Adaptive PMD equalizer and implementation

1. An apparatus for adaptive equalization of polarization mode dispersion in an optical signal, comprising:

an input configured to receive multiple input channels corresponding to digitized versions of in-phase and quadrature channels of horizontal and vertical polarity optical signals;

a plurality of substantially identical finite impulse response (FIR) filter modules, configured to receive the input channels, receive filter tap error updates, adjust FIR filter tap values based on the received tap error updates, partially filter one or more of the input channels based on the adjusted FIR filter tap values, and output one or more partially filtered input channels; and

an error calculation and output generation module configured to receive the partially filtered input channels from the FIR filter modules, combine the partially filtered input channels, output a polarization mode dispersion compensated version of each channel of the digitized optical signal, calculate filter tap error updates, and provide the tap error updates to the plurality of FIR filter modules.

2. The apparatus of claim 1 , wherein the plurality of FIR filter modules comprise:

a first subset of FIR filter modules configured to receive a first subset of the multiple input channels and to output a partially filtered first subset of the multiple input channels, each of the first subset of FIR filter modules including a first subset of FIR filter taps for one or more FIR filters; and

a second subset of FIR filter modules configured to receive the partially filtered first subset of the multiple input channels and to output a further partially filtered first subset of the multiple input channels, and each of the second subset of FIR filter modules including a second subset of FIR filter taps for each of the one or more FIR filters.

3. The apparatus of claim 2 , wherein each of the one or more FIR filters comprise a plurality of FIR filter taps, and wherein the first subset of FIR filter modules comprise a first subset of the plurality of FIR filter taps for each FIR filter, and the second subset of FIR filter modules comprise the remaining FIR filter taps for each FIR filter.

4. The apparatus of claim 1 , wherein the tap error updates calculated by the error calculation and output generation module are partial tap error updates.

5. The apparatus of claim 4 , wherein the FIR filter modules calculate partial tap error value updates that are combined with the tap error updates of the error calculation and output generation module to form filter tap values for FIR filters formed from the FIR filter modules.

6. The apparatus of claim 5 , wherein the error calculation and output generation module shifts the tap error updates when a metric indicating a bias of the taps exceeds a predetermined threshold relative to a tap value of a center tap.

7. The apparatus of claim 6 , wherein the error calculation and output generation module compensates the output based on the sum of prior tap values.

8. The apparatus of claim 1 , wherein the error calculation and output generation module comprises a constant modulus algorithm module that computes tap error updates to compensate for the effects of polarization mode dispersion.

9. The apparatus of claim 8 , wherein the constant modulus algorithm module provides a plurality of tap error updates for a plurality of FIR filters formed by the plurality of FIR filter modules.

10. The apparatus of claim 1 , wherein the plurality of FIR filter modules comprise a plurality of real-valued FIR filters.

11. The apparatus of claim 1 , wherein the plurality of FIR filter modules comprise a plurality of complex-valued FIR filters.

12. The apparatus of claim 1 , wherein the input is configured to receive a stream of digitized samples of the optical signal, and the error calculation and output generation module provides tap error updates for each sample based on one or more prior samples.

13. The apparatus of claim 12 , wherein the error calculation and output generation module provides tap error updates to generate updated tap values that each have a substantially equal distance to a centerpoint.

14. The apparatus of claim 12 , wherein the error calculation and output generation module provides tap error updates to generate updated tap values that each have a substantially equal distance to a centerpoint, and a radius of a circle containing output samples of the polarization mode dispersion compensated version of each channel of the digitized optical signal is substantially minimized.

15. The apparatus of claim 1 , wherein the error calculation and output generation module further computes an error signal comprising a positive or negative sign.

16. The apparatus of claim 1 , wherein the error calculation and output generation module further computes an error signal comprising a positive or negative sign and a computed magnitude of error.

17. The apparatus of claim 1 , wherein the input is further configured to receive initial timing offset information related to two or more channels in the received signal, and the filter module is initialized based on the initial timing offset information.

18. The apparatus of claim 1 , wherein the error calculation and output generation module is further configured to receive an indication that a polarization lock is invalid and provide the tap error updates to the plurality of FIR filter modules to set tap values to a default value.

19. A method for adaptive equalization of polarization mode dispersion in an optical signal, the method comprising:

receiving multiple input channels corresponding to digitized versions of in-phase and quadrature channels of horizontal and vertical polarity optical signals;

partially filtering one or more of the input channels at a plurality of substantially identical FIR filter modules, each of the plurality of FIR filter modules including a portion of the filter taps for one or more FIR filters;

receiving partially filtered input channels at an error calculation and output generation module;

calculating, at the error calculation and output generation module, tap error updates for each of the FIR filter modules;

outputting, from the error calculation and output generation module, the tap error updates to the plurality of FIR filter modules; and

outputting, from the error calculation and output generation module, a polarization mode dispersion compensated version of each channel of the digitized optical signal.

20. The method of claim 19 , further comprising:

collecting partial outputs from each of the FIR filter modules at the error calculation and output generation module,

compensating the partial outputs; and

outputting the polarization mode dispersion compensated version of each channel of the digitized optical signal.

21. The method of claim 19 , wherein the calculating comprises generating, at the error calculation and output generation module, a partial tap error update for each of a plurality of FIR filters formed from the plurality of FIR filter modules.

22. The method of claim 21 , wherein the FIR filter modules calculate partial tap error updates that are combined with the tap error updates of the error calculation and output generation module to form filter tap values for the FIR filters.

23. The method of claim 22 , further comprising:

calculating, at the error calculation and output generation module, a metric for prior filter tap values for the FIR filters; and

shifting the tap error updates when the metric indicates a bias of the filter tap values that exceeds a predetermined threshold relative to a tap value of a center tap.

24. The method of claim 19 , wherein the calculating comprises computing tap error updates based on a constant modulus algorithm.

25. The method of claim 19 , further comprising:

receiving, at the error calculation and output generation module, an indication that a polarization lock is invalid; and

outputting, from the error calculation and output generation module, the tap error updates to the plurality of FIR filter modules to set tap values to a default value.