【作者】 王双义；

【导师】 吕志伟； 林殿阳；

【作者基本信息】 哈尔滨工业大学 ， 物理电子学， 2008， 博士

【摘要】 重复频率、高功率、大能量激光可以广泛地应用于军事、科研和工业等领域。通常单束激光器的输出能量是从单个激活介质中得到的,因此增加单个激光器的输出能量受到单个激活介质体积、热效应等很多限制。为了克服这些限制,人们发展了激光组束技术。本文提出了基于布里渊放大的激光串行组束方案,针对方案中强信号共线和非共线布里渊放大、波矢失配对布里渊放大的影响、组束的结构设计等几个关键问题进行了研究。首先从受激布里渊散射(SBS)一维瞬态耦合波方程出发,给出了共线情况下布里渊放大的一般数学计算模型,并推导了波矢失配对布里渊放大的影响,给出了波矢失配影响布里渊放大的数学模型。在此基础上建立了非共线布里渊放大和波矢失配影响布里渊放大的一般数学计算模型。第二,对强信号布里渊放大进行了研究。先从实验和理论两方面对共线强信号布里渊放大进行了研究。研究发现,在不计介质吸收损耗的情况下,强信号布里渊放大受非聚焦后向散射的影响比较大,采用较短的介质池可以提高非聚焦后向散射的能量阈值,采用大口径、长脉宽的激光光束也可以提高非聚焦后向散射的能量阈值。在考虑介质的吸收损耗并且不考虑非聚焦后向散射的情况下,介质吸收损耗对强信号布里渊放大影响很大,通过缩短池长和选用吸收系数小的介质可以大大降低介质的吸收损耗对强信号布里渊放大的影响。研究结果表明,强信号布里渊放大高效实现的约束条件为:系统整体增益接近而不超过布里渊阈值(~25)。在系统整体增益值较为接近布里渊阈值,Stokes光能量为抽运光能量10倍以上时,能量提取效率超过85%。其次从实验和理论两方面对非共线情况下的强信号布里渊放大进行了研究,获得了Stokes光与抽运光交叉角对布里渊放大影响的规律。在Stokes光和抽运光的夹角为5°(~87mrad)时,能量提取效率和抽运光能量抽空率都随Stokes光能量的增加而增加。在抽运光与Stokes光能量都较大时,能量提取效率可达80%以上。Stokes光与抽运光的双光束耦合效率可达90%以上。第三,研究了波矢失配对布里渊放大的影响,进行了理论分析和实验验证。结果表明:在两束光相互作用长度不变,系统整体增益接近而不超过布里渊散射阈值的前提下,波矢失配角的容许度为400mrad以内。第四,实验实现了二束激光的组束。总的能量提取效率随着Stokes光能量的增加而增加,二束光与Stokes光的光束耦合效率亦随着Stokes光能量的增加而增加,在实验范围内最大可超过80%。当抽运光束能量分别为47.1mJ和41.5mJ,Stokes光能量为88.0mJ时,总的能量提取效率为61.3%,实现光束合成效率为80.7%。最后对串行激光组束方案的组束结构进行了设计和仿真计算,并分析了系统损耗对组束的影响。分别设计了基于共线结构和非共线结构的串行激光组束方案。对共线和非共线结构的设计方案进行了仿真,给出了供结构设计查询用的数据图表,并依据数据图表给出了几种组束方案的仿真结果和布局图,组束效率可达80%以上。研究表明:基于共线结构的组束方案可以使激光束之间具有较长的相互作用长度,适合于激光单光束能量较小时的应用;基于非共线结构的组束方案需要较大的光束口径来保证光束之间有足够的相互作用区域,适合于激光单光束能量较大时的应用。由光学元器件引入的系统损耗对组束方案中结构单元的效率影响很大,在选用吸收系数较小的介质,并且组束激光子光束能量确定的情况下,系统损耗决定了可组激光光束数的上限。通过本论文的研究,解决了基于布里渊放大串行激光组束方案中的若干关键技术问题,证明了方案的可行性。本文提出的方案获取的输出激光束在物理上是单束光,具有良好的相干性,光强空间分布匀滑;且不需要特殊光学元器件及控制器件,造价低,且结构相对简单,可操作性强;是一种完全相干组束方案。为激光组束的发展提出了新的技术路线。更多还原

