An integrated wavelength division multiplexer is described. The integrated wavelength division multiplexer may include a first waveguide core defining a first propagation axis and configured to guide light of a first wavelength and light of a second wavelength, and a second waveguide core defining a second propagation axis and configured to guide the light of the second wavelength. A first portion of the second propagation axis for which the first waveguide core and second waveguide core may be overlapping is oriented at a non-zero angle relative to the first propagation axis. The first waveguide core and second waveguide core may be configured relative to each other to adiabatically couple the light of the second wavelength between the first and second waveguide cores.