On-chip platform for a phased array with minimal beam divergence and wide field-of-view

Current silicon photonics phased arrays based on waveguide gratings enable beam steering with no moving parts. However, they suffer from a trade-off between beam divergence and field of view.Here,we showa platformbased on silicon-nitride/silicon that achieves simultaneously minimal beam divergence and maximum field of view while maintaining performance that is robust to fabrication variations. In addition, in order to maximize the emission from the entire length of the grating, we design the grating’s strength by varying its duty cycle (apodization) to emit uniformly. We fabricate a millimeter long grating emitter with diffraction-limited beam divergence of 0.089°. © 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement OCIS codes: (050.2770) Gratings; (280.3640) Lidar; (190.4390) Nonlinear optics, integrated optics; (250.5300) Photonic integrated circuits. References and links 1. J. K. Doylend, M. J. R. Heck, J. T. Bovington, J. D. Peters, L. A. 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Vol. 26, No. 3 | 5 Feb 2018 | OPTICS EXPRESS 2528 #309928 https://doi.org/10.1364/OE.26.002528 Journal © 2018 Received 25 Oct 2017; revised 11 Jan 2018; accepted 16 Jan 2018; published 24 Jan 2018 15. R. Waldhäusl, B. Schnabel, P. Dannberg, E.-B. Kley, A. Bräuer, and W. Karthe, “Efficient Coupling into Polymer Waveguides by Gratings,” Appl. Opt. 36, 9383 (1997). Current silicon photonics phased arrays based on waveguide gratings enable beam steering with no moving parts [1–7], however they suffer from a trade-off between beam divergence (critical for high resolution and long range) and field of view (critical for large steering angle). The beam divergence is determined by the grating’s length, which is proportional to the degree of light confinement in the waveguide. For example, in a highly confining platform such as silicon/silicon-dioxide the fully etched grating’s length is typically only a few μm [8–10], and even partial etch [11] also results in strong gratings with limited length due to high degree of light confinement. In contrast, in a platform with low degree of light confinement such as silicon-nitride/silicon-dioxide, the grating’s length can be as long as 4 mm [3,7]. The field of view (see ψ in Fig. 1(a)) is determined by the minimal spacing between adjacent waveguides required to avoid crosstalk, which is inversely proportional to the degree of light confinement. For example, in a silicon/silicon-dioxide platform the field of view is expected to be ±50° [3], while for silicon-nitride/silicon-dioxide it is expected to be ±25° [3] due to the large separation required between gratings. We include the coupling lengths of silicon and silicon-nitride based platforms in Fig. 5 in the appendix, showing that for a specific waveguide gap, cross-talk for silicon based platform is smaller than the silicon nitride one. Thus, using silicon as our waveguide enable narrower spacing between waveguides leading to large field of view. Fig. 1. Simulation of grating’s sensitivity to process variations. (a) Strength of grating formed by etching in to a 250 nm x 450 nm silicon waveguide (orange) and by etching a 120 nm silicon nitride overlay on the same silicon waveguide (blue). The period of both gratings is 650 nm. (b) Cross section and spatial mode distribution for the silicon waveguide and (c) for the same waveguide with a silicon nitride overlay and the thin Al2O3 between the silicon nitride and silicon. One can see that since the silicon waveguide tightly confines the light, the silicon-nitride overlay only slightly perturb the mode. Previous attempts to overcome the trade-off between beam divergence and field of view led to performance that is affected by fabrication variations. Hutchison et al. [2] used silicon/silicondioxide and was able to fabricate long gratings by reducing the grating’s strength using shallow etching of the silicon. Although theoretically this approach could overcome the aforementioned trade-off, in practice, the fabricated gratings are extremely sensitive to variations. Poulton et Vol. 26, No. 3 | 5 Feb 2018 | OPTICS EXPRESS 2529