【作者】 徐洪浩；

【导师】 张怀金； Dingyuan Tang；

【作者基本信息】 山东大学 ， 材料学， 2014， 博士

【摘要】 激光器作为20世纪最重要的发明之一,经过50多年的发展,已经在国家安全、科学研究、医疗卫生和日常生活中等各个领域发挥着重大的作用。固体激光器由于其体积小、稳定性高、结构简单等特点,成为激光器研究的重点,引领着激光器的研究方向。激光器工作模式可以分为连续输出模式和脉冲输出模式,其中脉冲激光由于具有大脉冲能量、高峰值功率、超短脉冲宽度等优点,有着很多重要的应用。例如当与物质相互作用时,由于脉冲能量很高,会产生一系列的新现象,例如强光自聚焦、受激拉曼散射和受激布里渊散射等一系列非线性光学效应；同时在精密加工、超快过程测量等方面有着重要的实际应用。获得脉冲激光的手段主要有调Q技术和锁模技术。调Q激光技术的提出是激光器的发展史上的一个重要突破,人们在压缩脉冲宽度和增加峰值功率上的实际要求,不断推动着调Q技术的发展。Q开关的种类由最初的机械开关逐渐发展为电光开关、声光开关和被动Q开关等,其中被动Q开关由于设计简单、便于小型化和成本低等特点,成为广泛使用的一类Q开关。最初人们采用有机染料作为被动Q开关,后来人们发现含有色心的r2：LiF和掺有吸收离子的Cr4+:YAG等晶体材料同样具有良好的可饱和吸收作用,近年来,新型的石墨烯、拓扑绝缘体和二硫化钼等材料也被用作被动Q开关,采用调Q技术,人们已经获得了脉冲能量为毫焦和峰值功率为兆瓦的脉冲激光。锁模技术是另一种获得脉冲激光的技术,锁模主要分为被动锁模和主动锁模,对于被动锁模,最初人们采用有机染料作为可饱和吸收体来获得锁模脉冲,克尔透镜锁模技术的发明为固体锁模提供了一种相对简单的方法。现在商业化的半导体可饱和吸收镜(SESAM)作为主要的可饱和吸收材料,在锁模激光器中得到广泛应用。同时一些新的材料例如碳纳米管和石墨烯也被发现具有可饱和吸收作用,并被证明可以用来锁模。人们在发展脉冲激光技术的同时,也在积极探索新型脉冲激光增益材料。首先人们对不同激活离子进行了研究,例如Tm3+离子具有较长的上能级寿命,有利于能量的存储,适合用作调Q激光器；而Ti3+、Cr3+和Yb3+等离子具有宽的发射光谱,理论上,脉冲宽度近似等于振荡线宽的倒数,所以这类激活离子适合超短脉冲的产生,其中Ti3+掺杂的蓝宝石就是一种重要的获得飞秒脉冲的增益材料。另外基质材料对激活离子的光谱的性质也有很大的影响,以Nd3+离子为例,采用有序晶体等有序材料作为基质,虽然热学性质良好,但是其发射谱线尖锐,不利于超短脉冲激光的产生；采用玻璃为基质,通常具有宽的发射谱线和大的荧光寿命,适合脉冲激光的产生,但是这类增益介质的热学性质较差。无序晶体集成了晶体和玻璃的优点,既保持晶体优良的热学性质,又具有了玻璃较宽的发射光谱,已经成为了脉冲激光材料研究的热点。混晶虽然在结构上具有晶体的长程有序的特点,但是激活离子在混晶中是随机取代的,占据的格位并不同,而位于不同格位的激活离子周围的晶格场不同,具有不同的荧光谱,不同格位激活离子谱线的相互叠加在宏观上造成了混晶荧光谱的非均匀加宽,所以混晶同样具有较宽的荧光谱线,适合用作脉冲激光材料。钒酸盐晶体一种优良的激光晶体材料,其中掺钕钒酸钇(Nd:YVO4)最早被人们发现,现在已经作为商业化的激光增益材料广泛应用于日常生活的各个方面,并且和掺钕钇铝石榴石(Nd:YAG)和掺钛蓝宝石(Ti:Al2O3)并成为三大基础激光晶体,钒酸镥(LuVO4)和钒酸钆(GdVO4)作为钒酸钇的同系物,也被证明具有优良的激光性能。他们都具有四方结构,都可以采用提拉法生长,并且熔点都在1800℃左右,基于这些相同点,人们将他们两两混合组成LuGdVO4、YGdVO4和LuYVO4等钒酸盐混晶,作为脉冲激光材料已经被人们广泛研究,其光谱具有明显的非均匀加宽,调Q性能证明了这类混晶具有较强的脉冲增强效应。钒酸镧(LaVO4)是钒酸盐家族的另一个成员,它在晶体结构上属于单斜晶系,由于其螺旋生长特性,采用提拉法不容易获得高质量晶体,晶体结构上的差异使得含镧的钒酸盐混晶在脉冲激光性能上更加具有优势,而当前人们对钒酸盐混晶的研究主要集中在同结构钒酸盐晶体的混合上,对含镧的钒酸盐混晶的研究反而较少,尤其是含镧钒酸盐混晶的脉冲激光方面的性能更是没有系统的研究。含镧钒酸盐混晶指的是LaxRn1-xVO4(Rn=Y、Lu和Gd)系列混晶,相比之前研究较多的LuGdVO4、YGdVO4和LuYVO4混晶,它具有自己的优势。第一,钒酸镧晶体属于单斜晶系,而钒酸钇、钒酸镥和钒酸钆晶体都属于四方晶系,不同晶系的晶体相互混合将对激活离子周围的晶格场造成更大的变化,进而造成光谱更大的非均匀加宽；第二,La3+相比于Y3+、Lu3+和Gd3+等离子具有最大的离子半径,这种大的离子半径的差异同样会引起激活离子周围晶格场大的变化,造成光谱更大的非均匀加宽。