  A novel class of photonic integrated circuits employs large-scale integration
of combined beam splitters and waveguides loaded with phase actuators to
provide complex linear processing functionalities that can be reconfigured
dynamically. Here, we propose and experimentally demonstrate a
thermally-actuated Dual-Drive Directional Coupler (DD-DC) design, integrated in
a silicon nitride platform, functioning both as a standalone optical component
providing arbitrary optical beam splitting and common phase as well as for its
use in waveguide mesh arrangements. We analyze the experimental demonstration
of the first integration of a triangular waveguide mesh arrangement, and the
first DD-DC based arrangement along with an extended analysis of its
performance and scalability.
