【作者】 李涛；

【导师】 赵圣之；

【作者基本信息】 山东大学 ， 光学工程， 2010， 博士

【摘要】 LD泵浦的高峰值功率、高稳定性、高光束质量调Q和锁模激光器具有体积小、重量轻、结构牢固、寿命长等诸多优点,在工业、通讯、军事、医疗等各领域被广泛应用,因而受到人们的极大关注。脉冲宽度、脉冲对称性、脉冲稳定性以及脉冲峰值功率是调Q和锁模激光的重要参数,针对这一问题,本文采用电光主动与饱和吸收体被动吸收的双损耗调Q及锁模机制,从理论和实验上研究了双调Q和双调Q锁模激光特性；利用半导体饱和吸收镜(SESAM)作锁模元件,研究并对比了Nd:YVO4,Nd:LuVO4和Nd:Y0.15Lu0.85VO4不同晶体的锁模激光特性；最后理论分析了脉冲激光器的动态温度分布及动态热效应对激光输出稳定性的影响。论文主要内容包括以下几点。Ⅰ.采用电光和Cr4+:YAG主被动双损耗调制,研究了LD泵浦YVO4／Nd:YVO4晶体双调Q 1.06μm激光特性和KTP内腔倍频0.53μm绿光特性；在不同泵浦功率和调制频率下测量了激光输出功率、脉冲宽度、单脉冲能量和峰值功率与泵浦功率的依赖关系,与单一电光调Q相比,双损耗调制的调Q激光脉冲变窄、峰值功率提高；通过考虑反转粒子密度对激光晶体吸收系数的影响,建立了高斯分布近似下描述EO／Cr4+:YAG双调Q激光基频及倍频输出特性的速率方程组,数值模拟方程组的理论值与实验结果相符。Ⅱ.利用GaAs双光子吸收对调Q脉冲后沿的压缩效应,研究了LD泵浦YVO4／Nd:YVO4晶体,EO／GaAs双损耗调制的1.06μm和KTP内腔倍频0.53μm调Q激光特性；与单一电光调Q激光相比,双损耗调Q机制能有效压缩脉冲宽度、改善脉冲波形的对称性、提高峰值功率；通过引入激光晶体中反转粒子密度纵向衰减对激光输出特性的影响,给出高斯分布近似下描述电光-GaAs双调Q激光基频及倍频输出特性的速率方程组,数值模拟方程组的理论值与实验结果相符。Ⅲ.将电光调Q和Cr4+:YAG被动锁模相结合,实现了LD泵浦双损耗调制的调Q锁模1.06μm激光运转,并研究了激光输出特性；与单一Cr4+:YAG被动调Q锁模激光相比,调Q包络的脉宽减小、峰值功率提高、锁模脉冲的调制深度增加、脉冲的稳定性增强；根据光强起伏机制结合调Q速率方程组,建立了描述电光-Cr4+:YAG双调Q锁模激光特性的速率方程,理论模拟再现了调Q锁模脉冲波形,并估算出锁模脉冲宽度。Ⅳ.采用四镜折叠腔,结合KTP腔内倍频技术,实现了LD泵浦EO/Cr4+:YAG双损耗调制的调Q锁模532 nm激光运转,获得不同泵浦功率和电光调制频率下输出功率、脉冲宽度及脉冲峰值功率的依赖关系。与Cr4+:YAG单一被动调Q锁模激光相比,双损耗调制的调Q锁模绿激光的脉冲输出更稳定、脉冲能量更大、峰值功率更高、锁模脉冲的调制度更深。Ⅴ.利用SESAM可饱和吸收体作为锁模器件,分别实现了Nd:YVO4、Nd:LuVO4、Nd:Lu0.15Y0.85VO4三种激光晶体的锁模运转。测量并比较了三者的平均输出功率、锁模脉冲宽度及峰值功率。与单一Nd:YVO4、Nd:LuVO4晶体相比,Nd:Lu0.15Y0.85VO4混晶锁模脉冲更窄、峰值功率更高。实验结果表明,不同晶体间相互掺杂,可以有效增加混合晶体的荧光谱线宽度,从而压缩锁模脉冲宽度,提高峰值功率。Ⅵ.实现了LD泵浦YVO4/Nd:YVO4晶体,KTP内腔倍频,SESAM连续锁模532nm绿光激光运转,获得了稳定的皮秒绿光脉冲输出,并研究了锁模绿光的运转规律。Ⅶ.理论分析了LD脉冲泵浦激光晶体的瞬时热效应,讨论了晶体热弛豫时间、泵浦脉冲峰值功率和重复频率对晶体中温度分布的影响。研究了脉冲泵浦激光器中,热焦距及热致损耗随时间的变化关系,为优化泵浦源及激光腔参数提供了理论依据。Ⅷ.建立了LD连续泵浦调Q激光器的动态热模型,通过求解热传导方程与调Q速率方程的耦合方程,详细研究了低重复频率下激光晶体中的动态温度分布。结果表明,热平衡建立的时间取决于晶体的热弛豫时间,晶体温度的上升值由受激吸收截面决定,温度振荡系数则主要受晶体荧光寿命的影响。论文的主要创新工作包括：Ⅰ.首次实现了LD泵浦YVO4/Nd:YVO4晶体EO/Cr4+:YAG双损耗调制的调Q 1.06μm和KTP腔内倍频0.53μm激光运转,与单一电光调Q激光相比,双损耗调制的调Q激光压缩了脉冲宽度、提高了峰值功率；建立了高斯分布近似下描述EO／Cr4+:YAG双调Q激光基频及倍频输出特性的速率方程组,数值模拟方程组的理论值与实验结果相符。Ⅱ.首次实现了LD泵浦YVO4／Nd:YVO4晶体电光-GaAs双损耗调制的1.06 gm和KTP腔内倍频0.53 gm调Q激光运转,获得了脉冲宽度为2.5 ns、峰值功率为300 kW的调Q激光输出；与单一电光调Q激光相比,双损耗调Q机制压缩了脉冲宽度、改善了脉冲波形的对称性、提高了峰值功率,脉冲宽度压缩比达68.7%,峰值功率的提高达86.8%；理论上,给出描述高斯分布近似下电光-GaAs双调Q激光特性的速率方程组,数值模拟方程组的理论值与实验结果相符。Ⅲ.首次实现了LD泵浦电光-Cr4+:YAG双损耗调制的调Q锁模激光运转；结合光强起伏机制理论,建立高斯分布近似下描述EO／Cr4+:YAG双损耗调Q锁模激光运转特性的速率方程组,理论模拟再现了调Q锁模运转的动力学过程。Ⅳ.首次实现了LD泵浦、电光-Cr4+:YAG双损耗调制、KTP内腔倍频双调Q锁模0.53 gm激光运转。与Cr4+:YAG单一被动调Q锁模激光器相比,双损耗调制激光脉冲的非稳定度从40%减小到4.8%,脉冲宽度压缩了62%,脉冲能量提高10倍,锁模脉冲峰值功率提高40倍。Ⅴ.利用SESAM饱和吸收体作锁模器件,首次实现了Nd:Lu0.15Y0.85VO4混合晶体的锁模激光运转。与单一激光晶体Nd:LuVO4和Nd:YVO4相比,由于混合晶体具有较宽荧光谱线,因而产生的锁模脉冲宽度更窄,峰值功率更高。Ⅵ.首次提出调Q激光器动态热模型,分析了连续光泵浦低重复率调Q激光晶体的温度演化过程和瞬时温度分布。探究了热弛豫时间、晶体荧光寿命,受激吸收截面以及调Q重复率对瞬时温度分布的影响。理论模拟结果为分析高泵浦功率、低重复频率的脉冲激光器热效应,优化激光器参数提供了支持。更多还原

