Circuit for multi-path interference mitigation in an optical communication system

1. A method of processing a plurality of samples of an optical signal having a pulse amplitude modulated (PAM) E-field, the method comprising:

for each sample, estimating a respective PAM level;

for each sample, subtracting the sample from the respective PAM level to generate a corresponding error sample;

low-pass filtering the error samples to produce estimates of multi-path interference (MPI);

for each sample, combining one of the estimates of MPI with the sample to produce an interference-mitigated sample.

2. The method of claim 1 wherein filtering comprises:

for a block of consecutive samples, filtering a block of corresponding error samples to produce a block estimate, and using the block estimate as the estimate of MPI that is combined with each sample in the block of consecutive samples.

3. The method of claim 2 wherein filtering the block of corresponding error samples to produce a block estimate comprises determining an average of the block of corresponding error samples.

4. The method of claim 2 further comprising:

configuring a size of the block of consecutive samples as a function of transmitter coherence.

5. The method of claim 1 wherein filtering comprises:

for each sample, for a respective block of error samples defined by a moving window, low-pass filtering the respective block of error samples to produce a respective block estimate, and using the respective block estimate as the estimate of MPI that is combined with the sample.

6. The method of claim 5 wherein filtering the block of respective error samples to produce a respective block estimate comprises determining an average of the respective block of error samples.

7. The method of claim 5 further comprising:

configuring a size of the block of consecutive samples as a function of transmitter coherence.

8. The method of claim 1 wherein combining the one of the estimates of multi-path interference with the sample to produce an interference-mitigated sample comprises subtracting the estimate from the sample to produce the interference-mitigated sample.

9. The method of claim 1 wherein for a given sample, combining one of the estimates of MPI with the sample to produce an interference-mitigated sample comprises:

producing a weighted estimate by multiplying the one of the estimates of MPI with a value proportional to a respective PAM level modulating the E-field estimated from the sample;

subtracting the weighted estimate from the sample to produce the MPI-mitigated sample.

10. The method of claim 1 further comprising:

delaying the sample before combining to account for a delay in determining the one of the estimates of MPI.

11. The method of claim 1 further comprising:

receiving the optical signal having the PAM modulated E-field;

performing direct detection of the optical signal to produce a direct detection output;

performing analog to digital conversion on the direct detection output to generate raw samples;

performing equalization on the raw samples to produce the plurality of samples.

12. The method of claim 11 further comprising:

estimating at least one interference component by estimating a respective delay and respective amplitude for each interference component;

wherein for each sample, producing the interference mitigated sample comprises combining the estimate of MPI and the estimated at least one electrical interference component with the sample;

for each interference-mitigated sample, performing PAM decision slicing.

13. The method of claim 1 further comprising:

for each interference-mitigated sample, performing PAM decision slicing.

14. An optical communications system comprising:

a plurality of network elements interconnected by optical paths that comprise optical fiber and optical interfaces;

at least one of the network elements comprising an optical module having an MPI-mitigation circuit according to claim 1 .

15. A circuit for processing a plurality of samples of an optical signal having a pulse amplitude modulated (PAM) E-field, the circuit comprising:

a slicer that, for each sample, estimates a respective PAM level of the sample;

a subtractor that, for each sample, subtracts the sample from the respective PAM level to generate a corresponding error sample;

a low-pass filter that filters the error samples to produce estimates of multi-path interference (MPI);

a combiner that for each sample, combines one of the estimates of MPI with the sample to produce an interference-mitigated sample.

16. The circuit of claim 15 wherein the low-pass filter comprises

a fixed block average component that determines an average of the error samples for a block of samples;

wherein the average is used as the estimate of MPI that is combined with each sample in the block of consecutive samples.

17. The circuit of claim 16 wherein:

a size of the block of consecutive samples is configured as a function of transmitter coherence.

18. The circuit of claim 15 wherein the low-pass filter comprises:

a moving window average component that determines, for each sample, an average of the error samples for a respective block of error samples defined by a moving window, wherein the average is used as the estimate of MPI that is combined with the sample.

19. The circuit of claim 18 further comprising:

a size of the block of consecutive samples is configured as a function of transmitter coherence.

20. The circuit of claim 15 wherein the combiner is a subtractor that combines one of the estimates of multi-path interference with the sample to produce an interference-mitigated sample by subtracting the estimate from the sample to produce the interference-mitigated sample.

21. The circuit of claim 15 wherein the combiner is a level-dependent subtractor that, for a given sample, combines one of the estimates of MPI with the sample to produce an interference-mitigated sample by producing a weighted estimate by multiplying the one of the estimates of MPI producing a weighted estimate by multiplying the one of the estimates of MPI with a value proportional to a respective PAM level modulating the E-field estimated from the sample, and subtracting the weighted estimate from the sample to produce the MPI-mitigated sample.

22. The circuit of claim 15 further comprising:

a delay element that delays the sample before combining to account for a delay in determining the one of the estimates of MPI.

23. The circuit of claim 15 further comprising:

a direct detection receiver that direct detects the optical signal to produce a direct detection output;

an analog to digital convertor that performs analog to digital conversion on the direct detection output to generate raw samples;

an equalizer that equalizes the raw samples to produce the plurality of samples.

24. The circuit of claim 23 further comprising:

an interference component estimator that estimates at least one interference component by estimating a respective delay and respective amplitude for each interference component;

wherein for each sample, the combiner combines the estimate of MPI and the estimated at least one electrical interference component with the sample;

a PAM decision slicer that performs PAM decision slicing for each interference-mitigated sample.

25. The circuit of claim 15 further comprising:

a PAM decision slicer that performs PAM decision slicing for each interference-mitigated sample.

26. The circuit of claim 15 configured to operate at a baud rate of greater than 25 GBaud, in which the low pass filter operates to filter frequencies below 100 MHz.

27. The circuit of claim 15 configured to operate at a baud rate of greater than 25 GBaud, in which the low pass filter operates to filter frequencies below 10 MHz.

28. An optical module comprising:

an optical IO (input/output) and an electrical IO;

a photo-diode for performing direct detection on an incoming optical signal received at the optical IO to produce a direct detection output;

a trans-impedance amplifier (TIA) that amplifies the direct detection output;

a PAM ASIC configured to perform PAM demodulation an output of the TIA to produce a signal at the electrical IO, the PAM ASIC comprising the circuit of claim 15 , the PAM ASIC further configured to perform PAM modulation based on an incoming electrical signal at the electrical IO;

a laser that outputs an optical signal at the optical IO having a PAM modulated E-field based on the output of the PAM modulation.