US Patent 7623789 Low loss WDM add drop node

In an optical fiber network bidirectional WDM traffic is sent on one optical main transmission fiber (1) between two nodes (A, B) connected in the path of the transmission fiber. The bidirectional traffic between a pair of nodes is carried on two different wavelength channels (Nos. 1, 2), one for each direction. The two wavelength channels are added to/dropped from the traffic in the ring fiber (1) in each node using a two-channel add/drop filter (5e1-2, 5w1-2), e.g. a band add/drop filter. In this way the number of add/drop filters connected in the path of the transmission fiber can be kept as small as possible.