Patent ID: 6917576

Claim:
An optical head device comprising: a semiconductor laser light source for emitting laser lights having two or more different wavelengths, an objective lens for converging the laser lights emitted from the semiconductor laser light source, an optical recording medium to which the laser lights are converged and introduced, a photodetector for receiving reflected light from the optical recording medium, and a broadband retarder located in an optical path from the laser light source to the optical recording medium, or an optical path from the optical recording medium to the photodetector to control a state of phase of the laser lights, the broadband retarder having such structure that a phase plate A to which any one of the laser lights is first incident and a phase plate B to which any one laser light is secondly incident are piled up so that respective fast axes of optical axes of the phase plates A and B are crossed wherein the ratio of the retardation value of the phase plate A to the retardation value of the phase plate B is 1.8-2.2.