【作者】 赵永光；

【导师】 许心光； 王正平； 于浩海；

【作者基本信息】 山东大学 ， 凝聚态物理， 2014， 博士

【摘要】 作为激光器的核心部件,激光材料一直是人们研究的热点。由于良好的光学特性、机械特性以及稳定的化学特性,钕掺杂的钒酸盐和硅酸盐晶体成为两大类比较重要的激光增益介质。对于钒酸盐晶体来说(Nd:YVO4, Nd:LuVO4和Nd:GdVO4),因为其内部掺杂的钕离子具有较大的吸收截面和受激发射截面,从而使得该类晶体成为中小功率全固态激光器的首选。然而,大的受激发射截面和相对较短的上能级寿命限制了该类材料在中小功率脉冲激光器中的应用。为了解决这一难题,近几年生长出了掺钕钒酸盐“混晶”(主要是Nd:GdxY1-xVO4、 Nd:LuxGd1-xVO4和Nd:LuxY1-xVO4)。研究证明,在该类“混晶”中基质不同阳离子的相互取代使得钕离子的受激发射截面减小且上能级寿命增长,相应的使得其能量存储能力增强。从本质上来说产生这一现象的主要原因是此类“混晶”中钕激活离子发射光谱的非均匀展宽,然而对于非均匀展宽的的直接测量国内外未见任何报道。针对这一研究空白,本论文第二章对上述三种钒酸盐混晶中(GdxY1-xVO4, LuxGd1-xVO4和LuxY1-xVO4)钕离子发射光谱的非均匀展宽情况进行了直接研究。结果表明,由于基质中相同晶格内不同阳离子的相互占据,“混晶”光谱有了明显的非均匀展宽。而这种展宽对于调Q和锁模脉冲激光器来说都是非常有益的。另外,根据我们测得的不同温度光谱发现,随着温度增加发射谱变宽且发射峰均出现红移。在低温77K下,通过与常规钕钒酸盐晶体(Nd:YVO4, Nd:LuVO4和Nd:GdVO4)半峰宽的比较,可以直接得到每种组分配比的非均匀展宽量。对三个系列的钒酸盐混晶来说,最大的非均匀展宽和最长的能级寿命分别来自Nd:Gd0.63Y0.37VO4、Nd:Lu0.5Gd0.5VO4以及Nd:Lu0.61Y0.39VO4。通过首次对非均匀展宽效应的直接测量,可以确信基质阳离子相互取代是一种用来调节激活离子发射特性从而满足不同激光需求的有效技术手段。近年来,大量的激光实验都证明Nd:GdxY1-xVO4和Nd:LuxGd1-xVO4晶体脉冲能量增强效应和更短脉冲锁模特性。本文结合直接测量的光谱非均匀展宽特性发展了新型Nd:LuxY1-xVO4晶体,并在第三章系统全面地研究了该系列晶体的调Q、锁模以及自受激拉曼激光特性。在调Q脉冲激光实验中,用c切Nd:Lu0.1Y0.9VO4晶体得到了脉宽为1.9ns、能量为104.7μJ以及峰值功率为55.1kW的1.06μm被动调Q脉冲输出。用a切Nd:Lu0.1Y0.9VO4晶体得到脉宽为23.8ns,脉冲能量为124μJ以及峰值功率5.21kW的1.3μm声光调Q脉冲输出。本章也首次实现了该系列晶体的声光自拉曼激光输出,用半径为800μm的大光斑激光二极管作为泵浦源有效减小了激光和拉曼过程中产生的热效应。结合c切晶体大的能量存储能力最终得到1175nm拉曼激光结果为：101μJ脉冲能量,1.3ns脉冲宽度和78kW峰值功率。通过比较a切Nd:LuxY1-xVO4系列晶体的声光自拉曼激光输出结果,发现2.3W的最大输出功率来自具有较大热导率的Nd:Lu0.26Y0.74VO4晶体。除此之外,本章也研究了该系列晶体的锁模激光特性,最短的锁模脉宽来自光谱最宽的组分(x=0.61),这也间接地证明了Nd:Lu0.61Y0.39VO4晶体中钕离子最强的非均匀展宽效应。对于硅酸盐晶体的研究,之前主要是集中在闪烁晶体和镱离子掺杂上。近年来由于该类晶体的各向异性激光发射特性,钕激活离子掺杂的研究也频繁出现。然而,目前的研究还停留在基础工作,包括材料的生长、非偏振发射谱的测量、单一切向激光输出等。对其偏振光谱、各向异性发射及应用方面研究甚少。针对这样的研究现状,本文第四章通过测量不同折射率主轴方向的偏振发射谱,首次计算了Nd:LYSO晶体受激发射截面的空间分布；并利用受激发射截面很好地解释了该晶体的各向异性激光输出,提出利用低对称性晶体不同切向发射特性的差异来满足不同实际需要。利用该晶体双波长输出特性,结合新型可饱和吸收材料石墨烯的宽波段调制特性成功实现了双波长同步调Q激光输出。所得到的最大脉冲平均输出功率为1.8W,单脉冲能量为11.3μJ,峰值功率为118W。比光纤激光器中相应结果大2～4个数量级,这也是目前石墨烯应用于全固态激光器中的最好输出结果。良好的的脉冲输出特性已能实现腔外非线性和频,这不但说明得到的双波长在时域上是同步的,另一方面也有助于石墨烯的进一步实用化。利用Nd:LYSO晶体作为激光增益介质,我们在简单紧凑的两镜腔内观察到了连续和脉冲光漩涡激光输出,详见本论文第五章。这是国际上首次直接产生光漩涡(没用任何其他辅助光学元件)。随着泵浦功率的增加,脉冲光漩涡的脉冲能量维持相对稳定但是其轨道角动量是增加的。用柱透镜对组成模式转换器,将得到的LG模式光漩涡转化为HG模式输出,从而确定我们得到的是LG01和LG02模式的光漩涡。为了更明确光漩涡的产生过程和变化原因,用量子亏损更严重的1.36μm的激光输出来进行验证。结果高阶LG模式出现得更早且最终得到了更高阶的LG1o模激光。结合在1.08μm的光漩涡输出情况我们初步进行了解释：在连续激光运转下,热透镜效应使得振荡光模式半径变小,然后通过与泵浦光的模式匹配来进行输出激光模式的选择。在脉冲激光运转中,除了热透镜效应之外还有高功率密度所引进的非线性效应,这就更加加剧了光漩涡的产生,因此在脉冲激光中可以较早地得到光漩涡。此外,在1.36μm的被动调Q实验中,我们所用的可饱和吸收体为石墨烯,该过程证实石墨烯在对激光进行脉冲调制时不影响拓扑电荷的变化,可以作为光漩涡脉冲调制器件。本章研究的这种结构紧凑、轨道角动量可连续改变的光漩涡激光脉冲将在很多领域有潜在的应用。更多还原

【Abstract】 As a key part of the laser, laser materials are always in the spotlight of scientific interest. Up to now, Nd-doped vanadate and silicate crystals have been demonstrated to be two prominent gains mediums, since they have excellent optical, mechanical and chemical properties. For the vanadate crystals, such as Nd:YVO4, Nd:LuVO4and Nd:GdVO4, they are the primary materials in the diode pumped solid state laser with middle to low output power, because of their large absorb and emission cross-sections. However, the relatively large emission cross-section and short up-level lifetime have constrained their application in Q-switched