Ultra-compact dual-band grating coupler for visible wavelengths with shape-optimized spot size converter

Here, we theoretically and numerically present, to the best of our knowledge, the first ultra-compact dual-band grating coupler for visible wavelengths based on a silicon nitride (Si₃N₄) platform. The device can be divided into two integrated components: fully etched one-dimensional (1D) grating coupler and shape-optimized spot-size converters (SSCs), enabling simultaneous 729 nm-λ and 860 nm-λ light coupling from a single-mode fiber to opposite single-mode dielectric waveguides. For the proposed 1D grating coupler, the optimized coupling efficiencies from fiber-to-chip are calculated to be -4.12 dB at 729 nm-λ and -5.5 dB at 860 nm-λ, with corresponding 3 dB bandwidths of ∼39 nm and ∼48 nm, respectively. Furthermore, by optimizing the spot-size converter using the adjoint method, modes converting between the wide grating waveguide and narrow single-mode waveguides are achieved with low insertion loss, large bandwidth and ultra-compact size. The region of the optimized fiber-grating-waveguide coupling device occupies only about 65 μm × 10 μm (length × width). The total coupling efficiencies are achieved as -4.28 dB and -5.57 dB for 729 nm-λ and 860 nm-λ, respectively. Such a coupling scheme could be applied in a CMOS-compatible integrated ion qubits surface-trap system to interface the external light source with in-plane multi-wavelength ion trap devices.