  Aluminum nitride (AlN)-on-insulator has emerged as a promising platform for
the realization of linear and non-linear integrated photonic circuits. In order
to efficiently route optical signals on-chip, precise control over the
interaction and polarization of evanescently coupled waveguide modes is
required. Here we employ nanophotonic AlN waveguides to realize directional
couplers with a broad coupling bandwidth and low insertion loss. We achieve
uniform splitting of incoming modes, confirmed by high extinction-ratio
exceeding 33dB in integrated Mach-Zehnder Interferometers. Optimized
three-waveguide couplers furthermore allow for extending the coupling bandwidth
over traditional side-coupled devices by almost an order of magnitude, with
variable splitting ratio. Our work illustrates the potential of AlN circuits
for coupled waveguide optics, DWDM applications and integrated polarization
diversity schemes.
