<s>
Photonic-crystal	B-Device
fiber	I-Device
(	O
PCF	O
)	O
is	O
a	O
class	O
of	O
optical	B-Architecture
fiber	I-Architecture
based	O
on	O
the	O
properties	O
of	O
photonic	O
crystals	O
.	O
</s>
<s>
Because	O
of	O
its	O
ability	O
to	O
confine	O
light	O
in	O
hollow	O
cores	O
or	O
with	O
confinement	O
characteristics	O
not	O
possible	O
in	O
conventional	O
optical	B-Architecture
fiber	I-Architecture
,	O
PCF	O
is	O
now	O
finding	O
applications	O
in	O
fiber-optic	B-Application
communications	I-Application
,	O
fiber	B-Device
lasers	I-Device
,	O
nonlinear	O
devices	O
,	O
high-power	O
transmission	O
,	O
highly	O
sensitive	O
gas	O
sensors	O
,	O
and	O
other	O
areas	O
.	O
</s>
<s>
More	O
specific	O
categories	O
of	O
PCF	O
include	O
photonic-bandgap	B-Device
fiber	I-Device
(	O
PCFs	O
that	O
confine	O
light	O
by	O
band	O
gap	O
effects	O
)	O
,	O
holey	B-Device
fiber	I-Device
(	O
PCFs	O
using	O
air	O
holes	O
in	O
their	O
cross-sections	O
)	O
,	O
hole-assisted	O
fiber	O
(	O
PCFs	O
guiding	O
light	O
by	O
a	O
conventional	O
higher-index	O
core	O
modified	O
by	O
the	O
presence	O
of	O
air	O
holes	O
)	O
,	O
and	O
Bragg	B-Device
fiber	I-Device
(	O
photonic-bandgap	B-Device
fiber	I-Device
formed	O
by	O
concentric	O
rings	O
of	O
multilayer	O
film	O
)	O
.	O
</s>
<s>
Photonic	B-Device
crystal	I-Device
fibers	I-Device
may	O
be	O
considered	O
a	O
subgroup	O
of	O
a	O
more	O
general	O
class	O
of	O
microstructured	B-Device
optical	I-Device
fibers	I-Device
,	O
where	O
light	O
is	O
guided	O
by	O
structural	O
modifications	O
,	O
and	O
not	O
only	O
by	O
refractive	O
index	O
differences	O
.	O
</s>
<s>
Optical	B-Architecture
fibers	I-Architecture
have	O
evolved	O
into	O
many	O
forms	O
since	O
the	O
practical	O
breakthroughs	O
that	O
saw	O
their	O
wider	O
introduction	O
in	O
the	O
1970s	O
as	O
conventional	O
step	O
index	O
fibers	O
and	O
later	O
as	O
single	O
material	O
fibers	O
where	O
propagation	O
was	O
defined	O
by	O
an	O
effective	O
air	O
cladding	O
structure	O
.	O
</s>
<s>
In	O
general	O
,	O
regular	O
structured	O
fibers	O
such	O
as	O
photonic	B-Device
crystal	I-Device
fibers	I-Device
,	O
have	O
a	O
cross-section	O
(	O
normally	O
uniform	O
along	O
the	O
fiber	O
length	O
)	O
consisting	O
of	O
one	O
,	O
two	O
or	O
more	O
materials	O
,	O
most	O
commonly	O
arranged	O
periodically	O
over	O
much	O
of	O
the	O
cross-section	O
.	O
</s>
<s>
Other	O
arrangements	O
include	O
concentric	O
rings	O
of	O
two	O
or	O
more	O
materials	O
,	O
first	O
proposed	O
as	O
"	O
Bragg	B-Device
fibers	I-Device
"	O
by	O
Yeh	O
and	O
Yariv	O
,	O
bow-tie	O
,	O
panda	O
,	O
and	O
elliptical	O
hole	O
structures	O
(	O
used	O
to	O
achieve	O
higher	O
birefringence	O
due	O
to	O
irregularity	O
in	O
the	O
relative	O
refractive	O
index	O
)	O
,	O
spiral	O
designs	O
which	O
allow	O
for	O
better	O
control	O
over	O
optical	O
properties	O
as	O
individual	O
parameters	O
can	O
be	O
changed	O
.	O
</s>
<s>
(	O
Note	O
:	O
PCFs	O
and	O
,	O
in	O
particular	O
,	O
Bragg	B-Device
fibers	I-Device
,	O
should	O
not	O
be	O
confused	O
with	O
fiber	B-Device
Bragg	I-Device
gratings	I-Device
,	O
which	O
consist	O
of	O
a	O
periodic	O
refractive	O
index	O
or	O
structural	O
variation	O
along	O
the	O
fiber	O
