Large aperture high-power laser drivers usually focus the high power laser beams in 2×2 quads to the target chamber center in order to increase the light intensity on the target plane. The large aperture wedged focus lenses are the core components in the focus system of quadruplets of beams, and it is thought possible to use four two-dimensional off-axis wedged focus lenses as four sub-lenses to make up a larger aperture wedged focus lens in form to focus the four beams. Given that the large aperture two-dimensional off-axis wedged focus lenses are processed and used difficultly, the wedged focus lenses are divided into three categories: the two-dimensional off-axis wedged focus lenses, the onedimensional off-axis wedged focus lenses, and the non-off-axis wedged focus lenses. On the basis of the three modes of the wedged focus lenses and the corresponding specific incidence angles of each sub-beam, the three focus schemes for the 2×2 beam array are put forward to comparatively research the light intensity distribution on the target plane. Research results show that from a perspective of the coherence among the four sub-beams, the phase factors of each sub-beam respectively introducing by the three focus systems with the two-dimensional off-axis, one-dimensional off-axis, and non-off-axis wedged focus lenses are asymmetric, asymmetric and symmetric inside each sub-beam, and symmetric, asymmetric and symmetric among the four sub-beams. Therefore, the wave front consistency of the four sub-beams decreases in the order of the focus systems with the non-off-axis, two-dimensional off-axis, and one-dimensional off-axis wedged focus lenses. The focus schemes with the non-off-axis wedged focus lenses for 2×2 beam array can get the narrowest main-lobe, the strongest peak-value intensity, the highest energy concentration ratio on the target plane, followed by the one-dimensional off-axis and two-dimensional off-axis wedged focus lenses. The off-axis mode of the wedged focus lenses not only increases the complexity in the course of processing and using, but also increases the main-lobe size, decreases the peak-value intensity and the energy concentration ratio, which obtains a weaker focusing characteristics than that of the non-off-axis mode of the wedged focus lenses. Research results can provide an important reference for the design of the focus system in the target area of high-power laser drivers.

The high-power laser beam in the final optics assembly of high-power laser facilities is often modulated by contamination particles, which may cause local high light intensity, thereby increasing the filamentary damage probability for optical components. To study the general design basis for a final optics assembly to decrease the risk of filamentary damage, different-sized contamination particles deposited on a component surface are simulated to modulate a 351-nm laser beam based on the optical transmission theory, and the corresponding simulation results are analyzed statistically in terms of the propagation characteristic and the light field intensity distribution of the modulated laser beam. The statistical results show that component thickness and distance between components can to some extent be optimized to reduce the appearance of local high light intensity, and the general design basis of component thickness and arrangement are given for different control levels of particle sizes. Moreover, the statistical results can also predict the laser beam quality approximately under the existing optics design and environmental cleanliness. The optimized design for final optics assembly based on environmental cleanliness level is useful to prolong the lifetime of optics and enhance the output power of high-power laser facilities.

Based on the optical transmission theory, the reason why front-surface particle contamination may induce the original damage of thin optical components is considered, and a damage mechanism is put forward: The localized thermal deformation of an optical element induced by the thermal effect of particle contamination together with the shading effect of it can disturb the laser beams. Simulated results show that for a high power laser, the localized thermal deformation of thin optical components, which disturbs the laser beam, is an important cause to produce strong light intensity modulations. The surface shape, phase delay, and thermal diffusion length of a localized thermal deformation are constantly changing with the increase of laser pulse shot number, so the highest light intensity modulation will be produced at different positions in the thickness direction or the xy direction on the rear-surface of an optical element. This not only can easily induce some damages on the rear-surface of the optical element, but also cause the interior damages scattered in the thickness direction.

为了实现高功率、高亮度的光纤激光输出,研究了新型多芯光纤组束技术,基于大模场倏逝波,采用时域有限差分数值模拟方法,对多芯光纤(包含多根微纳纤芯或包含多根微纳芯和大芯径纤芯)的模场特性进行了数值仿真和理论分析。结果表明,以一定方式排列的微纳光纤束阵列中多根微纳纤芯可以很好耦合;当多根微纳芯和大芯径纤芯组束时,微纳纤芯能够有效地平坦模场。这一结果对于高功率光纤激光器和放大器的进一步发展很有帮助。

为了研究微纳光纤的导波和远场辐射特性,采用模式理论和衍射理论分析了微纳光纤芯径与模场、z向能流密度、有效模面积和远场强度的关系,并通过拉锥光纤进行了实验验证。结果表明,通过合理设计,微纳光纤的大部分能量在包层中以倏逝波的形式传输,光纤的有效模面积可以超过1000μm2。拉锥光纤的实验证实了微纳光纤可以有效地传输导波并把其辐射出去。

Microstructure optical fibers with flat-top fundamental mode are first proposed by introducing a low-index inner core into the core of index-guiding microstructure optical fibers. The design guidelines and characteristics of beam-shaping microstructure optical fibers are demonstrated. The interrelationships of inner-core index with laser wavelength, air hole diameter and size of inner core are investigated. The influence of the relative size of inner core on the spatial profile of the fundamental mode is demonstrated. Moreover, sensitivity of the flat-top fundamental mode profile from the slight change of the optimum inner-core index value is studied. Starting from these results we deduce that it is possible to fabricate beam-shaping microstructure fibers with nowadays technique.

In this paper, a refractive index profile design enabling us to obtain a flat modal field around the fibre centre is investigated. The theoretical approach for designing such multilayer large flattened mode (LFM) optical fibres is presented. A comparison is made between the properties of a three-layer LFM structure and a standard step-index profile with the same core size. The obtained results indicate that the effective area of the LFM fibre is about twice as large as that of the standard step-index fibre, but the LFM fibre has less effective ability to filter out the higher order modes than the standard step-index fibre with the same bending radius.

光路自动准直系统用于惯性约束聚变的高功率激光装置的光束精密自动准直调整.针对神光Ⅲ原型装置四程放大光路对自动准直系统的调整时间和调整精度的要求,考虑到光路总体结构的要求,利用光纤像传递和光纤耦合技术,结合近场远场像传递原理,使用插入式光纤点光源实现了后腔镜的准直调整,充分考虑了四程放大的光路像传递特点,设计出一套优化合理的四程放大准直方案,并且在神光Ⅲ原型装置模拟实验平台上得到了充分的验证和考核.实验结果表明光路自动准直系统能够在15 min之内顺利完成系统的光路调整,光束近场调整精度优于近场光斑的±0.5%,光束远场调整精度≤±0.3",满足了原型装置总体的要求.

对B积分(自聚焦)的有效抑制和控制是高能激光系统发展过程中的一个焦点问题.传统的B积分控制技术(空间滤波器)在一定程度上提高了光束质量,但是随着激光功率的不断增大,空间滤波技术受到诸多因素限制,已经不能满足要求,因此还需发展新的B积分控制技术.基于非线性倍频过程的二阶级联非线性可等效于三阶非线性,它的符号可控(允许为负),量值可变且很大,并可消除双光子吸收.级联非线性实际上提供了一个新的自由度,它可以产生有用的非线性相移.初步实验表明,利用级联非线性产生的负非线性相移,在一定程度上可以补偿强激光束在非线性传输过程中积累的B积分,具有很好的应用前景.

在考虑自相位调制和高阶群速色散的情况下,分析了超短脉冲在准周期Fibonacci超晶格中传输时,入射光强对脉冲传输特性的影响.结果表明:入射光强度对脉冲传输特性影响很大,不但使频谱展宽,而且限制超晶格的有效长度,是超短脉冲进行频率转换的主要考虑因素之一.