Measuring signal to noise ratio of a WDM optical signal

1. A device to determine optical signal-to-noise ratio (OSNR) of a wavelength division multiplexing (WDM) optical signal, the device comprising:

a tunable optical filter to select an optical wavelength channel within the optical signal;

a polarization controller to adjust a polarization state of the selected optical wavelength channel; and

a control unit to:

perform a heuristic iteration over a plurality of control points for a target control point of the polarization controller wherein an optical power level of the selected optical wavelength channel is minimized; and

compute the OSNR of the selected optical wavelength channel based on the target control point.

2. The device of claim 1 , wherein the optical power level of the selected optical wavelength channel is a first optical power level, and wherein the OSNR is computed from a polarization extinction determined according to a ratio of a second optical power level to the first optical power level at the target control point, wherein the second optical power is of the selected optical wavelength channel bypassing the polarization controller.

3. The device of claim 2 , comprising an optical switch controllable to bypass the polarization controller and couple the tunable optical filter to a photodetector to measure the second optical power level.

4. The device of claim 1 , wherein the heuristic iteration comprises first and second iterations of adjusting the polarization controller, and the target control point is determined based on measurements of the optical power levels measured during the first iteration.

5. The device of claim 4 , wherein during the second iteration, a plurality of control points are selected based on the measurements of the optical power levels from the first iteration, and the target control point is selected from the plurality of control points.

6. The device of claim 5 , wherein the heuristic iteration comprises a Nelder-Mead search, and the plurality of control points form Nelder-Mead simplexes in a parameter space of the polarization controller.

7. The device of claim 1 , wherein the control unit is to perform the heuristic iteration for each wavelength channel of a plurality of wavelength channels within the optical signal during a single scan of all of the plurality of wavelength channels.

8. The device of claim 7 , wherein the control unit is to adjust the polarization controller to reach the target control point for each wavelength channel and the selected wavelength channel propagates through at least two independent optical phase shifters in sequence.

9. The device of claim 1 , wherein the control unit is to determine whether the target control point is reached at or prior to a maximum number of iterations of the heuristic iteration, and in response to a determination that the target control point is not reached, to re-measure the optical power level to determine the target control point.

10. The device of claim 9 , wherein the control unit is to periodically determine whether a polarization extinction at the target control point is higher than a threshold value, and in response to a determination that the polarization extinction at the target control point is not higher than the threshold value, to re-measure the optical power level at the target control point.

11. The device of claim 10 , wherein the threshold value is at least 20 dB, or wherein the maximum number of iterations is 25 iterations.

12. A device to determine OSNR of an optical signal, the device comprising:

a tunable optical filter to select an optical wavelength channel of a plurality of optical wavelength channels within the optical signal;

a polarization controller comprising at least two independent optical phase shifters to adjust a polarization state of the selected optical wavelength channel; and

a control unit to perform a scan of the plurality of optical wavelength channels, wherein for each of the plurality of optical wavelength channels the control unit is to:

tune the tunable optical filter to the optical wavelength channel;

perform an iterative search for a target control point of the polarization controller wherein an optical power level of the optical wavelength channel is minimized; and

compute the OSNR of the optical wavelength channel based on the target control point.

13. The device of claim 12 , wherein the iterative search comprises controlling the polarization controller to perform a plurality of adjustments of the polarization state of the optical wavelength channel and determine the optical power level for each adjustment to identify the target control point wherein the optical power level is minimized.

14. The device of claim 13 , wherein the iterative search comprises first and second iterations of the adjustments, and the target control point is determined based on measurements of optical power levels measured during the first iteration.

15. The device of claim 14 , wherein during the second iteration, a plurality of control points are selected based on the measurements of the optical power levels from the first iteration, and the target control point is selected from the plurality of control points.

16. The device of claim 13 , wherein, for each of the plurality of optical wavelength channels, the OSNR is computed from a polarization extinction determined according to a ratio of a second optical power level to a first optical power level at the target control point, wherein the second optical power level is of the optical wavelength channel bypassing the polarization controller.

17. A method of determining OSNR of an optical signal, the method comprising:

selecting an optical wavelength channel of a plurality of optical wavelength channels within the optical signal;

heuristically iterating over a plurality of polarization states for a polarization state of the selected optical wavelength channel wherein an optical power level of the selected optical wavelength channel is minimized; and

computing the OSNR of the selected wavelength channel based on a target control point.

18. The method of claim 17 , wherein the optical power level is a first optical power level, and wherein the heuristically iterating includes performing, using at least two independent optical phase shifters, a plurality of adjustments of the polarization state of the selected optical wavelength channel and determining the first optical power level for each adjustment, and computing the OSNR comprises:

determining the OSNR from a polarization extinction computed according to a ratio of a second optical power level to the first optical power level at the target control point, wherein the second optical power level is a measure of optical power of the selected optical wavelength channel without the selected optical wavelength channel passing through a polarization controller.

19. The method of claim 18 , wherein the heuristically iterating comprises:

performing a first iteration of the plurality of adjustments;

measuring optical power levels during the first iteration;

performing a second iteration of the plurality of adjustments; and

during the second iteration, selecting a plurality of control points based on the measured optical power levels from the first iteration, wherein the target control point is selected from the plurality of control points.

20. The method of claim 17 , wherein the heuristically iterating comprises heuristically iterating over the plurality of polarization states for each wavelength channel of the plurality of optical wavelength channels during a single scan of all of the plurality of optical wavelength channels.