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LD泵浦Nd:LYSO、Nd:LuGdAG激光特性研究
Study on Properties of LD Pumped Nd:LYSO、Nd:LuGdAG Lasers
【作者】 庄世栋;
【作者基本信息】 山东大学 , 凝聚态物理, 2012, 博士
【摘要】 根据工作物质的类型可以把激光器分为固体、液体、气体和半导体激光器四大类。总体上说,LD(激光二极管)泵浦的固体激光器(全固态激光器)结合了固体激光器和半导体激光器的优点,体积小、效率高、光束质量好、工作稳定可靠,在工业、医疗、国防、信息技术和科技前沿等领域有着重要的应用,已经成为固体激光器领域的主流研究方向。工作物质在激光器中具有核心地位,也是激光器领域基本的研究内容。目前已经在数百种固体工作物质中实现了激光运转,输出谱线有数千条。其中掺Nd3+材料倍受关注,从常见的全固态绿光激光笔到科技前沿的激光核聚变驱动器,已经在很多领域获得了实际应用。在诸多掺Nd3+晶体材料中,应用最普遍的莫过于Nd:YAG晶体和Nd:YVO4晶体。然而每种晶体材料都会存在一些固有的不足,比如Nd:YAG晶体的掺杂浓度低、生长成本高,Nd:YVO4晶体的热效应明显、光损伤阈值较低。因此探索新型激光材料、研究他们的运转特性是激光领域重要的研究内容。一般而言,正硅酸盐材料允许较高的掺杂浓度并且具有良好的机械和化学性能。其中Lu2Si05(LSO)和Y2Si05(YSO)都属于低对称性的单斜晶系,具有C2/c空间群结构,晶格中具有两个不对称的稀土格位。Nd:YSO由于具有生长性能好、自然双折射强、吸收和发射谱线宽、吸收截面大等优点而颇受关注,而LSO具有更好的热学性能。(LuxY1-x)2Si05(LYSO)则把它们的优点结合了起来,由于Lu3+离子和Y3+离子的半径差只有5%,因而Lu取代Y不会改变晶体的结构。重要的是这种取代将进一步增加晶体结构的无序性,导致激活离子的光谱具有更加显著的非均匀加宽,进而在某些应用中表现出更好的性能,例如Q调制中的能量增强效应、锁模中更短的脉冲宽度以及对泵浦源的光谱匹配要求不严格等。石榴石型晶体在众多的激光晶体材料中占有重要的地位,其中Lu3A15012(LuAG)晶体具有高的热导率和良好的物理和化学性能,而用半径更大的Gd3+离子取代Lu3+离子有助于Nd3+离子的掺杂,进而增加泵浦效率。本文研究近年出现的两种新型激光晶体Nd:(LuxY1-x)2Si05(Nd:LYSO)和Nd:(LuxGd1-x)3Al5O12(Nd:LuGdAG),涉及晶体的光学特性、LD泵浦激光的连续和被动调Q运转性能以及非线性频率变换等方面,主要内容如下:介绍了全固态激光器的研究历史和优势、掺Nd3+激光晶体的跃迁机制、被动调Q技术的特点和Cr4+:YAG晶体的可饱和吸收特性,综述了Nd:LYSO固体激光器和Nd:LuGdAG固体激光器的相关研究进展。利用偏光显微镜和棱镜耦合仪对Nd:LYSO晶体进行了光性方位和折射率的测定:b轴与X轴反向,(a,Y)=23.3。,(c,Z=-10.5。;nX=1.7915,nY=1.7933,nZ=1.8144(λ=632.8nm)。用分光光度计测得了Nd:LYSO晶体300-1000nm波段的室温偏振吸收谱,并利用Judd-Ofelt理论对光谱参数进行了计算,得到在Nd:LYSO晶体中Nd3+离子的辐射寿命为240μs,发现在811nm附近电场方向平行于X轴时吸收截面最大,平行于Y轴时吸收截面最小。利用荧光光谱仪在800-1400nm范围内测得了Nd:LYSO晶体的室温偏振吸收谱和发射谱,以及室温和低温(78K)的非偏振发射谱,并根据Fuchtbauer-Ladenbrug公式计算了Nd:LYSO晶体的偏振发射截面。发现Nd:LYSO晶体有望获得912nm、1076nm和1359nm附近的激光运转,室温时Nd:LYSO晶体以非均匀加宽为主。研究了LD端面泵浦固体激光器模型,写出了四能级系统的阈值和斜效率表达式。讨论了Cr4+:YAG被动调Q激光器的速率方程,给出了Q激光器脉冲能量、峰值功率和脉冲宽度的表达式。对LD泵浦Nd:LYSO(b切、0.5at.%)1075nm和1079nm双波长激光器进行了研究,当吸收泵浦功率3.87W时得到1.1W的连续双波长激光输出,光光转换效率28.4%,斜效率32.4%;用Cr4+:YAG晶体进行被动调Q得到的最大平均输出功率为294mW,最窄脉冲宽度为27.5ns,最大脉冲能量为34.3μJ,最高峰值功率为1.18kW。同时也研究了与Nd:LYSO结构相同性能相近的Nd:LSO晶体(b切、0.5at.%)的1075nm和1079nm双波长激光运转特性,在吸收泵浦功率3.86W时获得了1.09W连续运转双波长激光输出,光光转换效率为28.2%,斜效率为30.9%;被动调Q运转获得的最大平均输出功率为630mW、最窄脉冲宽度42.5ns、最大脉冲能量54.8μJ,最高峰值功率为1.16kW。