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题名	飞秒激光直写矩形凹陷包层光波导
作者	齐家
文献子类	硕士
导师	程亚
关键词	飞秒激光微加工 femtosecond laser micromachining 光波导 optical waveguide 光束空间整形 spatial beam-shaping 偏振独立 polarization-independent 二次谐波 second harmonic generation
其他题名	Fabrication of square-shaped depressed cladding waveguides with femtosecond laser pulses
英文摘要	光波导是光信号在光子芯片中的传播通道，它是由中心折射率较高的纤芯区域和外围折射率较低的包层区域共同构成的结构，由于全反射效应光信号会被约束在波导的导波区域进行定向传输。作为光子芯片的最基本元件，减小光波导的传输损耗、不同尺寸光波导之间的耦合损耗和光波导与其他元件之间的耦合损耗可以提高光子芯片的可靠性，集成度，所以高品质光波导的制备一直是集成光学中的一个重要方向。 利用飞秒激光直写技术可以在透明材料内部诱导折射率改变进而制备光波导。与传统的平面二维波导相比，飞秒激光可以通过聚焦后与材料的非线性效应将波导制备在透明基底材料内部，所以飞秒激光直写技术在光波导制备方面有着加工精确、集成度高、三维能力强等优势。在微流体、量子调控、光互联等方面，飞秒激光直写光波导有着广泛的应用前景。 飞秒激光直写光波导主要有两种方法，分别是纤芯直写和包层直写。纤芯直写是调节飞秒激光脉冲参数，使聚焦在透明材料内部的飞秒脉冲诱导焦斑区域折射率正致变，这样激光直写区域就构成了光波导纤芯区域。包层直写是调节飞秒激光脉冲参数，使聚焦在透明材料内部的飞秒脉冲诱导焦斑区域折射率负致变，这样激光直写区域构成光波导的包层区域，通过多焦点技术或多次直写可以形成直写包层对纤芯的包围。包层直写模式由于导波区域没有受到较强的直写激光作用，波导可以保留材料本身的特殊性质，所以在制备频率转换器件、小型波导激光、电光调制器件等方面包层直写模式更为合适。 但目前常规的两种包层结构都有各自明显的不足。双线包层直写加工方便但由于导波区域不封闭及应力不均匀它在很多透明材料中仅能实现一种偏振光的传输。大量增加直写次数或采用多焦点技术进行包层直写可以构成封闭区域，但由于其对加工条件要求苛刻、耗时长、尺寸难以控制等缺点并没有得到广泛的应用。 针对以上两种包层直写波导的不足，本论文中的实验利用对飞秒激光脉冲的空间整形技术研究了一种新型的矩形凹陷包层波导结构并在以下材料中完成了这种矩形压缩包层光波导的制备。 1. ZBLAN玻璃（ZrF4-BaF2-LaF3-AlF3-NaF）中矩形凹陷包层光波导的制备。使用空间整型后的飞秒激光在ZBLAN玻璃中制备了6 μm ? 6 μm与12 μm ? 12 μm两种尺寸的光波导，实现了红光（632.8nm He-Ne激光）的单模传输。 2. 铌酸锂晶体（LiNbO3）中矩形凹陷包层光波导的制备。使用空间整型后的飞秒激光在铌酸锂晶体（X切、10×5×3mm3）内部加工了矩形凹陷包层光波导。我们分别加工了两种尺寸（13×13μm， 18×18μm）的纵向双线光波导、横向双线光波导、由横向双线和纵向双线共同组合而成的矩形凹陷包层光波导，并且观察到纵向双线光波导仅支持TE模式导波，横向双线光波导仅支持TM模式导波，矩形凹陷包层光波导既支持TE模式也支持TM模式导波。矩形凹陷包层光波导在易于加工、模场尺寸易于调节的前提下第一次实现了封闭的包层结构及TE与TM两种偏振光的传输。 3. 偏硼酸钡晶体（BBO）中矩形凹陷包层光波导的制备。使用空间整形后的飞秒激光在偏硼酸钡晶体（10?2?1mm3，其中通光方向沿10mm方向，设为x方向，y-z面切割方向为θ=19.37°，φ=90°）内部加工了尺寸为（20μm ?20μm）的矩形凹陷包层光波导。基于之前发现的矩形凹陷包层光波导的偏振无依赖特性，它可以同时实现相对于波导o光和e光的导波，满足第一类相位匹配（o+o→e）产生二次谐波的要求。通过对波导二次谐波的研究发现，在1560nm连续激光的激励下，波导实现了1560→780nm二次谐波的产生。当连续基频光的输入功率在100mW时，二次谐波的输出功率为1.4μW，其归一化转换效率与先前研究基于飞秒激光加工相同尺度的硼酸盐包层波导基本一致。; Optical waveguide is the channel of optical signal in photonic chips, which consists of two parts, the central part so called core region has high refractive index and the surrounding part so-called cladding region has low refractive index. The input signal will be confined within the core region of the waveguide according to the total reflection principle. As waveguides are the most basic element of photonic chip, to reduce the transmission loss of the waveguide, to reduce the coupling loss between different waveguides, to reduce the coupling loss between waveguides and other photonic components can improve the reliability and integration of photonic chips. Therefore, the fabrication of high quality optical waveguides is always an important research area in integrated optics. Femtosecond laser direct writing (FLDW) technique can fabricate optical waveguide in transparent material since the refractive index can be changed under the laser spot irradiation. Compared with the traditional processing of planar waveguide, FLDW is based on nonlinear interaction between focus spot and substrate which means FLDW can fabricate waveguide inside the transparent substrate. Therefore, FLDW has a higher integration degree, more powerful three-dimensional fabrication ability and other advantages in optical waveguide processing. FLDW has a wide application prospect in micro fluid, quantum control, optical interconnection and other fields of photonic devices. The two main types of FLDW are core direct writing and cladding direct writing, respectively. Core