Mode-division multiplexing for visible photonic integrated circuits.

Visible wavelength photonic integrated circuits (PICs) are critical for a wide range of applications including quantum photonics, high-resolution imaging, optogenetics, and portable displays. These applications require functions such as wavefront structuring and dense optical routing on-chip to serve as compact optical interfaces for qubits and cells. The transverse spatial modes of a waveguide can provide the basis for these functions. However, the excitation of these modes in visible PICs has been limited due to fabrication challenges at shorter wavelengths. Here we demonstrate mode-division multiplexing of three higher-order waveguide modes at visible wavelengths (473 nm) with low crosstalk for the first time, to our knowledge. We use adiabatic linearly tapered asymmetric directional couplers that have high theoretical bandwidths of greater than 100 nm and fabrication tolerance to width variations of greater than 45 nm for future integration into large-scale visible PICs with operation across the red, blue, and green spectrum.