| 摘要 |
PURPOSE:To attain quick and assured pulling-in to the focus servo by providing a diaphragm mechanism on a laser optical axis reaching a condenser lens from a semiconductor laser to stop down the laser light flux and therefore increasing the depth of focus. CONSTITUTION:The laser light sent from a semiconductor laser 7 is condensed on a recording surface 3-1 through a collimator lens 8, a beam splitter 9, a 1/4 wavelength plate 10 and an objective lens 11. In a start mode a part of the parallel light 20 is cut off by an diaphragm surface 30-1, and the light 20 is irradiated to the lens 11 in the form of a spot diameter d1. Then the diaphragm is opened after it is pulled in the focus servo, and the parallel light is mase incident to the lens 11 as it is. The depth M of focus is adversely proportional to the square of the luminous flux diameter Dc of the parallel light made incident to the lens 11. Therefore the depth M of the lens 11 is increased up to 4 times as much as the depth obtained with no use of a diaphragm by stopping the parallel light down to a Dc/2 diameter, for example. |
