Laser beam scanning optical apparatus having means for generating moire

Abstract

A laser beam scanning optical apparatus which has Moire fringes generating elements which is located near a position optically equivalent to a scanning surface and modulates a laser beam emitted from a laser source to generate Moire fringes. The Moire fringes generating elements comprises, for example, a first filter which has spatial grating and a second filter which has spatial grating which slants slightly with respect to the spatial grating of the first filter, the first filter and the second filter being arranged upstream and downstream respectively in an optical path. The laser beam scanning optical apparatus further has a light receiving element for receiving the Moire fringes generated by the Moire fringes generating elements. Focusing means for correcting a position of an image point of the laser beam is driven in accordance with an output of the light receiving element. For example, the light receiving element is a photoelectric element with a plurality of light receiving surfaces, each of which generates an electric signal in accordance with the laser beam incident thereto, and from a phase difference between the electric signals, the position of the image point is detected.