| 摘要 |
PURPOSE:To make the light quantity un-change even when an objective lens moves in a wide range and to make the readout of signals from tracks of a wide extent possible by means of the operation of an actuator only by making the diameter of the luminous flux of laser beam larger than that of the objective lens. CONSTITUTION:The diameter of the luminous flux of the laser beam made incident on an object lens 1 is made larger than the diameter of the objective lens by 1.2-2 times. Therefore, the diameter of the luminous flux of the laser beam made incident on the lens 1 becomes larger than that of the lens 1 and, even when the lens 1 moves leftward and rightward, the light quantity does not change and a wide extent becomes accessible. The optimum value exists in the ratio between the diameters of the lens 1 and luminous flux and it is known from experiments that, when the diameter of the luminous flux is 1.2-2 times as large as that of the lens 1, excellent results are obtained. Therefore, the extent of the tracks which are accessible by means of the operation of an actuator 2 only is widened and the accessing time of a driving device can be shortened. |
