| 摘要 |
PURPOSE:To saturate the requisite precision in positioning and to facilitate the positioning by lowering the light quantity ratio of each beam split by a prism to the entire luminous flux at the time of detecting a focus by the Foucault method and increasing the spot diameter of each beam received by each photodetector. CONSTITUTION:The prism 14 capable of splitting a luminous flux into at least three beams is used as the prism for detecting a focus by the Foucault method. Photodetectors 10-12 are provided to the respective split beams, two photodetectors 10 and 11 are used for detecting the focus, and the quantity of light incident on the photodetectors 10 and 11 is controlled respectively to <=40% of that of the entire luminous flux. Accordingly, the spot diameter of the beam received by each photodetector can be increased. The precision in positioning when the photodetectors are assembled and positioned is saturated, and the allowable value in setting the direction of the optical axis of the photo detector is increased as the depth of the focus of the converged beam increases. Consequently, the precision of parts is eased, and the positioning precision is eased when parallel rays are formed with a semiconductor laser and a cou pling lens. |
