OADM stands for optical add/drop multiplexer. This is a type of degree-2
optical node, which is generally used for the construction of a ring-based
optical transport network.
An OADM consists of three stages, namely, optical demultiplexer, optical
switch or add/drop stage, and optical multiplexer. Optical demultiplexer
functions to separate wavelengths in an inlet fiber onto individual
fibers. These fibers are then either dropped, connected to 2x2 optical
switches, or directly connected to optical multiplexer. The last stage is
optical multiplexer which functions to multiplex all those wavelengths either
added, directly from optical demultiplexer, or 2x2 optical switches,
into an outlet fiber.
All the lightpaths that
directly pass an OADM are termed cut-through lightpaths, while those lightpaths
added/dropped at the OADM node termed added/dropped lightpaths. In
addition, depending on whether 2x2 optical switches are deployed in
the middle stage, OADM can be further classified
into reconfigurable OADM if optical switches are deployed, or fixed
OADM if everything is not reconfigurable.
Physically, there are several ways to realize an OADM. One can use the
traditional demultiplexer and multiplexer architecture to realize an OADM,
or use a combination of fiber grating and optical circulators to realize an
Although both have add/drop functionality, OADM is generally different from
ADM. The former functions in the optical domain under WDM
technique, while the latter is implicitly considered to functions in the
traditional SDH/SONET networks.
Note: I have copied and pasted this article to
Wikipedia; you can find this article there too.