【作者】 杨雨川；

【导师】 罗晖； 景峰；

【作者基本信息】 国防科学技术大学 ， 光学工程， 2011， 博士

【摘要】 在高能超高密度物理等科研需求的牵引下,提高单路短脉冲的输出能量到几千焦耳并进行多路高能短脉冲激光相干合成,创造前所未有的极端物态条件成为科研人员追求的目标。受限于非线性效应、光学元件损伤、热透镜效应、压缩器光栅尺寸等因素,单路激光器输出功率存在极限,而将输出光束进行相干合成是获得高能超高功率密度焦斑的有效途径之一,世界上主要的高能短脉冲研究机构均提出了在大型固体激光装置上实现相干合成的目标。目前的高能短脉冲激光装置普遍基于啁啾脉冲放大(CPA)技术,要求终端压缩器的光栅口径达到米量级,但制作大口径光栅的技术难度大而且代价昂贵,最经济有效的方法是采用阵列光栅替代单块大口径光栅,由于阵列器件的使用,在进行多路脉冲激光相干合成前,首先需要满足单路脉冲激光的相干合成输出,这也是高能短脉冲激光装置相干合成的显著特点。目前国际上对高能短脉冲激光相干合成的研究主要集中在概念探索和设计阶段。本论文对高能短脉冲激光相干合成技术展开了全面的研究,系统地分析了基于CPA技术脉冲激光装置单路和多路相干合成中的问题,建立了对应的控制模型,并针对实际情况进行了数值计算,得到主要影响因素的误差范围和补偿方法。在此基础上制定了多路高能短脉冲激光装置相干合成技术路线,并在离线实验平台上验证了技术路线中的部分关键技术。论文在以下几个方面取得了对大型高能短脉冲激光相干合成具有建设意义的结果:1、根据脉冲激光相干合成理论,考虑普遍的相位情况,对多路脉冲激光相干合成聚焦进行了系统的研究。理论上以衍射光学为基础,以锁相控制为目的,对影响相干合成效果的因素进行了分解,定量分析了光束近场分布、光束排布、束腰半径、系统f数、光束间相位差、光束指向性误差、波前畸变、脉冲宽度和光谱色散误差等对相干合成焦斑时空特性的影响,给出了其中某些重要因素的误差指标,建立了对应的时间和空间相干合成控制模型,为多路短脉冲的相干合成设计提供了理论基础和参数依据。2、根据误差范围和相干合成时空控制模型,给出了多路高能短脉冲激光装置相干合成技术路线,并对其中的相位监测和高精度光束相位锁定等关键技术进行了实验验证。在大型固体脉冲激光装置中由于脉冲激光的重复频率很低,在准直和调节脉冲激光时,往往采用连续激光模拟脉冲激光在激光链路中的传输,因此在验证实验中采用了连续激光光源,结合随机并行梯度下降(SPGD)算法,在振动实验环境下的离线光学平台上实现了2路连续激光较好的相干合成效果,这种相位调整和监测机构对脉冲激光装置上实现相干合成具有一定的借鉴意义。3、高能短脉冲激光装置的压缩器使用了阵列光栅,导致单光束经过压缩器后变成两支或多支并行的子光束,受到外界振动的影响,子光束之间的相位关系呈随机变化,因此需要解决单路光中子光束之间的相干合成。结合脉冲激光相干合成理论和复杂光栅压缩器的相位特点,建立了阵列光学元件的光学模型。通过光线几何追迹和惠更斯-菲涅尔衍射理论,计算了阵列光栅压缩器和阵列抛面镜的远场焦斑空间分布;在分析合成脉冲的时间特性时,从Treacy构型压缩器出发得到了阵列双光栅对压缩器输出脉冲群延迟和展宽的表达式,以上方法可以较为简便地处理阵列化光学元件相干合成问题。4、为了更好地开展阵列光学元件相位锁定工程化研究,建立了一个光机电集成模型,用以分析不同支撑结构在外界振动激励下对远场焦斑的影响和经过闭环控制后对远场焦斑的改善。在最终设计加工的2×2阵列光学支撑镜架上,实现了位移传感器和压电驱动器的闭环锁相控制和远、近场高速CCD视频采集相机和压电驱动器的自动锁相,锁相后的轴向错位均方根误差为几个纳米,锁相时间大于1小时,符合阵列化结构相干合成的工程化要求。5、Von Karman干扰谱能较好地描述实际高能短脉冲装置中激光束的波前畸变水平,光束相位关系则反映了光束的相干性,作为高能短脉冲相干合成研究的补充,建立了基于Von Karman干扰谱和随机相位屏的集成模型,用于分析高能短脉冲激光在畸变波前和不同合成方式条件下的远场焦斑,该模型的计算结果与文献上的实验报道结果吻合较好,建立集成模型对多路高能激光空间合成焦斑的仿真和优化提供了极大的便利。考虑到高能短脉冲激光装置目前的研究现状,论文在进行相干合成聚焦研究时更多地考虑了2×2阵列单元,因为此类大型激光装置的基本编组单元为一个Quad(2×2)阵列结构,从基本单元出发能与实际情况结合得更为紧密并具有较好的可扩展性和应用价值。更多还原

