Precise and diffraction-limited waveguide-to-free-space focusing gratings

Abstract: We present the design and characterization of waveguide grating devices that couple visible-wavelength light at $\lambda=674$ nm from single-mode, high index-contrast dielectric waveguides to free-space beams forming micron-scale diffraction-limited spots a designed distance and angle from the grating. With a view to application in spatially-selective optical addressing, and in contrast to previous work on similar devices, deviations from the main Gaussian lobe up to $25$ microns from the focus and down to the $5\times10^{-6}$ level in relative intensity are characterized as well; we show that along one dimension the intensity of these weak sidelobes approaches the limit imposed by diffraction from the finite field extent in the grating region. Additionally, we characterize the polarization purity in the focal region, observing at the center of the focus a low impurity $< 3 \times 10^{-4}$ in relative intensity. Our approach allows quick, intuitive design of devices with such performance, which may be applied in trapped-ion quantum information processing and generally in any systems requiring optical routing to or from objects 10s--100s of microns from a chip surface, but benefitting from the parallelism and density of planar-fabricated dielectric integrated optics.