Differential complex modulation optical receiver having compensation of Kerr effect phase noise
View Patent ↗A method and apparatus for reducing nonlinear phase noise that is induced in an optical transmission system by the interaction of optical amplifier noise and Kerr effect. The apparatus includes an intensity-scaled nonlinear phase noise compensator. The phase noise compensator reduces the nonlinear phase noise by applying a scaled differential signal strength estimate to estimated differential quadrature components or an estimated differential phase for providing a phase noise compensated representation.
1. An optical receiver for receiving an optical signal having nonlinear phase noise accumulated over one or more spans, comprising:
a complex signal estimator for estimating differential complex components for said optical signal; and
an intensity-scaled phase noise compensator using information for a differential signal strength of said optical signal for compensating for said phase noise in said differential complex components and providing a phase noise compensated representation of said differential complex components.
2. The receiver of claim 1 , wherein:
said phase noise is Kerr effect phase noise.
3. The receiver of claim 1 , wherein:
said phase noise is induced by fiber nonlinearity in said spans.
4. The receiver of claim 1 , wherein:
said phase noise is induced by additive noise in said spans.
5. The receiver of claim 1 , wherein:
the phase noise compensator uses information for a scale factor for scaling said differential signal strength in order to provide a scaled differential signal strength, said scaled differential signal strength used for compensating for said phase noise.
6. The receiver of claim 5 , wherein:
said scale factor is a function of a number N of said spans.
7. The receiver of claim 5 , wherein:
said scale factor is approximately one half a nonlinear coefficient of an optical fiber γ times an effective nonlinear length for one of said spans (L eff ) times a sum of one plus said N.
8. The receiver of claim 5 , wherein:
said scale factor is approximately
γ
L
eff
N
+
1
2
·
E
0
2
+
(
2
N
+
1
)
σ
2
/
3
E
0
2
+
N
σ
2
where said N is the number of said spans, said γ is a nonlinear coefficient of an optical fiber, said L eff is an effective nonlinear length for one of said spans, said E 0 is a calculated power expected of said optical signal without noise, and said σ 2 is a noise variance for one of said spans.
9. The receiver of claim 5 , wherein:
said scale factor is approximately one-half the ratio of a mean nonlinear phase shift and a mean signal power of said optical signal, said mean nonlinear phase shift due to Kerr effect.
10. The receiver of claim 1 , wherein:
said optical signal carries data in a form of differential phase modulation; and
said phase noise compensated representation represents said differential phase modulation.
11. The receiver of claim 1 , wherein:
said optical signal carries data in a form of differential phase and amplitude modulation; and
said phase noise compensated representation represents said differential phase and amplitude modulation.
12. The receiver of claim 1 , wherein:
the receiver includes a differential signal strength estimator for estimating said differential signal strength; and
the phase noise compensator includes a scaler for storing information for a scale factor and using said scale factor for scaling said differential signal strength; and a rotator for using said scaled differential signal strength for phase compensating said differential complex components for providing said phase noise compensated representation.
13. The receiver of claim 12 , wherein:
the phase noise compensator further includes a data detector using said phase noise compensated representation for detecting modulation data carried on said optical signal.
14. The receiver of claim 12 , wherein:
the complex signal estimator includes a differential phase estimator for estimating a differential phase for said optical signal; and
said rotator is a phase rotator for rotating said differential phase by said scaled differential signal strength for providing said phase noise compensated representation.
15. The receiver of claim 14 , wherein:
said phase rotator rotates said differential phase by adding said scaled differential signal strength to said differential phase for providing said phase noise compensated phase representation.
16. The receiver of claim 12 , wherein:
the complex signal estimator includes a differential quadrature estimator for estimating differential quadrature components for said optical signal; and
said rotator is a quadrature rotator for rotating said differential quadrature components by said scaled differential signal strength for providing said phase noise compensated representation.
17. The receiver of claim 16 , wherein:
said quadrature rotator rotates said differential quadrature components according to
Δ {circumflex over (x)} c =Δ{circumflex over (x)} R cos(αΔ {circumflex over (P)} N )−Δ ŷ R sin(αΔ {circumflex over (P)} N )
Δ ŷ c =Δ{circumflex over (x)} R sin(αΔ {circumflex over (P)} N )+Δ ŷ R cos(αΔ {circumflex over (P)} N )
where Δ{circumflex over (x)} R and Δŷ R are said differential quadrature components, Δ{circumflex over (P)} N is said differential signal strength, α is said scale factor, and Δ{circumflex over (x)} c and Δŷ c are said phase noise compensated representation.
