| 摘要 |
<p>PROBLEM TO BE SOLVED: To detect the displacement amount and the direction of a focal position with high accuracy by outputting a focal position signal expressing the relationship between the focal position and the prescribed focal position of a light beam according to difference in light beam diameter. SOLUTION: In a photodetector 10 to which a light beam at a starting end or a trailing end in a main scanning direction is inputted, the light beam is photoelectrically transduced 10a , 10b , and a signal according to quantity of incident light is output to signal processing circuits 12a , 12b . The circuits 12a , 12b process an input signal, and beam diameter signals Va , Vb are output. At this time, when the light beam has a beam diameter (a), the signals Va , Vb are in relationships of Va <V1 and Vb <V2 with reference to a reference beam diameter lower-limit value V1 and a reference beam diameter upper-limit signal V2 , and comparators 13a , 13b output comparison signals 0, 0. This expresses a far focal state that the focal position of the light beam is situated on the side opposite to a light source with reference to a reference focal position. A correction signal is output to a drive circuit 15, and a lens 2 is displaced to the direction of a laser 1. In the case of a beam diameter (b), the lens is not driven in the focused state of comparison signals 1, 0. In the case of a beam diameter (c), the lens 2 is displaced in a direction in which it is separated from the laser 1 in the near focal state of comparison signals 1, 1.</p> |
