A multiplexer/demultiplexer is provided comprising a capillary filter block, a capillary adhesive, a signal-routing block, and an index-matching adhesive. The capillary adhesive resides in the capillary interstices of the capillary filter block and the index-matching adhesive forms an optical and mechanical interface between the signal-routing block and the capillary filter block. The layer thickness of the index-matching adhesive accommodates for extra-planar surface irregularities in the bonding face of the signal routing block and extra-planar variations along the proximal ends of the component filter blocks of the capillary filter block. The capillary filter block can be formed from a plurality of component filter blocks by dicing multiple component filter blocks from a filter block substrate, placing the component filter blocks adjacent to one another, and using capillary force to draw adhesive between adjacent sidewalls of component filter blocks.