<s>
An	O
optical	B-Protocol
DPSK	I-Protocol
demodulator	I-Protocol
is	O
a	O
device	O
that	O
provides	O
a	O
method	O
for	O
converting	O
an	O
optical	O
differential	B-General_Concept
phase-shift	I-General_Concept
keying	I-General_Concept
(	O
DPSK	B-General_Concept
)	O
signal	O
to	O
an	O
intensity-keyed	O
signal	O
at	O
the	O
receiving	O
end	O
in	O
fiber-optic	B-Application
communication	I-Application
networks	O
.	O
</s>
<s>
It	O
is	O
also	O
known	O
as	O
delay	B-Device
line	I-Device
interferometer	I-Device
(	O
DLI	O
)	O
,	O
or	O
simply	O
called	O
DPSK	B-General_Concept
demodulator	O
.	O
</s>
<s>
The	O
DPSK	B-General_Concept
decoding	O
method	O
is	O
achieved	O
by	O
comparing	O
the	O
phase	O
of	O
two	O
sequential	O
bits	O
.	O
</s>
<s>
An	O
incoming	O
DPSK	B-General_Concept
optical	O
signal	O
is	O
first	O
split	O
into	O
two	O
beams	O
with	O
equal	O
intensities	O
,	O
in	O
which	O
one	O
beam	O
is	O
delayed	O
in	O
space	O
by	O
an	O
optical	O
path	O
difference	O
that	O
introduces	O
a	O
time	O
delay	O
corresponding	O
to	O
one	O
bit	O
.	O
</s>
<s>
A	O
typical	O
optical	O
system	O
for	O
such	O
a	O
purpose	O
is	O
Mach	O
–	O
Zehnder	O
interferometer	O
or	O
Michelson	O
interferometer	O
,	O
forming	O
an	O
optical	B-Protocol
DPSK	I-Protocol
demodulator	I-Protocol
.	O
</s>
<s>
For	O
instance	O
,	O
in	O
a	O
40	O
Gbit/s	O
system	O
,	O
one	O
bit	O
corresponds	O
to	O
25	O
picoseconds	O
,	O
and	O
light	O
travels	O
5mm	O
in	O
a	O
fiber	B-Architecture
optics	I-Architecture
or	O
7.5mm	O
in	O
free	O
space	O
within	O
that	O
period	O
.	O
</s>
<s>
DQPSK	B-General_Concept
is	O
the	O
four-level	O
version	O
of	O
DPSK	B-General_Concept
.	O
</s>
<s>
DQPSK	B-General_Concept
transmits	O
two	O
bits	O
for	O
every	O
symbol	O
(	O
bit	O
combinations	O
being	O
00	O
,	O
01	O
,	O
11	O
and	O
10	O
)	O
and	O
has	O
an	O
additional	O
advantage	O
over	O
conventional	O
binary	O
DPSK	B-General_Concept
.	O
</s>
<s>
DQPSK	B-General_Concept
has	O
a	O
narrower	O
optical	O
spectrum	O
,	O
which	O
tolerates	O
more	O
dispersion	O
(	O
both	O
chromatic	O
and	O
polarization-mode	O
)	O
,	O
allows	O
for	O
stronger	O
optical	O
filtering	O
,	O
and	O
enables	O
closer	O
channel	O
spacing	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
DQPSK	B-General_Concept
allows	O
processing	O
of	O
40	O
Gbit/s	O
data-rate	O
in	O
a	O
50GHz	O
channel	O
spacing	O
system	O
.	O
</s>
<s>
A	O
demodulator	O
for	O
optical	O
DQPSK	B-General_Concept
signals	O
can	O
be	O
constructed	O
using	O
two	O
matched	O
DPSK	B-General_Concept
demodulators	O
with	O
phase	O
off-set	O
at	O
.	O
</s>
