稀土离子掺杂铌酸钙超快激光晶体的生长及性能研究
文献类型：学位论文
| 作者 | 程艳 |
| 学位类别 | 博士 |
| 答辩日期 | 2010 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 周圣明 ; 徐晓东 |
| 关键词 | 激光晶体 提拉法生长 Yb:CaNb2O6 Nd:CaNb2O6 Yb:Ca2Nb2O7 |
| 其他题名 | Study of growth, structure, spectra and laser properties on Re3+ doped Calcium Niobate laser crystals |
| 中文摘要 | 寻求新型稀土离子掺杂的宽调谐超快激光晶体材料对发展新型全固态高功率和超快激光具有十分重要的意义。稀土Re3+掺杂的焦铌酸钙Ca2Nb2O7和正铌酸钙CaNb2O6 系列晶体因其独特的低对称性晶体结构和Re3+替代Ca2+出现电荷补偿造成的Re3+无序分布，均为Re3+提供了较强的晶体场和配位场，这有利于Re3+产生大的Stark能级分裂，导致发射带宽的非均匀加宽，是宽调谐超快稀土离子掺杂激光晶体的理想基质。本论文重点选择Yb/Nd掺杂的焦铌酸钙Ca2Nb2O7和正铌酸钙CaNb2O6作为研究对象，采用提拉法生长晶体，开展结构与基本物理性质、光谱、激光性能关系的探索性工作。主要内容如下： 1. 采用提拉法生长不同浓度的Yb:CaNb2O6，通过优化生长工艺，得到无色透明、高光学质量、无宏观缺陷的晶体。研究了不同浓度掺杂Yb:CaNb2O6晶体的晶格常数、晶体结构、Yb3+分凝系数、力学性能、热学性能、光谱性能（吸收光谱、发射光谱、荧光寿命）和激光性能。通过XRD分析计算了CaNb2O6和Re3+:CaNb2O6系列晶体的晶胞参数，确定它们的正交晶体结构。Yb:CaNb2O6晶体中Yb3+分凝系数km约为0.65。Yb:CaNb2O6断裂韧性和断裂强度分别为1.73MPam1/2和315MPa。Yb:CaNb2O6具有高热导率(沿c轴方向最大为6.05W/mK)、大的比热(0.45J/g K)。测试Yb:CaNb2O6晶体中Yb离子的吸收、发射光谱和荧光寿命，并通过光谱参数的计算，我们发现Yb:CaNb2O6具有很宽的吸收带宽，能与InGaAs LD抽运有效耦合，而且降低了对LD泵浦源温度控制的要求；宽的受激发射带宽，有利于LD泵浦超宽调谐激光输出。Yb3+基态2F7/2能级分裂达1014cm-1。首次对Yb:CaNb2O6晶体的激光性能进行探索，在 975nm LD泵浦下，通过合理设计谐振腔和散热系统，在吸收泵浦功率7.2W时，最优的激光输出为810mW，激光输出中心波长为1062nm，光光转换效率和斜效率分别为12%和19%，这是迄今为止Yb掺杂铌酸盐激光晶体最大输出功率。 2. Nd:CaNb2O6晶体中Nd 3+分凝系数约1.03，Nd:CaNb2O6具有高热导率(沿c轴方向最大为8.24W/mK)、大的比热（44.6cal/mol•K）。Nd:CaNb2O6晶体在808nm左右具有8nm的宽吸收带，吸收截面为5.04×10-20cm2。根据J-O理论，拟合了该晶体的晶体场参数并计算了部分光谱参数。Nd:CaNb2O6晶体在1062nm的发射截面积σem为8.75×10-20cm2，发射带宽为15nm。Nd3+离子的荧光寿命为152μs，量子效率为91%。首次报道了Nd:CaNb2O6晶体的有效二极管泵浦连续激光输出，激光输出功率1.86W，激光输出中心波长为1062nm，斜效率为19%，实验过程中没有出现饱和现象。Nd:CaNb2O6晶体具有宽的调谐特性，采用SESAM进行锁模激光实验，在Nd:CaNb2O6晶体中初步实现了锁模17.3ps激光脉冲输出，重复频率为88.4 MHz。多脉冲时得到了更短的脉冲宽度为13.9ps。用低功率单管LD得到的最大锁模平均功率为0.843W，激光斜效率为23%，实验结果表明Nd:CaNb2O6晶体是有希望的产生皮秒脉冲激光的有效增益介质。 3. 研究了Yb:Ca2Nb2O7晶体的生长、结构、Yb3+分凝系数、吸收光谱、发射光谱和荧光寿命。采用提拉法生长，通过优化生长工艺，得到无色透明、高光学质量、完整不开裂、无宏观缺陷的晶体。通过单晶X射线衍射和XRD分析计算了Ca2Nb2O7和Re3+:Ca2Nb2O7晶体的晶胞参数，确定它们属于正交晶体结构，其中Ca离子有4种不同格位，所以由于Re和Ca离子共同作用，在晶格中形成多格位特征，造成晶格无序度增加，有利于LD泵浦超宽调谐激光输出。Yb:Ca2Nb2O7晶体拥有宽的吸收和发射带宽，吸收带宽7.5nm，发射带宽57nm。根据吸收和发射光谱，估算了Yb:Ca2Nb2O7晶体的激光参数。通过良好的光谱和激光参数，可以预计该晶体是有希望的宽调谐超快激光增益介质。 |
| 英文摘要 | Developing new Re3+-doped laser materials for generation of efficient broadly tunable and ultra-short pulses lasers has attracted much attention in DPSSL. Calcium pyroniobate (Ca2Nb2O7) and Calcium Niobate (CaNb2O6) crystals possess a low-symmetry crystal structure and Re3+ ions are randomly distributed in Re:CaNb2O6 and Re:Ca2Nb2O7 crystals because there are various kinds of lattice site substitution of Re3+, Ca2+, Nb5+ and manifold charge compensation mechanism, which should be favorable for creating strong local crystal-field and coordination field strengths. Depending on the strong crystal-field and disorderd distribution of Re ions in Ca2Nb2O7 and CaNb2O6 crystals, it is expected to exhibit a large overall splitting of the ground-state manifold 2F7/2. As a consequence, the split levels, which