The Single Fiber Optical Multiplexer are available in 4 channels versions and are used to combine signals from the traffic cards on to a single fiber. The FRM220-MD40-5157 is 4 channels Single fiber MUX/DEMUX modular design card for CWDM wavelengths including 1510nm, 1530nm, 1550nm, 1570nm. The FRM220-MD40-5561 is 4 channels Single fiber MUX/DEMUX, modular design card for CWDM wavelengths including 1550nm, 1570nm, 1590nm, 1610nm. The Single fiber MUX/DEMUX cards provide the primary wave division and combination functions for CWDM. Line side wavelengths require translation to client side equipment via a transponder card.
Please read this Data Sheet