【Abstract】 LD pumped Q-switched and mode-locked lasers with high peak power, high stability and high beam quality witch become more and more interesting due to their merits of small size, light weight, firm structure and long life duration have been widely used in the scope of industry, coherent communication, military and medicine etc. Pulse duration, pulse symmetry, stability and peak power are key parameters for Q-switched and mode-locked lasers. To this question, some investigation and research have been executed. First, we experimentally and theoretically investigate the output performance of dual-loss Q-switched and mode-locked lasers by simultaneously adopting the EO and saturable absorbers (such as Cr4+:YAG and GaAs) in laser cavity as Q-switch. Second, the continuous-wave mode-locked characters of different laser crystals are also investigated with SESAM. Third, the transient temperature profiles in Q-switched laser and the back-reaction of the thermal dynamics to the laser one are discussed. The main content of this dissertation is list as follows:Ⅰ. The output performances of LD pumped EO/Cr4+:YAG dual-loss Q-switched Nd:YVO4 lasers at 1064 nm and 532 nm are demonstrated, and he average output power, pulse duration, peak power are measured experimentally. The dual-loss Q-switched laser can generate shorter pulse duration and high peak power when compared with that obtained by EO single Q-switched laser. A coupled equation is given to theoretically analyze the experimental results, in which the influences of population-inversion density (PID) on laser character are taken into account and leads to a good agreement with the experimental data.Ⅱ. By inserting GaAs saturable absorber in EO Q-switched laser cavity, the output performances of EO/GaAs dual-loss YVO4/Nd:YVO4 laser operating at 1064 nm and 532 nm with KTP as SHG crystal are studied. A stable pulse train with high peak and high pulse symmetry is obtained by EO/GaAs dual-loss laser. Considering the transversal and longitudinal distributions of the intracavity photon density and the population-inversion density, a rate equation model describing the actively and passively Q-switched laser is given and the solutions are agree well with the experimental results.Ⅲ.A diode-pumped dual-loss-modulated Q-switched and mode-locked (QML) YVO4/NdYVO4 laser is demonstrated successfully by simultaneously employing the EO and Cr4+:YAG saturaber absorber in a four mirrors fold cavity. In comparison with the singly passively QML laser with Cr4+:YAG, the dual-loss-modulated QML green laser with EO and Cr4+:YAG can generate a more stable pulse train with shorter pulse width, higher peak power and deeper modulation depth. By using a hyperbolic secant square function and considering the Gaussian distribution of the intracavity photon density, the coupled equations for diode-pumped dual-loss-modulated QML laser is given and the mode-locked pulse width is estimated accordingly.