Patent ID: 6891675

Claim:
An optical device for using a laser light having a predetermined wavelength to read information from an information recording medium, comprising: a laser light source for generating said laser light; a light receiving element substrate provided with a plurality of light receiving areas in the same plane; a 3-beam generating diffraction grating for branching the laser light from said laser light source to three emitted lights; and a holographic optical element, divided into at least a first area and a second area in the same plane, for branching a reflected light from said information recording medium and turning the light to said light receiving element substrate, wherein diffraction axes of said first area and the second area are formed in such a manner that a diffraction axis direction of ±1st order diffracted lights by the first area and the diffraction axis direction of the ±1st order diffracted lights by the second area form predetermined angles in opposite directions with respect to a radial axis crossing at right angles to a track of said information recording medium, and said first and second areas of said holographic optical element, and the 3-beam generating diffraction grating are constituted in such a manner that when one of the ±1st order diffracted lights branched by said 3-beam generating diffraction grating is a first side beam, and the other is a second side beam, the diffracted light of said first side beam by said first area is overlapped with the diffracted light of said second side beam by said second area on said light receiving element substrate, and a focus error signal is obtained from areas on which diffracted lights both diffracted by one of said first and second areas are not overlapped with each other, on said light receiving substrate.