Bias-control for optical Mach-Zehnder modulators with voltage-induced optical absorption
View Patent ↗A bias-control circuit that provides operating point control for a Mach-Zehnder modulator experiencing optical absorption at their interferometric arms. The bias control circuit generates compensation signals that are used to counter the thermally induced index shifts as a result of absorption. In addition, an operating point with desirable transmitter characteristics can also be arbitrarily chosen by over-compensating or under-compensating thermal effects.
1. An apparatus for providing operating point control for a Mach-Zehnder modulator experiencing optical absorption at its interferometric arms, comprising:
a bias control circuit for generating compensation signals that are used to counter thermally induced index shifts as a result of the optical absorption, wherein said compensation signals comprise a first compensation signal that is a direct current (DC) compensation signal that compensates for an average of the thermally induced index shifts as a result of a DC component of an electrical input signal, and a second compensation signal that is an alternating current (AC) compensation signal that compensates for an instantaneous one of the thermally induced index shifts as a result of an amplitude modulation (AM) component of the electrical input signal.
2. The apparatus of claim 1 , wherein an operating point with desirable transmitter characteristics is chosen for the Mach-Zehnder modulator so as to over-compensate or under-compensate thermal effects.
3. The apparatus of claim 1 , wherein the AC compensation signal has identical frequency and is either in-phase or out-of-phase with the AM component of the electrical input signal.
4. The apparatus of claim 1 , wherein the DC compensation signal is applied to a first arm of the MZM, while the AC compensation signal is a sinusoidal signal applied to the first arm or a second arm of the MZM.
5. The apparatus of claim 1 , wherein the amplitude of the AC compensation signal is proportional to an average photocurrent produced in the first arm of the MZM.
6. A method for providing operating point control for a Mach-Zehnder modulator experiencing optical absorption at its interferometric arms, comprising:
generating compensation signals in a bias control circuit that are used to counter thermally induced index shifts as a result of the optical absorption, wherein said compensation signals comprise a first compensation signal that is a direct current (DC) compensation signal that compensates for an average of the thermally induced index shifts as a result of a DC component of an electrical input signal, and a second compensation signal that is an alternating current (AC) compensation signal that compensates for an instantaneous one of the thermally induced index shifts as a result of an amplitude modulation (AM) component of the electrical input signal.
7. The method of claim 6 , wherein an operating point with desirable transmitter characteristics is chosen for the Mach-Zehnder modulator by over-compensating or under-compensating thermal effects.
8. The method of claim 6 , wherein the AC compensation signal has identical frequency and is either in-phase or out-of-phase with the AM component of the electrical input.
9. The method of claim 6 , wherein the DC compensation signal is applied to a first arm of the MZM, while the AC compensation signal is a sinusoidal signal applied to the first arm or a second arm of the MZM.
10. The method of claim 6 , wherein the amplitude of the AC compensation signal is proportional to an average photocurrent produced in the first arm of the MZM.
11. An apparatus for modulating optical signals, comprising:
a Mach-Zehnder modulator that in operation experiences optical absorption in interferometric arms thereof; and,
a bias control circuit for providing operating point control for the Mach-Zehnder modulator by generating compensation signals that are used to counter thermally induced index shifts resulting from the optical absorption, wherein in operation the bias control circuit generates a first compensation signal that is a direct current (DC) compensation signal that compensates for an average of the thermally induced index shifts as a result of a DC component of an electrical input signal, and a second compensation signal that is an alternating current (AC) compensation signal that compensates for an instantaneous one of the thermally induced index shifts as a result of an amplitude modulation (AM) component of the electrical input signal.
12. The apparatus of claim 11 , wherein an operating point with desirable transmitter characteristics is chosen for the Mach-Zehnder modulator by over-compensating or under-compensating thermal effects.
13. The apparatus of claim 11 , wherein the AC compensation signal has identical frequency and is either in-phase or out-of-phase with the AM component of the electrical input.
14. The apparatus of claim 11 , wherein the DC compensation signal is applied to a first arm of the MZM, while the AC compensation signal is a sinusoidal signal applied to the first arm or a second arm of the MZM.
15. The apparatus of claim 11 , wherein the amplitude of the AC compensation signal is proportional to an average photocurrent produced in the first arm of the MZM.
16. The apparatus of claim 11 , wherein the bias control circuit comprises:
a photocurrent measuring circuit for measuring a photocurrent in one of the interferometric arms of the Mach-Zehnder modulator; and,
a controller for controlling an amplitude of the AC compensation signal in dependence upon said photocurrent.
17. The apparatus of claim 16 , further comprising a circuit element electrically coupled between the controller and the one of the interferometric arms of the Mach-Zehnder modulator for setting at least one of the amplitude and phase of the AC compensation signal in response to a control signal from the controller.
18. The apparatus of claim 1 , wherein the bias control circuit comprises:
a photocurrent measuring circuit for measuring a photocurrent in an interferometric arm of the Mach-Zehnder modulator; and,
a control circuit for controlling an amplitude of the AC compensation signal in dependence upon said photocurrent.
19. The apparatus of claim 18 , further comprising a circuit element electrically coupled between the controller and the one of the interferometric arms of the Mach-Zehnder modulator for setting at least one of the amplitude and phase of the AC compensation signal in response to a control signal from the controller.
20. The method of claim 6 , comprising:
a) measuring a photocurrent in an interferometric arm of the Mach-Zehnder modulator; and,
b) controlling an amplitude of the AC compensation signal in dependence upon said photocurrent.
21. The method of claim 20 , wherein step b) comprises using stored calibration information to set a proportionality constant between the photocurrent measured in step a) and the amplitude of the AC compensation signal.
22. The method of claim 21 , wherein the proportionality constant is set for operating the Mach-Zehnder modulator away from a quadrature point of a transfer characteristic thereof.