Rare earth element-doped multiple-core optical fiber and optical systems using the same field of the invention

A rare earth element-doped multiple-core optical fiber, comprising:

• a plurality of cores each having a refractive index of nW doped with at least one rare earth element and Al therein;
• a plurality of primary cladding layers each having a refractive index of nP (nP<nW), and covering each of said cores;
• and
• an outer cladding layer having a refractive index of nC (nW>nC and nP.ltoreq.nC), and covering said plurality of cores each covered by said primary cladding layer;
• wherein said cores are positioned substantially on a central axis of said outer cladding layer, at least one of which is surrounded by remaining cores having a larger core diameter than said surrounded core.

A rare earth element-doped multiple-core optical fiber, comprising:

• a plurality of cores each having a refractive index of nW doped with at least one rare earth element and Al therein;
• a primary cladding layer having a refractive index of nP (nP<nW), and covering said plurality of cores;
• and
• an outer cladding layer having a refractive index of nC (nW>nC and nP.ltoreq.nC), and covering said primary cladding layer;
• wherein said cores are positioned substantially on a central axis of said outer cladding layer, at least one of which is surrounded by remaining cores having a larger core diameter than said surrounded core.

A rare earth element-doped multiple-core optical fiber, comprising:

• a plurality of cores each doped with at least one rare earth element therein;
• and
• an outer cladding layer having a lower refractive index than said cores, and covering said cores;
• wherein said cores are positioned substantially on a central axis of said outer cladding layer, at least one of which is surrounded by remaining cores having a higher refractive index than said surrounded core.

A rare earth element-doped multiple-core optical fiber, comprising:

• a plurality of cores each doped with at least one rare earth element and Al therein;
• a plurality of primary cladding layers each having a lower refractive index than said cores, and covering each of said cores;
• and
• an outer cladding layer having a lower refractive index than said primary cladding layers or the same refractive index as said primary cladding layers, and coveting said plurality of cores each covered by said primary cladding layer;
• wherein said cores are positioned substantially on a central axis of said outer cladding layer, at least one of which is surrounded by remaining cores having a higher refractive index than said surrounded core.

A rare earth element-doped multiple-core optical fiber, comprising:

• a plurality of cores each doped with at least one rare earth element therein;
• and
• an outer cladding layer having a lower refractive index than said cores, and covering said cores;
• wherein said cores are positioned substantially on a central axis of said outer cladding layer, at least one of which is surrounded by remaining cores containing a larger amount of rare earth element than said surrounded core.

A rare earth element-doped multiple-core optical fiber, comprising:

• a plurality of cores each doped with at least one rare earth element and Al therein;
• a plurality of primary cladding layers each having a lower refractive index than said cores, and covering each of said cores;
• and
• an outer cladding layer having a lower refractive index than said primary cladding layers or the same refractive index as said primary cladding layers, and covering said plurality of cores each covered by said primary cladding layer;
• wherein said cores are positioned substantially on a central axis of said outer cladding layer, at least one of which is surrounded by remaining cores containing a larger amount of rare earth element than said surrounded core.

A rare earth element-doped multiple-core optical fiber, comprising:

• a plurality of cores each having a first refractive index doped with at least one rare earth element;
• and
• a cladding layer having a second refractive index lower than said first refractive index for surrounding said plurality of cores, said plurality of cores being positioned in a central region of said cladding layer;
• wherein said plurality of cores comprises a first group of one or more cores, and a second group of cores, such that said second group of cores surrounds a periphery of said first group of one or more cores to equalize powers propagated through each core of said first and second groups.