Common mode rejection ratio control for coherent optical receivers

1. A method comprising:

receiving an optical signal at a coherent optical receiver;

converting the received optical signal to a first electrical signal and a second electrical signal through a first photodetector and a second photodetector, respectively;

applying a known dither to at least one of: the received optical signal, the first electrical signal or the second electrical signal;

inputting the first electrical signal into a first single input variable gain amplifier;

inputting the second electrical signal into to a second single input variable gain amplifier;

inputting the output of the first single input variable gain amplifier and the output of the second single input variable gain amplifier into a differential amplifier; and

obtaining a receiver output at an output of the differential amplifier;

analyzing the receiver output; and

controlling a gain of at least one of the first single input variable gain amplifier or the second single input variable gain amplifier based on the known dither and the receiver output to balance the output of the first single input variable gain amplifier and the output of the second single input variable gain amplifier.

2. The method of claim 1 , wherein applying the known dither comprises applying the known dither to the received optical signal.

3. The method of claim 1 , wherein analyzing the receiver output comprises measuring in the receiver output a second harmonic of the known dither.

4. The method of claim 1 , wherein applying the known dither comprises applying the known dither to a local oscillator of the coherent optical receiver.

5. The method of claim 4 , wherein analyzing the receiver output comprises measuring in the receiver output a second harmonic of the known dither.

6. The method of claim 1 , wherein applying the known dither comprises applying the known dither to each of the first single input variable gain amplifier and the second single input variable gain amplifier.

7. The method of claim 6 , wherein analyzing the receiver output comprises measuring in the receiver output a first harmonic of the known dither.

8. The method of claim 1 , wherein the first electrical signal and the second electrical signal are derived from a quadrature component of the optical signal.

9. The method of claim 1 , wherein the first electrical signal and the second electrical signal are derived from an in-phase component of the optical signal.

10. An apparatus comprising:

a coherent optical receiver comprising:

a first photodetector configured to convert a received optical signal to a first electrical signal;

a second photodetector configured to convert the received optical signal to a second electrical signal;

a first single input variable gain amplifier configured to receive the first electrical signal;

a second single input variable gain amplifier configured to receive the second electrical signal;

a differential amplifier configured to receive an output of the first single input variable gain amplifier and an output of the second single input variable gain amplifier; and

a controller configured to:

apply a known dither to at least one of: the received optical signal, the first electrical signal or the second electrical signal;

obtain a receiver output at an output of the differential amplifier; and

control a gain of at least one of the first single input variable gain amplifier or the second single input variable gain amplifier based on the known dither and the receiver output to balance the output of the first single input variable gain amplifier and the output of the second single input variable gain amplifier.

11. The apparatus of claim 10 , further comprising a local oscillator providing a local oscillator signal to the coherent optical receiver, wherein the controller is configured to apply the known dither to the local oscillator.

12. The apparatus of claim 10 , wherein the controller is configured to apply the known dither to each of the first single input variable gain amplifier and the second single input variable gain amplifier.

13. The apparatus of claim 10 , wherein the controller is configured to apply the known dither to the received optical signal.

14. The apparatus of claim 10 , wherein at least one of the first single input variable gain amplifier or the second single input variable gain amplifier comprises:

a transimpedance amplifier;

a first variable gain amplifier configured to receive a gain-adjusting control signal; and

a second variable gain amplifier configured to receive a known dither.

15. The apparatus of claim 10 , wherein the first electrical signal and the second electrical signal are derived from a quadrature component of the optical signal.

16. The apparatus of claim 10 , wherein the first electrical signal and the second electrical signal are derived from an in-phase component of the optical signal.

17. A method comprising:

converting a signal from an optical domain to an electrical domain by passing the signal through a coherent optical receiver;

applying a known dither to the signal;

analyzing an output of the coherent optical receiver; and

controlling, based on the output of the coherent optical receiver and the known dither, a gain of at least one of a first single input variable gain amplifier of the optical receiver or a second single input variable gain amplifier of the optical receiver to provide common mode rejection ratio control to the coherent optical receiver.

18. The method of claim 17 , wherein applying the known dither to the signal comprises applying the known dither through a local oscillator signal of the coherent optical receiver.

19. The method of claim 17 , wherein applying the known dither to the signal comprises applying the known dither to each of the first single input variable gain amplifier and the second single input variable gain amplifier.

20. The method of claim 17 , wherein applying the known dither to the signal comprises applying the known dither to an optical signal received by the coherent optical receiver.