Folded Shallow Grating Couplers With Minimal Back Reflection and Extended Coupling Bandwidth for Robust Coupling to Multimode Fibers

Grating couplers (GCs) applicable to isolator-free transceiver (TX) chips are desired. In this paper, we numerically and experimentally demonstrate that folded shallow GCs can be used for efficiently coupling isolator-free TX chips to multimode optical fibers to output high-speed optical signals since they can exhibit extended coupling bandwidths and small back reflection. Folded GCs include multiple grating segments connected to each other with waveguides and multimode interferometers. We investigated a single-center grating segment GC and a dual-center grating segment GC as two typical configurations of folded GCs. It was numerically shown that their detailed structures affect the near-field patterns and far-field patterns of light emitted from them, which determine the coupling properties of the GCs. Transient output characteristics of the GCs for a 25-Gb/s optical signal were numerically investigated. Our experimental results revealed that the coupling bandwidth of a folded GC was extended to the long-wavelength side. A back reflection from a folded GC was measured to be smaller than -20 dB. It is concluded that the single-center configuration is better than the other as long as a slight increase of signal distortion can be tolerated.