晶体结构和离子半径上的差异意味着相对少的La3+掺杂量就可以为光谱带来相同量的非均匀加宽,这样就可以最大限度的保证激光增益介质的热学性质。在晶体生长方面,加入较少的La3+,使含镧的钒酸盐混晶仍然保持四方结构,这样就可以避免了钒酸镧晶体的螺旋生长,生长出高质量晶体。本论文从探索脉冲激光材料出发,生长了钕掺杂的含镧钒酸盐混晶(Nd:LaxRn1-xVO4(Rn=Y、Lu和Gd)),并对其组分、晶体结构、热学性质、光谱性质和激光性能进行了表征,主要工作如下：一、晶体生长、晶体组分与结构采用提拉法,生长了Nd:LaxRn1-xVO4(Rn=Y、Lu和Gd)系列混晶,并对晶体提拉炉和提拉法的生长工艺进行了简单的介绍,讨论了生长过程中需要注意的问题。通过X射线荧光方法,确定了生长的混晶的具体组分,并计算了Nd3+在三种系列混晶中的分凝系数。利用X射线衍射,对生长的混晶结构进行了分析,确定晶体属于四方晶系,空间群为I41/amd,并计算出相应的晶胞参数。二、热学性质对生长的Nd:La0.11Y0.899VO4、Nd:La0.05Lu0.95VO4、Nd:La0.12Gd0.88VO4和Nd:Lao.25Gd0.75VO4四种混晶进行了详细的热学性质的测量,主要测量了热膨胀系数、比热、热扩散系数和热导率,以及这些热学参数随温度变化的规律。与Nd:YVO4、Nd:LuVO4和Nd:GdVO4三种单晶的热学性质相比有所下降,但是仍然能满足中小功率激光器的要求。三、光学性质对所生长的Nd:La0.11Y0.89VO4、Nd:La0.05Lu0.95VO4、Nd:La0.12Gd0.88VO4和Nd:La0.25Gd0.75VO4四种混晶的光学性质进行了研究。利用最小偏向角法,对Nd:La0.11Y0.89VO4、Nd:La0.12Gd0.88VO4和Nd:La0.25Gd0.75VO4三种混晶的折射率进行了测量,并拟合出色散曲线,为后续光谱的计算和晶体的镀膜奠定了基础；对Nd:La0.11Y0.89VO4、Nd:La0.05Lu0.95VO4、Nd:La0.12Gd0.88VO4和Nd:La0.25Gd0.75VO4四种混晶的偏振吸收光谱和偏振发射光谱进行了测量,计算出了相应的吸收截面、发射截面和受激辐射寿命等光谱参数。与Nd:YVO4、Nd:LuVO4和Nd:GdVO4三种单晶的光谱性质相比,含镧钒酸盐混晶具有较宽的吸收和发射谱线、小的发射截面和长的荧光寿命,适合在脉冲激光器中应用。通过对不同温度下荧光谱和荧光寿命的测量,发现随着温度的升高,荧光寿命逐渐变长,而谱线宽度逐渐变宽。四、激光性质采用激光二极管(laser diode, LD)泵浦,对生长的钒酸盐混晶进行了连续激光性能研究,发现a切的钒酸盐混晶相对于c切的钒酸盐混晶有更好的连续激光性能,其中Nd:La0.11Y0.89VO4,a切晶体的斜效率为46.5%,c切晶体的斜效率为43.9%; Nd:La0.05Lu0.95VO4两者的斜效率分别为43.7%和16.8%；Nd:La0.12Gd0.88VO4,两者的斜效率分别为38.8%和26.9%。另外c切的Nd:La0.11Y0.89VO4、Nd:La0.2Y0.8VO4和Nd:La0.05Lu0.95VO4等晶体具有双波长发射的特性。以Cr4+:YAG为饱和吸收体,对a切的Nd:La0.11Y0.89VO4、Nd:La0.05Lu0.95VO4和Nd:La0.12Gd0.88VO4混晶进行了被动调Q激光实验,获得的最大脉冲能量分别为97.3μJ、114.9μJ和124.7μJ,通过与Nd:YVO4、Nd:LuVO4和Nd:GdVO4的对比,证明了La3+的掺杂,确实起到了脉冲增强作用。通过对Nd:La0.12Gd0.88VO4、 Nd:La0.25Gd0.75VO4和Nd:La0.3Gd0.7VO4混晶调Q激光特性研究,在相同条件下,发现Nd:La0.3Gd0.7VO4获得了最大的脉冲能量与峰值功率,分别为131.5μJ和15.3KW。以SESAM为饱和吸收体,对a切的Nd:La0.11Y0.89VO4、 Nd:La0.12Gd0.88VO4混晶进行了被动锁模激光性能的研究,分别获得了脉冲宽度为6.8ps、7.6ps和3.4ps的锁模脉冲。利用c切的Nd:La0.11Y0.89VO4混晶,获得了稳定的σ偏振态的双波长锁模输出。针对a切钒酸盐晶体的偏振发射特性,通过对损耗的控制,分别获得π偏振态锁模、σ偏振态锁模和双偏振态锁模。通过以上的研究,我们证实了钕掺杂含镧钒酸盐混晶具有优良的热学和光谱性质,是一种优良的激光增益介质。其光谱具有明显的非均匀加宽,适合脉冲激光器的应用。更多还原

【Abstract】 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.更多还原