【Abstract】 Lasers of high repetition rate, high power and high energy have been found wide applications in defense, scientific research and industry, etc. The traditional way of achieving high energy is such that a laser uses a single active medium to produce a single high energy beam. This approach suffers from several restrictions, which include the volage of single active medium, thermooptic effects, etc., that limit the maximum energy achievable from a single beam laser.In order to walk around these restrictions, the technique of laser beam combination was developed and researched over the past decades. The scheme of serial laser beam combination based on Brillouin amplification was presented in this dissertation, and the main problems including strong signal Brillouin amplification, influence of wave-vector mismatch on the Brillouin amplification and the structural design of beam combination were investigated in detail Firstly, a general mathematical model of collinear Brillouin amplification was constructed from the classic one-dimension transient coupled-wave equation of stimulated Brillouin scattering (SBS). The equation for simulating the influence of wave vector mismatch on the Brillouin amplification was derived. Based on the equations, a general mathematical model of non-collinear Brillouin amplification and the influence of wave vector mismatch on the Brillouin amplification were established.Sceondly, strong signal Brillouin amplification was investigated. Preliminary theoretical analysis of the feasibility was performed, and results show that theoretically it is feasible to achieve collinear strong signal Brillouin amplification. Then the detailed investigation of strong signal Brillouin amplification was carried out both theoretically and experimentally. The results show that the influence of non-focus backward scattering on strong signal Brillouin amplification is significant when the medium absorption loss is not considered. Choosing shorter cell can increase the energy threshold. Also, the study on variation of laser pulse-width shows that choosing the laser beam with larger beam diameter and longer pulse-width will increase the energy threshold. When the non-focus backward scattering is not considered, the influence of medium absorption loss on strong signal Brillouin amplification was significant. It can be reduced by choosing the medium with small absorption coefficient and short medium cell. Therefore, it is important to choose a set of optimized working parameters in the applications of beam combination. Experimental study shows that when the value of total system gain is close to the gain of Brillouin threshold Gth (~25) and the Stokes beam energy is higher than 10 times of pump beam energy, the energy extraction efficiency is higher than 85%. Next, the non-collinear strong signal Brillouin amplification was analyzed in detail both experimentally and theoretically. Using the scheme that there was 50(~87mrad)-cross angle between Stokes beam and pump beam, the energy extraction efficiency and the depletion ratio of pump energy was found both increase with the increasing of the Stokes beam energy. The energy extraction efficiency can reach greater than 80% and the depletion ratio of pump energy greater than 90% when the energy of Stokes beam and pump beam are both larger. Then the influence of cross-angle between Stokes beam and pump beam on Brillouin amplification was studied.The above results show that the constraint condition of strong signal Brillouin amplification is efficiently realized is: the value of total system gain reaches but not exceeds the gain of Billouin threshold.Thirdly, the influence of wave vector mismatch on Brillouin amplification was analyzed theoretically using the mathematical model, and experimental verification was performed. The results show that when the interaction length between two beams is long enough, the tolerancethe of wave vector mismatch angle is smaller than 400 mrad.Fourth, the beam combination scheme consisted of two laser beams was studied. It was found that the energy extraction efficiency increased with the increasing of Stokes beam energy, and the coupling efficiency also increased. In the experiment, up to >80% coupling efficiency was observed. Using 47.1mJ and 41.5mJ pump beams and 88.0mJ Stokes beam, the total energy extraction efficiency of 61.3% was observed and the coupling rates of laser beams was 80.7%.Finally, the constructions of serial laser beam combination were designed, and the influence of system loss on beam combination was analyzed. The scheme of beam combination based on collinear construction and non-collinear construction were designed respectively. The designed schemes of collinear and non-collinear were simulated and the data plots and tables for the scheme design were given. Based on the data plots and tables, the simulation results and layout of corresponding schemes of beam combination were presented. The coupling rates of beam combination reaches to more than 80%. The results of investigation Show that the scheme of beam combination based on collinear construction is suitable to apply when the energy of single laser beam is small, since the interaction length between two beams is longer in it; while the scheme of beam combination based on non-collinear construction is suitable to apply when the energy of single laser beam is larger, since the larger caliber of beam is needed. The system loss due to the optical elements has large influence on the efficiency of construction unit, under the condition of medium with small absorption coefficient and confirmed energy of single laser beam, the number upper limit of laser beams that can be combined is decided by the system loss. In order to meet the need of scale in serial laser beam combination, the combined scheme of beam combination is designed based on the collinear construction module and non-collinear construction module.Through the investigation of this dissertation, some key problems of serial beam combination based on Brillouin amplification are solved, and the feasibility of the scheme is proved. The laser beam obtained from this scheme has such characteristics including single beam in physical domain, smooth space distribution of intensity, needlessness of special optical elements and control equipments, low cost, simple construction, and strong maneuverability. This scheme provides a novel technique route for the development of laser beam combination.更多还原