NOVELTY - The fiber has high refractive index core glass (1) made from silicon dioxide type glass, for containing rare earth elements in center portion of fiber structure. The high refractive index core glass is provided with low refractive index clad (2a), so that outer periphery of core glass does not contain rare earth elements. A gap (3a) is formed in greater portion of outer periphery of core glass. The additives are added to outer periphery of core glass. The outer periphery of silicon dioxide glass (4a,5a) does not contain additives.    USE - Rare-earth dope fiber for fiber type amplifier for fiber laser (all claimed) such as multi-mode semiconductor fiber laser and single mode fiber laser used in laser microfabrication process of hole and groove, scribing process of silicon and ceramic materials, marking process of surfaces of semiconductor, metal and resin materials, removal process of tin dioxide film of thin film solar cell, surface-modification process, processing operation of transparent material, welding process of conductive material such as metal foil, copper, silver, and gold materials or high reflection rate material, repairing process of wiring, medical laser application for performing surgery using laser scalpel, removal process of thin film, cutting process of hide loam tube for motor vehicle, brazing process of roof line of motor vehicle, welding process of industrial nail, heat processing operation, welding operation of medical nail, welding process of titanium panel used in aerospace application, depth welding process in shipbuilding industry, and steel and aluminum industries, and cutting process of cut end of stainless-steel thick board.    ADVANTAGE - The rare earth elements are easily added to the fiber structure, so that the efficiency of the fiber amplifier and fiber laser is improved. The relative refraction-index difference of the transmission line of the excitation light excited in core glass and clad portion, can be significantly enlarged according to the gap formed in greater portion of outer periphery of core glass. The excitation light can be efficiently sealed in fiber, and can be efficiently transmitted. The optimization of the excitation efficiency of the rare-earth ion by excitation light can be easily performed. The energy conversion efficiency from excitation light to signal optical amplification can be improved. The high output laser beam can be easily emitted from the emission end side of the fiber. The rare-earth dope fiber is easily manufactured.    DETAILED DESCRIPTION - The rare earth elements are made from erbium, ytterbium, neodymium, europium, praseodymium, thulium, holmium, lanthanum, samarium and cerium materials. The core glass is made from germanium dioxide, phosphorus pentoxide, titanium dioxide, aluminum oxide, barium oxide, diboron trioxide and zirconium dioxide materials. INDEPENDENT CLAIMS are included for the following:    (1) a fiber type amplifier; and    (2) a fiber laser.    DESCRIPTION OF DRAWING(S) - The drawing shows the cross-sectional view of the rare-earth dope fiber, and schematic view of the near-field pattern of the fiber.    High refractive index core glass (1)    Low refractive index clad (2a)    Gap (3a)    Silicon dioxide glasses (4a,5a)    Four places at outer periphery of clad (6-1-6-4)