Output stage for carrying out WDM message transmission and methods for exchanging full light sources in an output stage of this type

1. An output stage for wavelength division multiplex (WDM) information transmission, comprising: a plurality of information channel ports each providing a carrier wave modulated with an information signal at an output of the output stage, a first filling light source for providing a filling lightwave to the same output having a power corresponding to a desired power signal supplied to a control input of the first filling light source, at least one photodetector for detecting a total power of all filling lightwaves and carrier waves at the output of the output stage, a first control circuit for generating a first desired power signal such that the total power detected by the photodetector is held constant at a predetermined reference value, the first filling light source being part of a first assembly mounted replaceably at a first mounting location of the output stage, the output stage having a second mounting location at which a second assembly comprising a second filling light source is mounted, and the output stage including an auxiliary circuit for providing one of the two filling light sources with a continuously decreasing second desired power signal, the auxiliary circuit being a second control circuit, each control circuit having a filling light source associated with it and receiving the desired power signal generated by the respective control circuit, each control circuit including means for reducing the reference value individually assigned to them, the control circuits being switchable between two different reaction speeds for controlling the power of the filling light source.

2. The output stage of claim 1 , characterized in that the auxiliary circuit receives the desired power signal from the first control circuit and divides it into the second desired power signal and a third desired power signal, wherein the second and third desired power signals are representative for desired powers of the two filling light sources, the sum of which corresponds to the power represented by the first desired power signal.

3. The output stage of claim 2 , characterized in that the output is formed by a first output of a power splitter having two outputs, a second of which is connected to the photodetector.

4. The output stage of claim 1 , characterized in that the reaction speed with which one of the control circuits reacts to a change of the power detected by the photodetector is set to zero, and the means for reducing the reference value are operative to reduce the reference value gradually to zero.

5. The output stage of claim 1 , characterized in that each control circuit is combined with the filling light source controlled by it in one of said replaceable assemblies.

6. The output stage of claim 1 , characterized in that each assembly is a circuit board.

7. The output stage of claim 1 , and comprising an optical multiplexer having wavelength selective inputs, a first one of which is connected to an output of the first filling light source, and a second one of which is connected to an output of the second filling light source, and the first and second inputs being selective for adjacent frequencies of the wavelength multiplex.

8. The output stage of claim 1 , and comprising an optical multiplexer having wavelength selective inputs, one of which is connected to an output of the first filling light source and to an output of the second filling light source by a beam combiner.

9. The output stage of claim 8 , characterized in that the beam combiner is a 3 dB coupler.

10. The output stage of claim 1 , characterized in that the both mounting locations are equipped with an assembly.

11. A method of replacing filling light sources in an output stage for wavelength division multiplex (WDM) information transmission, comprising the steps of:

mounting an assembly comprising a new filling light source at a mounting location of the output stage;

controlling a power of a filling light source to be removed and of the new filling light source so as to keep constant a total power of carrier and filling lightwaves of the output stage, and at the same time continuously reducing the power of the filling light source to be removed, and continuously increasing the power of the new filling light source, until the power of the filling light source to be removed is zero;

removing the filling light source to be removed; and

predetermining a desired value for the total power of carrier and filling lightwaves of the output stage, and independently controlling the power of each filling light source based on a desired value of the total power and a detected effective total power, the power control of the new filling light source being aimed at minimizing a difference between the detected total power and the desired value, whereas the power control of the filling light source to be removed is aimed at minimizing a difference between the detected total power and a fraction of the desired value, the power control of the filling light source to be removed being carried out at a lower reaction speed than for the new filling light source.

12. The method of claim 11 , wherein a desired value for the powers of the filling lightwaves is derived from a detected total power of carrier and filling lightwaves of the output stage and is split between the filling light sources.

13. The method claim 11 , in which the reaction speed at which the power control of the filling light source to be removed reacts to a change of the total power is set to zero.