| 摘要 |
PURPOSE:To detect defocusing on an information recording surface with high sensitivity by causing the light beam of returning light to strike a quartered light detector by splitting. CONSTITUTION:Returning light reflected by information recording surface 1 is separated by half mirror 3 and halved by combined prism 4 differing in direction in the center of the beam; and the both are caused to strike convex lens 5 and further to strike quartered light detector 6 placed at the focal position of lens 5. Here, dotted lines show light polarized by prism 4 in the different direction as shown by solid lines. In this constitution, in case that the beam is completely focused on recording surface 1, no focusing control signal is generated. When recording surface 1 is displaced in the arrow (a) direction, spots of light in shapes shown by slanting lines are formed on detector 6. In this case, a positive focusing control signal is generated. When it is displaced in the opposite direction, a negative control signal is generated. Thus, the beam of returning light is split and then caused to strike detector 6, so that the positive or negative focusing control signal according to defocusing can be obtained. |
