<s>
An	O
optical	B-Protocol
add-drop	I-Protocol
multiplexer	I-Protocol
(	O
OADM	B-Protocol
)	O
is	O
a	O
device	O
used	O
in	O
wavelength-division	B-Architecture
multiplexing	I-Architecture
systems	O
for	O
multiplexing	B-Architecture
and	O
routing	B-Protocol
different	O
channels	O
of	O
light	O
into	O
or	O
out	O
of	O
a	O
single	B-Device
mode	I-Device
fiber	I-Device
(	O
SMF	O
)	O
.	O
</s>
<s>
An	O
OADM	B-Protocol
may	O
be	O
considered	O
to	O
be	O
a	O
specific	O
type	O
of	O
optical	B-Protocol
cross-connect	I-Protocol
.	O
</s>
<s>
A	O
traditional	O
OADM	B-Protocol
consists	O
of	O
three	O
stages	O
:	O
an	O
optical	O
demultiplexer	O
,	O
an	O
optical	O
multiplexer	B-Protocol
,	O
and	O
between	O
them	O
a	O
method	O
of	O
reconfiguring	O
the	O
paths	O
between	O
the	O
demultiplexer	O
,	O
the	O
multiplexer	B-Protocol
and	O
a	O
set	O
of	O
ports	O
for	O
adding	O
and	O
dropping	O
signals	O
.	O
</s>
<s>
The	O
reconfiguration	O
can	O
be	O
achieved	O
by	O
a	O
fiber	O
patch	O
panel	O
or	O
by	O
optical	O
switches	O
which	O
direct	O
the	O
wavelengths	O
to	O
the	O
multiplexer	B-Protocol
or	O
to	O
drop	O
ports	O
.	O
</s>
<s>
The	O
multiplexer	B-Protocol
multiplexes	B-Architecture
the	O
wavelength	O
channels	O
that	O
are	O
to	O
continue	O
on	O
from	O
demultiplexer	O
ports	O
with	O
those	O
from	O
the	O
add	O
ports	O
,	O
onto	O
a	O
single	O
output	O
fiber	O
.	O
</s>
<s>
All	O
the	O
lightpaths	B-Protocol
that	O
directly	O
pass	O
an	O
OADM	B-Protocol
are	O
termed	O
cut-through	O
lightpaths	B-Protocol
,	O
while	O
those	O
that	O
are	O
added	O
or	O
dropped	O
at	O
the	O
OADM	B-Protocol
node	O
are	O
termed	O
added/dropped	O
lightpaths	B-Protocol
.	O
</s>
<s>
An	O
OADM	B-Protocol
with	O
remotely	O
reconfigurable	O
optical	O
switches	O
(	O
for	O
example	O
1×2	O
)	O
in	O
the	O
middle	O
stage	O
is	O
called	O
a	O
reconfigurable	O
OADM	B-Protocol
(	O
ROADM	O
)	O
.	O
</s>
<s>
Ones	O
without	O
this	O
feature	O
are	O
known	O
as	O
fixed	O
OADMs	B-Protocol
.	O
</s>
<s>
While	O
the	O
term	O
OADM	B-Protocol
applies	O
to	O
both	O
types	O
,	O
it	O
is	O
often	O
used	O
interchangeably	O
with	O
ROADM	O
.	O
</s>
<s>
Physically	O
,	O
there	O
are	O
several	O
ways	O
to	O
make	O
an	O
OADM	B-Protocol
.	O
</s>
<s>
There	O
are	O
a	O
variety	O
of	O
demultiplexer	O
and	O
multiplexer	B-Protocol
technologies	O
including	O
thin	O
film	O
filters	O
,	O
fiber	B-Device
Bragg	I-Device
gratings	I-Device
with	O
optical	O
circulators	O
,	O
free	O
space	O
grating	O
devices	O
and	O
integrated	O
planar	O
arrayed	B-Protocol
waveguide	I-Protocol
gratings	I-Protocol
.	O
</s>
<s>
The	O
switching	O
or	O
reconfiguration	O
functions	O
range	O
from	O
the	O
manual	O
fiber	O
patch	O
panel	O
to	O
a	O
variety	O
of	O
switching	O
technologies	O
including	O
microelectromechanical	B-Architecture
systems	I-Architecture
(	O
MEMS	B-Architecture
)	O
,	O
liquid	B-Device
crystal	I-Device
and	O
thermo-optic	O
switches	O
in	O
planar	O
waveguide	O
circuits	O
.	O
</s>
<s>
Although	O
both	O
have	O
add/drop	O
functionality	O
,	O
OADMs	B-Protocol
are	O
distinct	O
from	O
add-drop	O
multiplexers	B-Protocol
.	O
</s>
<s>
The	O
former	O
function	O
in	O
the	O
photonic	O
domain	O
under	O
wavelength-division	B-Architecture
multiplexing	I-Architecture
,	O
while	O
the	O
latter	O
are	O
implicitly	O
considered	O
to	O
function	O
in	O
the	O
traditional	O
SONET/SDH	B-Protocol
networks	O
.	O
</s>