axis	O
,	O
as	O
opposed	O
to	O
variations	O
in	O
the	O
transverse	O
directions	O
as	O
in	O
PCF	O
.	O
</s>
<s>
Both	O
PCFs	O
and	O
fiber	B-Device
Bragg	I-Device
gratings	I-Device
employ	O
Bragg	O
diffraction	O
phenomena	O
,	O
albeit	O
in	O
different	O
directions	O
.	O
)	O
</s>
<s>
Generally	O
,	O
such	O
fibers	O
are	O
constructed	O
by	O
the	O
same	O
methods	O
as	O
other	O
optical	B-Architecture
fibers	I-Architecture
:	O
first	O
,	O
one	O
constructs	O
a	O
"	O
preform	O
"	O
on	O
the	O
scale	O
of	O
centimeters	O
in	O
size	O
,	O
and	O
then	O
heats	O
the	O
preform	O
and	O
draws	O
it	O
down	O
to	O
a	O
much	O
smaller	O
diameter	O
(	O
often	O
nearly	O
as	O
small	O
as	O
a	O
human	O
hair	O
)	O
,	O
shrinking	O
the	O
preform	O
cross	O
section	O
but	O
(	O
usually	O
)	O
maintaining	O
the	O
same	O
features	O
.	O
</s>
<s>
Most	O
photonic	B-Device
crystal	I-Device
fibers	I-Device
have	O
been	O
fabricated	O
in	O
silica	O
glass	O
,	O
but	O
other	O
glasses	O
have	O
also	O
been	O
used	O
to	O
obtain	O
particular	O
optical	O
properties	O
(	O
such	O
as	O
high	O
optical	O
non-linearity	O
)	O
.	O
</s>
<s>
There	O
is	O
also	O
a	O
growing	O
interest	O
in	O
making	O
them	O
from	O
polymer	O
,	O
where	O
a	O
wide	O
variety	O
of	O
structures	O
have	O
been	O
explored	O
,	O
including	O
graded	O
index	O
structures	O
,	O
ring	O
structured	O
fibers	O
and	O
hollow	B-Device
core	I-Device
fibers	I-Device
.	O
</s>
<s>
These	O
polymer	O
fibers	O
have	O
been	O
termed	O
"	O
MPOF	O
"	O
,	O
short	O
for	O
microstructured	O
polymer	B-Application
optical	I-Application
fibers	I-Application
.	O
</s>
<s>
Photonic	B-Device
crystal	I-Device
fibers	I-Device
can	O
be	O
divided	O
into	O
two	O
modes	O
of	O
operation	O
,	O
according	O
to	O
their	O
mechanism	O
for	O
confinement	O
:	O
index	O
guiding	O
and	O
photonic	O
bandgap	O
.	O
</s>
<s>
Index	O
guiding	O
photonic	B-Device
crystal	I-Device
fibers	I-Device
are	O
characterized	O
by	O
a	O
core	O
with	O
a	O
higher	O
average	O
refractive	O
index	O
than	O
that	O
of	O
the	O
cladding	O
.	O
</s>
<s>
These	O
photonic	B-Device
crystal	I-Device
fibers	I-Device
operate	O
on	O
the	O
same	O
index-guiding	O
principle	O
as	O
conventional	O
optical	B-Architecture
fiber	I-Architecture
—	O
however	O
,	O
they	O
can	O
have	O
a	O
much	O
higher	O
effective	O
refractive	O
index	O
contrast	O
between	O
core	O
and	O
cladding	O
,	O
and	O
therefore	O
can	O
have	O
much	O
stronger	O
confinement	O
for	O
applications	O
in	O
nonlinear	O
optical	O
devices	O
,	O
polarization-maintaining	O
fibers	O
.	O
</s>
<s>
Alternatively	O
,	O
one	O
can	O
create	O
a	O
photonic	O
bandgap	O
photonic	B-Device
crystal	I-Device
fiber	I-Device
,	O
in	O
which	O
the	O
light	O
is	O
confined	O
by	O
a	O
photonic	O
bandgap	O
created	O
by	O
the	O
microstructured	O
cladding	O
–	O
such	O
a	O
bandgap	O
,	O
properly	O
designed	O
,	O
can	O
confine	O
light	O
in	O
a	O
lower-index	O
core	O
and	O
even	O
a	O
hollow	O
(	O
air	O
)	O
core	O
.	O
</s>
<s>
The	O
term	O
"	O
photonic-crystal	B-Device
fiber	I-Device
"	O
was	O
coined	O
by	O
Philip	O
Russell	O
in	O
1995	O
–	O
1997	O
(	O
he	O
states	O
(	O
2003	O
)	O
that	O
the	O
idea	O
dates	O
to	O
unpublished	O
work	O
in	O
1991	O
)	O
.	O
</s>
