a wavelength division multiplexed optical communication system includes a plurality of optical line terminals which may be part of separate in service networks , each having a line interface and an all - optical pass - through interface including a plurality of pass - through optical ports , and each also including a plurality of local optical ports which are connectable to client equipment and an optical multiplexer / demultiplexer for multiplexing / demultiplexing optical wavelengths . the optical multiplexer / demultiplexer may include one or more stages for inputting / outputting individual wavelengths or bands of a predetermined number of wavelengths , or a combination of bands and individual wavelengths . at least one of the pass - through optical ports of an optical line terminal of one network may be connected to at least one of the pass - through optical ports of an optical line terminal of another network to form an optical path from the line interface of the optical line terminal of the one network to the line interface of the optical line terminal of the another network to form a merged network . the use of such optical line terminals allows the upgrading and merging of the separate networks while in service .