A conventional laser machining apparatus is described with reference to Japanese Patent Laid-Open Application H8-2511. FIG. 9 illustrates a configuration of the conventional laser machining apparatus.
Laser beam 902A emitted from laser oscillator 901 maintains its parallelism by means of aspherical lenses 903 and 904. In addition, the Gaussian distribution of laser beam 902A with respect to its cross-section is converted to a uniform distribution. Uniform laser beam 902B is first horizontally focused by convex cylindrical lens 905, and then spread. Convex cylindrical lens 906, which has a longer focal length than that of lens 905, produces parallel laser beam 902C which is broadened more horizontally than laser beam 902B. Laser beam 902C enters light-focusing optical apparatus 908 via reflecting mirror 907. Laser beam 902C is then focused by each of plano-convex lenses 911 on light-focusing optical apparatus 908, and finally irradiates target workpiece 909 as several beam spots. Target workpiece 909 is moved using X-Y table 910 to apply necessary machining. The use of aspherical lenses 903 and 904 achieves uniformity of the intensity distribution of laser beam 902A, and allows laser beam 902A to be focused on the plano-convex lenses, and then beam 902A is irradiated on target workpiece 909 in multiple spots. This makes the laser energy density uniform on machining points 912, enabling uniform machining from the center to the periphery.
However, the above laser machining apparatus has the following disadvantage.
During laser machining, laser oscillation conditions are adjusted depending on the size and material of the target workpiece so as to achieve optimal machining conditions. In addition, in some cases, a pulse-oscillated laser beam is emitted to the same position on the target workpiece for several pulses. In this case, machining takes place while changing the laser oscillation conditions for every shot. A pointing vector of laser beam 902A emitted from laser oscillator 901 often changes as a result of changes in the oscillation conditions due to the thermal lens effect of an optical system inside a resonator. In particular, the pointing vector in an unstable resonator typically of a slab laser and a laser oscillator having many optical elements such as a wavelength-converting element inside or outside the resonator often actually changes when the oscillation conditions are changed. If the pointing vector changes due to variations in the oscillation conditions, as described above, a point of the laser beam entering lens 903 shifts. As a result, the uniformity of the intensity distribution of the laser beam exiting from lens 904 breaks down, causing variations in machining among multiple machining spots.