Chromatic dispersion estimation for digital coherent optical receivers

1. An apparatus comprising:

a front end configured to:

receive an optical data signal; and

convert the optical data signal into a plurality of digital data signals; and

a processor coupled to the front end and configured to determine a best-match chromatic

dispersion (CD) estimate associated with the optical data signal by:

determining peaks of each component of the plurality of digital data signals;

accumulating the peaks to determine costs for CD candidate values, wherein the

costs are accumulations of the peaks;

minimizing the costs; and

setting the best-match CD estimate based on the minimizing.

2. The apparatus of claim 1 , wherein the processor is further configured to:

generate a plurality of digital filters, each configured to compensate the CD candidate values;

apply at least some of the plurality of digital filters to the plurality of digital data signals to produce filtered signals; and

compute a cost from among the costs for each CD candidate value from the filtered signals according to a cost function representing the accumulating.

3. The apparatus of claim 2 , wherein the CD candidate values range from a minimum CD candidate value to a maximum CD candidate value, and wherein the minimum CD candidate value and the maximum CD candidate value are associated with an optical network configuration.

4. The apparatus of claim 2 , wherein, to optimize the cost function, the processor is further configured to search for a minimum cost value over the costs, and wherein, to determine the best-match CD estimate, the processor is further configured to set the best-match CD estimate to a corresponding CD candidate value that produces the minimum cost value.

5. The apparatus of claim 2 , wherein the plurality of digital data

signals comprise one or more polarization components, each comprising an in-phase (I)

component and a quadrature-phase (Q) component, wherein each digital filter is applied to the I

component and the Q component of each polarization component, wherein the filtered signals

produced by each digital filter comprise a real component and an imaginary component, and

wherein, to compute the costs for the CD candidate values, the processor is further configured to

accumulate a maximum signal peak magnitude of each of the real component and the imaginary

component of each polarization component for each CD candidate value.

6. The apparatus of claim 5 , wherein the digital filters are frequency domain filters, and wherein the processor is further configured to:

transform each polarization component into frequency domain signals prior to applying the plurality of digital filters; and

transform each filtered signal into a time domain signal prior to computing the costs.

7. The apparatus of claim 5 , wherein the processor is further configured to compute an optimum filter for each polarization component according to the best-match CD estimate such that the optimum filter compensates a CD effect of the best-match CD estimate in a corresponding polarization component.

8. The apparatus of claim 1 , wherein the optical data signal comprises a binary phase shift keying (BPSK) signal, a quadrature phase shift keying (QPSK) signal, an eight quadrature amplitude modulation (8-QAM) signal, a sixteen quadrature amplitude modulation (16-QAM) signal, or combinations thereof.

9. The apparatus of claim 1 , wherein the optical data signal comprises a raised-cosine (RC) signal, a root-raised-cosine (RRC) signal, a non-return-to-zero (NRZ) signal, a return-to-zero with 67 percent duty cycle (RZ 67 ) signal, or combinations thereof.

10. The apparatus of claim 1 , wherein the processor is a digital signal processor (DSP).

11. The apparatus of claim 1 , wherein each digital data signal in the plurality of digital data signals is an electrical signal.

12. The apparatus of claim 1 , wherein the processor is further configured to determine the best-match CD estimate independent of a multiplication operation.

13. A coherent optical receiver comprising:

a chromatic dispersion (CD) sweep unit;

a first frequency domain equalizer (FDEQ) coupled to the CD sweep unit;

a first signal peak detector coupled to the first FDEQ;

a second signal peak detector coupled to the first FDEQ;

a first accumulator coupled to the first signal peak detector and the second signal peak detector;

a second FDEQ coupled to the CD sweep unit;

a third signal peak detector coupled to the second FDEQ;

a fourth signal peak detector coupled to the second FDEQ;

a second accumulator coupled to the third signal peak detector and the fourth signal peak detector;

a third accumulator coupled to the first accumulator and the second accumulator; and

a CD cost optimizer coupled to the third accumulator.

14. A method for estimating chromatic dispersion (CD) in an optical device coupled to an optical transmission link, the method comprising:

receiving an optical data signal via the optical transmission link;

converting the optical data signal into a plurality of digital data signals;

determining peaks of each component of the plurality of digital data signals;

accumulating the peaks to determine costs for CD candidate values, wherein the costs are accumulations of the peaks;

minimizing the costs; and

determining a best-match CD estimate associated with the optical data signal based on the minimizing.

