【作者】 王聪；

【导师】 张行愚；

【作者基本信息】 山东大学 ， 光学工程， 2014， 博士

【摘要】 受激拉曼散射(Stimulated Raman Scattering, SRS)是一种三阶非线性效应,是拓展现有激光波长的重要方法之一。受激拉曼散射分为斯托克斯(Stokes)拉曼散射和反斯托克斯(anti-Stokes)拉曼散射,拉曼散射光的频率取决于泵浦光的频率和由拉曼介质决定的拉曼频移,通过改变泵浦光波长和拉曼介质,获得的散射光光谱可遍及紫外到近红外。与气体和液体拉曼介质相比,固体拉曼介质具有体积小、增益高、热传导性好、机械特性好和粒子浓度大等优点。斯托克斯拉曼散射不需要相位匹配,且转换效率高,所以近年来,以晶体作为拉曼介质的固体斯托克斯拉曼激光器得到了广泛的研究。在许多应用中,比如激光雷达、激光测距等,高能量、高光束质量、高光谱纯度的拉曼光才能满足实际需要。此时仅靠拉曼激光器很难实现,拉曼放大器则成为获得高能量、高光束质量、高光谱纯度拉曼光的重要手段。首先由拉曼激光器产生满足要求的拉曼种子光脉冲,然后通过一级或多级拉曼放大器得到高能量拉曼光输出。与传统的激光放大器相比,拉曼放大器更难实现；第一,泵浦光必须有足够高的强度,以保证拉曼光得到充分放大；第二,泵浦光和拉曼种子光必须同时通过拉曼晶体,在时间和空间上要保证高的重合度。到目前为止,以光纤作为拉曼介质的拉曼放大器已经得到广泛研究。一般来说,光纤拉曼放大器适合于连续和低峰值功率拉曼光的产生,而高能量的固态拉曼放大器仅有少量的研究。为了增加波长的丰富性,反斯托克斯拉曼激光器的研究也是很有必要的。与斯托克斯拉曼散射不同,反斯托克斯拉曼散射是两个泵浦光光子、一个一阶斯托克斯光光子和一个一阶反斯托克斯光光子的四波混频过程,所以实现反斯托克斯激光的有效输出需要三束光满足相位匹配条件；同时反斯托克斯拉曼散射是一个频率上转换过程,转换效率相对比较低,因此反斯托克斯光的产生比斯托克斯光的产生更有难度。到目前为止,大部分反斯托克斯拉曼激光器的研究是基于气体拉曼介质,以晶体作为拉曼介质的反斯托克斯拉曼激光器仅有少量的研究。本论文中,我们主要研究了两个方面的内容：一是BaWO4拉曼放大器的理论和实验研究,二是关于反斯托克斯拉曼激光器的理论和实验研究。具体的研究内容如下：1.以α切BaWO4晶体作为拉曼介质首次实现了高效的1180nm BaWO4脉冲拉曼放大运转。拉曼放大器的泵浦光为氙灯泵浦的被动调Q单纵模激光器产生的1064nm激光,采用脉冲能量分别为8.0mJ,2.0mJ,200pJ和40μJ的拉曼种子光对拉曼放大器进行了研究。泵浦光能量为200mJ,种子光能量为40μJ时,得到的拉曼放大倍数为418倍；种子光能量8.0mJ时,得到的最大拉曼输出能量71.5mJ。通过测量剩余的泵浦光脉冲形状,观察到了明显的泵浦光损耗。2.通过简化外腔式拉曼激光器的辐射传输方程得到拉曼放大器的理论模型,已知输入的泵浦光和拉曼种子光强度的情况下,得到了描述拉曼放大器输出拉曼光强度的解析解。通过计算研究了拉曼放大器的输出能量、放大倍数及从泵浦光到放大后拉曼光的光-光转换效率随泵浦光能量的变化趋势。将理论结果与实验结果做了对比,理论与实验符合得很好。3.以α切BaWO4晶体作为拉曼介质首次实现了外腔泵浦的BaWO4反斯托克斯拉曼激光运转。BaWO4拉曼外腔的光轴相对于泵浦光的耦合波方向偏转的角度为34.1mrad时,实现了泵浦光、一阶斯托克斯光和一阶反斯托克斯光的相位匹配。泵浦光能量为128mJ时,得到的968nm反斯托克斯光的能量为2.2mJ,光-光转换效率为1.7%。同时得到了总能量为42.5mJ的一至三阶斯托克斯光的输出。4.平面波近似下,通过耦合波方程首次推导出外腔泵浦的反斯托克斯激光器的一阶反斯托斯光光强、泵浦光光强和一阶斯托克斯光光强之间的关系式,并首次用外腔拉曼激光器速率方程和推导出的关系式分析反斯托克斯激光器的性质,理论计算结果与外腔泵浦的BaWO4反斯托克斯拉曼激光器的实验结果基本一致。5.以α切SrWO4晶体作为拉曼介质,采用1064nm激光外腔泵浦的方式,首次实现了SrWO4反斯托克斯拉曼激光器运转。SrWO4拉曼外腔的光轴相对于泵浦光的耦合波方向偏转31.3mrad。泵浦光能量为120mJ时,得到的969nm反斯托克斯光的能量为0.74mJ,光-光转换效率为0.6%。同时得到了总能量为23.9mJ的一至三阶斯托克斯光的输出。6.以Nd:YAG作为激光介质,以α切BaWO4晶体作为拉曼介质,首次实现了LD侧面泵浦主动调Q的内腔式反斯托克斯拉曼激光器在968nm波长处的运转。拉曼耦合腔的光轴与基频光光轴偏离一个相位匹配角度,当泵浦功率为170W,重复频率为7.5kHz时,得到的最大反斯托克斯功率为0.94mW,最窄脉冲宽度为3.9ns。本论文的主要创新点如下：1.首次实现了单纵模1064nm泵浦的高效1180nm BaWO4拉曼放大运转。泵浦光能量为200mJ,种子光能量为40μJ时,得到的拉曼放大倍数为418倍；种子光能量8.0mJ时,得到的最大拉曼输出能量71.5mJ。2.首次实现了外腔泵浦的BaWO4反斯托克斯拉曼激光运转。泵浦光能量为128mJ时,得到的968nm反斯托克斯光的能量为2.2mJ,光-光转换效率为1.7%。3.平面波近似下,通过耦合波方程首次推导出外腔泵浦的反斯托克斯激光器中反斯托斯光光强、泵浦光光强和一阶斯托克斯光光强之间的四波混频关系式。4.首次用外腔拉曼激光器速率方程和推导出的反斯托克斯四波混频关系式分析外腔泵浦的反斯托克斯拉曼激光器的性质。5.首次实现了外腔泵浦的SrWO4反斯托克斯拉曼激光运转。泵浦光能量为120mJ时,得到的969nm反斯托克斯光的能量为0.74mJ,光-光转换效率为0.6%。6.首次实现了LD侧面泵浦的主动调Q Nd:YAG/BaWO4内腔式反斯托克斯拉曼激光器在968nm波长处的运转。当泵浦功率为170W,重复频率为7.5kHz时,得到的最大反斯托克斯功率为0.94mW,最窄脉冲宽度为3.9ns。更多还原

【Abstract】 Stimulated Raman Scattering (SRS) is a third nonlinear effect. It is one of important approaches for expanding the present laser lines. SRS includes Stokes Raman scattering and anti-Stokes Raman scattering. The frequency of the Raman scattering light depends on the pumping laser frequency and the Raman frequency shift of the Raman medium. By changing the pumping laser wavelength and the Raman medium, the laser spectrum with SRS can extend from the ultraviolet to the near infrared. Compared with the gas and liquid Raman media, crystalline Raman media have the advantages of small physical size, high gain coefficient, excellent thermal conductivity, good mechanical properties, high molecule density, and so on. Because the Stokes Raman scattering does not need phase-matching and has high conversion efficiency, the solid-state Raman lasers based on crystal Raman media have been widely investigated in recent years.In many