【作者】 郑光威；

【导师】 谭吉春；

【作者基本信息】 国防科学技术大学 ， 光学工程， 2011， 博士

【摘要】 惯性约束聚变、激光加工以及激光武器等用途,不仅要求激光器输出功率水平高,而且要求光束质量好。空间低通滤波是控制、改善激光器光束质量的关键措施之一。按是否对光束聚焦,空间低通滤波器分为聚焦型和非聚焦型。针孔滤波器属于聚焦型,体光栅、薄膜等滤波器属于非聚焦型。前一类滤波器在技术上较为成熟,后一类滤波器正处于研发的初期阶段。根据高功率激光空间滤波的特殊需求,本文研究光路中无透镜的新型滤波器――基于体光栅、多层介质薄膜以及Rugate薄膜的空间低通滤波器。本文采用Kogelnik耦合波理论、严格耦合波理论以及传输矩阵理论等三种工具,研究周期、准周期光学结构实现非聚焦空间滤波的原理,实验验证用非聚焦型空间滤波器替代针孔滤波器的可行性。研究内容和主要结论如下:1.基于体光栅的空间低通滤波技术在理论方面,分析了准单色平面波在体光栅中的衍射、传输模型;深入分析了超短脉冲高斯光束在透射型与反射型体光栅中的衍射、传输规律,得出了高效率、无失真衍射的条件,并修正了体光栅对超短脉冲高斯光束的衍射效率计算公式。设计体光栅非聚焦型空间低通滤波装置,分析了透射型与反射型体光栅的角谱选择性及滤波器件的优缺点,指出:反射型体光栅较透射型体光栅的工作性能更稳定、滤波波长可调谐等优点。制备透射型体光栅（对632.8nm激光相对衍射效率约90%）,测量其衍射角谱带宽随光栅周期以及光栅厚度的变化,实验测量结果与修正后的Kogelnik耦合波理论的计算结果相一致。利用光热折变玻璃（Photo-Thermal Refractive Glass, PTR Glass）透射型体光栅,完成了对空域畸变高斯光束的空间低通滤波实验。用针孔滤波与体光栅滤波的对比实验结果,验证了非聚焦器件实现空间低通滤波的实验可行性。实验中,用空间频率分别为7.2 mm^-1和3.0 mm^-1、占空比为1:1的周期振幅片生成两种畸变He-Ne激光束,经体光栅滤波后,输出的含高频调制(7.2 mm^-1)的衍射光束的空域光强分布与未畸变光束基本一致,而输出的含低频调制(3.0 mm^-1)的衍射光束的空域光强分布与未畸变光束相差较大。理论分析和实验结果一致表明:体光栅的衍射旁瓣严重影响其空间低通滤波性能。普通体光栅的相对介电常数呈正弦分布,其角谱选择性虽然能达到100μrad量级,然而受限于衍射旁瓣,其低通滤波远达不到100μrad量级的效果。针对体光栅的上述局限性,本文提出了用非倾斜型反射型体光栅相移组合器件改善角谱选择性、用切趾非倾斜反射型体光栅和相移非倾斜反射型体光栅抑制衍射旁瓣,降低体光栅空间截止频率等创新技术。2.基于多层介质薄膜的空间低通滤波技术用传输矩阵理论研究多层介质薄膜的两种基本类型——高反射膜和带通滤光片的衍射特性,分析了这两种薄膜的衍射效率以及角谱选择性与光谱选择性带宽随薄膜参量的变化规律。计算结果表明:高反射膜具有多级反射旁瓣,且角谱选择性带宽较宽,而带通滤光片无透射旁瓣,且角谱选择性带宽较窄。因此带通滤光片具有较好的空间低通滤波性能。本文提出一种利用串联级联两片四腔带通滤光片的空间低通滤波装置设计方案,并分析其二维空间低通滤波的性能。3.基于Rugate薄膜的空间低通滤波技术用传输矩阵理论研究Rugate薄膜的三种类型——啁啾Rugate膜系、切趾Rugate膜系以及相移Rugate膜系的衍射特性,分析这三种Rugate薄膜的衍射效率,以及角谱选择性与光谱选择性带宽随薄膜参量的变化规律,得出了切趾和相移Rugate薄膜可有效抑制旁瓣,相移Rugate薄膜较切趾Rugate薄膜具有更窄的角谱选择性带宽等结论。分别利用切趾Rugate薄膜和相移Rugate薄膜,组成空间低通滤波装置,并分析了这两种装置的空间低通滤波效果。综述了制备折射率渐变膜系的共相沉积法和斜向沉积法。根据高功率激光系统对空间低通滤波的需求,对以上三种候选技术进行比较,得出如下结论:现阶段的全息工艺在记录介质中形成正弦周期性折射率分布,全息法制备的透射型体光栅的角谱选择性可以达到100μrad量级,然而受限于其衍射旁瓣及“散射”,这类体光栅的角谱选择性指标在现阶段不能满足高功率激光器空间低通滤波的需求。切趾和相移反射型体光栅能满足高功率激光系统空间低通滤波的指标要求（≤20～40倍衍射极限）。增大光热折变玻璃的折射率调制度,制备大通光口径切趾、相移体光栅,是使光热折变体光栅满足高功率激光空间低通滤波需求的必要条件。基于带通滤光片的空间低通滤波器具有100μrad量级的角谱选择性带宽,适合用于准单色光的空间低通滤波。Rugate薄膜的周期（准周期）性结构具有多样化的特点,利用切趾和相移等膜系,能提高滤波器的角谱选择性。相移型Rugate薄膜具有比带通滤光片更高的抗光损伤阈值、更小膜层应力,以及更好的抑制旁瓣的效果,适合用于大口径、高功率激光空间低通滤波。高功率激光器中,针孔滤波器焦平面处光强>>10¹² W/cm²,容易诱发等离子体,产生等离子体堵孔等现象。针孔板的材料容易被激光烧蚀,作为硬边光阑,针孔容易形成低频衍射环。透镜与针孔的组合,起到了限制提升激光器输出功率水平的瓶颈作用。在空间低通滤波器中避免使用透镜,是克服针孔滤波技术上述缺点的有效措施。本文取得Rugate薄膜、多层介质薄膜和体光栅等三种新型空间低通滤波器的研究进展,是探索非聚焦型滤波器取代聚焦型滤波器的一次成功尝试。非聚焦型空间滤波器在超短脉冲滤波及热稳定性方面存在不足,在制备具有复杂函数分布的周期性光学结构的工艺方面存在困难,论文初步讨论了克服缺点、解决困难的技术措施。更多还原

【Abstract】 High-power laser with fine beam quality is significant for its applications in inertial confinement fusion system, laser manufacturing, laser weapon, and so on. Low-pass spatial filtering is one of the key techniques to control and refine the beam quality.Based on whether the beam must be focused or not, spatial filtering can be classified into two categories. One is the focusing method, and the other is the non-focusing method. The former method is very mature, while the latter one is in the preliminary situation.According to the special requirements of spatial filtering for high-power laser beam, three innovative spatial filtering techniques without focusing——volume grating, multilayer dielectric film, and Rugate coating are analyzed.Based on Kogelnik’s coupled wave theory, rigorous coupled wave theory, and matrix transfer method, the principles of periodic and quasi-periodic optical structures are investigated. The feasibility of the substitution of the non-focusing spatial filter for pin-hole filter is validated