Solid-state laser processing apparatus and laser welding process

A laser welding process, comprising:

• welding overlapped steel plates with a solid state laser processing apparatus, said apparatus comprising:
• a solid-state laser oscillator for oscillating a laser beam, said oscillator comprising an exciter, a partial reflection mirror, first and second lenses, and an aperture having a controlled diameter for controlling a beam diameter of the laser beam;
• an optical fiber for guiding the controlled laser beam;
• and
• a processing head comprising a transfer optical system for focusing the guided laser beam, and for irradiating the guided laser beam to a surface of a workpiece to be processed;
• the laser welding process comprising:
• controlling the beam diameter by controlling the controlled diameter of the aperture such that a beam quality M2 of the laser falls in a range of 85≦M2≦105,
• wherein the laser beam has a first incidence angle θ1 prior to entering the optical fiber, and a second incidence angle θ2 after exiting the optical fiber, wherein θ1=tan−1 (A/2L)=A/2L, and θ1≈θ2,
• wherein the beam quality M2 is defined as M2=π×D×(θ2/1.5)/(2×λ), with D being the diameter of the optical fiber,
• wherein λ is a wavelength of the laser beam,
• wherein A is a diameter of the aperture,
• wherein L is a distance from the aperture to the optical fiber,
• wherein the transfer optical system comprises two spherical lenses, and
• wherein the two spherical lenses are plane-convex lenses.