Patent ID: 7755991

Claim:
An optical information reproducing device, comprising: a light source for emitting light; a diffractive optical element for dividing light emitted from the light source into at least a main beam, a first sub-beam, and a second sub-beam; an objective lens for focusing the main beam, the first sub-beam, and the second sub-beam onto a same track of a read-only type optical recording medium; a photodetector for receiving light reflected by the read-only type optical recording medium, the photodetector including a first light receiving section for receiving the main beam in four divided portions, a second light receiving section for receiving the first sub-beam in four divided portions, and a third light receiving section for receiving the second sub-beam in four divided portions; circuitry for calculating a track error signal of the main beam using a differential phase detection method based on outputs from the first light receiving section, and for calculating a track error signal of the first and second sub-beams using the differential phase detection method based on outputs from the second and third light receiving sections, and for calculating a radial tilt signal based on the track error signal of the main beam and the track error signal of the first and second sub-beams; and correcting means for correcting a radial tilt of the optical recording medium; wherein the first light receiving section and the second and third light receiving sections output signals for calculating track error signals by the differential phase detection method; wherein the diffractive optical element is for generating the main beam and the first and second sub-beams with different light intensity distributions when making incidence to the objective lens; wherein the diffractive optical element outputs 0th-order light as the main beam and ±1st-order diffracted lights as the first and second sub-beams; wherein the circuitry is configured to calculate the track error signal of the main beam by calculating a phase difference between a sum of two outputs of the first light receiving section and a sum of two other outputs of the first light receiving section; and wherein the circuitry is configured to calculate the track error signal of the first and second sub-beams by calculating a first phase difference between a sum of two outputs of the second light receiving section and a sum of two other outputs of the second light receiving section, and by calculating a second phase difference between a sum of two outputs of the third light receiving section and a sum of two other outputs of the third light receiving section, and by calculating a sum of the first phase difference and the second phase difference.