direct writing is to overlap the center of focal spots of the writing beams. By adjusting the parameters of the femtosecond pulse, the area under the irradiation of the femtosecond laser spot is the light guiding region of the waveguide which has positive refractive index change. Cladding direct writing is to adjust the parameters of the femtosecond pulse to write the cladding which has negative refractive index change. Several tracks of claddings surround the guiding area of the waveguides by writing several times or using multi-spot technique. Since the guiding area does not under the direct irradiation of the femtosecond pulse, the waveguide fabricated by cladding direct writing can maintain the properties of the substrate itself. However, both traditional two types of cladding direct method have their own shortcomings. It is convenient to write so called double-line waveguides by just to write a pair of parallel line claddings. However, the guiding area is not enclosed and stress asymmetry, therefore, double-line waveguides can only support one polarization mode guiding. Depressed cladding waveguides is fabricated by writing many tracks of the cladding to realize the enclosed mode guiding area. But this method is hard to use in chip processing owing to harsh condition, time-consuming, uncontrollable mode size. To solve above problems, we invite a novel method to fabricate square-shaped depressed cladding (SSDC) waveguides by using spatial beam-shaping technique. We report on fabrication of SSDC waveguides in the following substrate. 1. SSDC waveguides were fabricated in ZBLAN glass (ZrF4-BaF2-LaF3-AlF3-NaF). By using spatial light modulator (SLM) shaped femtosecond laser beam we fabricate SSDC waveguides with two different size (6 μm ? 6 μm and 12 μm ? 12 μm). Both of the waveguides with different size can allow single-mode guiding (the wavelength of the signal is 632.8nm, He-Ne laser). 2. SSDC waveguides were fabricated in lithium niobate crystal（X-cut, 10×5×3mm3. By using SLM shaped femtosecond laser beam we fabricate vertical double-line cladding waveguides, horizontal double-double line cladding waveguides, SSDC waveguides respectively (13 μm ? 13 μm and 18 μm ? 18 μm). We present the performance of each waveguides that double-line waveguides in the vertical direction can only guide TE mode, whereas the double-line waveguides in the horizontal direction can only guide TM mode and SSDC waveguides can provide a capability of guiding both TE and TM modes. On the premise of controllable mode size and simple processing, we first ever realize single-mode and polarization-independent mode guiding by SSDC waveguides. 3. SSDC waveguides were fabricated in BiB3O6 (BBO) crystal. Femtosecond laser beam were shaped by SLM and focused inside the BBO substrate (10?2?1mm3, along 10mm direction set to x, crystal were θ=19.37°，φ=90°). Since SSDC waveguides provide polarization-independent mode guiding which satisfies TYPE-I phase-matching condition (o+o→e) of second harmonic generation (SHG). We get the SHG signal (1560nm→780nm) under CW laser pumping. SHG output is about 1.4μW when the input fundamental CW laser is 100mW. The normalized conversion efficiency of SHG at the same level of previous study of small-size BBO waveguide.
学科主题	光学
内容类型	学位论文
源URL	[http://ir.siom.ac.cn/handle/181231/30986]
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	齐家. 飞秒激光直写矩形凹陷包层光波导[D].

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