵浦的激光器中。3、首次对Nd：Lu_xGd_1-xVO₄系列晶体的光谱性质进行研究，发现Nd：Lu_xGd_1-xVO₄系列晶体具有偏振吸收和发射的特性，并具有比单晶更宽的吸收和发射线宽，我们认为这是由于Nd离子光谱的非均匀加宽引起的。（三）Nd：Lu_xGd_1-xVO₄和Nd：Y_xGd_1-xVO₄系列激光晶体连续激光特性的研究1、首次对Nd：Lu_xGd_1-xVO₄系列晶体1．06μm激光进行了理论上的优化和实验上的研究，得到了最大输出功率为7．4 W的激光输出，并对其有效发射截面进行了计算，讨论了发射光谱的非均匀加宽、热导率以及有效发射截面对连续激光输出特性的影响。2、首次对Nd：Lu_xGd_1-xVO₄系列晶体1．34μm激光进行了实验研究，发现由于荧光谱线的非均匀加宽和激发态吸收的共同作用使得发射截面随着x的增加呈现了一定规律性变化，讨论了热致损耗、激发态吸收以及能量上转换对连续激光输出特性的影响。3、首次对c切Nd：LuVO₄晶体1．06μm的连续激光特性进行了研究，并与a切Nd：LuVO₄晶体的连续激光特性进行了比较，计算了c切Nd：LuVO₄晶体1．06μm波段发射的有效发射截面。4、首次对c切对Nd：Y_xGd_1-xVO₄的1．34μm和1．06μm的连续激光特性进行了研究，并对其发射截面进行了计算，讨论了热致损耗和荧光谱线的非均匀加宽对连续激光输出特性的研究。（四）Nd：Lu_xGd_1-xVO₄和Nd：Y_xGd_1-xVO₄系列晶体调Q激光特性的研究1、首次对a切Nd：Lu_xGd_1-xVO₄晶体的调Q激光特性的进行研究，得到了输出能量为192．5μJ、脉冲宽度为6．2 ns以及峰值功率为31．1 kW的被动调Q脉冲激光，发现随着x趋向于0．5，其被动调Q激光特性逐渐增加，单脉冲能量逐渐增强。2、首次对c切Nd：Y_xGd_1-xVO₄晶体的调Q激光特性的进行研究，得到了输出能量为201．7μJ，脉冲宽度为6．6 ns以及峰值功率为30．6 kW的被动调Q脉冲激光，发现c切Nd：Y_xGd_1-xVO₄晶体具有更大的能量存储能力。3、首次对c切Nd：LuVO₄晶体的调Q激光特性进行研究，得到了输出能量为84．93μJ、脉冲宽度为12 ns以及峰值功率为7．02 kW的被动调Q脉冲激光。（五）Nd：Lu_xGd_1-xVO₄系列（x=1和0．5）晶体锁模激光特性的研究1、首次以SESAM为可饱和吸收体，对Nd：LuVO₄的被动锁模特性进行了研究，得到了平均输出功率为3．71 W，脉冲宽度为8．8 ps的锁模激光。2、首次以GaAs作为可饱和吸收体，对Nd：LuVO₄的被动锁模特性进行研究，得到了输出功率为3．11 W，脉冲宽度为7．1 ps的锁模激光。3、首次以SESAM为可饱和吸收体，对Nd：Lu_0．5Gd_0．5VO₄的被动锁模特性进行了研究，得到了平均输出功率为5．53 W，脉冲宽度为5．5 ps的锁模脉冲激光。4、首次以GaAs作为可饱和吸收体，对Nd：Lu_0．5Gd_0．5VO₄的被动锁模特性进行了研究，得到了平均输出功率为2．79 W，脉冲宽度为3．9 ps的锁模激光。这是我们所知道的、到目前为止用GaAs锁模所得到的最短的脉冲。（六）Cr：GdVO₄可饱和吸收和Nd：Cr：GdVO₄自调Q激光特性的研究1、合成了GdCrO₄的多晶料并首次生长了Cr：GdVO₄和Nd：Cr：GdVO₄晶体，其尺寸为Φ30 mm×20 mm，重约70 g。2．首次对Cr：GdVO₄和Nd：Cr：GdVO₄晶体的偏振吸收特性进行了研究，发现其π（E//c）偏振在1110 nm处的吸收使得Cr：GdVO₄和Nd：Cr：GdVO₄具有可饱和吸收和自调Q特性。3．首次对Cr：GdVO₄晶体的可饱和吸收特性进行了研究，得到了输出功率为122 mW、最短脉冲宽度为361 ns、最大脉冲能量为0．77μJ的调Q脉冲。4．首次对Nd：Cr：GdVO₄晶体的自调Q特性进行了研究，得到了最大输出功率为265 mW、最短脉冲宽度为230 ns、最大脉冲能量为1．12μJ的自调Q脉冲。通过以上的研究，我们验证了混晶系列晶体在脉冲激光器中良好的应用前景；发现在对于Nd：Lu_xGd_1-xVO₄和Nd：Y_xGd_1-xVO₄，其能量存储能力分别随x趋向于0．5和0．37逐渐变大以及其调Q特性逐渐变好；在锁模激光器的研究中，我们发现，与Nd：LuVO₄相比较，Nd：Lu_0．5Gd_0．5VO₄具有明显的脉宽变窄效应；对Cr⁵⁺：GdVO₄和Nd：Cr：GdVO₄的初步探索，验证了其饱和吸收特性以及在调Q激光中良好的应用。更多还原

【Abstract】 Laser is a simplified word of light amplification by stimulated emission of radiation. Actually, the laser equipment is just a cavity which makes the light oscillating and coherent by stimulated emission of radiation. It is monochromatic, directional, and coherent, and has become one of the important parts of military, medicine, scientific research, and our daily life. Recently, as the development of laser diodes, the diode pumped solid state lasers （DPSSLs） have taken a revolution in the laser field and have been applied in many fields, such as material processing, medicine, optical communications, lasers display and laser nuclear fusion, etc.As the most important part of lasers, laser gains ultimately determine the output performance and applications of the lasers. Among