function as terminal laser levels, will result in inhomogeneous broadening of emission band. This may support continuous tunability of the Yb:CaNb2O6 laser over a broad range of a smooth tuning curve. The study focused on the relationship between growth, structure, spectral and laser properties on the Yb3+ doped calcium pyroniobate (Ca2Nb2O7) and Yb3+, Nd3+ doped calcium niobate (CaNb2O6) crystals. Main contents are as follows: 1. Yb:CaNb2O6 crystals were grown by the Czochralski method after growth parameters had been optimized. The boules were colorless, high optical quality, cracking-free and no inclusions. The crystal lattice constants, Yb3+ segregation coefficient, thermal conductivity, absorption spectra, emission spectra and fluorescence lifetimes of Yb:CaNb2O6 were studied. The crystal structure of the as-grown Yb:CaNb2O6 crystals were examined by XRD, and the unit cell parameters were calculated. It demonstrated that Yb:CaNb2O6 crystal belongs to a low-symmetry orthorhombic syngony with the space group of . ICP experimental data show that the segregation coefficient of ytterbium ions in Yb:CaNb2O6 crystals is 0.65. The fracture toughness and fracture strength of Yb:CaNb2O6 single crystal are 1.73 MPam1/2 and 315 MPa, respectively. Yb:CaNb2O6 crystal possess a high thermal conductivity （about 6.05 Wm-1K-1 along c-axis）and a large specific heat (0.45 J/g K) for 1.5 at. % Yb doped CaNb2O6 crystal. Absorption, emission spectra and fluorescence decay curve of Yb3+ ions in Yb:CaNb2O6 at room temperature were also studied, and then spectroscopic parameters were calculated. Yb:CaNb2O6 has broad absorption band, which is well matched with the emission wavelength of efficient diode-pumping using high performance InGaAs laser diodes. Their particularly broad emission bandwidth is very suitable to be used for generating ultra-short pulses and as tunable laser medium. According to the absorption and emission spectra, it has been estimated that the overall splitting of the 2F7/2 manifold reaches about 1014 cm-1, which is one of the largest known values. Yb:CaNb2O6 laser properties were investigated for the first time. Under LD pumping at 975 nm, the laser experiments of Yb:CaNb2O6 crystals were performed with appropriate laser cavity and mode-locking elements. Optimal output power reached 810 mW at an absorbed pump power of 7.2 W near 1062 nm. The optical conversion efficiency and slope efficiency in this case were 12% and 19%, respectively. The output power is the highest reported so far for Yb-doped niobate crystals. 