18. A method for receiving an optical signal having nonlinear phase noise accumulated over one or more spans, comprising:
estimating differential complex components for said optical signal; and
using information for a differential signal strength for compensating for said phase noise in said differential complex components for providing a phase noise compensated representation of said differential complex components.
19. The method of claim 18 , wherein:
said phase noise is Kerr effect phase noise.
20. The method of claim 18 , wherein:
said phase noise is induced by fiber nonlinearity in said spans.
21. The method of claim 18 , wherein:
said phase noise is induced by additive noise in said spans.
22. The method of claim 18 , wherein:
the step of compensating for said phase noise includes steps of scaling said differential signal strength in order to provide a scaled differential signal strength; and
using said scaled differential signal strength for compensating for said phase noise.
23. The method of claim 22 , wherein:
said step of scaling said differential signal strength includes scaling by a function of a number N of said spans.
24. The method of claim 22 , wherein:
said step of scaling said differential signal strength includes scaling by approximately one half a nonlinear coefficient of an optical fiber γ times an effective nonlinear length for said spans (L eff ) times a sum of one plus said N.
25. The method of claim 22 , wherein:
said step of scaling said differential signal strength includes scaling by approximately
γ
L
eff
N
+
1
2
·
E
0
2
+
(
2
N
+
1
)
σ
2
/
3
E
0
2
+
N
σ
2
where said N is the number of said spans, said γ is a nonlinear coefficient of an optical fiber, said L eff is an effective nonlinear length for one of said spans, said E 0 is a calculated power expected of said optical signal without noise, and said σ 2 is a noise variance for one of said spans.
26. The method of claim 22 , wherein:
said step of scaling said differential signal strength includes scaling by approximately one-half the ratio of a mean nonlinear phase shift and a mean signal power of said optical signal, said mean nonlinear phase shift due to Kerr effect.
27. The method of claim 18 , wherein:
said optical signal carries data in a form of differential phase modulation; and
said phase noise compensated representation represents said differential phase modulation.
28. The method of claim 18 , wherein:
said optical signal carries data in a form of differential phase and amplitude modulation; and
said phase noise compensated representation represents said differential phase and amplitude modulation.
29. The method of claim 18 , wherein:
the step of compensating for said phase noise includes steps of scaling said differential signal strength; and using said scaled differential signal strength for phase compensating said differential complex components for providing said phase noise compensated representation.
30. The method of claim 29 , wherein:
said step of compensating for said phase noise further includes using said phase noise compensated representation for detecting modulation data carried on said optical signal.
31. The method of claim 29 , wherein:
the step of estimating differential complex components includes estimating a differential phase for said optical signal; and
the step of phase compensating includes rotating said differential phase by said scaled differential signal strength for providing said phase noise compensated phase representation.
32. The method of claim 31 , wherein:
the step of rotating said differential phase includes adding said scaled differential signal strength to said differential phase for providing said phase noise compensated phase representation.
33. The method of claim 29 , wherein:
the step of estimating differential complex components includes estimating differential quadrature components of said optical signal; and further comprising:
rotating said differential quadrature components by said scaled differential signal strength for providing said phase noise compensated representation.
34. The method of claim 33 , wherein:
said step of rotating said differential quadrature components including rotating according to
Δ {circumflex over (x)} c =Δ{circumflex over (x)} R cos(αΔ {circumflex over (P)} N )−Δ ŷ R sin(αΔ {circumflex over (P)} N )
Δ ŷ c =Δ{circumflex over (x)} R sin(αΔ {circumflex over (P)} N )+Δ ŷ R cos(αΔ {circumflex over (P)} N )
where Δ{circumflex over (x)} R and Δŷ R are said differential quadrature components, Δ{circumflex over (P)} N is said differential signal strength, α is said scale factor, and Δ{circumflex over (x)} c and Δŷ c are said phase noise compensated representation.