Closed loop quadrature bias control for an IQ phase modulator

1. An electro-optic device, comprising:

a Mach-Zehnder modulator (MZM); and

one or more components to:

apply a child DC bias with dither to arms of a first branch of the MZM and to arms of a second branch of the MZM;

determine a second harmonic of a first return signal associated with applying the child DC bias with dither to the arms of the first branch and to the arms of the second branch;

apply a child DC bias with phase-shifted dither to the arms of the first branch or to the arms of the second branch,

the phase-shifted dither being out of phase from the dither, and

the phase-shifted dither having a frequency that matches a frequency of the dither;

determine a second harmonic of a second return signal associated with applying the child DC bias with phase-shifted dither to the arms of the first branch or to the arms of the second branch;

determine, based on the second harmonic of the first return signal and the second harmonic of the second return signal, whether the first branch and the second branch are operating at quadrature; and

selectively adjust parent DC biases, applied to the first branch and the second branch, based on whether the first branch and the second branch are operating at quadrature.

2. The electro-optic device of claim 1 , where the one or more components, when determining whether the first branch and the second branch are operating at quadrature, are to:

determine that the first branch and the second branch are operating at quadrature when a difference between the second harmonic of the first return signal and the second harmonic of the second return signal is equal to zero.

3. The electro-optic device of claim 2 , where the one or more components, when selectively adjusting the parent DC biases, are to:

maintain application of the parent DC biases already being applied to the first branch and the second branch.

4. The electro-optic device of claim 1 , where the one or more components, when determining whether the first branch and the second branch are operating at quadrature, are to:

determine that the first branch and the second branch are not operating at quadrature when a difference between the second harmonic of the first return signal and the second harmonic of the second return signal is greater than or less than zero.

5. The electro-optic device of claim 4 , where the one or more components, when selectively adjusting the parent DC biases, are to:

increase a parent DC bias being applied to the first branch by a particular amount; and

decrease a parent DC bias being applied to the second branch by the particular amount.

6. The electro-optic device of claim 1 , where the one or more components, when applying the child DC bias with phase-shifted dither to the arms of the first branch or to the arms of the second branch, are to:

apply the child DC bias with phase-shifted dither to the arms of the first branch while continuing to apply the child DC bias with dither to the arms of the second branch.

7. The electro-optic device of claim 1 , where the one or more components, when applying the child DC bias with phase-shifted dither to the arms of the first branch or to the arms of the second branch, are to:

apply the child DC bias with phase-shifted dither to the arms of the first branch and to the arms of the second branch.

8. The electro-optic device of claim 1 , where the MZM comprises a planar portion formed from Indium Phosphide.

9. An electro-optic IQ phase modulator, comprising:

a Mach-Zehnder modulator (MZM); and

a controller to:

determine a second harmonic of a first return signal resulting from application of a child DC bias with dither to arms of a first branch of the MZM and to arms of a second branch of the MZM;

determine a second harmonic of a second return signal resulting from application of a child DC bias with phase-shifted dither to the arms of the first branch or to the arms of the second branch,

the phase-shifted dither being out of phase from the dither, and

the phase-shifted dither having a frequency that matches a frequency of the dither;

determine, based on the second harmonic of the first return signal and the second harmonic of the second return signal, whether the first branch and the second branch are operating at quadrature; and

selectively adjust parent DC biases, applied to the first branch and the second branch, based on whether the first branch and the second branch are operating at quadrature.

10. The electro-optic IQ phase modulator of claim 9 , where the controller, when determining whether the first branch and the second branch are operating at quadrature, is to:

determine whether the first branch and the second branch are operating at quadrature based on whether a difference between the second harmonic of the first return signal and the second harmonic of the second return signal differs from zero.

11. The electro-optic IQ phase modulator of claim 9 , where the controller, when selectively adjusting the parent DC biases, is to:

maintain application of the parent DC biases already being applied to the first branch and the second branch when the controller determines that the first branch and the second branch are operating at quadrature.

12. The electro-optic IQ phase modulator of claim 9 , where the controller, when selectively adjusting the parent DC biases, is to:

increase a parent DC bias being applied to the first branch by a particular amount and decrease a parent DC bias being applied to the second branch by the particular amount when the controller determines that the first branch and the second branch are not operating at quadrature.

13. The electro-optic IQ phase modulator of claim 9 , the phase-shifted dither is out of phase from the dither by approximately 180 degrees.

14. The electro-optic IQ phase modulator of claim 9 , where the controller is further to:

apply the child DC bias with dither to the arms of the first branch and to the arms of the second branch.

15. The electro-optic IQ phase modulator of claim 9 , where the controller is further to:

apply the child DC bias with phase-shifted dither to the arms of the second branch while continuing to apply the child DC bias with dither to the arms of the first branch.

16. The electro-optic IQ phase modulator of claim 9 , where the controller is further to:

apply the child DC bias with phase-shifted dither to the arms of the first branch and to the arms of the second branch.

17. The electro-optic IQ phase modulator of claim 9 , where the MZM comprises a planar portion formed from a material that exhibits non-linear behavior or changes to an absorption characteristic with a change in voltage applied to the MZM.

18. A method comprising:

applying, by a controller, a child DC bias with dither to arms of a first branch of a Mach-Zehnder modulator (MZM) and to arms of a second branch of the MZM;

determining, by the controller, a second harmonic of a first return signal associated with applying the child DC bias with dither to the arms of the first branch and to the arms of the second branch;

applying, by the controller, a child DC bias with phase-shifted dither to the arms of the first branch or to the arms of the second branch,

the phase-shifted dither being out of phase from the dither, and

the phase-shifted dither having a frequency that matches a frequency of the dither;

determining, by the controller, a second harmonic of a second return signal associated with applying the child DC bias with phase-shifted dither to the arms of the first branch or to the arms of the second branch;

determining, by the controller, whether the first branch and the second branch are operating at quadrature based on the second harmonic of the first return signal and the second harmonic of the second return signal; and

selectively adjusting, by the controller, parent DC biases, applied to the first branch and the second branch, based on whether the first branch and the second branch are operating at quadrature.

19. The method of claim 18 , where determining whether the first branch and the second branch are operating at quadrature comprises:

determining that the first branch and the second branch are operating at quadrature based on a difference between the second harmonic of the first return signal and the second harmonic of the second return signal.

20. The method of claim 18 , where selectively adjusting the parent DC biases comprises:

maintaining application of the parent DC biases already being applied to the first branch and the second branch when the first branch and the second branch are operating at quadrature; or

changing the parent DC biases being applied to the first branch or the second branch by a particular amount when the first branch and the second branch are not operating at quadrature.