Method and an apparatus for signal transmission

1. A method of transmitting a plurality of signals having different wavelengths through a length of signal transmission cable which exhibits a Raman effect of transferring energy to longer-wavelength signals from shorter-wavelength signals as the signals travel through the length of signal transmission cable, comprising the steps of:

a) providing a plurality of pump signals having wavelengths for providing energy for transfer to the plurality of signals traveling through the length of signal transmission cable;

b) providing a selected-wavelength probe signal at a wavelength different from the wavelengths of the plurality of pump signals, the wavelength of the probe signal constituting a wavelength of a supervisory channel;

c) transmitting the probe signal at the wavelength of the supervisory channel and the plurality of pump signals, one at a time, in turn, through the length of transmission cable;

d) determining changes in power level of the probe signal resulting from the pump signals;

e) setting power levels of the pump signals to provide a selected gain profile for the plurality of signals having different wavelengths, the power levels of the pump signals being determined in dependence on the changes in power level of the probe signal and in relation to differences in wavelengths among the probe signal, the pump signals and the plurality of signals having different wavelengths; and

f) continuing to transmit the pump signals at a ratio of set power levels along with the plurality of signals having different wavelengths through the length of signal transmission cable.

2. The method as claimed in claim 1 , wherein the ratio of the set power levels is selected to render the gain profile substantially flat for the plurality of signals having different wavelengths.

3. The method as claimed in claim 1 , wherein the wavelength of the probe signal lies outside a range of the wavelengths of the plurality of signals having different wavelengths.

4. The method as claimed in claim 1 , wherein a selected one of the plurality of signals having different wavelengths is used as the probe signal.

5. The method as claimed in claim 1 , wherein the step of providing the plurality of the pump signals is performed in a forward direction to a direction along which the plurality of signals travel through the length of signal transmission cable.

6. The method as claimed in claim 1 , wherein the step of providing the plurality of the pump signals is performed in a reverse direction to a direction along which the plurality of signals travel through the length of signal transmission cable.

7. An apparatus for transmitting a plurality of signals having different wavelengths, comprising:

a) a length of signal transmission cable which exhibits a Raman effect of transferring energy to longer-wavelength signals from shorter-wavelength signals as the signals travel through the length of signal transmission cable;

b) a plurality of signal transmitters connected to energize the length of transmission cable, for providing the plurality of signals having the different wavelengths to the length of transmission cable;

c) a plurality of pump signal transmitters connected to provide a plurality of pump signals having wavelengths for providing energy for transfer to the plurality of signals traveling through the length of signal transmission cable;

d) a selected-wavelength probe signal transmitter connected to energize the length of transmission cable, and being operable to provide a selected-wavelength probe signal at a wavelength different from the wavelengths of the plurality of pump signals, the wavelength of the probe signal constituting a wavelength of a supervisory channel;

e) means for transmitting the probe signal at the wavelength of the supervisory channel and the plurality of pump signals, one at a time, in turn, through the length of transmission cable;

f) means for determining changes in power level of the probe signal resulting from the pump signals;

g) means for setting power levels of the pump signals to provide a selected gain profile for the plurality of signals having different wavelengths, the power levels of the pump signals being determined in dependence on the changes in power level of the probe signal and in relation to differences in wavelengths among the probe signal, the pump signals and the plurality of signals having different wavelengths; and

h) means for continuing to transmit the pump signals at a ratio of set power levels along with the plurality of signals having different wavelengths through the length of signal transmission cable.

8. The apparatus as claimed in claim 7 , wherein the length of transmission cable is one of a plurality of lengths of transmission cable connected in series.

9. The apparatus as claimed in claim 8 , comprising a plurality of amplifier stages which separate the plurality of lengths of transmission cable from one another.

10. The apparatus as claimed in claim 7 , wherein the length of transmission cable is a length of optical cable.

11. The apparatus as claimed in claim 8 , wherein the plurality of lengths of transmission cable are lengths of optical cable.

12. The apparatus as claimed in claim 7 , wherein the plurality of pump signal transmitters are so connected as to provide energy in a forward direction for transfer in a direction with the plurality of signals traveling through the length of signal transmission cable.

13. The apparatus as claimed in claim 7 , wherein the plurality of pump signal transmitters are so connected as to provide energy in a reverse direction for transfer in a direction opposite to the plurality of signals traveling through the length of transmission cable.