基于啁啾反转和拼接光栅的激光脉冲高效压缩技术研究

【作者】 王逍；

【导师】 魏晓峰；

【作者基本信息】 中国工程物理研究院 ， 光学工程， 2013， 博士

【摘要】 高能、高功率激光技术的发展使人们能够在实验室内创造出前所未有的极端物态条件,为诸多领域(如天体物理、材料科学、激光核聚变、高能量密度物理等)的研究工作提供了无与伦比的手段；另一方面,众多前沿科学领域的持续发展也不断地对激光的输出能量和峰值功率提出新需求,对其品质的要求也越来越高。当前高能、高功率激光技术面临的几大主要任务有：极高的输出能量及功率、极高的聚焦功率密度、极高的信噪比。目前在短脉冲激光技术领域实现高能量、高功率输出的主要手段是采用啁啾脉冲放大(CPA)技术,其基本工作模式是：“正色散展宽-能量放大-负色散压缩”。提供正色散的展宽器往往需要使用大口径透镜或大口径凹面镜,这些元件不仅加工困难、成本高,色差及像差的控制及装调精度要求都很高,在元件面形及调整精度满足不了要求时往往会使输出光束质量变差,影响最终的聚焦功率密度,而且,这些元件的有限通光口径往往还会成为限制系统带通的主要因素。另外,在目前的高能短脉冲激光装置中,限制装置输出能力的瓶颈是压缩光栅的损伤阈值,采用拼接的办法扩大光栅口径是提高系统输出能力的重要途径。但是,要保证拼接好的光栅能够等效地替代单块大光栅,拼接精度要求很高,环境振动对焦斑形态的影响往往很严重,拼接误差的实时监测及稳定性控制一直以来就是这一技术领域的难题,目前发展起来的一些监测及控制方法通常比较复杂、不利于实施,发展简捷的拼接误差控制方法既具有战略意义也具有实用价值。如何应对前述的挑战、找到一些可行的方法和措施解决其中的难点问题,正是本论文研究的主要任务。论文的主要内容及创新点如下：(1)提出了一种展宽压缩仅用/共用光栅对的"CRAC"工作模式,并对其进行了原理性的实验验证。实验表明利用该工作模式简化展宽器结构、甚至“免去”单独的展宽器在原理上是可行的。(2)通过仿真计算,比较分析了传统工作模式与"CRAC"工作模式下系统输出能力及输出脉冲特性的异同。分析表明,与传统的工作模式相比,在共用光栅对的‘"CRAC"工作模式下,由于展宽压缩过程中的谱透过率能实现高度匹配,在一定程度上可以提高放大能量的利用效率,另外,由于该工作模式下可以增加全系统的截止带通,为信噪比的提高提供了必要条件。(3)分析了一般的CPA系统中采用拼接光栅压缩器时拼接误差对压缩后脉冲时空特性的影响,提出了“拼接误差等位相制约关系”,并将其应用于双光栅双程全拼接光栅压缩器的误差实时监控。(4)提出了基于"CRAC"工作模式、“啁啾反转结合近场反转降低拼接误差影响、提高聚焦稳定性”的新方案。理论分析表明,针对影响最大、最不易监测和控制的光栅拼接平移错位误差(Piston误差),该方案的实施使这种误差的影响能自动消除/降低,不需要对其进行特殊的控制便能显著提高聚焦稳定性、提高最终可获得的聚焦功率密度。论文分析了影响高能超强短脉冲激光输出能力及输出性能的一些因素,提出了针对短脉冲激光系统的一种新的工作模式,取得了一些理论及实验上的探索性成果,更重要的是,论文为当前高峰值功率激光技术中存在的一些难点问题提供了新的解决思路,也为将来高能、超强短脉冲激光技术的发展提供了可供选择的方案和措施。更多还原

