Compact and broadband silicon mode-order converter using bricked subwavelength gratings.

A compact and broadband silicon mode-order converter (MOC) scheme by employing reciprocal mode evolution between asymmetric input/output taper and bricked subwavelength gratings (BSWG) is proposed. In the proposed MOC, a quasi-TE mode is generated in the BSWG region, which can be regarded as an effective bridge between the two TE modes to be converted. Flexible mode conversion can be realized by only choosing appropriate structure parameters for specific mode transitions between input/output modes and the quasi-TE mode. By combing 3D finite difference time domain (FDTD) and particle swarm optimization (PSO) method, TE-TE and TE-TE MOCs are optimal designed, which can efficiently convert TE mode to TE and TE modes with lengths of 9.39 µm and 11.27 µm, respectively. Results show that the insertion losses of <1 dB and crosstalk of <-15 dB are achieved for both TE-TE and TE-TE MOCs, the corresponding working bandwidth are 128 nm (1511∼1639 nm) and 126 nm (1527∼1653 nm), respectively. Additionally, the MOCs can be fabricated with only single etch step with minimum feature size of 145 nm.