Patent ID: 7463805

Claim:
An optical fiber, comprising: a rare earth doped core with a first refractive index (n 1 ); a silica based inner cladding abutting and at least substantially surrounding said core of said fiber, said inner cladding having a refractive index n 2 , wherein n 2 <n 1 ; a silica based outer cladding surrounding said inner cladding, the outer cladding having a fourth refractive index (n 4 ), said outer cladding and said inner cladding defining a space having the refractive index n 3 of about 1, so n 3 <n 2 and n 3 <n 4 and said space substantially encloses said inner cladding; and at least one support structure situated between said outer cladding and said inner cladding of said fiber, wherein said support structure of said fiber is either hollow or gas filed and is not connected to any other support structure of said fiber.