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题名	飞秒激光直写在透明材料内部制备方形凹陷包层光波导
作者	王鹏
文献子类	硕士
导师	程亚
关键词	飞秒激光直写 femtosecond laser direct writing 包层光波导 depressed cladding waveguides 球差校正 spherical aberration correction
其他题名	Fabrication of square-shaped depressed cladding waveguides in transparent materials using femtosecond laser direct writing
英文摘要	集成光子器件具有功能多样化、处理效率高和稳定性好等特点，在信息处理、化学和生物分析等领域拥有广阔的应用空间。集成光子器件的基本组成部分是光波导，光信号可通过光波导在器件中进行定向的传递。同时，由于光波导可将光束缚在光波长量级尺寸的空间内，在这一狭小的区域内，光与材料的相互作用得到增强，有助于非线性频率转换效率的提高。 飞秒激光拥有极短的脉冲宽度和极高的峰值功率，它的出现为材料加工领域开辟了崭新的道路。作为一种新兴的加工方法，飞秒激光微加工既可以用来进行材料去除，也可以对材料进行改性。在过去的几十年间，在包括微流体加工以及细胞烧蚀等在内的诸多领域，该技术都有广泛的应用。使用飞秒激光微加工技术也可在透明材料上加工光波导，该方法不仅灵活度高，可单步完成，而且可用于构建任意形状的三维光学回路，进一步促进了光子器件的小型化和集成化。 对于许多玻璃材料来说，比如熔石英玻璃、硼硅酸盐玻璃等，飞秒激光的照射会使材料的折射率升高，因此照射区可作为光波导的中央导光区，与周围折射率未变化的区域组成光波导。此外，飞秒激光的照射会使一些材料的折射率降低，这些材料包括氟化物玻璃（ZrF4-BaF2-LaF3-AlF3-NaF，ZBLAN）、掺钕钇铝石榴石晶体以及铌酸锂晶体等。在这些材料中，飞秒激光照射区可作为光波导中折射率较低的包层区域。根据激光作用区域形状的不同，此类光波导可分为双线光波导和包层光波导。双线光波导的加工一般只需两次扫描，比较容易，但由于结构中先天缺少上下包层，无法对光进行完整的束缚，所以具有高度的偏振依赖性。包层光波导拥有完整的包层结构，所以可以进行偏振无关的光传输，但加工包层光波导需要进行大量的线扫描，整个过程既繁琐又耗时。 为了弥补以上光波导的不足，我们展示了一种新型的包层光波导：方形凹陷包层光波导。该光波导的包层区域由四部分组成：两条水平包层和两条竖直包层，所以加工过程仅需进行四次扫描即可完成。此外，通过调节加工条件，光波导模场尺寸及包层的厚度均可灵活调节，保证了该光波导在不同需求应用中的实用性。 论文的主要内容总结如下： 首先，介绍了方形凹陷包层光波导的几何结构、飞秒激光直写制备方形凹陷包层光波导的过程以及装置，对制备中所用到的狭缝整形技术做了简介，展示了ZBLAN玻璃、铌酸锂晶体以及光热折变（photo-thermo-refractive, PTR）玻璃中制备的方形凹陷包层光波导。 之后，讨论了如何在铌酸锂晶体中制备大深度方形凹陷包层光波导，随着加工深度增大，球差对加工精度的影响越来越大，使得在晶体深处的光波导很难加工。我们通过采用飞秒激光直写技术与球差校正技术相结合，在晶体内部不同深度尝试了光波导的制备，最深可至距表面1.4 mm处。这种不受加工深度限制的制备能力，有助于进一步提高光子芯片的纵向集成度。 最后，由于铌酸锂晶体是重要的光子学材料，同时又是双折射晶体，我们讨论了入射飞秒激光的偏振方向对加工方形凹陷包层光波导的影响。我们对比了当偏振方向垂直于晶体光轴以及平行于晶体光轴方向时得到的结构形状，所得结果有助于提高铌酸锂晶体中方形凹陷包层光波导的质量。; Integrated photonic devices have found broad range of applications including information processing, chemical and biological analyses thanks to their diverse functionalities and high operation efficiencies. One of the key elements of photonic integration is the optical waveguide, which provides predefined pathways for transferring light signals among different nodes in optical circuits. Furthermore, since the optical waveguides can confine the light fields in a tight space typically on the scale of wavelength of light, interaction of light with the optical materials can be enhanced in the waveguide, giving rise to enhanced efficiency of nonlinear frequency conversion. Due to the unique characteristics of ultrashort pulse widths and extremely high peak intensities, femtosecond lasers have opened up new avenues in materials processing. Femtosecond laser micromachining can be used either to remove materials or to change a material’s properties. Over the past decade, this technique has been used in a broad range of applications, from microfluidic fabrication to cell ablation. It has been demonstrated that formation of optical waveguides can be efficiently achieved by femtosecond laser direct writing in transparent materials. Writing waveguides with focused femtosecond laser pulses offers a continuous single-step processing approach with the capability of producing arbitrary three-dimensional (3D) optical circuits, which is desirable for miniaturization and integration. In many glasses such as fused silica and borosilicate glass, waveguides written by focused femtosecond laser beams