Optical systems for wavelength division multiplexing and wavelength division demultiplexing with a multi-core fiber, and methods of their fabrication, are disclosed. The systems include a coupling mirror with a plurality of angled surfaces that are configured to direct light to or from different cores of a multi-core fiber. In addition, the multiplexer systems can further include laser chips for generating light, while the demultiplexer systems can include photodiode arrays for detecting light from the multi-core fiber. The systems can also include a guiding structure comprising filters/micro-mirrors to direct light from the lasers or to the photodiodes.