| 题名 | 新型被动Q开关晶体材料的研究 |
| 作者 | 张丹 |
| 学位类别 | 硕士 |
| 答辩日期 | 2007 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 徐军 |
| 关键词 | 可饱和吸收 V:YAG 晶体生长 光谱性能 被动调Q |
| 其他题名 | Study on several New Passive Q-switchers |
| 中文摘要 | 窄脉宽、高峰值功率的脉冲激光在光测距、光通信、高速全息照相等领域有着广泛的应用。而被动调Q技术由于其结构简单、成本低、使用方便等优点，已成为实现这一功能最简单的装置之一。新型被动调Q材料V3+:YAG晶体由于其可饱和吸收波段宽、基态吸收截面大、激发态吸收损耗小、恢复时间短、饱和能量密度低、损伤阈值高等特点，已成功在0.78μm、1.06μm、1.3μm及1.44μm等波段实现调Q或锁模激光运转。但目前，使用射频加热法生长的V:YAG晶体仍存在晶体质量差、掺杂浓度低、吸收系数小等缺点，所以生长高质量、高掺杂浓度、高吸收系数的V:YAG晶体具有非常重要的意义。相比于V:YAG晶体，Cr4+:YAG晶体吸收截面更大，应用较广泛，但是其可饱和吸收波长范围为0.8~1.2μm，不再适合1.3μm等长波段的饱和吸收调Q。由于Cr4+属于过渡金属离子，电子¬－静态晶格相互作用同电子的库仑相互作用是可比的，所以其轨道能级分裂随着晶格场不同差异很大，可以想象把Cr4+掺杂进入不同的晶格场中使其能级分裂适用不同波段的可饱和吸收调Q。实际应用的调Q可饱和吸收体一般为四能级结构，大多数都存在有激发态吸收损耗，寻找无激发态损耗的可饱和吸收体意义将更加重要。 本论文首次采用石墨电阻加热的温度梯度法成功生长出了Ф75mm高质量的V:YAG晶体。对比分析了不同颜色V:YAG晶体的室温吸收光谱，推断出石墨发热体高温下扩散出来的C可以起到还原作用，有利于提高四面体格位中V3+离子的浓度。对不同颜色的V:YAG晶体进行了退火处理，在真空退火过程中处于八面体格位中的V3+离子在热激发作用下与近邻的四面体格位中Al3+离子存在置换反应，由此产生一定浓度的四面体格位V3+离子。同时，因缺氧形成的F心在退火过程中被完全消除，释放出来的自由电子被高价态的V离子捕获，可以进一步提高晶体中四面体格位V3+离子的浓度。在氢气氛退火下，不仅发生上述反应，同时H2起到的还原作用进一步提高了四面体格位中的V3+离子含量。采用温梯法、高浓度掺杂、原料预处理、晶体H2退火处理等一系列的措施，本论文已获得的V:YAG晶体中四面体格位V3+离子的浓度可达4.09×1017/cm3，1.342μm处的吸收系数为2.95cm-1。以V:YAG作为被动Q开关，实现了对Nd:YAG 1064nm激光和Nd:YVO4 1340nm激光的被动调Q。 首次利用中频加热的提拉法生长了Cr:Al2O3及Cr,Mg:Al2O3晶体，测试了晶体在室温下的吸收光谱，发现Cr,Mg:Al2O3晶体在1000~1600nm波段有一个宽的吸收带，通过对比研究氢气及氧气氛退火下的吸收光谱，证明了这个波段的吸收带属于Cr4+离子。Mg2+共掺Al2O3晶格中取代Al3+后，为了达到电荷平衡，首先形成阴离子空位，在弱氧化气氛下氧原子扩散进入晶格中，通过捕获电子填补Mg2+掺杂引入的O2-空位，同时Cr3+失去电子形成Cr4+。 采用温度提拉法生长了Na,Yb:CaF2晶体。研究了该晶体随1060nm入射光功率密度而变化的非线性透过率特性，结果表明掺Yb3+晶体材料是一种饱和透过率可达100%的可饱和吸收体。采用Na,Yb:CaF2晶体作被动Q开关，初步实现了对Yb:LYSO晶体的被动调Q 激光输出。 |
| 英文摘要 | Short, high-intensity laser pulses have a wide field of application in range finding, telecommunication and high speed hologram. Passive Q-switching is probably one of the most efficient and easy-to-realize methods for obtaining short pulses from diode-pumped lasers because of its simple construction, low cost and convenience. New passive Q switched absorber V3+:YAG crystal that has super advantages, such as wide absorber band, large absorption cross section, low exciting state loss, short recovery time, good heat-conducting and high damage threshold, can be effectively used for Q-switching and mode-locking at 0.78μm,1.06μm,1.3μm, 1.44μm. Usually, the V:YAG crystals were grown by Czochralski method which used radio-heating, but there were some disadvantages such as quantities, low doping concentration and little absorber coefficient. So it is really a valuable thing to grow large diameter, high doping concentration V:YAG crystals using another crystal growth method. Compared with V:YAG crystals, the Cr:YAG crystals have larger absorber coefficient, but its saturable absorption band in the near infrared extends from 800-1200nm, making it suitable for the 946nm and 1064nm Nd3+ transitions, however it is not suitable for longer wavelength transitions, such as the 1340nm transition of Nd3+. Because Cr4+ ions belong to transition metals, its electron-static crystal field interaction is comparable to electronic coulomb