lasers because it requires a large energy storage capacity, which in turn demands a small emission cross section and a long fluorescence lifetime. So, the vanadate mixed crystals (including Nd:GdxY1-xVO4、 Nd:Lux-Gd1-xVO4and Nd:LuxY1-xVO4) were grown and researched recently. Previous laser experiments have demonstrated that the stimulated emission cross-sections decreased and fluoresces lifetime increased in the mix crystals, which can enhance the energy storage capacity. In essence, it is caused by the inhomogeneous spectrum broadening of Nd3+ions in the mixed crystals. However, the direct research on the inhomogeneous spectrum broadening effect has never been reported. In chapter2, a study of inhomogeneous spectrum broadening in Nd3+-doped mixed crystals (GdxY1-xVO4, LuxGd1-xVO4and LuxY1-xVO4) is reported for the first time. Due to the random placement of different cations in the lattice, the spectra are inhomogeneously broadened, which is beneficial for both Q-switching and mode locking. For all the samples, as increasing the temperature, the spectrum broadened and emission peak presented "red shifted". Using traditional crystals (Nd:LuVO4, Nd:YVO4or Nd:GdVO4) as the reference, the inhomogeneous spectrum broadening caused by the random replacement can be obtained at77K. The largest spectrum width and longest lifetime are obtained by the same component. They are Nd:Gd0.63Y0.37VO4, Nd:Lu0.5Gd0.5VO4and Nd:Lu0.61Y0.39VO4crystals, respectively. All the results indicate that the random placement of different cations at the same lattice site is an efficient technology for the modification of emission properties that fulfill different requirements of laser applications.Recently, some research have demonstrated the large energy storage capacity and short mode-locking laser pulses of the Nd:GdxY1-xVO4and Nd:LuxGd1-xVO4mixed crystals. Combine with the property of the emission spectrum, we reported the Q-switching, mode-locking and self-stimulated Raman laser characteristics of the novel Nd:LuxY1-xVO4crystal in chapter3. For the1.06μm passively Q-switched laser, the shortest pulse width, the largest pulse energy and the highest peak power were found to be1.9ns,104.7μJ and55.1kW, respectively, using c-cut Nd:Lu0.1Y0.9VO4crystal as the laser gain and a Cr4+:YAG crystal as the saturable absorber. For the1.3μm acousto-optic (A-O) Q-switching laser operation, the shortest pulse width, largest pulse energy, and highest peak power were23.8ns,124μJ, and5.21kW, respectively, obtained by the a-cut Nd:Lu0.1Y0.9VO4crystal. In this chapter, the1175-nm A-0Q-switched self-Raman laser of Nd:LuxY1-xVO4crystal was also demonstrated for the first time. Combining the advantages of mixed vanadate crystal and c-cut orientation, the c-cut Nd:Lu0.1Y0.9VO4crystal exhibited prominent self-Raman laser characteristics, such as101μJ single pulse energy,1.3ns pulse width, and78kW peak power. With the largest thermal conductivity, excellent energy storage capability and moderate Raman gain coefficient, the a-cut Nd:Lu0.26Y0.74VO4crystal presented the largest average output power of2.3W. With a semiconductor saturable