Efficient coupling between optical fibers and on-chip photonic waveguides has long been a crucial issue for photonic chips used in various applications. Edge couplers (ECs) based on an inverse taper have seen widespread utilization due to their intrinsic broadband operation. However, it still remains a big challenge to realize polarization-insensitive low-loss ECs working at the O-band (1,260–1,360 nm), mainly due to the strong polarization dependence of the mode coupling/conversion and the difficulty to fabricate the taper tip with an ultra-small feature size. In this paper, a high-efficiency and polarization-insensitive O-band EC is proposed and demonstrated with great advantages that is fully compatible with the current 130-nm-node fabrication processes. By introducing an asymmetric bi-level dual-core mode converter, the fundamental mode confined in the thick core is evanescently coupled to that in the thin core, which has an expanded mode size matched well with the fiber and works well for both TE/TM-polarizations. Particularly, no bi-level junction in the propagation direction is introduced between the thick and thin waveguide sections, thereby breaking the critical limitation of ultra-small feature sizes. The calculated coupling loss is 0.44–0.56/0.48–0.61 dB across the O-band, while achieving 1-dB bandwidths exceeding 340/230 nm for the TE/TM-polarization modes. For the fabricated ECs, the peak coupling loss is ∼0.82 dB with a polarization dependent loss of ∼0.31 dB at the O-band when coupled to a fiber with a mode field diameter of 4 μm. It is expected that this coupling scheme promisingly provides a general solution even for other material platforms, e.g., lithium niobate, silicon nitride and so on.

中文翻译：

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非对称双电平双核模式转换器，用于高效和对偏振不敏感的 O 波段光纤芯片边缘耦合：打破临界尺寸限制


长期以来，光纤和片上光子波导之间的有效耦合一直是各种应用中使用的光子芯片的关键问题。基于反向锥度的边缘耦合器 （EC） 由于其固有的宽带操作而得到了广泛的应用。然而，实现在 O 波段 （1,260–1,360 nm） 工作的偏振不敏感的低损耗 EC 仍然是一个很大的挑战，这主要是由于模式耦合/转换的极化依赖性强，并且难以制造具有超小特征尺寸的锥形尖端。在本文中，提出并展示了一种高效且对偏振不敏感的 O 波段 EC，该 EC 具有巨大的优势，与当前的 130 nm 节点制造工艺完全兼容。通过引入非对称双电平双芯模式转换器，限制在厚芯中的基模与薄芯中的基模倏逝耦合，薄芯具有与光纤匹配良好的扩展模式尺寸，适用于 TE/TM 极化。特别是，在厚波导段和薄波导段之间没有引入传播方向的双能级结，从而打破了超小特征尺寸的临界限制。计算出的耦合损耗在整个 O 波段为 0.44–0.56/0.48–0.61 dB，同时在 TE/TM 极化模式下实现超过 340/230 nm 的 1 dB 带宽。对于制造的 EC，当耦合到模场直径为 4 μm 的光纤时，峰值耦合损耗为 ∼0.82 dB，在 O 波段的偏振相关损耗为 ∼0.31 dB。预计这种耦合方案有望为其他材料平台（例如铌酸锂、氮化硅等）提供通用解决方案。