A compact silicon-based mode converter using bricked subwavelength grating

Abstract

Facing with the increasing capacity requirements of on-chip optical interconnects, mode division multiplexing technology (MDM), which leverages the different spatial eigenmodes at the same wavelength as independent channels to transmit optical signals, has attracted tremendous interest. Mode-order converters that can convert the fundamental mode to high-order modes are key components in MDM systems. However, it is still very challenging to achieve compact mode-order converters with high performances. Subwavelength grating (SWG) can be equivalent to homogenous material, which has the prominent advantages such as control over birefringence, dispersion and anisotropy, enabling photonic devices with high performance. Wheras the conventional SWG only needs single-etch step, but the implementation of SWG structure usually requires a fabrication resolution of the order of 100 nm and below, which is difficult for current wafer-scale fabrication technology. The anisotropic response of SWG can be further engineered by introducing bricked topology structure, providing an additional degree of freedom in the design. Meanwhile, the requirement of fabrication resolution can also be reduced (>100 nm). In this work, we experimentally demonstrate compact TE0-TE1 and TE0-TE2 mode-order converters using bricked subwavelength grating (BSWG) based on silicon-on-insulator (SOI) with the minimum feature size of the BSWG is 145 nm. In the proposed mode-order converter, a quasi-TE0 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 structural parameters for specific mode transitions between input/output modes and the quasi-TE0 mode. By combing 3D finite difference time domain (FDTD) and particle swarm optimization (PSO) method, TE0-TE1 and TE0-TE2 mode-order converters are optimal designed. It can convert TE0 mode into TE1 and TE2 mode with conversion length of 9.39 μm and 11.27 μm. The simulation results show that the insertion loss of <1 dB and crosstalk of < ‒ 15 dB are achieved for both TE0-TE1 and TE0-TE2 mode-order converters, the corresponding working bandwidth are 128 nm (1511~1639 nm) and 126 nm (1527~1653 nm), respectively. The measurement results indicate that insertion loss and crosstalk are less than 2.5 dB and -10 dB in a bandwidth of 68 nm (1512~1580 nm, limited by the laser tuning range and grating coupler).