Counter-pumped distributed Raman amplification in wavelength division multiplex optical communication systems

1. A method of providing counter-pumped Raman amplification in a wavelength division multiplex (WDM) optical communication system that includes a transmission optical fiber for guiding WDM radiation having a transmission waveband and at least one Raman pump laser for generating pump radiation having a respective pump wavelength, the transmission fiber having a zero dispersion wavelength lying midway between the transmission waveband and the pump wavelength, the method comprising the step of: de-correlating longitudinal modes of the pump radiation before coupling the pump radiation into the transmission fiber in a counter propagating direction with respect to the WDM radiation.

2. The method according to claim 1 , in which the pump laser is a fiber Raman laser.

3. An optical wavelength division multiplex (WDM) communication system comprising:

a) a transmission optical fiber for guiding WDM radiation having a transmission waveband;

b) at least one Raman pump laser for generating pump radiation having a respective pump wavelength;

c) coupling means for coupling the pump radiation into the transmission optical fiber in a counter propagating direction with respect to a direction of propagation of the WDM radiation;

d) the transmission fiber having a zero dispersion wavelength lying midway between the transmission waveband and the pump wavelength; and

e) de-correlating means for de-correlating longitudinal modes of the pump radiation before it is coupled into the transmission fiber.

4. The communication system according to claim 3 , in which the pump laser is a fiber Raman laser.

5. The communication system according to claim 3 , in which the de-correlating means includes a length of optical fiber having dispersive or non-linear characteristics for de-correlating the longitudinal modes of the pump radiation.

6. The communication system according to claim 5 ; and further comprising a filter between a length of the fiber and the coupling means, for blocking spontaneous emission produced by the Raman effect in the length of the fiber.

7. The communication system according to claim 3 , in which the de-correlating means comprises a first optical coupler for dividing the pump radiation to propagate along first and second optical paths; a delay element included within one of the optical paths; and additional coupling means for re-combining the pump radiation from the first and second optical paths.

8. The communication system according to claim 7 , in which the delay element comprises a length of optical fiber whose length is sufficient to de-correlate the longitudinal modes of the pump radiation.

9. The communication system of claim 3 , in which the de-correlating means comprises a polarization divider for dividing the pump radiation to propagate along first and second optical paths; a delay element included within one of the optical paths; and a polarization combiner for re-combining the pump radiation from the first and second optical paths.

10. The communications system according to claim 9 , in which the delay element comprises a length of optical fiber whose length is sufficient to de-correlate the longitudinal modes of the pump radiation.

11. The communications system according to claim 3 ; and further comprising a plurality of Raman pump lasers each for generating pump radiation having a respective different pump wavelength; a respective de-correlating means for de-correlating the longitudinal modes of the pump radiation; and multiplexing means for combining the pump radiation.

12. A Raman pump module for generating pump radiation having a pump wavelength for coupling into a transmission fiber of an optical wavelength division multiplex (WDM) communication system to provide Raman amplification of WDM radiation counter propagating therethrough, the WDM radiation having a transmission waveband, the transmission fiber having a zero dispersion wavelength lying midway between the transmission waveband and the pump wavelength, the module comprising:

a) a Raman pump laser for generating the pump radiation; and

b) de-correlating means for de-correlating longitudinal modes of the pump radiation before it is coupled into the transmission fiber.

13. The Raman module according to claim 12 , in which the pump laser is a fiber Raman laser.

14. The Raman module according to claim 12 , in which the de-correlating means includes a length of optical fiber having dispersive or non-linear characteristics for de-correlating the longitudinal modes of the pump radiation.

15. The Raman module according to claim 14 ; and further comprising a filter for blocking spontaneous emission produced by the Raman effect in the length of fiber.

16. The Raman module according to claim 12 , in which the de-correlating means comprises a first optical coupler for dividing the pump radiation to propagate along first and second optical paths; a delay element included within one of the optical paths; and coupling means for re-combining the pump radiation from the first and second optical paths.

17. The Raman module according to claim 16 , in which the delay element comprises a length of optical fiber whose length is sufficient to de-correlate the longitudinal modes of the pump radiation.

18. The Raman module according to claim 12 , in which the de-correlating means comprises a polarization divider for dividing the pump radiation to propagate along first and second optical paths; a delay element included within one of the optical paths; and a polarization combiner for re-combining the pump radiation from the first and second optical paths.

19. The Raman module according to claim 18 , in which the delay element comprises a length of optical fiber whose length is sufficient to de-correlate the longitudinal modes of the pump radiation.