Embodiments described herein describe a sub-mount that is etched to include respective cavities with at least two adjacent sides that align optical filters and a mirror. Moreover, the cavities are arranged on the sub-mount such that when the filters and mirror are disposed in the cavities, they align in a manner that enables the performance of a multiplexing or demultiplexing function as part of, for example, a zigzag multiplexer/demultiplexer. In one embodiment, the filters and mirrors are aligned passively rather than actively. The sub-mount may then be placed on a substrate that includes other components of a ROSA or TOSA. In one embodiment, the substrate is also etched to include a cavity two adjacent sides to align the sub-mount so that sub-mount is passively aligned once disposed into the cavity.