15. The method of claim 14 further comprising:

generating a plurality of digital filters, each configured to compensate the CD candidate values;

applying at least some of the plurality of digital filters to the plurality of digital data signals to produce filtered signals; and

computing a cost from among the costs for each CD candidate value from the filtered signals according to a cost function representing the accumulating.

16. The method of claim 15 , wherein the plurality of digital data signals comprises one or more polarization components, each comprising an in-phase (I) component and a quadrature-phase (Q) component, wherein each digital filter is applied to the I component and the Q component of each polarization component, wherein the filtered signal produced by each digital filter comprises a real component and an imaginary component, and wherein computing the costs for the CD candidate values further comprises accumulating a maximum signal peak magnitude of each of the real component and the imaginary component of each polarization component for each CD candidate value.

17. The method of claim 15 , wherein the plurality of digital data signals comprises one or more polarization components, and wherein the plurality of digital filters are frequency domain filters, and wherein the method further comprises:

performing a fast Fourier transform (FFT) to convert in-phase (I) and quadrature-phase (Q) components of each polarization component into a frequency domain prior to applying the plurality of digital filters; and

performing an inverse FFT (IFFT) to convert each filtered signal into a time domain prior to computing the costs.

18. The method of claim 15 , further comprising searching for a minimum cost value over the costs, and wherein determining the best-match CD estimate comprises setting the best-match CD estimate to a CD candidate value that produces the minimum cost value.

19. The method of claim 14 , wherein each digital data signal in the plurality of digital data signals is an electrical signal.

20. A coherent optical receiver comprising:

a front end configured to:

receive an optical data signal; and

convert the optical data signal into a plurality of digital data signals;

one or more frequency domain equalizers (FDEQs) coupled to an output of the front end and configured to:

receive the plurality of digital data signals; and

compensate chromatic dispersion (CD) in the optical data signal; and

a CD estimator (CDE) coupled to the FDEQs and configured to:

determine peaks of each component of the plurality of digital data signals;

accumulate the peaks to determine costs for CD candidate values, wherein the costs are accumulations of the peaks;

minimize the costs; and

determine a best-match CD estimate associated with the optical data signal based on the minimizing,

wherein the FDEQs are further configured to use the best-match CD estimate to compensate the CD in the optical data signal.

21. The coherent optical receiver of claim 20 , wherein the CDE is further configured to configure the FDEQs with the CD candidate values, and wherein, for each CD candidate value, the CDE is further configured to:

compute coefficients for the FDEQs such that the FDEQs equalize a CD effect of a corresponding CD candidate value;

apply the FDEQs to the plurality of digital data signals to produce equalized signals; and

compute a cost from among the costs for the corresponding CD candidate value from the equalized signals according to a cost function representing the accumulating.

22. The coherent optical receiver of claim 21 , wherein to optimize the cost function, the CDE is further configured to search for a minimum cost value over the costs, and wherein, to determine the best-match CD estimate, the CDE is further configured to set the best-match CD estimate to a corresponding CD candidate value that produces the minimum cost value.

23. The coherent optical receiver of claim 21 , wherein the plurality of digital data signals comprises one or more polarization components, wherein each FDEQ equalizes one of the polarization components, wherein each equalized polarization component comprises a real component and an imaginary component, and wherein, to compute the cost for the corresponding CD candidate value, the CDE is further configured to accumulate a maximum signal peak magnitude of each of the real component and the imaginary component of each polarization component.

24. The coherent optical receiver of claim 21 , wherein the plurality of digital data signals comprises one or more polarization components, each comprising an in-phase (I) component and a quadrature phase (Q) component, and wherein each FDEQ comprises:

a fast Fourier transform (FFT) unit configured to transform the I component and the Q component of each polarization component into a frequency domain signal;

a frequency domain filter coupled to the FFT unit and configured to compensate CD in the frequency domain signal to create a CD compensated signal, wherein the coefficients correspond to frequency domain filter coefficients; and

an inverse FFT (IFFT) unit coupled to the frequency domain filter and configured to transform the CD compensated signal into a complex time domain signal.

25. The coherent optical receiver of claim 20 , wherein the CDE is further configured to configure the FDEQs according to the best-match CD estimate.

26. The coherent optical receiver of claim 20 , wherein each digital data signal in the plurality of digital data signals is an electrical signal.