A high-power laser system, comprising: a laser source outputting a single mode, SM, or low order mode kW-power light and having at least one fiber gain block, GB, which has an active fiber and an output SM passive fiber, wherein the laser source is configured to output single mode or low order mode light in a power range varying between 1 kW and 50 kW; and a passive delivery fiber (20, 22) spliced to the output SM passive fiber of the laser source and configured to have a double bottleneck-shaped core (24, 26) that is monolithic and includes: an input end core section receiving the kW power light from the laser source and an output end core section, the input and output end core sections each having a uniform diameter which is dimensioned to support a single mode; a uniformly dimensioned central core section located between the input and output end core sections and having a central core section diameter greater than each of the input and output end core section diameters, the central core section being dimensioned to support multiple modes; and input and output tapered core sections extending between respective opposite ends of the central core section and the respective input and output end core sections, wherein the passive delivery fiber (20, 22) is configured for increasing a threshold of nonlinear effects so that the passive delivery fiber (20, 22) has a fiber length at least twice the length of a standard passive delivery fiber with a uniformly dimensioned core, such that the passive delivery fiber has a length varying between about 2 m (meters) and about 20 m (meters), and wherein the high-power laser system is configured to emit the kW-power light having a beam quality factor M 2 less than 2 from the passive delivery fiber. a laser source outputting a single mode, SM, or low order mode kW-power light and having at least one fiber gain block, GB, which has an active fiber and an output SM passive fiber, wherein the laser source is configured to output single mode or low order mode light in a power range varying between 1 kW and 50 kW; and a passive delivery fiber (20, 22) spliced to the output SM passive fiber of the laser source and configured to have a double bottleneck-shaped core (24, 26) that is monolithic and includes: an input end core section receiving the kW power light from the laser source and an output end core section, the input and output end core sections each having a uniform diameter which is dimensioned to support a single mode; a uniformly dimensioned central core section located between the input and output end core sections and having a central core section diameter greater than each of the input and output end core section diameters, the central core section being dimensioned to support multiple modes; and input and output tapered core sections extending between respective opposite ends of the central core section and the respective input and output end core sections, wherein the passive delivery fiber (20, 22) is configured for increasing a threshold of nonlinear effects so that the passive delivery fiber (20, 22) has a fiber length at least twice the length of a standard passive delivery fiber with a uniformly dimensioned core, such that the passive delivery fiber has a length varying between about 2 m (meters) and about 20 m (meters), and wherein the high-power laser system is configured to emit the kW-power light having a beam quality factor M 2 less than 2 from the passive delivery fiber. an input end core section receiving the kW power light from the laser source and an output end core section, the input and output end core sections each having a uniform diameter which is dimensioned to support a single mode; a uniformly dimensioned central core section located between the input and output end core sections and having a central core section diameter greater than each of the input and output end core section diameters, the central core section being dimensioned to support multiple modes; and input and output tapered core sections extending between respective opposite ends of the central core section and the respective input and output end core sections, wherein the passive delivery fiber (20, 22) is configured for increasing a threshold of nonlinear effects so that the passive delivery fiber (20, 22) has a fiber length at least twice the length of a standard passive delivery fiber with a uniformly dimensioned core, such that the passive delivery fiber has a length varying between about 2 m (meters) and about 20 m (meters), and wherein the high-power laser system is configured to emit the kW-power light having a beam quality factor M 2 less than 2 from the passive delivery fiber.