基于动态工作点新机理的线状泵浦板条基模激光器研究

Diode-pumped slab lasers with good beam quality and high stability are broadly applied in a wide range of industrial, medical, military and other applications. Compared with lasers pumped by fiber-coupled diode sources, slab lasers driven by the lens-coupled pumping line have obvious advantages of low cost, compact structure and stable performance. However, most of line-pumped TEM00 slab lasers nowadays suffer from serious astigmatism, having asymmetric beam profile along horizontal and vertical directions in terms of waist radius, waist position and beam quality value. Furthermore, the stable resonator has its dynamic operating point located at thermally stable zone for matching the pumping line according to the conventional design method, leading to a narrow stable range and a bulky cavity volume. Thus it is the main challenge to match the pump line in a compact resonator while producing a near-diffraction-limit stable output with highly symmetric beam profile. This project will break through the traditional understanding of thermally stablized cavity, predict and verify for the first time that stable laser operation can be realized in the line-pumped slab laser with the dynamic operating point located in the critical region (at the edge of the stable zone). With regard to the unusual thermal effect of line-pumped slab laser, this project will present a new determination mechanism of dynamic operating point, based on which an efficient matching of the pump line will be demonstrated with short cavity length and the control technology of transverse mode will be developed, fully correcting the astigmatism of TEM00 output and realizing the symmetrization in waist radius, waist position and beam quality value.Finally the nonlinear frequency conversion will be carried out, achieving the intracavity frequency doubling and tripling with non-astigmatic beam profile.

高光束质量高稳定性的二极管泵浦板条激光器广泛应用于工业、医学、军事等领域。板条激光器采用透镜耦合的线状光斑泵浦，与光纤耦合相比具有成本低、结构简单、性能稳定的显著优势。然而现有的线状泵浦基模激光器普遍存在较严重的像散，表现为光腰尺寸、光腰位置和光束质量数值在水平和竖直两方向非对称；同时稳腔激光器按传统设计理论将工作点选取在稳定区，致使热稳区较窄而且谐振腔体积庞大。如何在短腔匹配线状泵浦光斑并实现高对称性的稳定基模输出是设计难点。本项目将突破传统热稳腔理论的认识，首次预测并验证线状泵浦板条激光器可以在临界区（热稳区的边沿）稳定工作。本项目将针对该型激光器的特殊热效应提出确定动态工作点的新机理，以此为指导实现短腔对线状泵浦光斑的高效匹配，并发展横模控制技术，全面矫正基模像散，实现光束的光腰尺寸、光腰位置和光束质量数值的对称化。最后开展非线性频率变换，获得腔内倍频的无像散基模输出。

本项目针对线状端面泵浦的板条激光振荡器开展研究，此类激光器的应用受到谐振腔体积大、热稳区窄、输出光斑像散严重等因素的制约。本项目通过采用准共心腔型、选取位于热稳区边沿的动态工作点，以短腔大模式体积高度匹配线状泵浦光斑，同时实现了具有高度圆对称性的稳定基模输出。本项目完成的主要研究工作包括：1）建立了线状端泵准共心腔板条激光器的热力学模型。热分析表明该腔型中晶体表现出的热透镜效应与腔内基模尺寸有关，高阶热效应造成的等效透镜效应是慢轴方向热透镜的重要组成部分；2）由于常规的动态工作点确定方法不适用于该型激光器特殊的热效应，因而提出了新机理研究横模控制机制。理论研究表明该型激光器的慢轴方向工作点选取在热稳区边沿时，可稳定工作并获得大基模体积；3）将高阶热效应视为对波振面的相位调制，提出了热致模式耦合的概念，并构建了基于多横模竞争和热致模式耦合的速率方程模型，精确描述慢轴方向不同基模尺寸下的输出特性；4）开展线状端泵准共心直腔振荡器的实验研究，对于线状泵浦光斑长度为2.57～9.44mm的情况下均获得基模输出，同时输出基模宽度对泵浦功率变化不敏感，实验表明热稳区边沿可稳定工作，验证了动态工作点新机理对横模控制的有效性；5）激光器在90W泵浦功率下获得基模连续输出24.5W，光光效率为27%，谐振腔长度为8.8cm，进一步基于横模控制技术实现了基模光束的光束质量数值、光腰尺寸和光腰位置三个参数在水平和竖直两个方向的对称化；6）振荡器实现了脉冲重频30kHz、脉冲宽度10ns、平均功率22.6W的基频脉冲输出，并获得了腔内二倍频和三倍频的基模输出。二倍频的最高输出功率为14.2W，二倍频转换效率为62.8%；三倍频的最高输出功率为7.5W，三倍频转换效率为33.1%。以上研究结果表明，本项目针对线状泵浦激光器给出了确定动态工作点的新机理，首次预测并验证线状泵浦板条激光器可选择临界区工作点获得稳定运转；同时本项目发展了线状泵浦板条激光器的横模控制技术，实现了基模光束的光腰尺寸、光腰位置和M2 因子三项参数的对称化。以上动态工作点的新机理和矫正像散的方法对于该型激光器的设计具有较大的指导意义，也可以推广用于其他类型的板条激光器中。