2. The segregation coefficience of Nd3+ in Nd:CaNb2O6 crystal is 1.03. Nd:CaNb2O6 crystal possesses a high thermal conductivity (about 8.24 Wm-1K-1 along c-axis) and a large specific heat (44.6 cal/mol•K) for 1.0 at. % Nd doped CaNb2O6 crystal. The absorption spectra, fluorescence spectra and fluorescence decay curve of Nd3+ ions in CaNb2O6 crystal were measured at room temperature. The peak absorption cross section was calculated to be 5.04×10-20 cm2 with a broad FWHM of 8 nm at 808 nm. The spectroscopic parameters of Nd3+ ions in CaNb2O6 crystal have been investigated based on Judd-Ofelt theory. The emission cross section at 1062 nm is 8.75×10-20 cm2 with a broad FWHM of 15 nm at 1062 nm. The radiative lifetime, the fluorescence lifetime and the quantum efficiency are 167 μs, 152 μs and 91%, respectively. We report what we believe to be the first demonstration of the continuous-wave laser operation of Nd:CaNb2O6 under diode pumping. Output power of 1.86 W at 1062 nm was obtained with a slope efficiency of 19% in the cw regime. No output saturation occured at the highest pump level applied in the experiment. We have experimentally demonstrated a diode-pumped passively mode-locked Nd:CaNb2O6 laser for the first time to our best knowledge. With a semiconductor saturable absorber mirror (SESAM) as a passive mode locker, the laser generated stable mode-locked pulses with a pulse duration of 17.3 ps and repetition rate of 88.4 MHz. The laser also generated numerous pulses with shorter pulse duration of 13.9 ps. With a single-emitter laser diode pumping, the maximum average output power of the mode-locked laser was 0.843 W, with a slope efficiency of 23%. The experimental results show the Nd:CaNb2O6 crystal is a promising laser gain medium for picosecond pulse generation. 3. The crystal growth, structure, Yb3+ segregation coefficient, absorption spectra, emission spectra and fluorescence lifetime of Yb:Ca2Nb2O7 were investigated. Yb:CaNb2O6 crystals were grown by the Czochralski method after growth parameters had been optimized. The boules are colorless, high optical quality, cracking-free and no inclusions. The crystal structure of the as-grown Ca2Nb2O7 and Yb:Ca2Nb2O7 crystals were examined by single crystal and powder X-ray diffraction, respectively. It is revealed that Yb:Ca2Nb2O7 crystal belongs to a low-symmetry orthorhombic syngony with space group of Pn21a. Ca2+ ions own four crystallographic sites in Yb:Ca2Nb2O7 crystal. Thus multi-crystallographic sites existed in the crystals resulting in disordered distribution of Re3+ and Ca2+ ions, which is favorable to LD-pumped ultra-wide tunable laser output. Yb:Ca2Nb2O7 crystal possesses broad absorption and emission band width. The values are 7.5 nm and 57 nm, respectively. The good spectroscopic and laser performance parameters indicate that Yb:Ca2Nb2O7 crystals are very promising laser gain medium in diode-pumped tunable solid state laser applications. |
| 语种 | 中文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15289]  |
| 专题 | 上海光学精密机械研究所_学位论文 |
| 推荐引用方式 GB/T 7714 | 程艳. 稀土离子掺杂铌酸钙超快激光晶体的生长及性能研究[D]. 中国科学院上海光学精密机械研究所. 2010. |
入库方式： OAI收割
来源：上海光学精密机械研究所
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