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Silicon nitride (SiN) optical phased arrays (OPAs) have emerged as a potential alternative to their silicon counterparts, due to their potential use in enhanced solid-state light detection and ranging. Grating antennas based on a SiN waveguide suffer from a limited beam steering efficiency, owing to the relatively lower effective refractive index of the waveguide. To mitigate this limitation, we propose and demonstrate a backward-emitting SiN OPA incorporating a reinforced grating vector, which provides efficient wavelength-tuned beam steering along the longitudinal direction. Two backward-emitting OPAs were primarily characterized by their steering efficiency and the spectral emission for the main lobe, and they were compared with a forward-emitting device using a weakened grating vector. The results indicated that strengthening the grating vector for the antenna improved the beam steering efficiency but adversely affected the dependence of its directionality on the wavelength. The directionality was particularly inspected, with respect to major design parameters for the grating antennas, including the pitch and etch depth of a SiN grating, as well as the thickness of a buried oxide and SiN core layer. The proposed backward-emitting OPA scheme is expected to significantly promote its feasibility as an advanced beam scanning device.

We have proposed and experimentally realized an ultra-compact and broadband silicon nitride edge-coupler that enables high coupling efficiency. The proposed coupler was realized by concatenating short tapers in four stages, whose angles were designed to minimize the footprint while preserving the coupling efficiency. The constituting taper segments were designed by carefully sectioning a long adiabatic taper while adapting to an appropriate taper angle for each segment. The designed coupler exhibited an extremely short footprint of 76 μm. A coupling efficiency of 92% was experimentally attained at 1550 nm wavelength when coupled to a single-mode fiber having a mode field diameter of ~4 μm. Further, an efficiency of over 90% throughout the C and L bands was observed. A 3-dB bandwidth of 965 nm, spanning λ = 1015-1980 nm, was achieved in the simulation. Additionally, the fabricated device exhibited an enhanced cleaving tolerance by virtue of its elongated tip, along with relaxed alignment tolerances ranging up to 3.5 μm. The proposed design was also found to comply with the waveguides having widths between 1 μm and 4 μm without affecting the overall footprint and efficiency. This work is anticipated to provide a promising foundation for the development of compact photonic devices.