Split-band amplifying apparatus using a depressed-profile fiber amplifier

1. A split-band amplifying apparatus comprising:

a) a first section for amplifying a long wavelength band of an optical signal;

b) a second section having a fiber amplifier for amplifying a short wavelength band of said optical signal, said fiber amplifier comprising:

1) a core having a core cross-section and a refractive index n o ;

2) an active material doped in said core;

3) a depressed cladding surrounding said core, said depressed cladding having a depressed cladding cross-section and a refractive index n 1 ;

4) a secondary cladding surrounding said depressed cladding, said secondary cladding having a secondary cladding cross-section and a refractive index n 2 ;

5) a pump source for pumping said active material to a high relative inversion D, such that said active material exhibits positive gains in said short wavelength band and high gains in said long wavelength band;

wherein said core cross-section, said depressed cladding cross-section, and said refractive indices n o , n 1 , and n 2 are selected to produce a roll-off loss curve about a cutoff wavelength λ c , said roll-off loss curve yielding losses at least comparable to said high gains in said long wavelength band and losses substantially smaller than said positive gains in said short wavelength band.

2. The split-band amplifying apparatus of claim 1 , wherein said active material is Erbium such that said fiber amplifier is a first Erbium-doped fiber amplifier.

3. The split-band amplifying apparatus of claim 2 , wherein said short wavelength band comprises at least a portion of the S-band and said long wavelength band comprises at least a portion of the C-band, and said cutoff wavelength λ c is at a crossover wavelength between said S-band and said C-band.

4. The split-band amplifying apparatus of claim 3 , wherein said long wavelength band further comprises at least a portion of the L-band.

5. The split-band amplifying apparatus of claim 3 , wherein said cutoff wavelength is at about 1530 nm.

6. The split-band amplifying apparatus of claim 1 , wherein said first section comprises a second Erbium-doped fiber amplifier.

7. The split-band amplifying apparatus of claim 6 , wherein said active material is Erbium such that said fiber amplifier is a first Erbium-doped fiber amplifier.

8. The split-band amplifying apparatus of claim 7 , further comprising a common pump source for delivering a pump radiation to said first Erbium-doped fiber amplifier and to said second Erbium-doped fiber amplifier.

9. The split-band amplifying apparatus of claim 8 , wherein said common pump source comprises a laser diode delivering said pump radiation at about 980 nm.

10. The split-band amplifying apparatus of claim 1 , wherein said first section and said second section share an overlapping segment.

11. The split-band amplifying apparatus of claim 10 , wherein said overlapping segment comprises a second Erbium-doped fiber amplifier for amplifying said long wavelength band.

12. A method for split-band amplification using a split-band amplifying apparatus, said method comprising:

a) providing a first section for amplifying a long wavelength band of an optical signal;

b) providing a second section having a fiber amplifier for amplifying a short wavelength band of said optical signal, said fiber amplifier being constructed by:

1) providing a core having a core cross-section and a refractive index n o ;

2) doping said active material into said core;

3) providing a depressed cladding around said core, said depressed cladding having a depressed cladding cross-section and a refractive index n 1 ;

4) providing a secondary cladding around said depressed cladding, said secondary cladding having a secondary cladding cross-section and a refractive index n 2 ;

5) selecting said core cross section, said depressed cladding cross-section, and said refractive indices n o , n 1 , and n 2 to produce a roll-off loss curve about a cutoff wavelength λ c , said roll-off loss curve yielding losses at least comparable to said high gains in said long wavelength band and losses substantially smaller than said positive gains in said short wavelength band.

13. The method of claim 12 , further comprising selecting Erbium as said active material such that said fiber amplifier is a first Erbium-doped fiber amplifier.

14. The method of claim 13 , further comprising counter-pumping said a first Erbium doped fiber amplifier.

15. The method of claim 12 , further comprising providing said first section with a second Erbium-doped fiber amplifier for amplifying said long wavelength band.

16. The method of claim 15 , further comprising selecting Erbium as said active material such that said fiber amplifier is a first Erbium-doped fiber amplifier.

17. The method of claim 16 , further comprising co-pumping said first Erbium-doped fiber amplifier and said second Erbium-doped fiber amplifier from a common pump source.

18. The method of claim 17 , wherein said common pump source delivers pump radiation at about 980 nm.

19. The method of claim 12 , wherein said short wavelength band comprises at least a portion of the S-band and said long wavelength band comprises at least a portion of the C-band, and said cutoff wavelength λ c is set at a crossover wavelength between said S-band and said C-band.

20. The method of claim 19 , wherein said long wavelength band further comprises at least a portion of the L-band.

21. The method of claim 19 , wherein said cutoff wavelength λ c is set at about 1530 nm.