Compact double-part grating coupler for higher-order mode coupling.

We propose and demonstrate a compact and efficient grating coupler for first-order mode fiber-to-chip coupling. The coupler is configured by a double-part grating structure combined with a curved Y-junction by means of mode diversity. Compared with a traditional grating coupler, the designed structure takes advantage of minimizing the taper lengths of gratings, while performing higher coupling efficiency and lower crosstalk. At a mere 20 μm taper length, we measured a peak coupling efficiency of -3.68  dB with a 1 dB bandwidth of 35 nm. The coupling performance for the fundamental mode is also investigated. A reduced crosstalk below -15  dB within the whole band with reasonable coupling efficiency can be experimentally observed.