Nd：YVO₄和Nd：GdVO₄晶体自激活自拉曼转换激光器

【作者】 贾鹏；

【导师】 张行愚；

【作者基本信息】 山东大学 ， 光学工程， 2007， 硕士

【摘要】 受激拉曼散射是一种重要的变频方法，可以获得新的激光波长，大大拓宽激光的光谱范围。转换后的激光波长由抽运光的波长和拉曼介质决定。拉曼激光具有散射光光束质量好、脉宽窄、不需要相位匹配以及高的转换效率等多种优点。与传统的气体和液体拉曼介质相比，固体拉曼介质具有粒子浓度大、体积小、拉曼增益系数大及热导性好等优点。已经有多种晶体被发现是优良的拉曼晶体，例如Ba（NO₃）₂、LiIO₃、SrWO₄、BaWO₄、KGd（WO₄）₂、YVO₄和GdVO₄晶体等。以晶体为拉曼介质的固体拉曼激光器结构紧凑，效率高，稳定性好，在信息、交通、测量、医疗、国防和工农业等领域都有重要的应用，是激光器件领域的研究热点。Nd：YVO₄晶体和Nd：GdVO₄晶体是同构体，两种晶体具有受激发射截面大、吸收带宽比较宽、吸收系数高、线偏振输出等优点，已经被证明是优良的激光晶体。2001年，Kaminskii等人发现YVO₄晶体和GdVO₄晶体还是优良的拉曼晶体，提出Nd：YVO₄晶体和Nd：GdVO₄晶体可以作为自激活自拉曼晶体并可应用于可见光和近红外区域。本文以Nd：YVO₄晶体和Nd：GdVO₄晶体为自激活自拉曼晶体，实验方面系统地研究了这两种晶体的自激活自拉曼转换激光器的输出特性，研究了不同的脉冲重复率、输出镜透过率、饱和吸收体的小信号透过率、拉曼晶体的切割方向对拉曼光输出特性的影响；理论方面使用速率方程对主动调Q和被动调Q的自激活自拉曼转换激光器的特性进行了理论模拟，理论结果与实验结果大致相符。具体内容为：系统地研究了主动调Q的a-cut Nd：YVO₄晶体自激活自拉曼转换激光器的特性。Nd：YVO₄晶体同时作为激光晶体和拉曼晶体，使用不同透过率的输出镜，在不同的脉冲重复率下，得出了平均输出功率、脉冲宽度、脉冲能量和峰值功率随抽运功率的变化关系。典型的1064nm基频光和1176nm拉曼光脉冲的脉冲宽度分别为26.3ns和9.0ns。在脉冲重复率为20kHz，抽运功率为8.46W时，产生了平均功率为0.38W的1176nm光的输出，光-光转化效率为4.5％。使用速率方程对自激活自拉曼Nd：YVO₄主动调Q激光器的输出特性进行了理论研究，把脉冲重复率为10kHz，20kHz，30kHz时的拉曼光的单脉冲能量和脉冲宽度的实验结果与理论结果进行比较，理论结果与实验结果大致地相符。系统地研究了主动调Q的c-cut Nd：YVO₄晶体自激活自拉曼转换激光器的特性。使用不同透过率的输出镜，得出了不同的脉冲重复率下，平均输出功率、脉冲宽度、脉冲能量和峰值功率随抽运功率的变化关系。利用速率方程对c-cut Nd：YVO₄晶体的拉曼光输出特性进行了分析比较。首次实现了主动调O的a-cut Nd：GdVO₄晶体自激活自拉曼转换激光器在1173nm的运转，系统研究了LD抽运的主动调O的a-cut Nd：GdVO₄晶体自激活自拉曼转换激光器的输出特性。Nd：GdVO₄晶体同时作为激光晶体和拉曼晶体，利用声光调Q技术，产生了1173nm的拉曼激光。使用不同透过率的输出镜，在不同的脉冲重复率下，得出了平均输出功率、脉冲宽度、脉冲能量和峰值功率随抽运功率的变化关系。典型的1063nm基频光和1173nm拉曼光脉冲的脉冲宽度分别为33.0ns和13.8ns。在脉冲重复率为30kHz，抽运功率为9.6W时，产生了平均功率为0.8W的1173nm拉曼光，相应的光-光转化效率为8.3％。在脉冲重复率为10kHz，抽运功率为9.6W时，获得的最大单脉冲能量为70.7μJ，相应的峰值功率为5.2kW。利用速率方程对a-cut Nd：GdVO₄晶体的拉曼光输出特性进行了分析比较。 系统地研究了被动调O的c-cut Nd：GdVO₄晶体自激活自拉曼转换激光器的特性。分别利用两块不同初始透过率的Cr⁴⁺：YAG晶体，研究和比较了Nd：GdVO₄晶体自激活自拉曼转换激光器的输出特性。使用不同透过率的输出镜，得出了平均输出功率、脉冲宽度、脉冲重复率、脉冲能量和峰值功率随抽运功率的变化关系。当Cr⁴⁺：YAG的初始透过率为91％，抽运功率是5.7W时，得到拉曼光的最高平均输出功率为0.24W，相应的转换效率为4.2％。利用速率方程对被动调Q的c-cut Nd：GdVO₄晶体自激活自拉曼转换激光器的输出特性进行研究，理论结果与实验结果大致相符。更多还原

