Silicon-Based TM0-to-TM3 Mode-Order Converter Using On-Chip Shallowly Etched Slot Metasurface

Mode-order converters drive the on-chip applications of multimode silicon photonics. Here, we propose a TM0-to-TM3 mode-order converter by leveraging a shallowly etched slot metasurface pattern atop the silicon waveguide, rather than as some previously reported TE-polarized ones. With a shallowly etched pattern on the silicon waveguide, the whole waveguide refractive index distribution and the corresponding field evolution will be changed. Through further analyses, we have found the required slot metasurface pattern for generating the TM3 mode with high conversion efficiency of 92.9% and low modal crosstalk <−19 dB in a length of 17.73 μm. Moreover, the device’s working bandwidth and the fabrication tolerance of the key structural parameters are analyzed in detail. With these features, such devices would be beneficial for the on-chip multimode applications such as mode-division multiplexing transmission.

Comparison of Fiber-to-Waveguide Couplers in Point Diffraction Interferometer Based on Waveguide Reference Wavefront Source

To improve the power of reference wavefront generated by reference wavefront source (RWS) based on silicon nitride (SiN) waveguide in point diffraction interferometer (PDI), we design the Y-branch coupler and grating coupler, and then compare the maximum coupling efficiency, 3 dB fabrication tolerance, 3 dB alignment tolerance and polarization dependent loss of two couplers. Our results show that grating coupler has higher coupling efficiency, lower etching difficulty and alignment difficulty, while Y-branch coupler has lower coating difficulty. To get the maximum efficiency, mode in the fiber must be TE mode no matter for grating coupler or Y-branch coupler. This paper improves the power of the wavefront by selecting appropriate fiber-to-waveguide couplers for waveguide RWS. PDI based on the power improved waveguide RWS is expected to be used in many measurement fields.