【Abstract】 With the scientific needs for the high-energy super-high density physics research, the coherent combination of multi-way short pulses with hundreds kilojoule output energy to obtain extreme physical state has become the goal of the researchers. The ultimate output power of single laser is limited by nolinear effect, facet fracture, thermal lens and compressor grating size. Meanwhile, coherent combination of multiple laser beams provides an effective approach for achieving high-energy super-high density focal spot, and the goal for coherent combination on the large-scale solid-state laser facility has been proposed by the main high-energy short-pulse research institute in the worldwide range. The size and threshold of the compression gratings determine the maximum output energy for all short-pulse laser systems using the chirped-pulse-amplification (CPA) technology. Production of gratings with big size is technically difficult and financially impractical, so most researchers consider that the most efficient and financial method is using phased array grating to increase the grating size. Because the arrayed grating is applied here, the single laser beam divided into two ways should be coherently combined firstly, which is the notable character of high-energy short-pulse laser facility.At present, the coherent combination research of high-energy short-pulse laser is focusing on concept exploring and preliminary design. The general researches on the issue are studied in this paper, the coherent combination based on the CPA technology is systematically analyzed and the corresponding physical model is setup. The numerical simulation results with practical considerations give the error ranges of chief influencing factors, which is the foundation for designing the coherent combination technology programme. The key technology of phase-locking is demonstrated in the laser coherent combination. As the results, the following progresses have been made:1. Systematic study about the coherent combined focusing of mulit-way high-energy short-pulse lasers is made by theoretical method. Based on diffractive optics, the influencing factors, including near-field amplitude distribution, beam layout, laser waist radius, f number, phase difference between beams, beam directivity, wavefront distortion, pulse spectrum width and dispersion, on the spatial and temporal coherence characteristics are analyzed both qualitatively and quantitatively, and the corresponding error tolerances and physical models are also given which is very helpful for designing multi-channel short-pulse coherent combination.2. The coherent combined technical route is described to produce ultra-high peak power density. Because of the low repetitive frequency of pulse laser, the pulse laser soure is instead by countinuous laser source when collimating and adjusting laser line. In the experimental demonstration, the key techniques of phase-locking between beams have been tested to achieve 2-way countinuous laser beams coherent combination using stochastic parallel gradient descent (SPGD) algorithm under the vibrating laboratory condition, which can be drawn lessons by engineering facility.3. The high-energy short-pulse laser compressor uses the array grating, which makes the single beam seperate into two-way beams. The phase between the beams stochastically changes by the external vibrating effect. Therefore, the coherent combination of single beam lines should be solved firstly. Combining the coherent combinatino theory and grating compressor phase character, the model for array optical elment is setup. The geometric tracing and the Huygens - Fresnel diffraction theory are used to handle the far-field spatial distribution of array grating compressor and array parabolic mirror. The array grating compressor is dispersion compensation component, and the analytical expressions of a pulse delay and pulse broadening are deviated from Treacy compressor configuration to describe the temporal characteristics of the combined waveform,4. In order to better carry out engineering studies on optical array phase-locking, an optical-electromechanical integrated model is established to analyze the far-field focal spot under vibration motivation of different supporting structures. The focal-spot shape is improved in the closed-loop feedback mode. Finally, the closed-loop system consisted of the displacement sensors and piezoelectric actuators and the automatic phase-locking by the far-field CCD camera and actuators are achieved on the optical supporting shelf. After phase locking, the root-mean-square error is limited within several nanometers and the stablization time is over one hour, which meets the engineering requirements for arrayed system.5. The Von Karman disturbing spectrum is preferably used to describe the wavefront distortion, and the coherence between beams is determined by phase relationship. For the supplement of coherent combination cases of large-size high-energy laser beams, the Von Karman disturbing spectrum and random phase screens are used to build an integrated model to simulate coherent combination, non-coherent combination and partial-coherent combination under the distorted wavefronts, and the simulations result in good agreement with the reported experimental results. The integrated model is very convenient for simulating and optimizing the multi-way short-pulse combined focal spot.Considering the current study of the high-energy short-pulse laser facility, the studies on combined focusing and tiling technique in this paper more focuses on 2×2 array unit. Because the Quad (2×2) structure is taken as basic grouping unit in such large-scale laser facility, starting from the basic unit can be more closely combined with the actual situation and has good scalability and application value.更多还原