多次反射的简化模式方法及其在高密度光栅中的应用
文献类型：学位论文
| 作者 | 孙文婷 |
| 学位类别 | 博士 |
| 答辩日期 | 2013 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 周常河 |
| 关键词 | 多次反射的模式方法 菲涅耳反射 消零级相位板 宽带高效率透/反射式光栅 浸入式分束器 |
| 其他题名 | Multiple Reflection Incorporated Simplified Modal Method and Its Application in High-Density Gratings |
| 中文摘要 | 光栅作为一种非常重要的衍射光学元件，在光信息处理、激光脉冲压缩、光学加工、集成光学等领域中得到了广泛的应用。近年来，随着微电子刻蚀加工工艺的成熟和有关光栅设计的数值计算方法的发展，周期具有波长或亚波长量级的高密度光栅由于其偏振特性和光谱特性等都显示出与传统光栅截然不同的特征而受到研究者关注。基于目前高密度光栅的研究现状，本博士论文围绕如何在不同折射率对比度高密度光栅中引入界面反射的模式分析这一问题，介绍了以下几个方面的研究工作： 1)低对比度熔融石英透射光栅的模式分析。为了减小光栅的反射损失，设计了浸入式熔融石英光栅分束器，其透射效率接近100%。在简化模式方法只考虑光栅模激发和传播的基础上，通过引入光栅模在界面处的等效菲涅耳系数来描述其在界面处的反射行为。在一定占空比和周期下，只考虑一个光栅模在界面处的反射，该浸入式光栅可等效为标准的法布里－珀罗(F-P)干涉腔，当同时满足F-P干涉极大值条件和简化模式方法的分束条件时，光栅即可实现高效率分束功能。优化设计得到的浸入式光栅分束器0级和-1的衍射效率均为49.89%。对一般的(非浸入式的)宽带熔融石英光栅，由于简化模式方法无法解释其宽带特性，故提出了多次反射的模式分析方法。该分析方法考虑了多个光栅模在界面处的反射，同时引入了衍射级次的有效折射率，进而定义了出射界面处的等效菲涅耳系数。光栅模被界面反射，在光栅区域多次往返传播后，在出射界面处的复振幅表达式类似平行平板干涉的结果。相比于简化模式方法，利用多次反射模式方法的数值模拟结果整体上更符合严格耦合波分析的计算结果，因而能够有效地解释光栅的宽带衍射特性。以一定带宽内光栅-1级衍射效率的平均值和标准差来衡量光栅的宽带特性，优化设计得到的光栅在126nm带宽范围内-1衍射效率均高于93.9%，可以实现宽带功能。利用全息曝光技术和感应耦合等离子体刻蚀工艺制作了光栅，实验测得的结果与理论值相吻合。 2)中对比度HfO2消零级相位板光栅的模式分析。当光栅周期略大于入射波长时，标准的融熔石英相位板无法消除透射0级。设计了光栅材料为HfO2，基底材料为融熔石英的消零级相位板结构。800nm脉冲激光垂直入射下，优化的相位板结构在光栅周期1055nm-1150nm范围内0级衍射效率均小于0.1%，可用于制作光纤布拉格光栅。垂直入射下，入射光在光栅区域只激发两个偶模。利用多次反射的模式方法分析了光栅模的激发、往返传播和耦合出射过程。并详细分析了该相位板光栅实现消零级的物理机制，推导了实现消零级的解析式条件。利用该解析式条件可指导设计消零级相位板光栅，并为严格耦合波法所证实。 3)高对比度Si反射光栅的模式分析。当光栅折射率对比度较低时，光栅模之间在界面处的耦合较小，可以被忽略。认为每一个光栅模在光栅区域的传播和界面处的反射相对于其他光栅模都是独立的。但对于高折射率对比度光栅，模式之间在界面处的耦合对光栅的衍射特性有很大影响。引入界面处的反射矩阵来描述光栅模在界面处的反射及相互之间的耦合，此时反射矩阵是一个非对角矩阵。分析了非对称光栅实现100%反射率的物理机制，推导了不同入射方向实现高反的解析式条件。优化得到的高对比度Si光栅，在中心波长1550nm垂直入射下，Δλ/λ>10%范围内，反射率高于99%；Δλ≈400nm范围内，反射率高于90%，可以实现宽带高效率反射功能。另外得到的解析式条件对于宽带反射器的设计具有指导意义。 本博士论文包括了不同折射率对比度高密度光栅中考虑界面反射的模式分析方法，浸入式光栅分束器、消零级相位板光栅和宽带高效率透/反射式光栅的设计与制作等方面的主要研究成果。这些工作将增进人们对光栅衍射过程以及光栅实现各种功能物理机制的了解，从而加深对高密度深刻蚀光栅这一新兴学科方向的认识，有益于高密度光栅的实用化。 |
| 英文摘要 | As important diffractive optical elements, gratings are widely used in the fields of optical information processing, laser pulse compression, optical fabrication, integrated optics, etc. Recently, with the mature of microelectronics etching technology and the development of numerical algorithm on the grating design, high-density gratings whose period is of the order of wavelength or subwavelength have attracted extensive attention because of their novel polarization and spectrum properties. Based on the present research status of high-density gratings, multiple reflection at interfaces incorporated modal analysis method in different index contrast high-density gratings is studied in this doctoral dissertation. 1) Modal analysis of low-contrast fused-silica transmission gratings. In order to reduce the reflection losses, a buried fused-silica grating with close to 100% diffraction efficiency for beam splitting is designed. Based on the simplified modal method only considering the excitation and propagation of grating modes, the neglected reflection of grating modes at interfaces can be described by introducing the equivalent Fresnel coefficients. In the given period and duty cycle range, only the reflection of one propagating mode are given consideration, therefore, the buried fused-silica grating can be equivalent to a standard Fabry-Perot cavity. When the resonance condition of the Fabry-Perot cavity and the beam splitting condition from the simplified modal method are satisfied simultaneously, the buried fused-silica grating enables the suppression of any reflection losses and thus can function as a beam splitter with high efficiency. With optimum grating parameters, diffraction efficiencies of the transmitted 0th and -1st orders of the buried fused-silica grating are both 49.89% under Littrow mounting. For general (non-embedded) wideband fused-silica gratings, as the simplified modal method can not explain the wideband diffraction characteristics, here, a multi-reflection modal method is proposed. In this method, reflection of several grating propagating modes at the interfaces are considered, meanwhile, the effective indices of diffracted orders are introduced and therefore the equivalent Fresnel coefficients at emergent interfaces can be defined similarly. As grating propagating modes will be reflected multiple times in the grating