In recent years, in a field of materials processing with high-power lasers, attention is paid to processing by a laser apparatus using an optical fiber to which rare earth elements are doped. The laser emits a wavelength of 1 μm band, and an optical-fiber delivery is possible unlike CO2 laser used in the related art. Additionally, since this laser can be focused to a smaller spot than CO2 laser, it is suitable for high-speed processing or fine micro processing, such as cutting, welding, or the like. Regarding this laser apparatus, there are already a number of well-known techniques (for example, refer to Patent Document 1).
An example of a laser materials processing apparatus 100 of the related art is schematically shown in FIG. 13. The laser materials processing apparatus 100 is generally constituted by a laser apparatus 101, an optical fiber 122, and an irradiation optical system (a collimating lens 105 and a condenser lens 106). A laser beam 104, which is generated in the laser apparatus 101, is guided and transmitted through the optical fiber 122, and exits the emitting end 103, passes through the collimating lens 105 and the condenser lens 106, and reaches an irradiation point α on a workpiece 107.
A related-art example of the above laser apparatus 101 is shown in FIG. 14. The laser apparatus 101 is generally constituted by an active fiber 120 that is a double-clad fiber in which rare earth elements are doped to a parent material of a core, fiber Bragg gratings (hereinafter abbreviated as FBGs) 121 that are formed near both ends of this active fiber 120 to act as mirrors of laser resonators, a multi-coupler 123, a plurality of semiconductor laser beam sources 126, and an optical fiber 122 that is connected to the active fiber 120 via a connecting point 129.
The double-clad fiber is an optical fiber having double claddings around a core. In the related art, the double-clad fiber is used as a laser medium by doping rare earth elements or the like to the parent material of the core. Inner cladding has two kinds of actions. One action confines the light within the core. The other action confines pumping light that pumps the laser medium within the core. Outer cladding has an action that confines the pumping light.
The pumping light emitted from the plurality of semiconductor laser beam sources 126 is coupled into the inner cladding of the active fiber 120 via the optical fiber 125 and the multi-coupler 123. The laser beam generated in the core of the active fiber 120 is amplified while going back and forth between both the FBGs 121, and a portion thereof is taken out from one FBG 121 (FBG 121 on the right of the plane of the sheet). This laser beam is coupled into the optical fiber 122 located downstream of the connecting point 129, and is emitted to exterior space from an output end 103. In addition, a passive optical fiber for transmission with single-layer cladding is used as the optical fiber 122 located downstream of the connecting point 129. As described above, the laser apparatus of the configuration using an active fiber as the laser medium is referred to as a fiber laser.
The power density distribution at a cross-section of the optical fiber 122 may be a single mode close to a Gaussian shape or a multi-mode close to a top-hat shape.
A laser beam of an output of 2 kW or more is used for macro processing referred to as cutting of steel of a thickness of 1 mm or more. The single mode output in these region causes, the large optical loss due to nonlinear effects such as stimulated Brillouin scattering or Raman scattering within the core of the optical fiber. Therefore, the distance by which fiber transmission is allowed is limited to about several meters. Thus, in the laser materials processing apparatus, generally, a fiber laser of a multi-mode where a larger fiber transmission distance is secured is used.
The power density distribution of the laser beam at the irradiation point α is an image of the outlet (the emitting end 103 of the laser apparatus 101) of the optical fiber 122, formed by an irradiation optical system including the collimating lens 105 and the condenser lens 106. In the case of the fiber laser where the power density distribution in the optical fiber 122 being multi-mode, the power density distribution at the irradiation point α becomes a substantially uniform top-hat shape, as shown in FIG. 15, which is uniform and almost the same distribution within the optical fiber 122. In addition, in FIG. 15, X represents the distance from the center O of a laser beam at the irradiation point α, and I represents the power density of the laser beam.