Abstract: A shield tunnelling apparatus is provided that includes a first tubular shield body in which a cutter assembly for excavation and a drive mechanism for driving the cutter assembly are disposed; a second shield body connected to the rear of the first shield body so as to have a common axis therewith; a target disposed in the first shield body so as to receive a light ray directed along the axis; first optical path changing means disposed in the second shield body and capable of changing the optical path of the light ray directed toward the target such that the light ray travels along an optical path displaced away from the axis; and second optical path changing means disposed in the second shield body and capable of changing the optical path of the light ray passing through the first optical path changing means such that the light ray is again directed along the axis.

Abstract: The present invention is a laser system that includes an acousto-optic device (24B) having an optical medium (40B) that maintains a substantially uniform index of refraction even when subjected to nonuniform thermal or mechanical stress. The acousto-optic device provides substantially distortion-free polarized output light that can be directed accurately to strike an intended target.