| 摘要 |
A monitoring optical system of the present invention is used in a scanning optical device in which a deflected laser beam by a polygonal mirror is separated from an incident laser beam in an auxiliary scanning direction, and the deflected laser beam is converged onto a drawing surface through a curved surface mirror and a drawing anamorphic lens, which corrects an effect of skew of the laser beam. The monitoring optical system includes a monitoring cylindrical lens for converging a monitor beam separated from the deflected laser beam without passing the drawing anamorphic lens, and a monitoring sensor for detecting the monitor beam converged by the monitoring cylindrical lens to generate synchronizing signal. A main meridian that indicates a main refractive power of the monitoring cylindrical lens is inclined with respect to a z-direction on a plan perpendicular to the monitor beam so that wavefront aberration on the monitoring sensor is minimized. The z-direction is perpendicular to a scanning direction of the monitor beam on the plane.
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