experimentally. The main contents and conclusions are as follows.1. non-focusing spatial filtering technique based on volume gratingThe diffraction and propagation characteristics of quasi-monochromatic plane wave by volume grating are analyzed theoretically. And the diffraction and propagation characteristics of ultra-short Gaussian laser pulse by transmitting and reflecting volume grating are investigated intensively. The condition of the diffraction with high efficiency and fidelity is achieved. The diffraction efficiency equation for the ultra-short laser pulse by transmitting volume grating is rectified.The configuration of spatial filtering without focusing based on volume grating is designed. The angular spectrum selectivity and filtering performances between transmitting and reflecting volume grating are compared with each other. The results show that reflecting volume grating has the advantages of high stability and wavelength-tunability over transmitting one.The transmitting volume grating is fabricated. And it has the relative diffraction efficiency up to 90% at 632.8 nm. The bandwidths of its diffractive angular spectrum with the change of the grating’s period and thickness are measured. The results show that the measured data are in good accordance with those by refined Kogelnik’s coupled wave theory. Using the transmitting volume grating recorded in Photo-Thermal Refractive glass （PTR glass）, the low-pass spatial filtering of the spatially-deformed laser is realized. Compared with the results by pin-hole filter, the reliability of the non-focusing spatial filter is validated experimentally. In the experiment, the spatially-deformed laser beams are produced by He-Ne laser modulated by the amplitude plates, which has duty cycle of 1: 1 and the frequency of 7.2 mm^-1 and 3.0 mm^-1, respectively. The spatial profile of the deformed beam with the high frequency modulation (7.2 mm^-1) diffracted by the volume grating is similar to that of the undeformed one, while that with the low frequency modulation (3.0 mm^-1) is not.The coincidence between the theoretical analysis and the experimental results show that the diffractive side-lobes of the volume grating may greatly influence its filtering performance. The relative permittivity of the common volume grating is sinusoidally-modulated. Though its angular spectrum bandwidth may be up to 100μrad, its spatial filtering performance can not achieve the fine performance of the 100μrad due to the side-lobes. Aiming at the limitations, the phase-shifted configuration of the combination of two pieces of non-slanted volume grating is conceived to improve the angular selectivity. The apodized and phase-shifted non-slanted reflecting volume grating is put forward to suppress the side-lobes in order to enhance the filtering performance.2. non-focusing spatial filtering technique based on multilayer dielectric filmBy the matrix transfer method, the diffraction efficiencies, angular spectrum bandwidths, and the light spectrum bandwidths of two basic categories of multilayer dielectric film——high reflectance film and bandpass filter are analyzed. The results show that high reflectance film has lots of side-lobes and broad bandwidth, while bandpass filter does not. So bandpass filter has better spatial filtering performance than high reflectance film. In this thesis, the configuration of two pieces of four-etalon bandpass filter cascaded is