consist of cores with increased refractive index that overlap the center of focal spots. Besides, femtosecond laser can also induce negative refractive index change in several kinds of glass and crystals, such as ZBLAN glass, YAG crystal and lithium niobate crystal. In these materials, waveguides are generally produced by forming a cladding region of reduced refractive index by irradiation of femtosecond laser pulses. Both double-line waveguides and depressed cladding waveguides have been produced using femtosecond laser direct writing. The former type waveguides are easy to fabricate, whereas such waveguides show strong polarization dependence owing to the fact that the top and bottom claddings of the waveguides are missing. The latter type of waveguides can guide the light waves polarized in both horizontal and vertical directions, whereas forming such depressed cladding waveguides can often be a time-consuming process which requires overlapping large numbers of lines inscribed by the focused femtosecond laser to form the enclosed cladding. In this dissertation, we demonstrated fabrication of square-shaped depressed cladding (SSDC) waveguides buried in transparent materials based on transverse femtosecond laser direct writing. Unlike the conventional depressed cladding waveguides with a circular contour, our SSDC waveguides are composed of only four sides. Therefore, only four scans are required to form the SSDC waveguides. In addition, the mode-field size as well as the thickness of cladding can be easily tuned in the SSDC waveguides by adjusting the writing conditions, facilitating its effectiveness in various applications. The main contents of the dissertation are as follows: First, the geometric structure of the SSDC waveguides is introduced. We show the fabrication process of the SSDC waveguides using femtosecond laser direct writing as well as the slit beam shaping method. SSDC waveguides fabricated in ZBLAN glass, lithium niobate and photo-thermo-refractive glass are demonstrated, and their properties are discussed, respectively. Next, we focus on how to fabricate a deeply buried SSDC waveguide in lithium niobate crystal using femtosecond laser direct writing. The fabrication at large depths are always hampered by rapidly degraded axial resolution caused by optical aberration. We show that by making use of the correction of the spherical aberration, polarization-independent waveguides can be inscribed in the crystal at depths up to 1400 ?m, which is more than one order of magnitude deeper than the waveguides written with aberration uncorrected femtosecond laser pulses. Finally, the influence of incident femtosecond laser polarization on the structure of SSDC waveguides fabricated in lithium niobate crystal is systematically investigated. We compare the written structures produced in lithium niobate with the laser polarization being perpendicular and parallel to the optical axis of the crystal. The results are beneficial for improving the fabrication quality of the SSDC waveguides.
学科主题	光学
内容类型	学位论文
源URL	[http://ir.siom.ac.cn/handle/181231/30974]
专题	中国科学院上海光学精密机械研究所
作者单位	中国科学院上海光学精密机械研究所
推荐引用方式 GB/T 7714	王鹏. 飞秒激光直写在透明材料内部制备方形凹陷包层光波导[D].

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