<s>
Modal	B-Device
dispersion	I-Device
is	O
a	O
distortion	O
mechanism	O
occurring	O
in	O
multimode	B-Device
fibers	I-Device
and	O
other	O
waveguides	O
,	O
in	O
which	O
the	O
signal	O
is	O
spread	O
in	O
time	O
because	O
the	O
propagation	O
velocity	O
of	O
the	O
optical	O
signal	O
is	O
not	O
the	O
same	O
for	O
all	O
modes	O
.	O
</s>
<s>
Other	O
names	O
for	O
this	O
phenomenon	O
include	O
multimode	B-Device
distortion	I-Device
,	O
multimode	B-Device
dispersion	I-Device
,	O
modal	B-Device
distortion	I-Device
,	O
intermodal	B-Device
distortion	I-Device
,	O
intermodal	B-Device
dispersion	I-Device
,	O
and	O
intermodal	B-Device
delay	I-Device
distortion	I-Device
.	O
</s>
<s>
In	O
the	O
ray	O
optics	O
analogy	O
,	O
modal	B-Device
dispersion	I-Device
in	O
a	O
step-index	B-Device
optical	I-Device
fiber	I-Device
may	O
be	O
compared	O
to	O
multipath	O
propagation	O
of	O
a	O
radio	O
signal	O
.	O
</s>
<s>
Rays	O
of	O
light	O
enter	O
the	O
fiber	O
with	O
different	O
angles	O
to	O
the	O
fiber	O
axis	O
,	O
up	O
to	O
the	O
fiber	O
's	O
acceptance	B-Device
angle	I-Device
.	O
</s>
<s>
Modal	B-Device
dispersion	I-Device
limits	O
the	O
bandwidth	B-Algorithm
of	O
multimode	B-Device
fibers	I-Device
.	O
</s>
<s>
For	O
example	O
,	O
a	O
typical	O
step-index	B-Device
fiber	I-Device
with	O
a	O
50μm	O
core	O
would	O
be	O
limited	O
to	O
approximately	O
20MHz	O
for	O
a	O
one	O
kilometer	O
length	O
,	O
in	O
other	O
words	O
,	O
a	O
bandwidth	B-Algorithm
of	O
20MHz·km	O
.	O
</s>
<s>
Modal	B-Device
dispersion	I-Device
may	O
be	O
considerably	O
reduced	O
,	O
but	O
never	O
completely	O
eliminated	O
,	O
by	O
the	O
use	O
of	O
a	O
core	O
having	O
a	O
graded	B-Device
refractive	I-Device
index	I-Device
profile	O
.	O
</s>
<s>
However	O
,	O
multimode	O
graded-index	B-Device
fibers	I-Device
having	O
bandwidths	B-Algorithm
exceeding	O
3.5GHz	O
·km	O
at	O
850nm	O
are	O
now	O
commonly	O
manufactured	O
for	O
use	O
in	O
10	O
Gbit/s	O
data	O
links	O
.	O
</s>
<s>
Modal	B-Device
dispersion	I-Device
should	O
not	O
be	O
confused	O
with	O
chromatic	O
dispersion	O
,	O
a	O
distortion	O
that	O
results	O
due	O
to	O
the	O
differences	O
in	O
propagation	O
velocity	O
of	O
different	O
wavelengths	O
of	O
light	O
.	O
</s>
<s>
Modal	B-Device
dispersion	I-Device
occurs	O
even	O
with	O
an	O
ideal	O
,	O
monochromatic	O
light	O
source	O
.	O
</s>
<s>
A	O
special	O
case	O
of	O
modal	B-Device
dispersion	I-Device
is	O
polarization	O
mode	O
dispersion	O
(	O
PMD	O
)	O
,	O
a	O
fiber	O
dispersion	O
phenomenon	O
usually	O
associated	O
with	O
single-mode	O
fibers	O
.	O
</s>
<s>
In	O
multimode	B-Device
optical	I-Device
fiber	I-Device
with	O
many	O
wavelengths	O
propagating	O
,	O
it	O
is	O
sometimes	O
hard	O
to	O
identify	O
the	O
dispersed	O
wavelength	O
out	O
of	O
all	O
the	O
wavelengths	O
that	O
are	O
present	O
,	O
if	O
there	O
is	O
not	O
yet	O
a	O
service	O
degradation	O
issue	O
.	O
</s>
