Abstract: A single core 35, 50 or a plurality of cores arranged in rings 21, 28, 29 around a central core 20, 27, or in an array 42, are provided with either or both of (a) a modal discrimination characteristic, including gain, index of refraction and cross sectional area, which is greatest in the center of the core or the array, and lowers outwardly therefrom, and (b) an oblong cross section, thereby to provide either or both of (c) a bright laser beam of the fundamental in-phase supermode, and/or (d) a linearly polarized output beam.

Abstract: Efficient delivery of large amounts of pump laser power distributed along the cladding (11, 54) of a single core (6) or multiple core (7, 51) laser fiber, without degrading the fiber integrity or compromising the fiber's waveguiding property, is accomplished by injecting the power into the cladding via delivery fibers (18, 30, 42, 62) permanently affixed to a peripheral wall (20, 56) at an angle that satisfies the condition for total internal reflection of the pump radiation so that it is confined within the inner cladding of the laser fiber. In one embodiment, the laser fiber (51) is wrapped around a drum (53). Each delivery fiber has a numerical aperture (NA) less than half the NA of the laser fiber, and a core 21 having a refractive index substantially the same as that of the inner cladding (11) of the laser fiber.

Abstract: A fiber laser 10 with square inner cladding 12, 29 may have a single core 11 codoped either with Ytterbium or Erbium or with Thulium and Holmium at a ratio of at least 10:1 operating in a single mode to provide eye-safe radiation with wavelengths above 1.5 micron. The single core laser has a pump clad cross sectional area about 2(10).sup.3 greater than the cross sectional area of the core. A multi-core laser has a plurality of single mode cores 28 doped with any rare earth ions, the cores equally spaced by at least two core diameters in an isometric array, in a cavity having a finesse of greater than ten, to produce a single, very bright phase-locked beam in the fundamental supermode. A method starts with hexagonal cladded-core rods 35, 36 in an isometric array, which are then fused and drawn down.