Structures for a wavelength-division multiplexing filter and methods of forming a structure for a wavelength-division multiplexing filter. A waveguide core of the wavelength-division multiplexing filter includes a first bend having a first curvature and a second bend having a second curvature different than the first curvature. The structure further includes a waveguide core region having a first end surface, a second end surface, and a bend arranged between the first and second end surfaces. The bend is positioned over the first bend of the waveguide core in an overlapping relationship.