the laser materials, neodymium doped vanadate crystals, such as Nd:YVO₄, Nd:GdVO₄ and Nd:LuVO₄, have been identified to be excellent gains due to their broad absorption bandwidths, large absorption and emission cross-sections, polarized emission and high chemical stability. As the development of pulsed lasers, the gain with broad emission linewidth has attracted more and more attention. Because of same ZrSiO₄ structure, with a tetragonal space group I4₁/amd, and close melting points, nearing 1800℃, of the three vanadate crystals, the mixed Nd:Y_xGd_1-xVO₄, Nd:Lu_xGd_1-xVO₄ and Nd:Y_xGd_1-xVO₄ （0≤x≤1） can be grown as single crystal. In those mixed crystals, due to the variation of the local crystal fields surrounding the Nd ions, the spectra, including absorption and fluorescence lines, are inhomogeneous broadened and the emission cross-sections are reduced. In the laser field, smaller emission cross-sections and broadened fluorescence spectra are favorable by the Q-switched and mode-locked lasers, respectively. Besides the laser crystals, the saturable absorber is also an important part in the passive Q-switching and even mode-locking. In the passive Q-switching regime, chromium (Cr⁴⁺) doped YAG crystals have domained. However, due to the substitution of Cr⁴⁺ ions for a fraction of Al³⁺ ions at Al-ion sites in this crystal, Ca²⁺ or Mg²⁺ 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. In this work, we studied the characteristics of Nd:Lu_xGd_1-xVO₄ and Nd:Y_xGd_1-xVO₄ from three aspects, including preparations, basic properties measurements, and laser applications. Based on the growth of the vanadates, we also primarily investigated the saturable absorber Cr⁵⁺:GdVO₄ and self-Q-switched laser materials Nd:Cr:GdVO₄, in order to overcome the drawbacks of conventional Cr⁴⁺:YAG in the preparations and applications.The contents of this dissertation is outlined as following:1. Nd:Lu_xGd_1-xVO₄ and Nd:Y_xGd_1-xVO₄ （0≤x≤1） with different components were grown by the Czochralski method. In order to apply the crystals in the high pumped power level, Nd concentrations in the crystals are determined to be as low as 0.5at.%. During the preparation, different segregation coefficients of Nd ion in YVO₄, LuVO₄ and GdVO₄ were considered. By the X-ray fluorescence method, Nd concentrations in all crystals are determined to be about 0.5at% and x ranges from 0 to 1, as expected.2. Thermal properties of the crystals are measured. It has been found that the thermalproperties, including average linear thermal expansion coefficients, thermal diffusion coefficients, specific heats and thermal conductivities show obviously variations as x approaching 0.5 for Nd:Lu_xGd_1-xVO₄, and 0.37 for Nd:Y_xGd_1-xVO₄. By considering the