Gain control for an optical modulator

1. An optical modulation system for transmitting modulated optical light, comprising:

an electro-optic modulator having at least two arms through which light is transmitted;

a voltage bias electrode located on each arm for applying bias voltage to that arm for modifying the phase of light passing through that arm;

an imbalance electrode located on at least one arm;

a current source for injecting current into the imbalance electrode for modifying the phase of light passing through the arm;

a dither generator for modulating the injected current or bias voltage with a dither signal;

a phase sensitive detector for detecting an error in the phase of light emitted by the modulator; and

an operating point controller for monitoring the detected phase error and adjusting the current injected into the imbalance arm so as to compensate for the detected error and thereby control an operating point of the modulator, the required current adjustment being determined by multiplying the detected phase error by a gain;

wherein the gain is modified by a compensation function selected to compensate for a non-linear relationship between the phase in the arm and the current supplied to the imbalance electrode, and

the compensation function is selected to reduce the gain at low imbalance electrode currents compared to the gain at high imbalance electrode currents.

2. The system of claim 1 , wherein the relationship between the phase of light passing through the arm and the current supplied to the imbalance electrode is modelled by a polynomial function, and wherein the compensation function is an inverse of the polynomial function.

3. The system of claim 1 , wherein the compensation function is selected to compensate for the rate of change of the phase in the arm with respect to the current supplied to the imbalance electrode.

4. The system of claim 1 , wherein the compensation function is preselected and stored in firmware associated with the operating point controller.

5. The system of claim 1 , configured to adapt the compensation function dynamically in response to changes over time in the relationship between the phase in the arm and the current supplied to the imbalance electrode.

6. An electro-optic system for transmitting a phase modulated optical signal comprising the modulation system of claim 1 .

7. The electro-optic system of claim 6 configured to operate phase shift keying, wherein the modulation system is included in an inner modulator, outer modulator or pulse carver of the electro-optic system.

8. A method for controlling current injected into an imbalance electrode located on an arm of an electro-optic modulator, the injected current modifying the phase of light passing through the imbalance arm, the method comprising:

modulating the injected current or a bias voltage applied to the arm with a dither signal;

detecting an error in the phase of light emitted by the modulator; and

adjusting the current injected into the imbalance arm so as to compensate for the detected error and thereby control an operating point of the modulator, the required current adjustment being determined by multiplying the detected phase error by a gain;

wherein the gain is modified by a compensation function selected to compensate for a non-linear relationship between the phase in the arm and the current supplied to the imbalance electrode, and

the compensation function reduces the gain at low imbalance electrode currents compared to the gain at high imbalance electrode currents.

9. The method of claim 8 , wherein the relationship between the phase of light passing through the arm and the current supplied to the imbalance electrode is modelled by a polynomial function, and wherein the compensation function is an inverse of the polynomial function.

10. The method of claim 8 , wherein the compensation function compensates for the rate of change of the phase in the arm with respect to the current supplied to the imbalance electrode.

11. The method of claim 8 , further comprising preselecting the compensation function and storing it in firmware.

12. The method of claim 8 , further comprising adapting the compensation function dynamically in response to changes over time in the relationship between the phase in the arm and the current supplied to the imbalance electrode.

13. A non-transitory computer-readable storage medium storing executable computer program instructions for controlling current injected into an imbalance electrode located on an arm of an electro-optic modulator, the injected current modifying the phase of light passing through the imbalance arm, the instructions executable to perform steps comprising:

modulating the injected current or a bias voltage applied to the arm with a dither signal;

detecting an error in the phase of light emitted by the modulator; and

adjusting the current injected into the imbalance arm so as to compensate for the detected error and thereby control an operating point of the modulator, the required current adjustment being determined by multiplying the detected phase error by a gain;

wherein the gain is modified by a compensation function selected to compensate for a non-linear relationship between the phase in the arm and the current supplied to the imbalance electrode, and

the compensation function reduces the gain at low imbalance electrode currents compared to the gain at high imbalance electrode currents.