Grating couplers are a fundamental building block of integrated optics as they allow light to be coupled from free-space to on-chip components and vice versa. A challenging task in designing any grating coupler is represented by the need for reducing back reflections at the waveguide-grating interface, which introduce additional losses and undesirable interference fringes. Here, we present a design approach for focusing TM grating couplers that minimizes these unwanted reflections by introducing a modified slot that fulfills an anti-reflection condition. We show that this antireflection condition can be met only for the Bloch mode of the grating that concentrates in the dielectric. As a consequence the light is scattered from the grating coupler with a negative angle, referred to as "backscattering design". Our analytic model shows that the anti-reflection condition is transferrable to grating couplers on different waveguide platforms and that it applies for both TE and TM polarizations. Our experimentally realized focusing grating coupler for TM-modes on the silicon photonics platform has a coupling loss of (3.95 ± 0.15) dB at a wavelength of 1.55 µm. It has feature sizes above 200 nm and fully etched slots. The reflectivity between the grating coupler and the connected waveguide is suppressed to below 0.16%.

中文翻译：

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聚焦光栅耦合器的后向散射设计，具有用于横向磁模式的完全蚀刻槽。

光栅耦合器是集成光学器件的基本构建模块，因为它们允许光从自由空间耦合到片上组件，反之亦然。设计任何光栅耦合器的一项艰巨任务是需要减少波导光栅界面处的背反射，这会引入额外的损耗和不希望的干涉条纹。在这里，我们提出了一种聚焦TM光栅耦合器的设计方法，该方法通过引入满足抗反射条件的改进缝隙来最大程度地减少这些有害反射。我们表明，只有集中在电介质中的光栅的Bloch模式才能满足这种抗反射条件。结果，光以负角从光栅耦合器散射，这被称为“反向散射设计”。我们的分析模型表明，抗反射条件可以转移到不同波导平台上的光栅耦合器，并且适用于TE和TM偏振。我们在硅光子平台上通过实验实现的TM模式聚焦光栅耦合器在1.55 µm波长处的耦合损耗为（3.95±0.15）dB。它具有超过200 nm的特征尺寸和完全蚀刻的槽。光栅耦合器和连接的波导之间的反射率被抑制到0.16％以下。它具有超过200 nm的特征尺寸和完全蚀刻的槽。光栅耦合器和连接的波导之间的反射率被抑制到0.16％以下。它具有超过200 nm的特征尺寸和完全蚀刻的槽。光栅耦合器和连接的波导之间的反射率被抑制到0.16％以下。