Optical information recording and reproducing apparatus and optical information recording and reproducing method

An optical information recording and reproducing apparatus which branches a light beam into reference light and signal light to cause interference and records an obtained interference fringe as a hologram on an optical information recording medium and also reproduces the recorded hologram by applying reference light onto the optical information recording medium, the apparatus comprising:

• a pickup having a light source which emits the light beam, a branching element which branches the light beam into the reference light and the signal light, a spatial light modulator for adding information to the signal light, an aperture through the signal light added with the information, an objective lens which applies the signal light passing through the aperture onto the optical information recording medium, an optical system which applies the reference light onto the optical information recording medium, an image pickup element which applies the reference light onto a hologram of a reproduction target formed in the optical information recording medium and detects reproduction light passing through the aperture in the obtained reproduction light and generate a reproduced signal, and a photo detector different from the image pickup element, the photo detector which detects the reproduction light applied on periphery of the aperture in the reproduction light;
• and
• a computing unit which computes a position error signal indicating a deviation in position of the optical information recording medium with respect to the objective lens, based on an output from the photo detector different from the image pickup element.