Ⅳ. By adopting a KTP as frequency doubling crystal, an EO/Cr4+:YAG doubly Q-switched and mode-locked laser at 532 nm is realized with four mirrors cavity for the first time. The average output power and pulse peak power is measured under different pump power and repetition rate. Compared with singly passively QML green laser, the dual-loss-modulated QML green laser can generate more stable pulse train with deeper modulation depth, shorter pulse width, greater pulse energy and higher peak power.Ⅴ. The mode-locking characteristics of diode-pumped lasers with Nd:YVO4, Nd:LuVO4 and Nd:Lu0.15Y0.85VO4 as laser mediums are demonstrated with a semiconductor saturable absorber mirror. The output power, mode-locked pulse duration and peak power are experimentally measured. In comparison with Nd:YVO4 and Nd:LuVO4 single crystals, Nd:Lu0.15Y0.85VO4 can generate even shorter pulses and even higher peak powers because of its broader fluorescence linewidth.Ⅵ. A diode-pumped passively mode-locked YVO4/Nd:YVO4 composite crystal green laser with a semiconductor saturable absorber mirror (SESAM) and a intracavity frequency doubling KTP crystal is realized. Stable mode-locked pulse train with picoseconds duration is obtained. The experimental results given a significant direction for further investigation of this kind of laser.Ⅶ. The dynamics of thermal buildup and temperature distribution in laser mediuml which is end pumped by pulsed operation laser diode is analyzed by the finite difference method. The influences of thermal time constant, pump peak power and the repetition rate on the thermal dynamics process are obtained. The simulation results give a primary theoretical support on designing and optimizing of pump source and laser parameters.Ⅷ. The transient temperature profiles in continuous-wave end-pumped Q-switched lasers is investigated by solving the coupling of the transient heat conduction equation and the rate equations. The interaction of temperature profile in laser medium and the laser output performance is discussed in detail. The numerical results reveal that the thermal buildup time of the quasi-steady-state temperature is mainly determined by the thermal time constant which depends on the dimension of crystal. When the thermal time constant is fixed at a certain value, the temperatures rise is primary influenced by the absorption cross section and the thermal oscillation contrast is principally determined by the fluorescence lifetime. In addition, we also find that the repetition rate and the pump power also have significant effects on the temperature characters in Q-switched lasers.The main innovations of this dissertation are as followsⅠ. By employing a YVO4/Nd:YVO4 composite crystal as laser medium, simultaneously using an EO modulator and a Cr4+:YAG saturable