Transmission link with multiple order Raman pumps

1. A system comprising:

first and second pump assemblies optically coupled to a first end and a second end of an optical fiber span, respectively, the first and second pump assemblies each comprising:

a single-wavelength or multi-wavelength first-order Raman pump for providing first-order Raman pump light for amplifying an optical signal; and

first and second higher-order Raman pumps for providing higher-order Raman pump light for amplifying the first-order Raman pump light; and

a controller operationally coupled to a first photodetector, a second photodetector, and each Raman pump of the second pump assembly, wherein

the first-order and the first and second higher-order Raman pumps are configured for operation at such emission wavelengths and non-zero optical power levels that a peak-to-peak variation of an optical power of the optical signal along the optical fiber span is less than 5 dB,

the controller stores a first look-up table of values of a first electrical signal, generated by the first photodetector, and associated power levels of each Raman pump of the second pump assembly and a second look-up table of values of a second electrical signal, generated by the second photodetector, and associated power levels of each Raman pump of the second pump assembly, and

the controller is configured to adjust, using the first look-up table and the second look-up table, power levels of each Raman pump of the second pump assembly based the first electrical signal and the second electrical signal to lessen the peak-to-peak variation of the optical power of the optical signal.

2. The system of claim 1 , wherein

the optical signal includes a plurality of wavelength channels spanning a wavelength range of at least 25 nm,

the first and second pump assemblies each comprise a single-wavelength first-order Raman pump and an additional single-wavelength first-order Raman pump, and

each Raman pump, of the first and second pump assemblies, is configured for operation at such emission wavelengths and non-zero optical power levels that a peak-to-peak variation of an optical power of each one of the wavelength channels along the optical fiber span is less than 5 dB.

3. The system of claim 1 , wherein the first and second higher-order Raman pumps, of the first and second pump assemblies, comprise a second-order Raman pump and a third-order Raman pump, respectively.

4. The system of claim 1 , wherein the first and second pump assemblies each further comprise an additional second-order Raman pump configured for operation at such emission wavelengths and non-zero optical power levels that a peak-to-peak variation of the optical power of each one of a plurality of wavelength channels along the optical fiber span is less than 4 dB.

5. The system of claim 4 , wherein the optical fiber span is less than 100 km long.

6. A system comprising:

first and second WDM combiners optically coupled to a first end and a second end of an optical fiber span, respectively, for coupling pump light;

first and second pump subassemblies optically coupled to the first and second WDM combiners, respectively, for providing the pump light, the first and second pump subassemblies each comprising:

first and second first-order Raman pumps for providing first-order Raman pump light for amplifying an optical signal;

first and second second-order Raman pumps for providing second-order Raman pump light for amplifying the first-order Raman light; and

a first third-order Raman pump for providing third-order Raman pump light for amplifying the second-order Raman light; and

a controller operationally coupled to a first photodetector, a second photodetector, and each Raman pump of the second pump subassembly, wherein

the first and second first-order and second-order Raman pumps and the first third-order Raman pump of the first and second pump subassemblies are configured for operation at such emission wavelengths and non-zero optical power levels that a peak-to-peak variation of optical power of each one of a plurality of wavelength channels along the optical fiber span is less than 4 dB,

the controller stores a first look-up table of values of a first electrical signal, generated by the first photodetector, and associated power levels of each Raman pump of the second pump subassembly and a second look-up table of values of a second electrical signal, generated by the second photodetector, and associated power levels of each Raman pump of the second pump subassembly, and

the controller is configured to adjust, using the first look-up table and the second look-up table, power levels of each Raman pump of the second pump subassembly based the first electrical signal and the second electrical signal to lessen the peak-to-peak variation of the optical power of the optical signal.

7. The system of claim 6 , wherein the first and second pump subassemblies each further comprise:

a second third-order Raman pump, for providing the third-order Raman pump light for amplifying the second-order Raman light, configured for operation at such emission wavelengths and non-zero optical power levels that a peak-to-peak variation of the optical power of each one of the plurality of wavelength channels along the optical fiber span is less than 3 dB.

8. The system of claim 6 , wherein the first and second pump subassemblies each further comprise:

a third first-order Raman pump for providing the first-order Raman pump light for amplifying the optical signal; and

a third second-order Raman pump for providing the second-order Raman pump light for amplifying the first-order Raman pump light,

wherein the third first-order and the third second-order Raman pumps are configured for operation at such emission wavelengths and non-zero optical power levels that a peak-to-peak variation of the optical power of each one of the plurality of wavelength channels along the optical fiber span is less than 2 dB.

9. The system of claim 6 , wherein

the emission wavelengths of each Raman pump in the first and second pump subassemblies differ from each other by at least 5 nm,

the second pump subassembly further comprises:

a first WDM splitter optically coupled to the second end of the optical fiber span for splitting off a portion of residual pump light propagated through the optical fiber span from the first pump subassembly, and

the first photodetector optically is coupled to the first WDM splitter for receiving the residual pump light portion and generating a first electrical signal in response to the residual pump light portion.