designed. And its two-dimensional spatial filtering performance is studied.3. non-focusing spatial filtering technique based on Rugate coatingBy the matrix transfer method, the diffraction characteristics of chirped, apodized and phase-shifted Rugate coatings are investigated. Their diffraction efficiencies, angular spectrum bandwidths and light spectrum bandwidths with the change of the parameters are analyzed. The results show that apodized and phase-shifted Rugate coatings can suppress side-lobes efficiently. And the angular spectrum bandwidth of the phase-shifted coating is narrower than that of the apodized one. Using apodized and phase-shifted Rugate coatings, the configurations of low-pass spatial filter are designed, respectively. And their performances are analyzed. Two kinds of the fabrication of Rugate coating——codoposition and glancing-angle decomposition method are reviewed.According to the special requirements of the high-power laser system, three aforementioned techniques are compared with each other. The results are as follows.Currently, the holography techniques can fabricate sinusodially-modulated refractive index easily. Though the angular spectrum bandwidth of the transmitting volume holographic gratings may be up to 100μrad, they can not meet the requirements of the high-power system due to their diffractive side-lobes and dispersion. Apodized and phase-shifted reflecting volume grating can meet the requirements （≤20～40 diffraction limit）. Improving the refractive index modulation amplitude of the PTR glass, and fabricating apodized and phase-shifted volume grating with large cross section, are the requisite conditions to make the PTR volume grating facilitate its application for high-power laser system.The angular spectrum bandwidth of bandpass filter is up to 100μrad. It is very useful for the spatial filtering of the laser with narrow light spectrum. Rugate coating has lots of diversified kinds of periodic and quasi-periodic structures. The angular spectrum selectivity can be improved by the apodized and phase-shifted coating. Phase-shifted Rugate coating has the higher laser-damage threshold, lower stress, and more efficient side-lobe suppression than bandpass filter. So it facilitates the low-pass spatial filtering of high-power laser beam with large cross section.In the high-power laser system, the intensity at the focal plane of the pin-hole filter is more than 10¹² W/cm². So the plasma closure may be probably caused. And the material of the pin-hole plate is easily burned. As a hard-edge diaphragm, the low-frequency diffractive rings are likely to appear. The combination of lens and pin-hole has the performance to limit the enhancement of the output power of the laser. Low-pass spatial filtering without focusing can surmount the drawbacks of the pin-hole filter. The low-pass spatial filters based on Rugate coating, multilayer dielectric film, and volume grating are analyzed. And their filtering performances are validated. The non-focusing low-pass spatial filter is successfully explored to substitute for focusing one.There are several disadvantages of the non-focusing filter, such as the limitation of its usage for the ultra-short laser pulse and limited thermal stability. And it is difficult to fabricate the periodic optical structure with complex function. The techniques to overcome the drawbacks and solve the problem are discussed preliminarily.更多还原