region, at emergent interfaces contributions of complex amplitudes of grating modes to diffracted orders are similar to the results of interference of a plane-parallel plate. Compared with the simplified modal method, simulation results from the proposed multi-reflection modal method show better agreement with with those from the rigorous coupled-wave analysis, which demonstrates the significance of the multi-reflection effect in modulating the wideband diffraction behavior of the grating. Mean and standard deviation of diffraction efficiency over a given bandwidth are used as the criteria for the wideband diffraction property, the optimized grating exhibits diffraction efficiency of greater than 93.9% for the TE polarization over a bandwidth of 126 nm, so it can be used as a wideband and high-efficiency diffractive optical element. The designed wideband fused-silica transmission grating is fabricated by using holographic interference recording and inductively coupled plasma etching technology. Experimental results are in agreement with the theoretical values. 2) Modal analysis of HfO2 zeroth-order nulled phase masks. Standard fused-silica binary phase masks cannot extinguish the zeroth order in a situation where the period is only slightly larger than the exposure wavelength. A phase mask structure comprising of a HfO2 grating layer and a fused-silica substrate is designed. With an 800nm femtosecond source, for normal incidence and the TE polarization, the optimized phase mask structure has an extremely low diffraction efficiency of the transmitted 0th order of less than 0.1% over a period range from 1055nm to 1150nm. Therefore, this phase mask structure can be used for fabricating fiber Bragg gratings. Under normal incidence, only the two symmetrical(even) modes can be excited in the grating region. By using the multi-reflection modal method, process of excitation, propagation and multi-reflection of grating modes are analyzed. Physical mechanism of this zeroth-order nulled phase mask is discussed in detail, and the conditions for canceling the transmitted 0th order are obtained. These conditions can be used a guideline for designing zeroth-order nulled phase masks, which is confirmed by the rigorous coupled-wave analysis. 3) Modal analysis of high-contrast silicon reflection gratings. When the refractive index contrast of gratings is relatively low, coupling between the propagating modes at grating interfaces is very weak and can be neglected. The propagation in grating region and reflection at grating interfaces of every grating mode is considered independent from that of other modes. But for high-contrast gratings, coupling between the grating modes at interfaces has an important effect on the grating diffraction behavior. Here, reflection matrices are introduced to describe the coupling between the grating modes at different interfaces, and now the reflection matrices are non-diagonal matrices. Physical mechanism of 100% reflection efficiency of asymmetric high-contrast gratings is analyzed and the the analytical conditions for 100% reflection are given. The optimized silicon grating centered at wavelength of 1550nm exhibits reflectivity of greater than 99% with Δλ/λ>10% and of greater than 90% with Δλ≈400nm for the TE polarization under normal incidence. It can be function as a wideband high-efficiency reflector and the obtained analytical conditions provide a guideline for the design of wideband reflector. This dissertation includes reflection at interfaces incorporated modal analysis method in different index contrast high-density gratings, the design, fabrication and physical mechanism of buried grating splitters, zeroth-order nulled phase masks and wideband transmission /reflection gratings. These work provides a clear description of the diffraction process and physical mechanism of high-density gratings. Thereby, it would enhance the knowledge on this burgeoning subject of high-density gratings, which should be helpful for practical applications. |
| 语种 | 中文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15750]  |
| 专题 | 上海光学精密机械研究所_学位论文 |
| 推荐引用方式 GB/T 7714 | 孙文婷. 多次反射的简化模式方法及其在高密度光栅中的应用[D]. 中国科学院上海光学精密机械研究所. 2013. |
入库方式： OAI收割
来源：上海光学精密机械研究所
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