Proposal of mosaic-based 2 x 2 3-dB couplers for prospective on-chip communication in mid-infrared wavelengths around 2.1 μm

We propose two types of mosaic-based 2 x 2 3-dB couplers for wavelengths around 2.1 mu m. For type I, the proposed device provides excess loss of <0.22 dB and imbalance of <+/- 0.07 dB over the wavelength range from 2.06 to 2.14 mu m. Compared with the standard multimode-interference-based 2 x 2 3-dB coupler, the imbalance is improved by similar to 0.4 dB. For type II, coupling length in the multimode-interference region is about three times shorter than the one based on multimode-interference theory. It provides excess loss of <0.49 dB and imbalance of <+/- 0.23 dB over the wavelength range from 2.06 to 2.14 mu m. Both proposed devices show high fabrication tolerance to hole diameter variation. Especially for type II, the fabrication-tolerant direct binary search was effective to obtain such a structure. These mosaic-based 2 x 2 3-dB couplers will play an important role in highly integrated photonic circuits. (C) 2022 Optica Publishing Group