A laser amplifier comprising: - a laser gain medium having an active core with a high aspect ratio cross-section, the core including top and bottom wide surface interfaces, and first and second narrow edge interfaces (44, 46), the core adapted to receive and amplify a laser input signal; - waveguiding means for mode control of the laser input signal along a fast-axis direction of the core, the wave guiding means including signal claddings (36, 38) disposed in contact with the top and bottom wide surface interfaces (40, 42) of the core, the fast-axis direction being perpendicular to the top and bottom wide surface interfaces of the core; and - a material (50, 52) adjacent to the first and second narrow edge interfaces (44, 46) of the core, the material (36, 38) having a refractive index n MIM unequal to a refractive index n core of the core and unequal to the refractive index n sc of the signal claddings, the refractive index of the material substantially matching an effective mode index of a particular mode within the core to minimize reflections of this effective index mode at the narrow edge interfaces (44, 46), and the refractive index n MIM of the material being: ∘ smaller than the refractive index of the core n core, ∘ higher than the refractive index n sc of the signal claddings, to increase leakage loss and thereby discriminate against higher order modes in favor of a fundamental mode of the core, a slow-axis direction being perpendicular to the first and second narrow edge interfaces of the core. - a laser gain medium having an active core with a high aspect ratio cross-section, the core including top and bottom wide surface interfaces, and first and second narrow edge interfaces (44, 46), the core adapted to receive and amplify a laser input signal; - waveguiding means for mode control of the laser input signal along a fast-axis direction of the core, the wave guiding means including signal claddings (36, 38) disposed in contact with the top and bottom wide surface interfaces (40, 42) of the core, the fast-axis direction being perpendicular to the top and bottom wide surface interfaces of the core; and - a material (50, 52) adjacent to the first and second narrow edge interfaces (44, 46) of the core, the material (36, 38) having a refractive index n MIM unequal to a refractive index n core of the core and unequal to the refractive index n sc of the signal claddings, the refractive index of the material substantially matching an effective mode index of a particular mode within the core to minimize reflections of this effective index mode at the narrow edge interfaces (44, 46), and the refractive index n MIM of the material being: ∘ smaller than the refractive index of the core n core, ∘ higher than the refractive index n sc of the signal claddings, to increase leakage loss and thereby discriminate against higher order modes in favor of a fundamental mode of the core, a slow-axis direction being perpendicular to the first and second narrow edge interfaces of the core. ∘ smaller than the refractive index of the core n core, ∘ higher than the refractive index n sc of the signal claddings, to increase leakage loss and thereby discriminate against higher order modes in favor of a fundamental mode of the core, a slow-axis direction being perpendicular to the first and second narrow edge interfaces of the core.