Polarization-insensitive and low-loss O-band edge coupler for silicon photonics.

Fiber-to-chip edge couplers (ECs) are critical for integrated photonic systems. Currently, the ECs are often designed with tiny structure sizes to achieve high coupling efficiency, which, however, are hindered by the lithography resolution limitation in foundry fabrication capabilities. In this work, we propose and experimentally demonstrate a low-loss and polarization-insensitive EC that overcomes the fabrication constraints in the feature size. This is enabled by an ultra-low-loss mode conversion happening in a specially developed bi-level taper with an ultra-tiny tip, which is formed by introducing a shallowly etched tilted taper on a fully etched inverse taper, showing an ultra-large overlay fabrication tolerance. The simulated coupling loss for TE- and TM-polarizations are 0.53-0.57 dB and 0.45-0.55 dB in the wavelength range of 1260-1360 nm, respectively, when coupled to a fiber with a mode field diameter of ∼4 µm. It is experimentally shown that the minimum coupling loss is ∼0.54 dB and the polarization-dependent loss is as low as ∼0.18 dB. The proposed method assisted by the overlay technology enables the easy fabrication of the structure with tiny feature sizes not only for high-efficiency edge coupling but also for other on-chip structures, making it a promising solution for a wide range of applications in photonic integration.