applications, e.g. lidar and range-meter, Raman laser radiations with high energies, high beam qualities, and high spectral purities are needed. In this situation, they are hard to be realized only by Raman lasers. Raman amplifiers become necessary alternatives. First, satisfactory Raman seed pulses are generated in advance by a Raman laser. And then, the Raman seed radiations are amplified by one-or multi-stage Raman amplifiers to generate high-energy Raman outputs. Compared with the traditional laser amplifiers, the realization of the Raman amplifiers are more difficult. First, the pumping radiation must have high intensity to ensure that the Raman seed is fully amplified. Second, the pumping laser pulses and the Raman seed pulses must pass through one Raman gain medium simultaneously, with good spatial and temporal overlaps. So far, Raman amplifiers using optical fibers as the Raman gain media have been widely investigated. Generally speaking, fiber Raman amplifiers are suitable for continuous-wave (CW) lasers and low peak-power pulsed lasers. However, very few researches with regard to the crystalline Raman amplifiers have been reported.To make full use of SRS to obtain more wavelengths, the anti-Stokes generation is necessary. In contrast to Stokes generation, it is a four-wave mixing (FWM) process of two pumping photons, one first Stokes photon, and one first anti-Stokes photon. So, to realize the effective anti-Stokes generation, the three laser fields need to satisfy phase-matching condition. Meanwhile, the anti-Stokes scattering is an up-conversion process with low conversion efficiency. The anti-Stokes generation is harder than the Stokes generation. So far, most of the researches with regard to the anti-Stokes generation are based on gas Raman media. Only a few reports related to the anti-Stokes Raman lasers use crystals as the Raman media.In this dissertation, we mainly investigated two aspects. First, we theoretically and experimentally studied crystalline pulsed Raman amplifier based on BaWO4crystal. Second, we theoretically and experimentally studied the anti-Stokes Raman lasers. The concrete research contents of this thesis are as follows.1. An a-cut BaW04pulsed Raman amplifier at1180nm was realized for the first time. The pumping source was a flash-lamp pumped passively Q-switched single-longitudinal-mode laser at1064nm. Four Raman laser seeds with the pulse energies of8.0mJ,2.0mJ,200μJ and40μJ were used to investigate the properties of the Raman amplifier. In the case of200mJ pumping laser energy, the highest Raman amplification ratio of418was obtained with the Raman seed energy of40μJ. The largest amplified Raman energy of71.5mJ was obtained with the Raman seed energy of8.0mJ. Serious depletion of the pumping laser pulse was detected.2. A theoretical model of the Raman amplifier was deduced by simplifying the radiative transfer equations of the extracavity pumped Raman laser. An analytic solution that depicted the output Raman intensity of the Raman amplifier was obtained with known incident pumping and Raman seed intensities. The theoretical output energy, the amplification ratio and the optical-optical conversion efficiency with regard to the pumping energy were analyzed. The theoretical results were in well accordance with the experimental ones.3. An extracavity pumped a-cut BaW04anti-Stokes Raman laser was demonstrated for the first time. By separating the optical axis of the Raman external cavity from the pumping coupled wave