【Abstract】 Successful application of Chirped pulse amplification (CPA) technology in the field of laser had dramatically improved the output capacity (energy and power) of short-pulse laser facilities, which has been kept in a stagnant level for almost20years before. And this realized unprecedented extreme conditions, which provided impact and opportunities to materials science, plasma physics, laser fusion and other research fields.Large-scale CPA laser facilities usually work in a mode as "pulse being stretched in a positive dispersion stretcher-energy amplification-compressed in a negative dispersion compressor." The stretchers which provide positive dispersion often use large aperture lens or concave mirrors, which can’t be processed easily and be of high cost. In addition, aberration control and alignment accuracy requirements for these components are severe. If the requirements can not be met, it tends to distort the beam quality and decrease the final focused intensity. Therefore, it’s meaningful to simplify the structure of the stretcher to avoid or reduce the impact of the large-aperture components, to reduce costs, and to improve the output laser beam quality.In addition, the gratings for pulse-compression with limit size and limit damage threshold are bottleneck of the output capacity for a high-energy short-pulse laser facility. Expanding the grating’s aperture in a tiling manner to increase the output capacity is almost an inevitable choice. However, to ensure a tiled grating alternative to a single one, tiling errors must be controlled in a high precision, and environmental vibration may seriously distort the distribution of the focal spot. Real-time monitoring of tiling errors and stability control has always been a difficult problem, and some of the developed monitoring and control methods are very complex and expensive. So a simplified, practical and inexpensive error-control-method is important and valuable.An operation mode especially adaptable for CPA system with TGCs is proposed in this paper. The laser pulse is designed to sequentially pass through a parallel-grating compressor, which is essentially a stretcher with negative dispersion, parametric process for chirp reversal, energy amplification, and finally the compressor again to complete the CPA process (denoted as CRAC mode). That means we can use chirp reversal to simplify the structure of a stretcher, or even "remove" the independent stretcher. In addition, combination of chirp reversal and near-field inversion may improve the stability of the tiled gratings. Moreover, theoretical analysis showed that this CRAC operation mode will also be benefit to improve the band-pass of the overall system and to increase the utilization efficiency of the all amplified energy.The main contents in this paper are:(1) Chirped-pulse amplification system based on chirp reversal in optical parametric chirped-pulse amplification is proposed and experimentally demonstrated. The operation of this system can be described as "negative stretching-temporal chirp reversal-energy amplification-negative compression", named as CRAC mode, in which the pulse is stretched and compressed with the same gratings. Stand-alone stretcher adopting lenses or concave mirrors with large aperture can be omitted.(2) Calculation and analysis of chirped pulse transmitting in the amplifier chain is made. Simulation showed that with CRAC operation mode, the cut-off band-pass of the whole system can be increased and the spectral transmittance matching between stretcher and compressor can be better. Based on that, the SNR of the pulse and utilization ratio of the energy from amplifier chain can be improved.(3) Influence of the tiling errors from mosaic gratings used in a CPA system is analyzed. A group of "match relation equations" for the work laser and the monitor laser is proposed, which has been successfully used in a "two-pass tiled grating compressor". Experiment showed that, with wavelength and incident angle of the diagnostic beam being specially chosen, real-time monitoring and alignment can be achieved without disturbing the work beam. The far-field of the main laser beam and that of the diagnostic beam can vary in the same way with the tiling errors between the sub-aperture gratings. Rotational and translational errors can be controlled and compensated according to the far-field of the diagnostic beam.(4) A method,"combination of CRAC mode and near-field reversal", to diminish or remove the influence of tiling error and to improve the focused stability is proposed. Theoretical analysis shows that this method can increase the focused intensity to a great extent even if the piston error is not accurately controlled. So the monitoring scheme with this method can be simplified with improved stability.In general, some new ways and methods to solve the troublesome problems in a high-energy-short-pulse laser facility are proposed. A new operation mode--named as "CRAC" mode--is proposed and demonstrated for a CPA laser system. Methods for real-time monitoring and control of tiled-grating-compressor are also proposed and demonstrated.更多还原