A filter module is provided for an optical communications system. The filter module includes a lens, three optical fibers, an optical filter, and a mirror. The three optical fibers are arranged on a single side of the lens. The lens includes a first face coated with the optical filter and that faces the mirror. The optical fibers are arranged on a second face of the lens. A cascade of the filter modules are configured to connect and form a multiplexing/demultiplexing unit.