<s>
In	O
fiber	B-Architecture
optics	I-Architecture
,	O
polarization-maintaining	B-Device
optical	I-Device
fiber	I-Device
(	O
PMF	O
or	O
PM	B-Device
fiber	I-Device
)	O
is	O
a	O
single-mode	B-Device
optical	I-Device
fiber	I-Device
in	O
which	O
linearly	O
polarized	O
light	O
,	O
if	O
properly	O
launched	O
into	O
the	O
fiber	O
,	O
maintains	O
a	O
linear	O
polarization	O
during	O
propagation	O
,	O
exiting	O
the	O
fiber	O
in	O
a	O
specific	O
linear	O
polarization	O
state	O
;	O
there	O
is	O
little	O
or	O
no	O
cross-coupling	O
of	O
optical	O
power	O
between	O
the	O
two	O
polarization	O
modes	O
.	O
</s>
<s>
Polarization-maintaining	B-Device
fibers	I-Device
work	O
by	O
intentionally	O
introducing	O
a	O
systematic	O
linear	O
birefringence	O
in	O
the	O
fiber	O
,	O
so	O
that	O
there	O
are	O
two	O
well	O
defined	O
polarization	O
modes	O
which	O
propagate	O
along	O
the	O
fiber	O
with	O
very	O
distinct	O
phase	O
velocities	O
.	O
</s>
<s>
Optical	B-Device
fiber	I-Device
connectors	I-Device
used	O
for	O
PM	B-Device
fibers	I-Device
are	O
specially	O
keyed	O
so	O
that	O
the	O
two	O
polarization	O
modes	O
are	O
aligned	O
and	O
exit	O
in	O
a	O
specific	O
orientation	O
.	O
</s>
<s>
Note	O
that	O
a	O
polarization-maintaining	B-Device
fiber	I-Device
does	O
not	O
polarize	O
light	O
as	O
a	O
polarizer	O
does	O
.	O
</s>
<s>
Rather	O
,	O
PM	B-Device
fiber	I-Device
maintains	O
the	O
linear	O
polarization	O
of	O
linearly	O
polarized	O
light	O
provided	O
that	O
it	O
is	O
launched	O
into	O
the	O
fiber	O
aligned	O
with	O
one	O
of	O
the	O
fiber	O
's	O
polarization	O
modes	O
.	O
</s>
<s>
The	O
fiber	O
may	O
be	O
geometrically	O
asymmetric	O
or	O
have	O
a	O
refractive	O
index	O
profile	O
which	O
is	O
asymmetric	O
such	O
as	O
the	O
design	O
using	O
an	O
elliptical	O
cladding	B-Device
as	O
shown	O
in	O
the	O
diagram	O
.	O
</s>
<s>
Alternatively	O
,	O
stress	O
permanently	O
induced	O
in	O
the	O
fiber	O
will	O
produce	O
stress	O
birefringence	O
;	O
this	O
may	O
be	O
accomplished	O
using	O
rods	O
of	O
another	O
material	O
included	O
within	O
the	O
cladding	B-Device
.	O
</s>
<s>
It	O
is	O
possible	O
to	O
create	O
a	O
circularly	O
birefringent	O
optical	B-Architecture
fiber	I-Architecture
just	O
using	O
an	O
ordinary	O
(	O
circularly	O
symmetric	O
)	O
single-mode	B-Device
fiber	I-Device
and	O
twisting	O
it	O
,	O
thus	O
creating	O
internal	O
torsional	O
stress	O
.	O
</s>
<s>
Polarization-maintaining	B-Device
optical	I-Device
fibers	I-Device
are	O
used	O
in	O
special	O
applications	O
,	O
such	O
as	O
in	O
fiber	B-Algorithm
optic	I-Algorithm
sensing	I-Algorithm
,	O
interferometry	O
and	O
quantum	O
key	O
distribution	O
.	O
</s>
<s>
They	O
are	O
also	O
commonly	O
used	O
in	O
telecommunications	O
for	O
the	O
connection	O
between	O
a	O
source	O
laser	O
and	O
a	O
modulator	B-Application
,	O
since	O
the	O
modulator	B-Application
requires	O
polarized	O
light	O
as	O
input	O
.	O
</s>
<s>
They	O
are	O
rarely	O
used	O
for	O
long-distance	O
transmission	O
,	O
because	O
PM	B-Device
fiber	I-Device
is	O
expensive	O
and	O
has	O
higher	O
attenuation	O
than	O
singlemode	B-Device
fiber	I-Device
.	O
</s>
<s>
Another	O
important	O
application	O
is	O
fiber-optic	B-Device
gyroscopes	I-Device
,	O
which	O
are	O
widely	O
used	O
in	O
the	O
aerospace	O
industry	O
.	O
</s>
<s>
The	O
output	O
of	O
a	O
PM	B-Device
fiber	I-Device
is	O
typically	O
characterized	O
by	O
its	O
polarization	O
extinction	O
ratio	O
(	O
PER	O
)	O
—	O
the	O
ratio	O
of	O
correctly	O
to	O
incorrectly	O
polarized	O
light	O
,	O
expressed	O
in	O
decibels	O
.	O
</s>
<s>
The	O
quality	O
of	O
PM	O
patchcords	B-Device
and	O
pigtails	O
can	O
be	O
characterized	O
with	O
a	O
PER	O
meter	O
.	O
</s>
<s>
Good	O
PM	B-Device
fibers	I-Device
have	O
extinction	O
ratios	O
in	O
excess	O
of	O
20	O
dB	O
.	O
</s>
