Ultra-compact and efficient 1 × 2 mode converters based on rotatable direct-binary-search algorithm

In this paper, we innovatively demonstrate a rotatable direct-binary-search algorithm. Based on this unique inverse design method, the coupling region of nanophotonic device can be realized with multi-shape and multi-rotation pixels. In addition, the novel 1× 2 mode converters with multipurpose design goals on a 220 nm-thick top silicon-on-insulator platform are proposed by utilizing this enhanced algorithm, which can simultaneously achieve power splitting and mode conversion. By 3D fine difference time domain solutions, the 1 × 2 mode converter that converts TE₀ mode into TE₁, with a footprint of 2.7 µm × 2.4 µm, exhibits the excess loss of 0.1 - 0.2 dB (TE₁ mode), crosstalk of lower than -20.6 dB (TE₀ mode) and reflection loss of lower than -19.5 dB (TE₀ mode) from 1500 nm to 1600 nm. The 1 × 2 mode converter that transforms TE₀ into TE₂ occupies the footprint of 3.6 µm × 3 µm. The excess loss is 0.3 - 0.4 dB (TE₂ mode) in the wavelength range of 1500 - 1600 nm. The crosstalks are lower than -17.5 dB (TE₁ mode) and -25.1 dB (TE₀ mode), and the reflection loss is lower than -18.3 dB (TE₀ mode). Besides, the fabrication tolerances caused by both expansion or contraction of etched pattern contour and round corner effect are also investigated.