Ultra wide-range frequency offset estimation for digital coherent optical receivers

1. A method for receiving an optical signal, including:

coherently detecting an optical signal, including:

compensating for a first laser frequency offset between a transmitting laser and a local oscillator laser by determining a maximum phase error (MPE) in the optical signal; and

compensating for a residual laser frequency offset between the transmitting laser and the local oscillator laser; and

decoding data stored in the optical signal.

2. The method of claim 1 , wherein said compensating for the first laser frequency offset includes:

determining the MPE in the optical signal;

converting the MPE to a coarse derotation value; and

derotating the signal using the coarse derotation value.

3. The method of claim 2 , wherein said converting includes using a lookup table to find the coarse derotation value that corresponds to the maximum phase error.

4. The method of claim 2 , wherein determining an MPE in the optical signal includes:

sampling the optical signal at twice a symbol rate to produce an inphase and quadrature signal;

finding a phase error U(k) of the inphase and quadrature signals across a plurality of timing offsets as

U t (2 k )= I x (2 k− 1)[ I x (2 k )− I x (2 k− 2)]+ Q x (2 k− 1)[ Q x (2 k )− Q x (2 k− 2)],

where 2k is a particular sample, I x (n) is a particular sample of the inphase signal and Q x (n) is a particular sample of the quadrature signal; and

determining which timing offset produces the maximum phase error.

5. The method of claim 1 , wherein said compensating for a fine laser frequency offset includes:

determining a fine derotation value; and

derotating the optical signal using the fine derotation value.

6. The method of claim 5 , wherein the fine derotation value is determined using a fast-fourier transform (FFT)-based approach.

7. The method of claim 6 , wherein the fine derotation value is determined using an Mth power approach.

8. The method of claim 1 , wherein said compensating for the first laser frequency offset can accommodate a frequency offset range of at least ±0.9 times a symbol rate of the optical signal.

9. The method of claim 1 , wherein said compensating for a fine laser frequency offset can accommodate a frequency offset range of at least [−R s /2M, R s /2M], where M is a modulation order and R s is a symbol rate.

10. A receiver, including:

a digital signal processor configured to detect an optical signal, including:

a coarse frequency offset estimator (FOE) configured to compensate for a first laser frequency offset between a transmitting laser and a local oscillator laser by determining a maximum phase error (MPE) in the optical signal; and

a fine FOE configured to compensate for a residual laser frequency offset between the transmitting laser and the local oscillator laser; and

a digital signal processor configured to decode data stored in the optical signal.

11. The receiver of claim 10 , wherein said coarse FOE includes:

an MPE sweeper configured to determine a maximum phase error in the optical signal;

a lookup table configured to convert the maximum phase error to a coarse derotation value; and

a rotator configured to derotate the optical signal using the coarse derotation value.

12. The receiver of claim 11 , wherein the MPE sweeper is configured to sample the optical signal at twice a symbol rate to produce an inphase and quadrature signal, find a phase error U(k) of the inphase and quadrature signals across a plurality of timing offsets as

U t (2 k )= I x (2 k− 1)[ I x (2 k )− I x (2 k− 2)]+ Q x (2 k− 1)[ Q x (2 k )− Q x (2 k− 2)],

where 2k is a particular sample, I x (n) is a particular sample of the inphase signal and Q x (n) is a particular sample of the quadrature signal, and determine which timing offset produces the maximum phase error.

13. The receiver of claim 10 , wherein said fine FOE is configured to determine a fine derotation value and derotate the optical signal using the fine derotation value.

14. The receiver of claim 13 , wherein the fine FOE is configured to determine the fine derotation value using a fast-fourier transform (FFT)-based approach.

15. The method of claim 14 , wherein the fine FOE is configured to determine the fine derotation value using an Mth power approach.

16. The receiver of claim 10 , wherein said coarse FOE is configured to accommodate a frequency offset range of at least ±0.9 times a symbol rate of the optical signal.

17. The receiver of claim 10 , wherein said fine FOE is configured to accommodate a frequency offset range of at least [−R s /2M, R s /2M], where M is a modulation order and R s is a symbol rate.

18. The receiver of claim 10 , wherein the coarse FOE and the fine FOE comprise a cascaded FOE configured to provide wide-range frequency offset estimation.

19. A method for receiving an optical signal, including:

coherently detecting an optical signal, including:

compensating for a first laser frequency offset between a transmitting laser and a local oscillator laser by:

determining a maximum phase error (MPE) in the optical signal by sweeping a timing offset;

converting the MPE to a coarse derotation value by comparing the MPE to a lookup table; and

derotating the signal using the coarse derotation value; and

compensating for a residual laser frequency offset between the transmitting laser and the local oscillator laser by:

determining a fine derotation value; and

derotating the optical signal using the fine derotation value; and

decoding data stored in the optical signal.

20. The method of claim 19 , wherein determining an MPE in the optical signal includes:

sampling the optical signal at twice a symbol rate to produce an inphase and quadrature signal;

finding a phase error U(k) of the inphase and quadrature signals across a plurality of timing offsets as

U t (2 k )= I x (2 k− 1)[ I x (2 k )− I x (2 k− 2)]+ Q x (2 k− 1)[ Q x (2 k )− Q x (2 k− 2)],

where 2k is a particular sample, I x (n) is a particular sample of the inphase signal and Q x (n) is a particular sample of the quadrature signal; and

determining which timing offset produces the maximum phase error.