  Traditional photonic integrated circuit (PIC) inherits the mature CMOS
fabrication process from the electronic integrated circuit (IC) industry.
However, this process also limits the PIC structure to a single-waveguide-layer
configuration. In this work, we explore the possibility of the
multi-waveguide-layer PIC by proposing and demonstrating a true 3-D optical
phased array (OPA) device, with the light exiting from the edge of the device,
based on a multi-layer Si3N4/SiO2 platform. The multi-waveguide-layer
configuration offers the possibility of utilizing edge couplers at both the
input and the emitting ends to achieve broadband high efficiency. This
uniqueness provides the potential for a more extended detection range in the
Lidar application. The device has been studied by numerical simulation, and
proof-of-concept samples have been fabricated and tested with a CMOS-compatible
process. To the best of our knowledge, this is the first experimental
proof-of-concept of a true 3-D OPA with a multi-waveguide-layer configuration
all over the device.
