Demodulation of an optical carrier
View Patent ↗A method of transmitting a plurality n data streams comprises modulating an optical carrier using differential M-ary phase shift key (DMPSK) signaling in which M=2 n . Advantageously the method comprises using differential quaternary phase shift keying in which n=2. A particular advantage of the method of the present invention is that since the data is differentially encoded in the form of phase changes rather than absolute phase values this enables the modulated optical carrier to be demodulated using direct detection without requiring a phase-locked local optical oscillator. The invention is particularly applicable to WDM communication systems.
1. An optical demodulator for demodulating a differential quaternary phase shift key (DQPSK) optical signal comprising:
a splitter for splitting the DQPSK optical signal into first and second components;
a first interferometer coupled to receive the first component, the first interferometer having first and second arms and first and second outputs; and
a second interferometer coupled to receive the second component, the second interferometer having first and second arms and first and second outputs,
wherein a delay between the two respective arms of each of the first and second interferometers is substantially one symbol period, and
wherein one of the first and second interferometers is configured to introduce a relative phase shift of pi/4 radians between its respective arms, and the other of the first and second interferometers is configured to introduce a relative phase shift of −pi/4 radians between its respective arms.
2. The optical demodulator of claim 1 , further comprising:
a first balanced optical detector coupled to the outputs of the first interferometer; and
a second balanced optical detector coupled to the outputs of the second interferometer.
3. The optical demodulator of claim 1 , wherein the DQPSK optical signal is a modulated carrier signal.
4. A system comprising:
a splitter for splitting a differential quaternary phase shift key (DQPSK) optical signal into first and second components;
a first interferometer coupled to receive the first component, the first interferometer having first and second arms and first and second outputs;
a second interferometer coupled to receive the second component, the second interferometer having first and second arms and first and second outputs;
a first balanced optical detector coupled to the outputs of the first interferometer; and
a second balanced optical detector coupled to the outputs of the second interferometer,
wherein a delay between the two respective arms of each of the first and second interferometers is substantially one symbol period, and one of the first and second interferometers is configured to introduce a relative phase shift of pi/4 radians between its respective arms and the other of the first and second interferometers is configured to introduce a relative phase shift of −pi/4 radians between its respective arms.
5. The system of claim 4 , wherein the first and second interferometers are unbalanced Mach Zehnder interferometers.
6. An optical demodulator arrangement for demodulating and detecting a differential quaternary phase shift key (DQPSK) optical signal comprising:
a splitter for splitting the DQPSK optical signal into first and second components;
a first interferometer coupled to receive the first component, the first interferometer having first and second arms; and
a second interferometer coupled to receive the second component, the second interferometer having first and second arms,
wherein a delay between the two respective arms of each of the first and second interferometers is substantially one symbol period, and
wherein one of the first and second interferometers is configured to introduce a relative phase shift of φ radians between its respective arms, and the other of the first and second interferometers is configured to introduce a relative phase shift of φ±pi/2 radians between its respective arms.
7. The optical demodulator arrangement of claim 6 , wherein the first interferometer imparts a relative phase shift between its respective arms of pi/4 radians and the second interferometer imparts a relative phase shift between its respective arms of −pi/4 radians.
8. The optical demodulator arrangement of claim 6 wherein each of the first and second interferometers has a first and a second output.
9. The optical demodulator arrangement of claim 8 further comprising:
a first balanced optical to electrical converter coupled to outputs of the first interferometer; and
a second balanced optical to electrical converter coupled to outputs of the second interferometer.
10. The optical demodulator arrangement of claim 6 further comprising:
an optical filtering means for filtering the DQPSK optical signal before it is split by the splitter, wherein the optical filtering means has a characteristic which is selected to minimize inter-symbol interference at the sample time for each symbol.
11. The optical demodulator arrangement of claim 10 , wherein the optical filtering means has a characteristic which is substantially a squared raised cosine.
12. The optical demodulator arrangement of claim 10 , wherein the optical filtering means has a transmission characteristic G(f) given by:
G
(
f
)
=
{
1
for
0
≤
f
-
f
0
≤
1
-
α
2
T
1
4
(
1
-
sin
[
π
T
α
(
f
-
f
0
-
1
2
T
)
]
)
2
for
1
-
α
2
T
≤
f
-
f
0
≤
1
+
α
2
T
0
for
f
-
f
0
>
1
+
α
2
T
where T is the sampling period, a is the excess bandwidth factor of the filter, and f 0 is the center frequency of a wavelength division multiplex (WDM) channel.
13. The optical demodulator arrangement of claim 10 , wherein the filter has an excess bandwidth factor of between 0.2 and 0.8.
14. The optical demodulator arrangement of claim 10 , wherein the optical filtering means comprises a respective optical filter for each wavelength channel.
15. The optical demodulator arrangement of claim 10 , wherein the filter comprises a grating structure defined within an optical waveguide.
16. The optical demodulator arrangement of claim 10 , wherein the filter comprises an optical fiber Bragg grating.
17. A method of demodulating and detecting an optical signal comprising:
receiving a differential quaternary phase shift key (DQPSK) optical signal;
splitting the received DQPSK optical signal into two signal components;
introducing a first component to a first interferometer, the first interferometer having first and second arms; and
introducing a second component to a second interferometer, the second interferometer having first and second arms,
wherein a delay between the two respective arms of each of the first and second interferometers is substantially one symbol period, and one of the first and second interferometers is configured to introduce a relative phase shift of φ radians between its respective arms and the other of the first and second interferometers is configured to introduce a relative phase shift of φ±pi/2 radians between its respective arms.
18. The method of claim 17 , wherein the first interferometer is configured to introduce a relative phase shift between its respective arms of pi/4 radians and the second interferometer is configured to introduce a relative phase shift between its respective arms of −pi/4 radians.
19. The method of claim 17 , wherein the DMPSK optical signal is a modulated carrier signal.
20. The method of claim 17 , further comprising detecting the binary output signal using a balanced optical detector.
21. The method of claim 17 , further comprising filtering, via an optical filter, the DQPSK optical signal before it is split by the splitter, wherein the optical filter has a characteristic which is selected to minimize inter-symbol interference at the sample time for each symbol.