Optical systems for wavelength division multiplexing and wavelength division demultiplexing with a multi-core fiber, and methods of their fabrication, are disclosed. In accordance with one method for fabricating an optical system for wavelength division demultiplexing, a resin is molded to define an element with a plurality of angled surfaces. Further, a plurality of filters are formed around the element, where each of the filters is configured to filter at least one respective wavelength and transmit at least one other respective wavelength. In addition, reflective surfaces are formed on the plurality of angled surfaces. Moreover, an additional resin is formed over the element and over the plurality of filters to complete a guiding structure. Further, arrays of photodiodes are affixed to the guiding structure below the plurality of filters. Alternatively, for demultiplexing embodiments, sets of laser chips are affixed to the guiding structure.