Patent ID: 8432591

Claim:
A light irradiation device comprising: a focus servo control unit, which includes a light source configured to irradiate light on a hologram recording medium having a recording layer where information is recorded with an interference pattern between signal light and reference light, a spatial light modulating unit configured to subject light from said light source to spatial light modulation to execute generation of at least said signal light or said reference light, and generation marker light at a position away from an optical axis within an input surface of the light from said light source, and a light irradiating unit configured to irradiate light subjected to spatial light modulation by said spatial light modulating unit on said hologram recording medium via a relay lens system and an objective lens, said focus servo control unit being configured to execute focus servo control so that an ideal focal position of light to be irradiated via said objective lens is set so as to satisfy a condition that distance between said ideal focal position and a hologram recording medium surface is smaller than distance from said hologram recording medium surface to a lower layer side face of said recording layer, and also a focal position of light to be irradiated via said objective lens is constant at said ideal focal position; an objective-lens/relay-lens distance adjusting unit configured to adjust distance between said objective lens and a relay lens disposed closer to said objective lens of relay lenses making up said relay lens system on the optical axis; a light reception unit configured to receive said marker light via said hologram recording medium; and a constant distance control unit configured to control said objective-lens/relay-lens distance adjusting unit based on error between an ideal light reception position of said marker light, and an actual light reception position of said marker light by said light reception unit.