System and method for spectral loading an optical transmission system

1. A system comprising:

at least one transmitter configured to transmit information signals on different respective signal wavelengths within a band of system wavelengths associated with channels of a wavelength division multiplex (WDM) system;

an optical load generator configured to generate dummy tones at loading wavelengths within said band of system wavelengths, wherein said loading wavelengths are associated with only a subset of unutilized channels in said WDM system and a majority of said loading wavelengths are within a lower half of said band of system wavelengths, said unutilized channels being channels of said WDM system that are not initially loaded with one of said information signals, and wherein said loading wavelengths are selected and spaced to provide a flattened gain spectrum across said band of system wavelengths compared to the gain spectrum of said system without said loading wavelengths; and

at least one combiner configured to combine said information signals and said dummy tones to provide an output comprising said information signals on utilized channels of the WDM system and said dummy tones on said subset of unutilized channels of the WDM system.

2. The system of claim 1 , wherein said loading wavelengths are selected and spaced based on a distortion in a gain spectrum for a partially loaded WDM system.

3. The system of claim 1 , wherein said optical load generator is configured to generate at least three dummy tones.

4. The system of claim 1 , wherein a majority of said dummy tones are generated at wavelengths below about 1550 nm.

5. The system of claim 1 , wherein one of said dummy tones is generated at a wavelength in the middle of said band of system wavelengths.

6. The system of claim 1 , further comprising at least one power adjustment device coupled to at least said optical load generator and configured to adjust a power level of said dummy tones.

7. The system of claim 1 , wherein said optical load generator comprises continuous wave laser sources configured to operate at said loading wavelengths.

8. The system of claim 7 , wherein said optical load generator further comprises at least one polarization scrambler coupled to said laser sources.

9. The system of claim 1 , wherein said optical load generator comprises: a broadband noise source configured to generate optical noise; and at least one filter configured to filter said optical noise to produce said dummy tones.

10. The system of claim 9 , wherein said noise source comprises an amplified spontaneous emission (ASE) noise source.

11. The system of claim 1 , further comprising an optically amplified transmission line coupled to said output of said at least one combiner.

12. A method of transmitting a WDM signal on an optical transmission path, said method comprising:

generating a plurality of information signals at different respective signal wavelengths within a band of system wavelengths associated with channels of a partially loaded WDM system;

generating dummy tones at loading wavelengths within said band of system wavelengths, wherein said loading wavelengths are associated with only a subset of unutilized channels in said WDM system and a majority of said loading wavelengths are within a lower half of said band of system wavelengths, said unutilized channels being channels of said WDM system that are not initially loaded with one of said information signals, and wherein said loading wavelengths are selected and spaced to provide a flattened gain spectrum across said band of system wavelengths compared to the gain spectrum of said system without said loading wavelengths; and

imparting said information signals and said dummy tones on said optical transmission path such that said information signals are provided on utilized channels of said WDM system and said dummy tones are provided on said subset of unutilized channels of said WDM system.

13. The method of claim 12 , further comprising selecting said loading wavelengths based on a distortion in a gain spectrum for said partially loaded WDM system.

14. The method of claim 12 , wherein generating said dummy tones comprises generating at least three dummy tones.

15. The method of claim 12 , wherein generating said dummy tones comprises generating a majority of said dummy tones at wavelengths below about 1550 nm.

16. The method of claim 12 , wherein generating said dummy tones comprises generating at least three noise tones.

17. The method of claim 12 , wherein generating said dummy tones comprises filtering an amplified spontaneous emission (ASE) noise source to generate at least three noise tones.

18. A method of spectral loading to suppress spectral hole burning, said method comprising:

identifying a distortion in a gain spectrum for a partially loaded WDM system including utilized and unutilized channels across a band of system wavelengths, each of said utilized channels having an associated information signal loaded thereon;

selecting loading wavelengths within said band of system wavelengths based on said distortion in said gain spectrum for said WDM system, said loading wavelengths associated with some of said unutilized channels in said WDM system and a majority of said loading wavelengths being within a lower half of said band of system wavelengths, said unutilized channels being channels of said WDM system that are not initially loaded with one of said information signals; and

imparting dummy tones at said loading wavelengths only on a subset of said unutilized channels in said WDM system to change said distortion and provide a flattened gain spectrum across said band of system wavelengths compared to the gain spectrum of said system without said loading wavelengths.

19. The method of claim 18 , wherein selecting said wavelengths includes selecting at least three wavelengths for generating at least three dummy tones.

20. The method of claim 18 , wherein selecting wavelengths includes selecting loading wavelengths unevenly spaced across said band of system wavelengths.

21. The method of claim 18 , wherein selecting loading wavelengths includes selecting one of said loading wavelengths in the middle of said band of system wavelengths.

22. The method of claim 18 , wherein selecting loading wavelengths includes selecting a majority of said loading wavelengths below about 1550 nm.