Optical FSK receiver having compensation for 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 non-linear phase noise compensator. The phase noise compensator provides a phase noise compensated representation of a differential frequency by combining a measured differential frequency with a scaled differential signal strength estimate. The scale factor is derived from the number of spans in the transmission system.
1. An optical receiver for receiving an optical signal having nonlinear phase noise accumulated over one or more spans, comprising:
a differential frequency estimator for estimating a differential frequency for said optical signal;
a differential signal strength estimator for estimating a differential signal strength for said optical signal; and
an intensity-scaled phase noise compensator using said differential signal strength for compensating for said phase noise in said differential frequency and providing a phase noise compensated representation of said differential frequency.
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 , further comprising:
a frequency data detector using said phase noise compensated representation for detecting data carried on said optical signal.
6. The receiver of claim 1 , wherein:
the phase noise compensator uses information for a scale factor for determining a scaled differential signal strength and uses said scaled differential signal strength for compensating said differential frequency for said phase noise.
7. The receiver of claim 6 , wherein:
said scale factor is a function of a number N of said spans.
8. The receiver of claim 6 , wherein:
said scale factor is approximated by 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.
9. The receiver of claim 6 , wherein:
said scale factor is approximated by
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 the noise variance for one of said spans.
10. The receiver of claim 6 , 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 phase shift due to Kerr effect.
11. The receiver of claim 6 , wherein:
the phase noise compensator includes a scaler using information for said scale factor and a symbol time period for scaling said differential signal strength for providing said scaled differential signal strength; and a frequency shifter for compensating said differential frequency for said phase noise by shifting said differential frequency by said scaled differential signal strength.
12. The receiver of claim 1 , wherein:
said optical signal carries data in a form of frequency modulation; and
said phase noise compensated representation represents said frequency modulation.
13. The receiver of claim 1 , wherein:
said optical signal carries data as frequency shift key (FSK) modulation; and
said phase noise compensated representation represents said FSK modulation.
14. The receiver of claim 1 , wherein:
the phase noise compensator includes a scaler for scaling said differential signal strength by a scale factor and dividing by a symbol time period for providing a scaled differential signal strength, a frequency shifter for shifting said differential frequency by said scaled differential signal strength, and a frequency data detector using said shifted differential frequency for detecting modulation data carried on said optical signal.
15. A method for receiving an optical signal having nonlinear phase noise accumulated over one or more spans, comprising:
estimating a differential frequency of said optical signal;
estimating a differential signal strength of said optical signal; and
using said differential signal strength for compensating for said phase noise in said differential frequency for providing a phase noise compensated representation of said differential frequency.
16. The method of claim 15 , wherein:
said phase noise is Kerr effect phase noise.
17. The method of claim 15 , wherein:
said phase noise is induced by fiber nonlinearity in said spans.
18. The method of claim 15 , wherein:
said phase noise is induced by additive noise in said spans.
19. The method of claim 15 , further comprising:
using said phase noise compensated representation for detecting data carried on said optical signal.
20. The method of claim 15 , wherein:
the step of compensating for said phase noise includes steps of using information for a predetermined scale factor for scaling said differential signal strength for providing a scaled differential signal strength; and
using said scaled differential signal strength for compensating for said differential frequency.
21. The method of claim 20 , wherein:
said step of scaling said differential signal strength includes scaling by a function of a number N of said spans.
22. The method of claim 20 , 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 one of said spans (L eff ) times a sum of one plus said N.
23. The method of claim 20 , 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 the noise variance for one of said spans.
24. The method of claim 20 , 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 phase shift due to Kerr effect.
25. The method of claim 20 , further comprising:
using information for said scale factor and a symbol time period for providing said scaled differential signal strength; and shifting said differential frequency by said scaled differential signal strength for compensating said differential frequency for said phase noise.
26. The method of claim 15 , wherein:
said optical signal carries data in a form of frequency modulation; and
said phase noise compensated representation represents said frequency modulation.
27. The method of claim 15 , wherein:
said optical signal carries data as frequency shift key (FSK) modulation; and
said phase noise compensated representation represents said FSK modulation.
28. The method of claim 15 , wherein:
the step of compensating for said phase noise includes steps of scaling said differential signal strength by a scale factor and dividing by a symbol time period for providing a scaled differential signal strength; shifting said differential frequency by said scaled differential signal strength; and using said shifted differential frequency for detecting modulation data carried on said optical signal.