【作者】 陈英；

【导师】 钱列加；

【作者基本信息】 复旦大学 ， 光学， 2010， 博士

【摘要】 宽带激光既有助于提高大口径(-300 mm)、高能量(-kJ)的钕玻璃激光驱动器自身的综合性能,如避免大口径元件的破坏、改善光束质量、提高放大器的储能的提取效率等；也有助于提高物理实验的综合效果,激光的宽频带有利于提高靶丸目标对激光的有效吸收,获得更高的靶丸压缩比；有助于抑制激光一等离子作用中有害的超热电子的产生等,从而提升高能量密度物理实验的成功率。用于惯性约束聚变的高功率激光驱动器今后的一个重要发展趋势是采用宽带激光脉冲进行传输和放大。钕玻璃激光的三倍频成分是用于惯性约束核聚变的理想波段,受非线性KDP晶体色散特性的影响,高效钕玻璃激光脉冲的三倍频仅限于于单纵模的窄带情况,高效率的三倍频转换成为宽带激光驱动器技术发展中少数几个核心难题之一。长期以来,国内外从事钕玻璃激光驱动器的专业单位先后尝试了许多实现宽带三倍频的思想和技术,但都存在技术复杂或支持带宽不够等不足。因此,宽带激光的高效率谐波转换是一项具有重要实际意义的研究课题,需要有突破常规的思路及技术去解决这一难题。本文提出了一种新颖的宽带激光三倍频技术方案,指出可以利用传统的窄线宽钕玻璃激光来推动宽带钕玻璃激光脉冲的高效三倍频产生,称为组合激光工作模式(宽带激光+窄带激光)的宽带三倍频方案；该方案无需应用大口径光栅等色散元件,并可以与目前使用成熟的双和频晶体方案结合,可以支持目前钕玻璃激光装置能达到的最大带宽-5 nm的高效三倍频(理论上效率-80%),有望突破宽带型钕玻璃激光驱动器发展的技术瓶颈。另外,本文提出的组合激光工作模式也为超短脉冲激光的高效谐波转换提供了新的思路。论文的主要内容和创新点如下：一、提出了利用窄线宽的钕玻璃激光脉冲与宽带钕玻璃激光脉冲组合进行频率转换,高效得到紫外波段的宽带激光脉冲的三倍频技术方案。研究发现,利用窄线宽的激光脉冲,可以减缓宽带激光谐波转换过程中群速度失配对效率的限制,从而提高宽带激光的三倍频转换效率。在小信号情况下分别通过数学推导和数值模拟研究了其相位匹配带宽的性质,证实了与窄线宽激光有关的群速度失配不影响转换过程的相位匹配带宽。二、对于中等宽带的钕玻璃激光脉冲(-1.2 nm,通过对脉冲进行时间相位调制获得)的三倍频,双和频晶体级联的方案虽然在效率上基本可以达到实用的标准,但并没有从根本上消除群速度失配的影响；导致输入时间相位调制的基频光,其产生的三倍频脉冲顶部会出现较强振荡。利用钕玻璃激光在KD*P晶体Ⅱ类和频过程中两个群速度失配值差别较大的特性,在和频过程中引入窄线宽的钕玻璃激光脉冲,可以大大减缓群速度失配值的限制,从而可以很好地抑制基频光的相位调制向三倍频脉冲强度调制的转移,改善了输出脉冲形状的均匀性及对称性。三、针对目前高能钕玻璃激光装置能达到的最大带宽-5 nm,采用窄线宽激光脉冲来推动其纳秒级(109s)啁啾脉冲的三倍频转换,结合目前使用成熟的双和频晶体级联技术,理论上转换三倍频效率可达80%。对生成的宽带紫外啁啾脉冲进行压缩,可得到功率为拍瓦(1015 W)量级的亚皮秒(1012 s)超强紫外脉冲,为强场物理的研究提供新的手段。产生的紫外激光的一部分能量来自窄线宽激光,所以其拍瓦脉冲的能量比钕玻璃拍瓦脉冲的能量更高,再加上它有着更短的脉冲,其峰值功率将比现有的钕玻璃拍瓦脉冲提高约2.5倍。四、分析了组合激光工作模式下三倍频转换中输入脉冲光强的变动及晶体失谐对转换效率的影响。研究发现采用两块级联的倍频晶体可提高谐波转换效率的动态范围,同时,还大幅度降低晶体失谐对效率的影响。更多还原