movement of phonons in crystals, the reduction of thermal conductivities were explained. The thermal fracture limits have been estimated and found to be comparable with Nd:YVO₄, which means that the mixed crystals can be used in the high pump power level.3. The absorption and fluorescence spectra, and fluorescence lifetimes of the crystalshave been measured. With J-O theory and absorption spectra, fluorescence lifetimes of Nd:Lu_xGd_1-xVO₄ have been calculated in theory, which agree well with the experimental results. It was found that due to the variation of the crystal field neighboring the Nd ions, the spectra are inhomogenously broadened, and the energy storability are improved as x approaching 0.5 for Nd:Lu_xGd_1-xVO₄, and 0.37 for Nd:Y_xGd_1-xVO₄. The results indicated that the mixed crystals should have promising properties in the pulsed lasers.4. The continuous-wave laser performance of Nd:Y_xGd_1-xVO₄ and Nd:Lu_xGd_1-xVO₄ hasbeen demonstrated. With Nd:Lu_xGd_1-xVO₄, and Nd:Y_xGd_1-xVO₄, the maximum output power of 7.4 W was obtained at 1.06μm, and 2.56 W at 1.34μmn. 5. With Cr⁴⁺:YAG as saturable absorber, the passive Q-switching performance of a-cutNd:Lu_xGd_1-xVO₄, and c-cut Nd:Y_xGd_1-xVO₄ and Nd:LuVO₄ was demonstrated. Compared with Nd:GdVO₄, the energy of a-cut Nd:Lu_xGd_1-xVO₄ was found to be enhanced in the same conditions. The maximum pulse energy was obtained to be 192.5μJ with highest peak power of 31.06 kW and shortest pulse width of 6.2 ns. With c-cut Nd:Y_0.37Gd_0.63VO₄, the maximum pulse energy of 201.7μJ was obtained with peak power of 30.6 kW and pulse width of 6.6 ns. With c-cut Nd:LuVO₄, the maximum pulse energy, shortest pulse width, and highest peak power were measured to be 84.93μJ, 12 ns,and 7.02 kW, respectively,6. The passive mode-locking performance of a-cut Nd:Lu_xGd_1-xVO₄ for x=0.5 and 1,was demonstrated with SESAM and GaAs as saturable absorbers. The pulse width of shorter than 10 ps was obtained. Compared with Nd:LuVO₄, the pulse width was found to be shortened with Nd:Lu_0.5Gd_0.5VO₄.7. For the first time, the saturable absorber of Cr⁵⁺:GdVO₄ and self-Q-switchingmaterial Nd:Cr:GdVO₄ were grown. Their passive Q-switching performance wasprimarily investigated.Based on the above research, the proming applications of the mixed crystals have been proved. It has also been found that the energy storability can be improved as x approaching 0.5 and 0.37 for Nd:Lu_xGd_1-xVO₄ and Nd:Y_xGd_1-xVO₄, respectively. In the mode-locking, the Nd:Lu_0.5Gd_0.5VO₄ was found to have pulse width shortening property, compared with Nd:LuVO₄. The primarly investigation on the Cr⁵⁺:GdVO₄ and Nd:Cr:GdVO₄ has proved their saturable properties and promising applications in the Q-swithched lasers.更多还原