Patent ID: 8139912

Claim:
Optical fiber apparatus having a wavelength of operation at which said optical fiber apparatus propagates optical energy, comprising: an optical fiber, said optical fiber including a core; a pump cladding disposed about said core, said pump cladding for receiving pump optical energy having a pump wavelength; a second cladding disposed about said pump cladding for tending to confine pump optical energy to said pump cladding for propagation thereby; said core comprising a rare earth material for providing optical energy having the wavelength of operation responsive to said optical fiber receiving the pump optical energy having the pump wavelength; at least one ring core spaced from said core, said ring core defined by inner and outer diameters and comprising the cross sectional area therebetween, said ring core further comprising an absorbing material for absorbing optical energy having the wavelength of operation; wherein said optical fiber is configured and arranged such that at said wavelength of operation said optical fiber comprises a fundamental mode that is primarily a mode of said core and at least one higher order mode (HOM) that is a mixed mode of a selected mode of said core and of selected mode of said ring core, said selected modes being of the same azimuthal order; and wherein said mixed mode is suppressed relative to said fundamental mode.