Design of ultra-long waveguide grating antennas with uniform emission and high directionality for optical phased arrays on a silicon-on-insulator waveguide platform

We propose a novel type of waveguide grating antennas (WGAs) for silicon photonics based integrated optical phased arrays (OPAs) that require an ultra-large aperture. Our proposed WGAs can realize grating strength low enough for extending the effective length to centimeter level while still keep the feature sizes large enough to fulfill the design rules of mainstream foundry fabrication on a silicon-on-insulator (SOI) waveguide platform. Moreover, due to its unique structure, our proposed WGA has a large design flexibility, allowing easy realization of uniform-emission or/and unidirectional emission by employing chirped gratings, apodized gratings and compounded gratings. To evaluate the nearfield and farfield radiation profile of ultra-long WGAs, we establish a simple but helpful analytical model, which can provide information including beam width and beam angle. For demonstrations, we present a design example of a centimeter-long WGA with uniform emission. We theoretically show that a beam width as small as 0.008 degrees can be obtained with the proposed WGA, which has minimum feature sizes as large as 150 nm . In another example, by involving two etch depths, we demonstrate that our proposed WGA can be designed to emit around 90% of the optical power upwards, i.e. with high upward directionality.