Modulator control system

1. An optical communications system comprising:

an optical input and an output,

a source of input signals in an asymmetric format and having a duty cycle,

a modulator for modulating the input signals, to produce an asymmetric signal, the modulator comprising a Mach-Zehnder interferometer;

drive means for driving the modulator at a predetermined modulator drive voltage;

bias means for applying a bias voltage to the modulator;

adjustment means for adjusting a first variable chosen from one of the bias voltage, gain voltage, or duty cycle in response to the relationship existing between changes in the amplitude of the asymmetric signal and changes in the first variable, characterised in that the system further comprising varying means for varying a second variable, different to the first variable and chosen from one of the bias voltage, gain voltage or duty cycle in response to adjustments made to the first variable; and

a monitor for recording the average optical output power.

2. A communication system according to claim 1 wherein the first variable comprises the bias voltage, and the second variable comprises the gain voltage.

3. A system according to claim 2 wherein the adjustment means comprises a first dither for applying a dither to the bias voltage.

4. A system according to any one of the preceding claims wherein the adjustment means increases the bias voltage if the output decreases when the bias voltage is increased, and decreases the bias voltage if the output, increases when the bias voltage increases.

5. A system according to any one of claims 1 to 3 wherein the varying means increases the modulator drive voltage when the bias voltage increases, and decreases the modulator drive voltage when the bias voltage decreases.

6. A system according to any one of claims 1 to 3 wherein the adjustment means decreases the bias voltage if the output decreases when the bias voltage is increased, and increases the bias voltage if the output increases when the bias voltage increases.

7. A system according to any one of claims 1 to 3 wherein the varying means decreases the modular drive voltage when the bias voltage increases, and increases the modular drive voltage when the bias voltage decreases.

8. A system according to any one of claims 1 to 3 wherein the varying means comprises a second dither for applying a dither to the modulator drive voltage.

9. A method of transmitting an optical signal in asymmetric format, the method comprising modulating an input signal having an asymmetric format and a duty cycle using a Mach-Zehnder modulator (MZM);

applying a predetermined modular drive voltage to the MZM;

applying a bias voltage to the MZM;

adjusting a first variable chosen from one of the bias voltage, gain voltage, or duty cycle in response to the relationship existing between changes in the amplitude of the asymmetric signal and changes in the first variable;

varying a second variable, different to the first variable and chosen from one of the bias voltage, gain voltage or duty cycle in response to adjustments made to the first variable;

monitoring the average optical power output of the Mach-Zehnder modulator.

10. A modulator comprising a Mach-Zehnder interferometer, which modulator produces asymmetric signals in an optical communication system.