direction at34.1mrad, the phase-matching between the pumping, the first Stokes, and the first anti-Stokes laser beams was achieved. When the pumping energy was128mJ, the output energy of the anti-Stokes radiation at968nm obtained was2.2mJ and the corresponding optical-optical conversion efficiency was1.7%. Meanwhile, three orders of Stokes radiations were obtained with the total energy of42.5mJ.4. Under the plane-wave approximation, by using coupled wave equations, the relational equation between the pumping, the first Stokes, and the first anti-Stokes intensities of the extracavity pumped anti-Stokes Raman lasers were deduced for the first time. Together with this equation, the rate equations of extracavity Raman lasers are were used to simulate the properties of the anti-Stokes laser. The theoretical predictions agreed well with the measured data.5. An extracavity pumped a-cut SrWO4anti-Stokes Raman laser was studied for the first time. The optical axis of the Raman external cavity was deviated from the pumping coupled wave direction at31.3mrad. When the pumping energy was120mJ, the output energy of the anti-Stokes radiation at968nm obtained was0.74mJ and the corresponding optical-optical conversion efficiency was0.6%. Meanwhile, three orders of Stokes radiations were obtained with the total energy of23.9mJ.6. With an Nd:YAG laser medium and an a-cut BaWO4Raman medium, a LD-side-pumped actively Q-switched intracavity anti-Stokes Raman laser at968nm was demonstrated for the first time. A Raman coupled cavity was placed in the fundamental cavity. The optical axis of the Raman coupled cavity was separated from the optical axis of the fundamental oscillating direction at the phase-matching angle. When the pumping energy was170W and the repetition rate was7.5kHz, the obtained maximum anti-Stokes power was0.94mW with the shortest pulse width of3.9ns.The main innovations of this thesis are summarized as follows:1. For the first time, the single-longitunidal-mode1064nm laser pumped highly efficient BaWO4Raman amplifier at1180nm was demonstrated. In the case of200mJ pumping laser energy, the highest Raman amplification ratio of418was obtained with the Raman seed energy of40μJ. The largest amplified Raman energy of71.5mJ was obtained with the Raman seed energy of8.0mJ.2. The extracavity pumped BaWO4anti-Stokes Raman laser was realized for the first time. When the pumping energy was128mJ, the output energy of the anti-Stokes radiation at968nm obtained was2.2mJ and the corresponding optical-optical conversion efficiency was1.7%.3. For the first time, by using coupled wave equations, the relational equation between the pumping, the first Stokes, and the first anti-Stokes intensities of the extracavity pumped anti-Stokes Raman lasers were deduced under the plane-wave approximation.4. The rate equations of extracavity Raman laser were used to simulate the properties of the anti-Stokes lasers for the first time.5. The extracavity pumped anti-Stokes Raman laser at969nm was realized based on SrWO4crystal for the first time. Under the pumping energy of120mJ, the anti-Stokes energy obtained was0.74mJ and the corresponding optical-optical conversion efficiency was0.6%.6. A LD-side-pumped actively Q-switched Nd:YAG/BaWO4intracavity anti-Stokes Raman laser at968nm was demonstrated for the first time, a. When the pumping energy was170W and the repetition rate was7.5kHz, the obtained maximum anti-Stokes power was0.94mW with the shortest pulse width of3.9ns.更多还原