Compact devices for multiplexing applications

Disclosed is a compact device for wavelength-division multiplexing applications. In particular, disclosed is a device that includes a housing and a core at least partially positioned within the housing. The core includes a first single fiber stub, a second single fiber stub, and at least one functional layer positioned between a first fiber of the first single fiber stub and a second fiber of the second single fiber stub. The at least one functional layer is configured to: (i) permit routing of a transmission signal of a multiplexed signal along an optical path from the first fiber stub to the second fiber stub, and (ii) prevent routing of a non-transmission signal of the multiplexed signal along the optical path from the first fiber stub to the second fiber stub. A distance between a first ferrule of the first fiber stub and a second ferrule of the second fiber stub is less than about 0.05 mm.