Silicon-based optical phased arrays have received a lot of research due to their small size, high reliability, low cost, and so on. It has  broad prospects in the fields like free-space optical communication [1], [2], lidar [2], [3], [4], [5], and projection systems [6], [7]. For optical phased arrays, optical antennas with excellent performance can greatly improve the performance, such as large field of view, small divergence angle, high emission directionality, and low loss.

Among them, the emission directionality characterizes the proportion of the upward emission of the antenna. The far-field beam of an optical phased array is formed based on the interference of multiple light sources. According to a study reported in 2017, there will be blind points in the far-field of antennas with low upward emission directionality compared to antennas with high upward emission directionality [8]. The downward emission of the antenna is reflected upward by the substrate and interferes with the upward emission. There will be an interference reduction in certain directions. There is higher downward emission for a low upward directionality antenna. So that the interference reduction is more significant, causing blind spots. Therefore, antennas with high upward emission directionality have been researched [8], [9], [10], [11], [12]. In addition, a small spot is required for high resolution. This requires a large aperture to achieve a small divergence angle. Large-scale antennas can be realized by reducing the perturbation of the grating. But the emission field intensity decays exponentially along the antenna. In this case, the effective aperture of the antenna is smaller than the actual aperture. Therefore, uniform emission antennas are designed [8], [12], [13], [14], [15], [16], [17], [18].

However, to the best of our knowledge, there are only two studies that have both high directionality and uniform emission [8], [12]. The two antennas designed are shown in Fig. 1, both based on a dual-layer grating structure for high upward emission directionality and by etching the waveguide to fabricate the grating. As shown in Fig. 1(g), etching the waveguide has a strong effect on the mode in the waveguide. The antenna in Fig. 1(a) uses a small perturbation value to achieve a 3 mm long uniform emission antenna. The perturbation value is less than 50 nm in the length of 2/3 of the antenna. This is difficult for many foundries [19]. The antenna of Figs. 1(c) and 1(e) are limited by the process precision. The characteristic dimensions of these antennas exceed 150 nm, which limits their uniform emission lengths to about 800μm.

The antenna we designed is based on a three-layer SiN structure of grating–waveguide–grating. The grating layer interacts with evanescent waves outside the waveguide (Fig. 1(h)). Since 3D FDTD requires a lot of computing resources, the simulation of the millimeter-level antenna is difficult, so we designed a 500μm uniform emission antenna. Antennas with lengths of several millimeters can be designed by adjusting the grating structure and the gap between the grating and the waveguide. Multilayer silicon nitride-SOI platforms already exist [20]. And the characteristic dimension is larger than 300 nm. This will facilitate faster and more stable fabrication in foundries.

Two gratings are positioned above and below the waveguide and both generate upward and downward emissions. By adjusting the relative offset of the two gratings and optimizing their structure, the upward emission coherence is enhanced and the downward emission coherence is reduced, thereby achieving high upward emission directionality. Then, based on the directionality optimization result, a uniform emission antenna is proposed (Fig. 2). It is designed by adjusting the waveguide width Wwg and etched portion P. Its full length is 500μm, and it has a flat near-field and far-field with a side mode suppression ratio of less than 0.2 from 1540 nm to 1620 nm. The upward directionality exceeds 80% and reaches 92% at 1540 nm. The divergence angle is about 0.17° and the field of view is approximately 5.54°.