Abstract

We present a 416 μm × 416 μm waveguide hologram that can focus light from single-mode waveguides to a 50 μm × 50 μm narrow spot located 10 mm above the chip for three distinct wavelengths. To the best of our knowledge, this waveguide hologram stands as the most expansive grating coupler achieved on a photonic integration platform with a high refractive index contrast. To achieve nearly uniform emission across its expansive area, we etch the hologram's nanostructures into a relatively low refractive index layer atop the waveguide. Additionally, we have developed a novel semi-analytical model that enables us to efficiently calculate the emission pattern of large-area waveguide holograms with varying nanostructure sizes. This model enables us to optimize the nanostructure's sizes across the hologram to achieve a Gaussian intensity profile in the far-field. Leveraging this approach, we design and fabricate a tri-wavelength focusing emitter on the InP Membrane On-Silicon (IMOS) Platform, targeting wavelengths of 1310 nm, 1450 nm, and 1600 nm. Our calculated far-field intensity profiles closely align with experimental measurements. We observe the coupler achieves near-vertical focusing at 9.4 mm across the target wavelengths. Though an initial design mistake resulted in an imbalance in coupling efficiency across different wavelengths. Nevertheless, our model clearly predicts this imbalance. Our model allows one to conveniently optimize the intensity profile of various waveguide holograms used in optical sensing, Augmented Reality or optical imaging.