Patent ID: 8493830

Claim:
An information reproducing apparatus comprising: a laser source; a splitting unit to split a luminous flux emitted from the laser source into a first luminous flux as a signal light and a second luminous flux as a reference light, the second luminous flux being not collected on an optical information recording medium; a detector to detect an interfering light of the signal light and the reference light; a unit to make the signal light optically interfere with the reference light on the detector in a state in which a phase relationship between the signal light and the reference light is different from each other; a defocusing unit to defocus one of the first luminous flux and the second luminous flux in front or behind with respect to other of the first luminous flux and the second luminous flux focused on a predetermined layer of the optical information recording medium; and a unit to find a focus error signal from a difference signal between interfering lights defocused in front and behind.