action, So that the orbital level splitting changes obviously along with different crystal fields. So Cr4+ doped different crystals can be used as absorber in different wavelength bands. As a fact, nearly all absorbers have excited state absorption (ESA) loss. So it is important to find a new absorber which has no excitated loss. In this paper, transparent and diameter 75mm V:YAG single crystals were grown by the temperature gradient technique (TGT) with graphite heating elements. From the absorption spectrum of the yellow-brown part with peaks at 370, 820 and 1320nm, we can deduce that the reducing atmosphere of carbon diffused from the heating elements can increase the concentration of tetrahedral V3+ ions. All three samples exhibited light-green color after annealing in vacuum or H2 atmospheres. In the vacuum annealing process, the V3+ ions in tetrahedral positions were enhanced through two methods: one method is the exchanging of octahedral V3+ and tetrahedral Al3+ ions in neighboring sites under thermal excitation, the other is that F color centers which were brought by anoxiced were thoroughly eliminated and the escaped free electrons could be captured by V ions with higher valance states to further improve the concentration of tetrahedral V3+ ions. Besides the two mechanisms, the H2 annealing process greatly improved the V3+tetra ions through the reduction effect of H2. Used a series of steps such as TGT, high doped concentration, material pretreatment, H2 annealing et al, the V:YAG crystals was abtained in which the tetrahedral V3+ ions up to 4.09×1017/cm3, absorption coefficient up to 2.95cm-1 at 1.342μm. The Q-switched laser at 1.06μm was obtained use V:YAG crystal as absorber which pumped by 808nm diode laser pumped Nd:YAG, at the same time, Q-switched laser at 1.34μm was obtained pumped by Nd:YVO4. Cr-doped and Mg,Cr-codoped Al2O3 crystals were grown by Czochralski method. The samples were polished mechanically for recording optical absorption spectra in the room temperature. The Mg, Cr-codoped Al2O3 crystals have a broad absorption peak in 900~1600nm region which enhanced along with the increase of Cr concentration. It can be interpreted that when Mg substituted Al in growing process, the VO2- were formed in order to keep charge balance. In the oxidation atmosphere, through capture electron fill up the O2- vacancies, at the same time the Cr3+ formed Cr4+. Na,Yb:CaF2 crystal was grown by TGT technique. The nonlinear transmission character of this crystal as a function of incident pump intensity at 1060nm indicated that the nonlinear transmission of Yb3+ doped crystals can be up to 100%. Passive Q-switching of a diode-pumped Yb:LYSO laser at 1060nm with Na,Yb:CaF2 crystal without the excited-state absorption was demonstrated. |
| 语种 | 中文 |
| 内容类型 | 学位论文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/16608]  |
| 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 | 张丹. 新型被动Q开关晶体材料的研究[D]. 中国科学院上海光学精密机械研究所. 2007. |
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