10. The system of claim 9 , wherein

the second pump subassembly further comprises:

a second WDM splitter optically coupled to the second end of the optical fiber span for splitting off a portion of a Raman-generated co-propagated ASE generated in the optical fiber span, and

the second photodetector is optically coupled to the second WDM splitter for receiving the Raman-generated co-propagated ASE portion and generating a second electrical signal in response to the ASE portion.

11. The system of claim 10 , wherein the second pump subassembly further comprises:

a third WDM splitter optically coupled to the second end of the optical fiber span for splitting off a portion of the optical signal; and

a third photodetector optically coupled to the third WDM splitter for receiving the optical signal portion and generating a third electrical signal in response to the portion of the optical signal, wherein

the controller is operationally coupled to the third photodetector;

the controller stores a third look-up table of values of the third electrical signal and associated power levels of each Raman pump of the second pump subassembly, and

the controller is configured to adjust the power levels of each Raman pump of the second pump subassembly in dependence upon the third electrical signal generated by the third photodetector based on the third look-up table, so as to further lessen the peak-to-peak variation of the optical power of the optical signal.

12. The system of claim 6 , wherein

an emission wavelength of each second-order Raman pump is least 10 nm shorter than an emission wavelength of each first-order Raman pump, and

an emission wavelength of each third-order Raman pump is at least 10 nm shorter than the emission wavelength of each second-order Raman pump.

13. The system of claim 12 , wherein

the optical signal includes wavelengths between 1535 nm and 1615 nm,

the emission wavelength of each first-order Raman pump is between 1420 nm and 1520 nm,

the emission wavelength of each second-order Raman pump is between 1300 nm and 1420 nm, and

the emission wavelength of each third-order Raman pump is between 1200 nm and 1300 nm.

14. The system of claim 6 , wherein a noise figure of each wavelength channel, of the plurality of wavelength channels, is at least −6 dB.

15. The system of claim 6 , wherein at least one of the first or second pump subassemblies comprises a multi-wavelength Raman pump including a light source having a continuous emission band having a 3 dB bandwidth of at least 25 nm.

16. The system of claim 6 , wherein the optical fiber span is absent Bragg gratings for reflecting pump light.

17. A method comprising:

optically coupling, to a first end and a second end of an optical fiber span, first and second pump subassemblies, respectively, the first and second pump subassemblies each comprising N 1 first-order Raman pumps and N 2 higher-order Raman pumps, wherein N 1 is an integer ≧1, and N 2 is an integer ≧2;

coupling an optical signal to the first end of the optical fiber span;

using pump controllers to operate the N 1 first-order Raman pumps and the N 2 higher-order Raman pumps at such emission wavelengths and optical power levels that a peak-to-peak variation ΔP P2P of an optical power P of the optical signal along the optical fiber span is less than 5 dB;

storing a first look-up table of values of a first electrical signal, generated by a first photodetector, and associated power levels of each Raman pump of the second pump subassembly and a second look-up table of values of a second electrical signal, generated by a second photodetector, and associated power levels of each Raman pump of the second pump subassembly; and

adjusting, using the first look-up table and the second look-up table, power levels of each Raman pump of the second pump subassembly based the first electrical signal and the second electrical signal to lessen the peak-to-peak variation ΔP P2P of an optical power P of the optical signal.

18. The method of claim 17 , further comprising

running a numerical optimization of propagation of the optical signal in the optical fiber span pumped with the N 1 first-order Raman pumps and the N 2 higher-order Raman pumps,

wherein emission wavelengths and optical power levels of the N 1 first-order and the N 2 higher-order Raman pumps are varied to determine optimal emission wavelengths and optical power levels, at which the peak-to-peak variation ΔP P2P of an optical power P of the optical signal along the optical fiber span is less than 5 dB, and

wherein the N 1 first-order and the N 2 higher-order Raman pumps are operated at the optimal emission wavelengths and optical power levels based on running the numerical optimization.

19. The method of claim 18 , wherein

the optical signal includes a plurality of wavelength channels spanning a wavelength range of at least 25 nm, and

N 1 is ≧2, and wherein the N 2 higher-order Raman pumps of the first and second pump subassemblies each include a second-order Raman pump and a third-order Raman pump.

20. The method of claim 18 , wherein

the optical signal includes a plurality of wavelength channels spanning a wavelength range of at least 25 nm;

N 1 is ≧2,

N 2 ≧3,

the N 2 higher-order Raman pumps of the first and second pump subassemblies each include two second-order Raman pumps and one third-order Raman pump, and

ΔP P2P ≦4 dB.