  Optical phased arrays (OPAs) implemented in integrated photonic circuits
could enable a variety of 3D sensing, imaging, illumination, and ranging
applications, and their convergence in new LIDAR technology. However, current
integrated OPA approaches do not scale - in control complexity, power
consumption, and optical efficiency - to the large aperture sizes needed to
support medium to long range LIDAR. We present the serpentine optical phased
array (SOPA), a new OPA concept that addresses these fundamental challenges and
enables architectures that scale up to large apertures. The SOPA is based on a
serially interconnected array of low-loss grating waveguides and supports fully
passive, two-dimensional (2D) wavelength-controlled beam steering. A
fundamentally space-efficient design that folds the feed network into the
aperture also enables scalable tiling of SOPAs into large apertures with a high
fill-factor. We experimentally demonstrate the first SOPA, using a 1450 - 1650
nm wavelength sweep to produce 16,500 addressable spots in a 27x610 array. We
also demonstrate, for the first time, far-field interference of beams from two
separate OPAs on a single silicon photonic chip, as an initial step towards
long-range computational imaging LIDAR based on novel active aperture synthesis
schemes.
