| 摘要 |
PROBLEM TO BE SOLVED: To reduce a coma aberration on an information recording medium by setting within a specified range a ratio of a tertiary coma aberration component of the wave front aberration generating when an objective lens is inclined a unit angle to the tertiary coma aberration component of the wave front aberration generating when a parallel flat plate is inclined the unit angle, in the objective lens forming a light spot on the information recording medium consisting of the parallel flat plate. SOLUTION: A condensing spot is formed on an optical disk 4 consisting of a parallel flat plate through an objective lens 3 by a light beam 100 emitted from a semiconductor laser 1. The objective lens 3 is a single lens having an aspheric surface and is formed so as to satisfy the relation of 0.5<CL/CD<1. Where CL expresses a least square error of a tertiary coma aberration component of a wave front aberration generating when the objective lens is inclined one degree and is set to e.g. 60mλ using a wavelength λ as reference of unit, and CD expresses the least square error of the tertiary coma aberration component of the wave front aberration generating when the parallel flat plate is inclined a unit angle and is set to e.g. 75mλ. |
