| 摘要 |
PROBLEM TO BE SOLVED: To reduce the size of the optical system, which employs two semiconductor lasers emitting light beams having different wavelengths, by providing the hologram optical element, which eliminates the coma aberration and the spherical aberration of the light beams that pass a recording surface and an objective lens, and generates an astigmatic aberration having a prescribed amount. SOLUTION: The light beams form semiconductor lasers LD1 and LD2 pass an objective lens 4 and optical spots are formed on the recording surfaces of an optical disk. The beams returned from the spots are converged by the lens 4 and guided to a quadrisected light receiving surface PD1 of a photodetector by a common optical system. A diffraction relief 50a of a hologram optical element 50 passes the light beams from the lasers LD1 and LD2 with no other operation but the diffraction operation, eliminates the coma and the spherical aberrations against the returned light beams and generates the astigmatic aberration having a proscribed amount. The common element 50 is used for the lasers LD1 and LD2 to form the optical path and the optical system is made simpler. |