发现对于这两种晶体,非均匀加宽的影响导致使用透过率较小的输出镜可以获得更高的效率。对沿不同主轴方向切割Nd:LYSO晶体,分别研究了它们的4F3/2→4I11/2跃迁的激光特性。由于Y切的晶体对泵浦光的吸收最强,再加上在1.08μm附近不同偏振方向的发射截面相近,所以获得了最低的阈值和最高的效率。连续运转时用X切Nd:LYSO晶体得到7.56W输出,用入射泵浦功率计算其最大光光转换效率和斜效率分别为26.5%和33.1%(用吸收泵浦功率计算为52.5%和55.8%);用Y切晶体得到10.3W输出,最大光光转换效率和斜效率分别为36.4%和45.9%(用吸收泵浦功率计算为50.2%和54.8%);Z切晶体得到的结果为7.61W、26.7%和32.3%(49.1%和51.5%)。这三种切向的晶体在高泵浦下输出激光均含1076nm和1079nm谱线。X切晶体的激光光谱以1076nm为主,1079nm所占比例甚小,Y切和Z切晶体的输出均以1079nm为主。就偏振特性而言,除X切晶体输出的1076nm激光谱线沿Y轴的振动明显强于沿Z轴的振动外,得到的其它激光谱线均基本为沿某一主轴方向的线偏振光。利用Cr4+:YAG晶体研究了沿不同主轴方向切割Nd:LYSO晶体4F3/2→4I11/2跃迁激光的被动调Q运转特性。用X切晶体获得的最高平均输出功率为2.64W(光光转换效率9.3%,斜效率14.9%)、最窄脉冲宽度为10.9ns、最大脉冲能量为120.9gJ,最高峰值功率为7.8kW。对Y切晶体上述参数分别为4.36W(16.8%,24.8%)、8.3ns、150.8μJ和14.7kW。Z切晶体得到3.17W(12.2%,18.4%)、10.1ns、52.1μJ和4.7kW。可能是由于Cr4+:YAG晶体的热效应和Fabry-Perot标准具效应的影响,输出光谱较为复杂。但在最高泵浦功率时,每种组合最强的输出波长基本相对应于连续运转时的最强波长。研究了沿不同主轴方向切割Nd:LYSO晶体4F3/2→4113/2跃迁激光的连续运转特性,用X切晶体得到最大输出功率2.22W,最大光光转换效率和斜效率分别为9.0%和12.5%(用吸收泵浦功率计算为18.3%和24.8%);Y切晶体得到2.61W、13.2%和17.1%(19.6%和24.7%);Z切晶体得到3.05W、11.7%和16.1%(22.1%和27.2%)。发现就运转效率而言Y切晶体仍然占有较明显的优势,但Z切晶体的输出功率最高,应该是由Z切晶体在该波段的峰值受激发射截面较大造成的。对Nd:LuGdAG晶体800-1400nm波段的室温荧光光谱进行了测量,发现对应于Nd3+离子4F3/2→4I9/2、4F3/2→4I11/2和4F3/2→4113/2跃迁的三组发射峰中心波长分别位于946.5nm、1063.5nm和1338nm附近,随之用LD泵浦成功实现了基于这三种跃迁的激光运转。在LD泵浦1064nmNd:LuGdAG激光器的研究中,当最大入射泵浦功率为31.4W时,得到6.88W的最大连续功率输出,光光转换效率为21.9%,斜效率为26.3%(用吸收泵浦功率计算为75.6%)。利用Cr4+:YAG晶体进行了Q调制得到3.94W的最大平均功率输出,相应的光光转换效率和斜效率分别为12.5%和15.1%(用吸收泵浦功率计算为43.4%)。获得的最窄脉冲宽度为5.8ns,最大脉冲能量为36.4μJ,最高峰值功率为6.1kW。在LD泵浦1.3μmNd:LuGdAG激光器实验中得到了3.31W的最大功率输出,最高光光转换效率和最大斜效率分别为10.5%和13.0%(用吸收泵浦功率计算为37.4%)。分析了准三能级系统理论模型,对LD泵浦948nmNd:LuGdAG激光器进行了实验研究。最大泵浦功率26.3W时得到3.03W的连续功率输出,相应的光光转换效率为11.5%,斜效率为15.6%(用吸收泵浦功率计算为44.8%)。利用Cr4+:YAG晶体进行被动调Q,在26.3W入射泵浦功率下得到1.01W的最大平均功率输出,获得的最窄脉冲宽度、最大脉冲能量和最高峰值功率分别为20.7ns,124.0μJ和5.8kW。介绍了倍频的基本理论,利用LBO晶体(Ⅱ类相位匹配)进行腔内倍频,成功获得了LD泵浦474nmNd:LuGdAG/LBO蓝光激光输出。连续运转时得到802mW的蓝光激光输出,光光转换效率为3.0%(用吸收泵浦功率计算为8.6%)。在Cr4+:YAG被动调Q运转中得到331mW的平均功率输出,光光转换效率为1.3%(用吸收泵浦功率计算为3.7%),获得的最窄脉冲宽度、最大脉冲能量和最高峰值功率分别为38.0ns、6.4μJ和146W。
【Abstract】 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.