Single step-index fibers with large cores— typically 400 um to 1 mm— can be used to guide a laser beam from the laser to a target or industrial workpiece. The large diameter serves two purposes. First, it can collect power from the laser more efficiently than a smaller core fiber. In addition, it spreads the laser power over a larger area at the ends of the fiber and through a larger volume within the fiber. This is important because some laser power inevitably is lost at the surfaces and within the fiber. If the beam must be focused tightly to concentrate it in the fiber, the power density (power per unit area) may reach levels so high it can damage exposed ends of the fiber.