Abstract

We propose two types of mosaic-based ${{2}} \times {{2}}$ 3-dB couplers for wavelengths around 2.1 µm. For type I, the proposed device provides excess loss of ${\lt}{0.22}$ dB and imbalance of ${\lt}{\rm{\pm 0}.{07}}\; {\rm dB}$ over the wavelength range from 2.06 to 2.14 µm. Compared with the standard multimode-interference-based ${{2}} \times {{2}}$ 3-dB coupler, the imbalance is improved by ${\sim}{0.4}\;{\rm{dB}}$. 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 ${\lt}{0.49}\;{\rm{dB}}$ and imbalance of ${\lt}{\rm{\pm 0}.{23}}\;{\rm{dB}}$ over the wavelength range from 2.06 to 2.14 µm. 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 ${{2}} \times {{2}}$ 3-dB couplers will play an important role in highly integrated photonic circuits.

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