Fiber preform with weld volumes welding inner preform tubes and an outer preform cladding together, and method of making the same using laser preheating and laser welding steps

A fiber preform comprising:

• a first end, a second end, a longitudinal axis extending from the first end to the second end;
• an outer preform cladding through which the longitudinal axis extends, the outer preform cladding comprising an inner surface disposed radially around the longitudinal axis;
• inner preform tubes welded to the outer preform cladding at the inner surface of the outer preform cladding, wherein each of the inner preform tubes comprises (i) a tube axis extending therethrough parallel to the longitudinal axis of the fiber preform, (ii) an inner surface extending radially around the tube axis, and (iii) an outer surface facing away from the tube axis, and wherein the inner preform tubes collectively define a preform core that extends radially from the longitudinal axis and that is tangential to the outer surfaces of the inner preform tubes;
• and
• first weld volumes, each of the first weld volumes welding a different one of the inner preform tubes to the outer preform cladding proximate the first end of the fiber preform;
• wherein, each of the first weld volumes has a vortex or conical shape.