Light irradiation device and control method

A control method for a light irradiation device including

• 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 said signal light and/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 the 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 said hologram recording medium surface is smaller than distance from said surface to the lower layer side face of said recording layer, and also the focal position of light to be irradiated via said objective lens is constant at said ideal focal position, and
• 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;
• said method comprising the steps of:
• receiving of said marker light via said hologram recording medium;
• and
• controlling of said objective-lens/relay-lens distance adjusting unit based on error between the ideal light reception position of said marker light, and the actual light reception position of said marker light, in said receiving;
• wherein control is executed to set the distance between said objective lens and said relay lens to be constant.