<s>
The	O
core	O
of	O
a	O
conventional	O
optical	B-Architecture
fiber	I-Architecture
is	O
the	O
part	O
of	O
the	O
fiber	O
that	O
guides	O
the	O
light	O
.	O
</s>
<s>
The	O
core	O
is	O
surrounded	O
by	O
a	O
medium	O
with	O
a	O
lower	O
index	O
of	O
refraction	O
,	O
typically	O
a	O
cladding	B-Device
of	O
a	O
different	O
glass	O
,	O
or	O
plastic	O
.	O
</s>
<s>
Light	O
travelling	O
in	O
the	O
core	O
reflects	O
from	O
the	O
core-cladding	O
boundary	O
due	O
to	O
total	O
internal	O
reflection	O
,	O
as	O
long	O
as	O
the	O
angle	O
between	O
the	O
light	O
and	O
the	O
boundary	O
is	O
greater	O
than	O
the	O
critical	O
angle	O
.	O
</s>
<s>
The	O
limiting	O
angle	O
is	O
called	O
the	O
acceptance	B-Device
angle	I-Device
,	O
and	O
the	O
rays	O
that	O
are	O
confined	O
by	O
the	O
core/cladding	O
boundary	O
are	O
called	O
guided	B-Device
rays	I-Device
.	O
</s>
<s>
The	O
core	O
is	O
characterized	O
by	O
its	O
diameter	O
or	O
cross-sectional	B-Application
area	I-Application
.	O
</s>
<s>
In	O
most	O
cases	O
the	O
core	O
's	O
cross-section	B-Application
should	O
be	O
circular	O
,	O
but	O
the	O
diameter	O
is	O
more	O
rigorously	O
defined	O
as	O
the	O
average	O
of	O
the	O
diameters	O
of	O
the	O
smallest	O
circle	O
that	O
can	O
be	O
circumscribed	O
about	O
the	O
core-cladding	O
boundary	O
,	O
and	O
the	O
largest	O
circle	O
that	O
can	O
be	O
inscribed	O
within	O
the	O
core-cladding	O
boundary	O
.	O
</s>
<s>
Another	O
commonly	O
quoted	O
statistic	O
for	O
core	O
size	O
is	O
the	O
mode	B-Device
field	I-Device
diameter	I-Device
.	O
</s>
<s>
For	O
single-mode	B-Device
fiber	I-Device
,	O
the	O
mode	B-Device
field	I-Device
diameter	I-Device
is	O
larger	O
than	O
the	O
physical	O
diameter	O
of	O
the	O
core	O
,	O
because	O
the	O
light	O
penetrates	O
slightly	O
into	O
the	O
cladding	B-Device
as	O
an	O
evanescent	O
wave	O
.	O
</s>
