Dielectrically loaded thin-film lithium niobate (LiNbO₃) on insulator (LNOI) platforms have enabled a range of photonic integrated circuit components, such as high-speed optical modulators, switches, and nonlinear devices, while avoiding the direct etching of the LiNbO₃ thin film. Silicon nitride (Si₃N₄) is one of the most attractive dielectric loading materials as it has a similar refractive index and transparency window to LiNbO₃ and can be deposited and patterned by mature fabrication processes. The patterning of Si₃N₄ opens the opportunity to fabricate grating couplers in the same fabrication step, providing efficient optical interfaces for wafer-scale testing. In this paper, we investigate and demonstrate single-step etched grating couplers on a Si₃N₄-LNOI (X-cut) platform. The grating couplers (straight and curved) are designed and fabricated for TE-polarized modes along the Y and Z crystallographic directions, considering the LiNbO₃ crystal’s birefringence. The experimentally demonstrated coupling losses are as low as 4.02 and 4.24 dB along the crystallographic Y and Z directions, respectively. The corresponding peak wavelengths are 1609 and 1615 nm, respectively. The measured 3-dB bandwidths are wider than 70 nm for both crystallographic directions. We also numerically investigated the influence of fabrication variations and the fiber angle on the transmission. To the best of our knowledge, this work is the first demonstration of grating couplers with different light propagation directions on the Si₃N₄ loaded LNOI platform.

中文翻译：

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绝缘体平台上氮化硅负载铌酸锂的单步蚀刻光栅耦合器

绝缘体 (LNOI) 平台上的介电负载薄膜铌酸锂 (LiNbO ₃ ) 使一系列光子集成电路组件成为可能，例如高速光调制器、开关和非线性器件，同时避免了对 LiNbO ₃的直接蚀刻薄膜。氮化硅 (Si ₃ N ₄ ) 是最具吸引力的介电负载材料之一，因为它具有与 LiNbO ₃相似的折射率和透明度窗口，并且可以通过成熟的制造工艺进行沉积和图案化。Si ₃ N ₄的构图开启了在同一制造步骤中制造光栅耦合器的机会，为晶圆级测试提供高效的光学接口。在本文中，我们研究并演示了 Si ₃ N ₄ -LNOI（X 切割）平台上的单步蚀刻光栅耦合器。考虑到 LiNbO ₃晶体的双折射。实验证明，沿晶体 Y 和 Z 方向的耦合损耗分别低至 4.02 和 4.24 dB。相应的峰值波长分别为 1609 和 1615 nm。测得的 3-dB 带宽在两个晶体学方向都大于 70 nm。我们还数值研究了制造变化和光纤角度对传输的影响。据我们所知，这项工作是首次在加载了 Si ₃ N _{4 的}LNOI 平台上展示了具有不同光传播方向的光栅耦合器。