Method and device for in-band optical performance monitoring

1. A method of determining in-band noise of an optical signal, the method comprising:

splitting the optical signal to produce a first optical signal component and a second optical signal component of distinct polarisation to the first optical signal component;

converting the first optical signal component to the electrical domain to produce a first electrical signal component;

converting the second optical signal component to the electrical domain to produce a second electrical signal component;

equalising a signal power of the first electrical signal component and a signal power of the second electrical signal component; and

subtracting the equalised first electrical signal component and second electrical signal component.

2. The method of claim 1 wherein the second optical signal component is of sufficiently distinct polarisation to the first optical signal component that the noise to be measured in the second optical signal component is substantially uncorrelated with the noise to be measured in the first optical signal component.

3. The method of claim 1 , further comprising direct in-band measurement of the result of the subtracting.

4. The method of claim 1 wherein the equalising comprises passing at least one of the first optical signal component and the second optical signal component through a respective variable optical attenuator prior to conversion to the electrical domain.

5. The method of claim 4 further comprising controlling the or each respective variable optical attenuator by feedback from the electrical domain.

6. The method of claim 1 wherein the equalising comprises passing the optical signal through a polarisation controller configured at substantially 45° to the input of a polarisation beam splitter such that, upon splitting, substantially equal signal power is carried by the first optical signal component and the second optical signal component.

7. The method of claim 1 wherein the equalising comprises monitoring the signal power of the first and second electrical signal components, and waiting for polarisation changes in the optical signal to cause equalisation from time to time.

8. The method of claim 1 further comprising passing the optical signal through a rotating quarter wave plate prior to splitting.

9. The method of claim 1 wherein the equalising comprises electrical domain signal processing of at least one of the first electrical signal component and the second electrical signal component, prior to subtraction of the first electrical signal component and the second electrical signal component.

10. The method of claim 1 wherein the equalising comprises:

splitting the second optical signal component to obtain a third optical signal component and a fourth optical signal component having a polarisation at substantially 45 degrees to the third optical signal component;

converting the third optical signal component to the electrical domain to obtain a third electrical signal component;

converting the fourth optical signal component to the electrical domain to obtain a fourth electrical signal component; and

selecting at least one of the third electrical signal component and the fourth electrical signal component to serve as the second electrical signal component for subtraction with the first electrical signal component.

11. The method of claim 1 , wherein the equalising comprises depolarising the optical signal prior to splitting the optical signal.

12. The method of claim 1 , wherein the first optical signal component and the second optical signal component undergo distinct delays, such that the output of the subtractive combination of the first electrical signal and the second electrical signal is substantially noise only at particular frequencies defined by the delay difference.

13. A device for determining in-band noise of an optical signal, the device comprising:

a splitter for splitting the optical signal to produce a first optical signal component and a second optical signal component of distinct polarisation to the first optical signal component;

a first photodetector for converting the first optical signal component to the electrical domain to produce a first electrical signal component;

a second photodetector for converting the second optical signal component to the electrical domain to produce a second electrical signal component;

means for equalising a signal power of the first electrical signal component and a signal power of the second electrical signal component; and

a combiner for subtracting the equalised first electrical signal component and second electrical signal component.

14. The device of claim 13 wherein the splitter is a polarisation beam splitter.

15. The device of claim 14 further comprising a polarisation controller configured at substantially 45° to the input of the splitter such that, upon splitting, substantially equal signal power is carried by the first optical signal component and the second optical signal component.

16. The device of claim 13 wherein the splitter is a power splitter, and wherein the device further comprises at least one polariser to produce the distinct polarisation between the first optical signal component and the second optical signal component.

17. The device of claim 13 , further comprising an OSNR meter for direct in-band measurement of the output of the combiner.

18. The device of claim 13 further comprising at least one variable optical attenuator for variably attenuating at least one of the first optical signal component and the second optical signal component for equalising the first optical signal component and the second optical signal component prior to conversion to the electrical domain.

19. The device of claim 18 further comprising a feedback circuit from the electrical domain for controlling the or each respective variable optical attenuator.

20. The device of claim 13 further comprising a rotating quarter wave plate through which the optical signal is passed prior to splitting.

21. The device of claim 13 further comprising an electrical domain signal processor for at least one of the first electrical signal component and the second electrical signal component, to equalise the first electrical signal component and the second electrical signal component prior to subtraction of the first electrical signal component and the second electrical signal component.

22. The device of claim 13 further comprising:

a splitter for splitting the second optical signal component to obtain a third optical signal component and a fourth optical signal component having a polarisation at substantially 45 degrees to the third optical signal component;

a third photodetector for converting the third optical signal component to the electrical domain to obtain a third electrical signal component;

a fourth photodetector for converting the fourth optical signal component to the electrical domain to obtain a fourth electrical signal component; and

a processor to select at least one of the third electrical signal component and the fourth electrical signal component to serve as the second electrical signal component for subtraction with the first electrical signal component.

23. The device of claim 13 , further comprising a depolariser to depolarise the optical signal prior to splitting.

24. The device of claim 13 , wherein the first optical signal component and the second optical signal component undergo distinct delays, such that the output of the subtractive combination of the first electrical signal and the second electrical signal is substantially noise only at particular frequencies defined by the delay difference.