Fiber amplifiers with depressed cladding and their uses in Er-doped fiber amplifiers for the S-band

A method for designing a fiber amplifier using an active material pumped to a high relative inversion D, said active material exhibiting positive gains in a short wavelength band and high gains in a long wavelength band, said method comprising:

• a) providing a core having a core cross-section and a refractive index no;
• b) doping said active material into said core;
• c) providing a depressed cladding around said core, said depressed cladding having a depressed cladding cross-section and a refractive index n1;
• d) providing a secondary cladding around said depressed cladding, said secondary cladding having a secondary cladding cross-section and a refractive index n2;
• e) selecting said core cross section, said depressed cladding cross-section, and said refractive indices no, n1, and n2 to produce a roll-off loss curve about a cutoff wavelength λc, 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.

A method for pumping a fiber amplifier in a W-profile fiber, said method comprising:

• a) providing a core having a core cross-section and a refractive index no;
• b) doping an active material into said core;
• c) providing a depressed cladding around said core, said depressed cladding having a depressed cladding cross-section and a refractive index n1;
• d) providing a secondary cladding around said depressed cladding, said secondary cladding having a secondary cladding cross-section and a refractive index n2;
• e) selecting said core cross section, said depressed cladding cross-section, and said refractive indices no, n1, and n2 to produce a roll-off loss curve about a cutoff wavelength λc;
• and
• f) pumping said active to a relative inversion D≧0.7, such that said active material exhibits positive gains in a short wavelength band and high gains in a long wavelength band.

A fiber amplifier comprising:

• a) an erbium-doped region;
• b) a mechanism for providing a distributed loss by engineering an index profile in said fiber;
• and
• c) a pump source producing a high inversion, wherein the gain at a wavelength below 1525 nm exceeds the distributed lossat said wavelength below 1525 nm by at least 5 dB, and wherein the distributed loss in a wavelength band longer than 1525 nm exceeds the gain in said wavelength band longer than 1525 nm.

A fiber amplifier comprising:

• a) an erbium-doped region;
• b) a mechanism for providing a distributed loss;
• and
• c) a pump source producing a high inversion, wherein the gain at a wavelength below 1525 nm exceeds the distributed loss at said wavelength below 1525 nm by at least 5 dB, and wherein the distributed loss in a wavelength band longer than 1525 nm exceeds the gain in said wavelength band longer than 1525 nm.