System and method using differential loop gain for fault identification in line monitoring equipment

1. A line monitoring system for an optical communication system comprising:

a test signal transmitter configured to provide a test signal for transmission on an optical communication system, the optical communication system comprising

a first optical fiber path for receiving said test signal and carrying said test signal in a first direction,

a second optical fiber path for carrying signals in a second direction opposite from said first direction, and

a plurality of loop back paths, each of said loop back paths coupling said test signal to said second optical fiber path as an associated returned test signal; and

a correlator configured to calculate differential loop gain data associated with each of said loop back paths from said associated returned test signals and provide an output identifying a fault in the optical communication system in response to said differential loop gain data, said differential loop gain data comprising, for each one of said loop back paths,

a loop gain imparted to said test signal through said each one of said loop back paths, minus

a previous loop back path loop gain imparted to said test signal through one of said loop back paths preceding said each one of said loop back paths.

2. A line monitoring system according to claim 1 , wherein said correlator is configured to compare said differential loop gain data with a predetermined differential loop gain fault signature corresponding to said fault in the optical communication system.

3. A line monitoring system according to claim 1 , wherein said fault is an extra fiber loss fault in said optical communication system.

4. A line monitoring system according to claim 1 , wherein said fault is an extra pump loss fault in said optical communication system.

5. A line monitoring system according to claim 1 , wherein said differential loop gain data comprises differential loop gain tilt data.

6. A line monitoring system according to claim 1 , wherein a said test signal comprises a short LME wavelength signal and a long LME wavelength signal, said long LME wavelength signal having a wavelength longer than a wavelength of said short LME wavelength signal.

7. A line monitoring system according to claim 6 , wherein said wavelength of said short LME wavelength signal is at a short wavelength end of a data signal transmission band of the optical communication system and said wavelength of said long LME wavelength signal is at a long wavelength end of a said data signal transmission band of the optical communication system.

8. A line monitoring system according to claim 1 , wherein said test signal transmitter comprises a polarization scrambler for scrambling the state of polarization of said test signal.

9. A line monitoring system according to claim 1 , wherein each of said loop back paths is disposed in an associated repeater coupled to said first and second fiber paths, each of said repeaters comprising a first amplifier for amplifying signals on said first optical fiber path and a second amplifier for amplifying signals on said second optical fiber path.

10. An optical communication system comprising:

a test signal transmitter configured to provide a test signal;

a first optical fiber path for receiving said test signal and carrying said test signal in a first direction;

a second optical fiber path for carrying signals in a second direction opposite from said first direction;

a plurality of loop back paths, each of said loop back paths coupling said test signal to said second optical fiber path as an associated returned test signal; and

a correlator configured to calculate differential loop gain data associated with each of said loop back paths from said associated returned test signals and provide an output identifying a fault in the optical communication system in response to said differential loop gain data, said differential loop gain data comprising, for each one of said loop back paths,

a loop gain imparted to said test signal through said each one of said loop back paths, minus

a previous loop back path loop gain imparted to said test signal through one of said loop back paths preceding said each one of said loop back paths.

11. An optical communication system according to claim 10 , wherein said correlator is configured to compare said differential loop gain data with a predetermined differential loop gain fault signature corresponding to said fault in the optical communication system.

12. An optical communication system according to claim 10 , wherein said fault is an extra fiber loss fault in said optical communication system.

13. An optical communication system according to claim 10 , wherein said fault is an extra pump loss fault in said optical communication system.

14. An optical communication system according to claim 10 , wherein said differential loop gain data comprises differential loop gain tilt data.

15. An optical communication system according to claim 10 , wherein a said test signal comprises a short LME wavelength signal and a long LME wavelength signal, said long LME wavelength signal having a wavelength longer than a wavelength of said short LME wavelength signal.

16. An optical communication system according to claim 15 , wherein said wavelength of said short LME wavelength signal is at a short wavelength end of a data signal transmission band of the optical communication system and said wavelength of said long LME wavelength signal is at a long wavelength end of a said data signal transmission band of the optical communication system.

17. An optical communication system according to claim 10 , wherein said test signal transmitter comprises a polarization scrambler for scrambling the state of polarization of said test signal.

18. An optical communication system according to claim 10 , wherein each of said loop back paths is disposed in an associated repeater coupled to said first and second fiber paths, each of said repeaters comprising a first amplifier for amplifying signals on said first optical fiber path and a second amplifier for amplifying signals on said second optical fiber path.

19. A method of monitoring an optical communication system comprising a plurality of loop back paths coupling a first fiber optic path for carrying signals in a first direction and a second fiber optic path for carrying signals in a second direction opposite to the first direction, said method comprising:

transmitting a test signal on the first fiber optic path;

receiving an associated returned test signal from each of the loop back paths from the second fiber optic path;

calculating differential loop gain data associated with each of said loop back paths from the returned test signals; and

identifying a fault in the optical communication system in response to said differential loop gain data, said differential loop gain data comprising, for each one of said loop back paths,

a loop gain imparted to said test signal through said each one of said loop back paths, minus

a previous loop back path loop gain imparted to said test signal through one of said loop back paths preceding said each one of said loop back paths.

20. A method according to claim 19 , wherein said identifying a fault comprises comparing said differential loop gain data with a predetermined differential loop gain fault signature corresponding to said fault.

21. A method according to claim 19 , wherein said fault is an extra fiber loss fault in said optical communication system.

22. A method according to claim 19 , wherein said fault is an extra pump loss fault in said optical communication system.

23. A method according to claim 19 , wherein said differential loop gain data comprises differential loop gain tilt data.

24. A method according to claim 19 , wherein a said test signal comprises a short LME wavelength signal and a long LME wavelength signal, said long LME wavelength signal having a wavelength longer than a wavelength of said short LME wavelength signal.

25. A method according to claim 24 , wherein said wavelength of said short LME wavelength signal is at a short wavelength end of a data signal transmission band of the optical communication system and said wavelength of said long LME wavelength signal is at a long wavelength end of a said data signal transmission band of the optical communication system.

26. A method according to claim 19 , said method further comprising scrambling the state of polarization of said test signal.

27. A method according to claim 19 , wherein each of said loop back paths is disposed in an associated repeater coupled to said first and second fiber paths, each of said repeaters comprising a first amplifier for amplifying signals on said first optical fiber path and a second amplifier for amplifying signals on said second optical fiber path.

28. A method of monitoring an optical communication system comprising a plurality of loop back paths coupling a first fiber optic path for carrying signals in a first direction and a second fiber optic path for carrying signals in a second direction opposite to the first direction, said method comprising:

transmitting a test signal on the first fiber optic path;

receiving an associated returned test signal from each of the loop back paths from the second fiber optic path;

calculating gain data associated with each of said loop back paths from the returned test signals;

comparing said gain data with a predetermined gain fault signature representative of a fault in said optical communication system; and

identifying said fault in the optical communication system in response to said comparing of said gain data with said predetermined fault signature.