Method and apparatus for providing light having a selected polarization with an optical fiber

2 2 Optical apparatus for providing light having a selected linear polarization having a polarization ratio, the apparatus comprising a length of optical fiber comprising a rare earth for providing light having a first wavelength responsive to receiving pump light having a second wavelength that is different than said first wavelength, wherein if the length of optical fiber were placed in a first position between the length of fiber is substantially linearly oriented the fiber could propagate at the first wavelength a fundamental mode and a plurality of higher order modes and the apparatus could provide light having a first polarization ratio for the selected linear polarization and an Mparameter, and wherein the length of fiber is positioned in a second position that increases the bend loss of the fiber relative to the first position such that, responsive to the increased bend loss, the apparatus can provide light having a reduced Mparameter as well as a second polarization ratio for the selected polarization that is increased relative to the first polarization ratio, the increase being at least 6 dB greater than the first polarization ratio, and wherein when the length of fiber is in the second position the apparatus can provide a slope efficiency that is at least 50% of the ratio of the second wavelength to the first wavelength.