Information recording and retrieval method, and its apparatus

An information recording and retrieval method for a multi-layer optical disk including a plurality of recording layers and a single servo layer which layers together form a set, servo areas for detecting a track error signal and a focus error signal, which areas are formed discretely along a track in the servo layer, non-recording areas which are formed discretely in the recording layer, and in the optical disk, the non-recording areas in the recording layer and the servo areas in the servo layer being arranged so as to overlap with each other when viewed from the light incident side, the method comprising the steps of:

• making a first laser beam and a second laser beam enter the multi-layer optical disk along the optical axis of an objective lens, and making the resultant beams converge respectively in positions on the optical axis, the positions being different from each other;
• detecting the first laser beam which interacts with the servo layer;
• generating a sampling logic signal from the first laser beam detection signal;
• turning on alternately the first laser beam and the second laser beam in accordance with the sampling logic signal;
• deriving a track error signal and a focus error signal for the servo layer while the first laser beam is turned on;
• controlling the relative position of an objective lens to the multi-layer optical disk by use of the track error signal and the focus error signal, and making the spot of the first laser beam follow a desired track in the servo layer;
• detecting the second laser beam while the second laser beam is turned on, the second laser beam interacting with one of the plurality of recording layers, and deriving a focus error signal of the second laser beam for the recording layer;
• and
• focusing the spot of the second laser beam on the recording layer by use of a focus error signal for the second laser beam.

An information recording and retrieval apparatus for a multi-layer optical disk including a plurality of recording layers and a single servo layer which layers together form a set, servo areas for detecting a track error signal and a focus error signal, which areas are formed discretely along a track in the servo layer, non-recording areas which are formed discretely in the recording layer, and in the optical disk, the non-recording areas in the recording layer and the servo areas in the servo layer being arranged so as to overlap with each other when viewed from the light incident side, the apparatus comprising:

• a first laser beam generating means;
• a second laser beam generating means;
• an optical system for linking the first laser beam and the second laser beam on a single axis;
• an objective lens for converging the linked first laser beam onto the servo layer, and for converging the second laser beam onto one of the plurality of recording layers;
• an actuator for driving the objective lens;
• a photodetector having a focus error detector and a track error detector;
• a timing generation circuit for generating a timing signal for irradiating alternately the first laser beam and the second laser beam in accordance with the output of the track error detector;
• a first focus servo block for driving the actuator in the optical axis direction in accordance with the output of the track error detector while the first laser beam is turned on;
• a tracking servo block for driving the actuator in a direction perpendicular to the optical axis in accordance with the output while the first laser beam is turned on;
• and
• a second focus servo block for driving the spot of the second laser beam in the optical axis direction in accordance with the output of the focus detector while the second laser beam is turned on.