【Abstract】 A broadband bandwidth is key factor for the performance of the laser-aperture (～300 mm) and high energy (-kJ) Nd:glass laser facility, such as avoiding damage to optical components, improving laser-beam uniformity, improving extracted efficiency from energy-storage of laser amplifer. A broadband laser is also helpful to the experiment in high energy density science (HEDS), such as improving absorption of laser by a target and obtaining a higher target compression, suppressing the production of hot electron in the laser-plasma interaction. An important trend of the high-power laser delivers for inertial confinement fusion (ICF) is to apply broadband laser light.A frequency doulber and tripler has become a standard auxiliary device in high-power laser facilities because a the third-harmonic component of Nd:glass laser is thought to be and ideal wavelength for ICF. Efficient third-harmonic generation (THG) is limited to narrow bandwidth cases because of the dispersion of KDP crystal, so the need of efficient broadband frequency-tripling conversion become more urgent for the development of broadband laser delivers. Several approaches have benn proposed to increase the acceptance bandwidth, however, they are too complicated to be adopted in fusion lasers or still insufficient. Therefore, the efficient broadband frequency conversion technique is an urgent and difficult research subject.We report an efficient frequency tripling scheme for 351 nm broadband pulses by use of broadband and narrowband Nd:glass lasers. Avoiding any additional large dispersive optical element (i.e. diffraction grating) caused energy loss or beam distortion, with the assist of a narrowband laser in the sum-frequency generation (SFG) process and the addition of a second tripler, results in a large bandwidth acceptance (over 5 nm) as well as high tripling conversion efficiency (>80%). It is expected to overcome the obstacle of broadband Nd:glass laser delivers.The major research work can be listed as follows:1. We propose and numerically study a simple and efficient broadband THG scheme for Nd:glass laser system based on mixing narrowband and broadband laser pulses in the SFG process. The group-velocity mismatching (GVM) effects of the SFG process is substantially alleviated with the assist of a narrowband laser, which is the dominate obstacle to efficient broadband THG. Under the small signal the phase-matching bandwidth of the proposed frequency conversion scheme are studied analytically and numerically, respectively. The results show that the narrowband pulse-related GVMs have little effect on phase-matching bandwidth.2. A dual-tripler scheme has been successfully demonstrated and seems to be more promising, and the acceptance bandwidth of 1.2 nm almost can satisfy the need of practical application. But it does not allivated the GVM effects substantially, result in a conversion from the frequency modulation (FM) of fundamental pulse to amplitude modulation (AM) of output third-harmonic pulse. Two GVM values with considerable difference exit in the SFG process, the bigger one has little effect on frequency convrsion if the long wavelength laser is chosen as narrowband laser in the proposed scheme. As a result, the FM-to-AM conversion is substantial suppressed and a pulse with a well-temporal shape is obtained.3. Since a typical Nd:glass petawatt laser delivers has a bandwidth of 4～5 nm, the ultimate solution for frequency tripling needs to exploit the whole bandwidth of 5 nm efficiently. Based on the frequency-mixing of broadband chirped-pulse with a narrowband laser and a dual-tripler scheme, a broadband Nd:galss chirped-pulse with a bandwidth of 5 nm can be efficiently converted to ultraviolet pulse (efficiency >80%). The generated ultraviolet pulse can be compressed to a duration shorter than that of the initial Nd:glass petawatt laser pulse and ultraviolet petawatt peak power can be increased by 2.5 times as high as that of the Nd:glass petawatt laser. The demonstrated THG scheme may provide a promising route to efficient generation of high-power lasers at short wavelengths.4. The sensitivities of conversion efficiency on both the intensity variation and crystal-orientation offset in the proposed scheme are also discussed for practical applications. Designs that involve two doublers offer a high dynamic range of conversion efficiency versus intensity variation, at the same time, a dual-doubler is quite insentive to input angle offset.更多还原