Coherent receiver with mixed signal processing

1. A receiver system, comprising:

a polarization beam splitter configured to split a received coherent Pulse Amplitude Modulation (PAM) encoded signal into two polarized PAM encoded signals;

two hybrids configured to combine each of the two polarized PAM encoded signals with a local oscillator (LO) signal;

an analog equalizer configured to:

perform polarization recovery between the received coherent PAM encoded signal and the LO signal;

perform phase recovery between the received coherent PAM encoded signal and the LO signal; and

a digital signal processor (DSP) configured to provide equalizer coefficients to the analog equalizer for polarization and phase recovery.

2. The receiver system of claim 1 , wherein the analog equalizer is a 4×2 multi-input multi-output equalizer.

3. The receiver system of claim 1 , wherein the analog equalizer is further configured to receive four input signals.

4. The receiver system of claim 3 , wherein the four input signals include a detected in-phase component in x-polarization (Ix), a detected quadrature component in x-polarization (Qx), a detected in-phase component in y-polarization (Iy), and a quadrature component in y-polarization (Qy).

5. The receiver system of claim 3 , wherein the analog equalizer receives the four input signals from four photodetectors and transimpedance amplifiers.

6. The receiver system of claim 1 , wherein the equalizer coefficients are determined by the DSP using one or more control loops.

7. The receiver system of claim 6 , wherein at least one of the one or more control loops is a feedback control loop.

8. The receiver system of claim 1 , wherein the equalizer coefficients are determined by the DSP by comparing two outputs of the analog equalizer with an expected PAM signal level distribution.

9. The receiver system of claim 8 , wherein comparing the two outputs of the analog equalizer with the expected PAM signal level distribution includes using a gradient descent based algorithm.

10. The receiver system of claim 9 , wherein the gradient descent based algorithm is a least mean square algorithm.

11. A method, comprising:

receiving, by a receiver system, a coherent Pulse Amplitude Modulation (PAM) encoded signal;

splitting, by the receiver system, the coherent PAM encoded signal into two polarized PAM encoded signals;

receiving, by the receiver system, a local oscillator (LO) signal;

combining, by the receiver system, each of the two polarized PAM encoded signals with the LO signal;

performing, by the receiver system using an analog equalizer, polarization recovery between the received coherent PAM encoded signal and the LO signal; and

performing, by the receiver system using the analog equalizer, phase recovery between the received coherent PAM encoded signal and the LO; and

providing, by the receiver system, equalizer coefficients to the analog equalizer for polarization and phase recovery.

12. The method of claim 11 , further comprising determining, by a digital signal processor (DSP), the equalizer coefficients using a control loop.

13. The method of claim 12 , wherein the control loop is a feedback control loop.

14. The method of claim 11 , further comprising determining, by a digital signal processor (DSP), the equalizer coefficients by comparing two outputs of the analog equalizer with an expected PAM signal level distribution.

15. The method of claim 14 , wherein comparing the two outputs of the analog equalizer with the expected PAM signal level distribution includes using a gradient descent based algorithm.

16. The method of claim 11 , wherein when performing the polarization and phase recovery, the method further comprises receiving, by the analog equalizer, four input signals.

17. The method of claim 16 , wherein the four input signals include a detected in-phase component in x-polarization (Ix), a detected quadrature component in x-polarization (Qx), a detected in-phase component in y-polarization (Iy), and a quadrature component in y-polarization (Qy).