【Abstract】 The stimulated Raman scattering （SRS） can shift the laser frequency over a fixed frequency gap to produce new laser lines. The Raman laser wavelength is determined by the wavelength of the fundamental pump laser and the Raman shift of the Raman-active crystal. SRS possesses many attractive features such as beam clean-up, pulse compression, no need for phase matching, and high conversion efficiency. Compared with the traditional gas and liquid Raman media, the Raman crystals offer high gain, high molecule density, good thermal and mechanical properties. A good number of crystals have been found to be Raman-active, such as Ba（NO₃）₂, BaWO₄, KGd（WO₄）₂, YVO₄ and GdVO₄ crystals. The Raman lasers by using crystal media have the advantages of compactness, high efficiency and high stability and have wide applications in such ares as Information, Communication, Medicine, National defence, and so on.Nd:YVO₄ and Nd:GdVO₄ crystals are isomorphs. They have the advantages of high absorption coefficient, large emission cross-section, high absorption over a wide pumping wavelength bandwidth and polarization emitting feature, and have been recognized as excellent laser media for diode pumping. In 2001 Kaminskii et al. found YVO₄ and GdVO₄ were Raman-active, predicted that Nd:YVO₄ and Nd:GdVO₄ would be promising self-Raman laser crystals in the visible and near infrared spectral regions.By using Nd:YVO₄ and Nd:GdVO₄ crystals as both gain media and Raman media, we make a sysmetic research on the characteristics of the self-Raman lasers with Nd:YVO₄ and Nd:GdVO₄ crystals. The rate equations are used to describe the Raman pulse characteristics, the theoretical results are in agreement with the experimental results on the whole. The main content of this dissertation is as follows:1. A 1176 nm Raman laser is obtained by use of self-Raman conversion in a diode-pumped actively Q-switched a-cut Nd:YVO₄ 1064 nm laser. The characteristics including the average output power, pulse width, pulse energy and peak power versus the incident pump power and pulse repetition rate are measured. The widths of the typical fundamental pulses at 1064 nm and the Raman pulses at 1176 nm are 26.3 ns and 9.0 ns, respectively. At a repetition rate of 20 kHz and an incident pump power of 8.46 W, an average output power of 0.38 W at 1176 nm and an optical-to-optical conversion efficiency of 4.5% are obtained. The rate equations are used to describe the performance of the self-Raman actively Q-switched a-cut Nd:YVO₄ laser. Experimentally measured and theoretically calculated pulse energies and pulse widths at 1176 ran for the pulse repetition rates of 10, 20, and 30 kHz are compared, the theoretical results are in agreement with the experimental results on the whole.2. A 1179 nm Raman laser is obtained by use of self-Raman conversion in a diode-pumped actively Q-switched c-cut Nd:YVO₄ laser. The characteristics including the average output power, pulse width, pulse energy and peak power versus the incident pump power and pulse repetition rate are measured. The rate equations are used to describe the performance of the self-Raman actively Q-switched c-cut Nd:YVO₄ laser.3. A 1173 nm Raman laser is obtained for the first time by use of the self-Raman conversion in a diode-pumped actively Q-switched a-cut Nd:GdVO₄ 1063 nm laser. The characteristics including the average output power, pulse width, pulse energy and peak power versus the incident pump power and pulse repetition rate are measured. The widths of the typical fundamental pulses at 1063 nm and the Raman pulses at 1173 nm are 33.0 ns and 13.8 ns, respectively. At a repetition rate of 30 kHz and an incident pump power of 9.6 W, an average output power of 0.8 W at 1173 nm is obtained and the corresponding optical-to-optical conversion efficiency is 8.3%. The maximum pulse energy is 70.7μJ with an incident pump power of 9.6 W and a pulse repetition rate of 10 kHz, the corresponding peak power is 5.2 kW. The rate equations are used to describe the performance of the self-Raman actively Q-switched a-cut Nd:GdVO₄ laser.4. By using c-cut Nd:GdVO₄ as the gain medium and the Raman medium simultaneously, and using two Cr⁴⁺:YAG crystals with different initial transmissions as the saturable absorbers, the passively Q-switched operation of the self-Raman c-cut Nd:GdVO₄ laser at 1176 nm is realized. The characteristics including the average output power, pulse width, pulse repetition rates, pulse energy and peak power versus the incident pump power are measured. The obtained maximum output average power is 0.24 W with respect to the incident power of 5.7 W and the corresponding conversion efficiency is 4.2% with saturable absorber initial transmission of 91%. The rate equations are used to describe the performance of the self-Raman passively Q-switched c-cut Nd:GdVO₄ laser. The theoretical results are in agreement with the experimental results on the whole.更多还原