absorber as Q-switchers a 1064 nm dual-loss Q-switched YVO4/Nd:YVO4 laser and a 532 nm intracavity frequency doubled laser with KTP crystal are presented. Shorter pulse duration and higher peak power are generated by the dual-loss laser when compared with that of EO singly Q-switched one. In addition, we introduce a rate equation model under Gaussian distribution approximation to simulate the performance of those lasers and leads to a good agreement with the experimental results.Ⅱ. Using both EO modulator and GaAs saturable absorber as Q-switch, a LD end-pumped dual-loss Q-switched laser as well as its frequency doubling performance are demonstrated. A 2.5 ns pulse with peak power of 300 kW is obtained, compared with that from EO singly Q-switched laser, the pulse compression is 68.7% and peak power improvement is 86.8%. A rate equation model under Guassian distribution describing the dual-loss Q-switched laser with EO/GaAs is given out and the solutions are agree well with the experimental results.Ⅲ. By employing EO modulator and Cr4+:YAG saturable absorber simultaneously, a diode-pumped dual-loss-modulated QML YVO4/NdYVO4 laser is demonstrated for the first time. By using a hyperbolic secant square function and considering the Gaussian distribution of the intracavity photon density, the coupled equations for diode-pumped dual-loss-modulated QML laser is given and the numerical solutions of the equations are in good agreement with the experimental results.Ⅳ. A diode-pumped dual-loss-modulated Q-switched and mode-locked (QML) YVO4／NdYVO4／KTP green laser is presented. In comparison with the singly passively QML green laser with Cr4+:YAG, the dual-loss-modulated QML green laser with EO and Cr4+:YAG can generate a more stable pulse train with deeper modulation depth, shorter pulse width and higher peak power. For the dual-loss-modulated QML green laser, the pulse width compression is 62% and the QML peak power increasing is 40 times when compared with that of the singly passively QML green laser.Ⅴ. The mode-locking performance of a diode-pumped Nd:Lu0.15Y0.85VO4 laser is first demonstrated with a semiconductor saturable absorber mirror. In comparison with Nd:YVO4 and Nd:LuVO4 single crystals, Nd:Lu0.15Y0.85VO4 can generate even shorter pulses and even higher peak powers because of its broader fluorescence linewidth.Ⅵ. By solving a numerical model coupling the rate equation and transient heat equation, the temperature distribution in actively Q-switched laser operating at low repetition rate is investigated for the first time. The knowledge of the transient temperature distribution as a function of pump power, repetition rate and thermal time constant in different laser crystals make it possible to predict thermal distortion for a large variety of operation parameters.更多还原