Low-loss and broadband cascaded SiN RGB coupler with dual-mode interference

We design and demonstrate a cascaded SiN-based RGB coupler with dual-mode interference (DMI) for a micro laser scanning image projector. The DMI configuration in SiN-based waveguides mitigates the adverse effects of self-image point deviation caused by wavelength dispersion, achieving decreased device length and high transmission efficiency. The underlying mechanisms are discussed based on coupled-mode theory and 3D finite difference time domain simulation. A small footprint of 3µm×70µm is achieved for the RGB coupler device without input/output circuits, and the insertion losses are less than 0.56 dB at RGB wavelengths. There are two orders of magnitude reduction in device length as compared with the conventional S i O ₂-based RGB coupler, which greatly promotes the miniaturization of the couplers and displays integration advantages with a laser diode and waveguide photodetector. In addition, the 3 dB bandwidth is over 50 nm for the coupler, and it demonstrates good fabrication tolerance. This design can further be integrated into visible light communication systems and applied to visible light integrated photonics.