| 摘要 |
PURPOSE:To obtain an optical system having focus detection function by providing a positive single lens, for which the condition of a diffraction factor (n)>=1.5 and prescribed condition are satisfied while being inclined to an optical axis in the luminous flux of reflected light from an optical disk to detecting/light receiving part. CONSTITUTION:The reflected light from an optical disk 1 of a semiconductor laser 8 is received through a beam splitter 10 to an optical part 3 for detection. A positive single lens 7 is arranged by inclining it to the optical axis 11 of the light receiving luminous flux. The diffraction factor (n) is selected to be (n)>=1.5 and astigmatism required for focus detection is generated while suppressing the generation of coma abberation to be generated by the inclination. Further, in order to improve the performance of a lens 7, when a paraxial curvature radius is defined as r1 in a side distance from the light receiving part and as r2 in a side close to the light receiving part, the radius is selected so as to establish the condition of -3<r1/r2<3. Further, in order to obtain a satisfactory spot, the condition of -1<r2/r1<1 is preferable. When a peripheral lens not to use a non-spherical surface or a plane concave lens is used, the satisfactory performance can be obtained by selecting the diffraction factor to satisfy the condition of (n)>1.7. |
