Compensation of gain variations in a multistage optical amplifier

1. A method for compensating a variation in gain in an optical amplifier for amplifying an optical Wavelength Division Multiplexing signal, comprising:

providing a plurality of series-connected amplifier stages with at least two pumping devices;

adjusting a pumping power of the pumping devices to keep a amplifier gain essentially constant when an abrupt change of an input power occurs;

adjusting the pumping power of a first amplifier stage when the abrupt change in the input power occurs;

determining an expected change in the input power at a succeeding second amplifier stage;

determining a new pumping power for the pumping device associated to a second amplifier stage based upon the determined expected change in the input power at the second amplifier stage; and

adjusting the new pumping power for the second amplifier stage based upon a derivative time before the abrupt change in power occurs at the input of the second stage.

2. The method as claimed in claim 1 , wherein the abrupt change is based upon an event selected from the group consisting of:

adding a channel of the WDM signal transmitted,

removing a channel of the WDM signal transmitted,

coupling a channel of the WDM signal transmitted, and

decoupling a channel of the WDM signal transmitted.

3. The method as claimed in claim 2 , wherein a output power of the second amplifier stage is determined after the abrupt change in power has occurred at the input of the second amplifier stage.

4. The method as claimed in claim 2 , wherein the output power of the second amplifier stage is determined immediately after the abrupt change in power has occurred at the input of the second amplifier stage.

5. The method as claimed in claim 3 , wherein a new pumping power of the pumping device associated to the second amplifier stage is calculated based upon the output power of the second amplifier stage.

6. The method as claimed in claim 5 , wherein the new pumping power is set.

7. The method as claimed in claim 1 , wherein the output power of the first amplifier stage is determined immediately after the abrupt change in power at the input of the first amplifier stage.

8. The method as claimed in claim 1 , wherein the output power of the first amplifier stage is determined after the abrupt change in power at the input of the first amplifier stage.

9. The method as claimed in claim 8 , wherein a new pumping power for the pumping device associated with the first amplifier stage is calculated based upon the determined output power of the first amplifier stage.

10. The method as claimed in claim 6 , wherein the output power of the first amplifier stage is determined after the abrupt change in power at an input of the first amplifier stage.

11. The method as claimed in claim 10 , wherein a new pumping power for the pumping device associated with the first amplifier stage is calculated based upon the determined output power of the first amplifier stage.

12. The method as claimed in claim 1 , wherein the pumping power of the second amplifier stage is adjusted based upon a control operation.

13. A multistage optical amplifier to amplify an optical Wavelength Division Multiplexing signal, comprising:

a plurality of series-connected amplifier stages with at least two pumping devices;

a sensor for determining a power of the amplifier stages;

a control unit to monitor the signal power of the Wavelength Division Multiplexing signal and to adjust the pumping power of at least one of the pumping devices in respect to an abrupt change in an input power to the multistage optical amplifier to keep an amplifier gain essentially constant;

a first amplifier stage, wherein a pumping power of the first amplifier stage is changed via the control unit in response to the abrupt change in input power;

a second amplifier stage, wherein the control unit determines an expected abrupt change in power at an input of a second amplifier stage;

a new pumping power value for a pumping device associated to the second amplifier stage based upon the determined expected change in power at an input of the second amplifier stage; and

a derivative time to set the new pumping power value before the abrupt change in power occurs at the input of the second amplifier stage.

14. The multistage optical amplifier as claimed in claim 13 , wherein the change in power is compensated based upon a counter variation.

15. The multistage optical amplifier as claimed in claim 13 , wherein the sensor determines an input power.

16. The multistage optical amplifier as claimed in claim 13 , wherein the sensor determines an output power.

17. The multistage optical amplifier as claimed in claim 13 , wherein each of the amplifier stages has an erbium-doped fiber.

18. The multistage optical amplifier as claimed in claim 13 , wherein a dispersion-compensating fiber is located between the first amplifier stage and the second amplifier stage.

19. The multistage optical amplifier as claimed in claim 13 , wherein the control device controls the pumping power and wherein the amplifier has no closed loop control to control the amplifier gain.

20. The multistage optical amplifier as claimed in claim 13 , wherein the multistage optical amplifier has three amplifier stages.