Laser beam scanning optical apparatus having means for generating moire fringes

A laser beam scanning optical apparatus in which a laser beam emitted from a laser source is scanned on a scanning surface linearly at a substantially constant speed by a scanner and an optical element, said laser beam scanning optical apparatus comprising:

• focusing means for correcting a position of an image point of the laser beam;
• Moire fringes generating means which is located near a position optically equivalent to the scanning surface and modulates the laser beam to generate Moire fringes, the Moire fringes generating means comprising:
• a first filter which is located in an upstream side of an optical path of the laser beam, the first filter having spatial grating;
• and
• a second filter which is located in a downstream side of the optical path of the laser beam, the second filter having spatial grating which slants slightly with respect to the spatial grating of the first filter;
• a light receiving element which receives the Moire fringes generated by the Moire fringes generating means;
• operating means for operating an amount of a drive of the focusing means from an output of the light receiving element;
• and
• control means for driving the focusing means in accordance with an output of the operating means to correct the position of the image point.