absorber mirror (SESAM), the passively mode-locking of this series crystal was carried out under the same condition. The shortest pulse was obtained by the Nd:Lu0.61Y0.39VO4crystal, which indirectly manifested the widest line-width of this component and its excellent properties for pulse laser applications.Previous researches about LYSO have been focused on scintillator applications and Yb-doped lasers. Research about the Nd-doped LYSO was mainly focus on the crystal growth and simple laser experiment. Chapter4mainly discussed the the anisotropy of laser emission in the monoclinic, disordered crystal Nd:LYSO. According to the polarized emission spectrum, we calculated the spatial distribution of the stimulated emission cross-sections for the first time. Then the output characteristic is well explained by a theoretical analysis on the stimulated emission cross-section. This work reveals that the intrinsic anisotropy in disordered laser crystal can be utilized to elevate the emission controllability. Accordantly, the material’s application scopes can be extended. Using multilayered graphene as the saturable absorber (SA), Nd:LYSO crystal as the laser material, we demonstrated a laser-diode (LD) pumped, dual-wavelength passively Q-switched solid-state laser. The maximum average output power is1.8W, the largest pulse energy and highest peak power is11.3μJ,118W, respectively. The results are2-4orders of magnitude of the corresponding values that obtained in the fiber laser. As we have known, they are also the best results for passively Q-switched operation of grapheme in all solid-stated lasers. The pulse laser is strong enough to realize extra-cavity frequency conversions, which manifests the synchronous modulation to the dual-wavelength with multi-layered grapheme, and the possibility of its practical.Chapter5demonstrated the direct generation of1.08μm optical vortex pulses in a simple and compact two-mirror laser cavity for the first time, with Nd:LYSO crystal as laser gain material. Single Laguerre-Gaussian (LG0,1) laser modes were directly generated using a laser diode with output intensity profile of doughnut distribution. With passive Q-switching, vortex pulses with stable energy were obtained. Moreover, the topological charge was changeable by variation of the pump power. By a mode-converter and second harmonic generation, the LG0,21mode was identified. In order to know the generation of the vortex and its changing, we demonstrated the continuous-wave (cw) and pulse optical vortex operation at1.36μm. During the lasing process, the topological charges were changeable by the thermal-induced lens and selected by the mode-matching between the pump and oscillating beams. With a graphene sample as the saturable absorber, the pulsed optical vortex was achieved, which identified that graphene could be used as a pulse modulator for the generation of pulsed optical vortex. It could be believed that the thermally driven cw and pulsed optical vortex should have various promising applications based on the compact structure, changeable topological charges and specific wavelength.更多还原