Patent ID: 8594476

Claim:
A semi-guiding optical fiber, comprising: a laser active core having a high aspect ratio elongated cross-section along a slow axis direction of the fiber, the laser active core having a first refractive index; first and second signal claddings positioned in contact with and sandwiching the laser active core, the first and second signal claddings having a second refractive index; a third cladding substantially surrounding at least side edges of the laser active core, the third cladding having a third refractive index greater than the first refractive index of the laser active core; wherein either the first refractive index of the laser active core, the third refractive index of the third cladding, or the second refractive index of the first and second signal claddings, or two or more thereof, are selected so as to maximize a differential loss between a lowest order mode propagating in the optical fiber and next order modes so as to discriminate in favor of the lowest order mode and against the next order modes; and wherein the gain within the laser active core has a spatial variation in the slow-axis direction such that the gain has a maximum value along the fiber axis, a minimum value that is substantially zero near the slow-axis core edges, and the gain is thereby mostly concentrated in the central half of the total slow-axis dimension.