LASER BEAM INCIDENCE OPTICAL SYSTEM

PURPOSE:To improve incidence efficiency by using at least three lenses and moving them automatically in an optical-axis direction while holding their relative positions and focal lengths in specific relations. CONSTITUTION:At least three lenses are used and the lens L3 which is arranged most closely to the side of a hollow waveguide 1 among them is fixed its focal length f3 closer to the side of a laser light source 2 than the incidence end of the hollow waveguide 1. The lens L2 which is arranged closer to the side of the laser light source 2 than the lens L3 and the lens L1 is arranged closer to the side of the laser light source 2 than the lens L2 are moved automatically in the optical axis direction according to variations in the beam waist of a laser beam incident on the lens L1 and the distance l1 to the lens L1 so that equations I and II hold, where l12 is the distance between the lenses L1 and L2, l23 is the distance between the lens L2 and L3, and f1 and f2 are the focal lengths of the lenses L1 and L2. Consequently, the beam diameter of the incident beam is converted optionally over a wide range and the incidence efficiency is improved.