A bi-directional multiplexer/demultiplexer comprises a common port, an upgrade (UPG) port, a plurality of input/output ports, a plurality of wavelength division multiplexing (WDM) filters arranged in a zig-zag fashion, and a single bi-directional tap. The single bi-directional tap directs a tapped signal to a demultiplexing tap port in demultiplexing mode and a multiplexing tap port in multiplexing mode. The bi-directional tap may be a thin-film tap filter.