| 摘要 |
PURPOSE:To obtain a large modulation degree for read signals with even a prepit depth of lambda/8 by reading with the out-of-focus detecting means an information previously recorded on an information recording medium using an extracted light in focus in an asymmetric condition. CONSTITUTION:The other laser beam L (for the out-of-focus detection system) separated with a half-mirror 7 comes to a knife edge 15 (light extracting member) where components of light that pass through an off-axis region only are extracted. The extracted light, after passing through the second projector lens 16, comes to photodetector cells 17a and 17b of the second photodetector 17. The said knife edge 15 stops the light on either front or rear side with respect to the direction a tracking guide extends (or front or rear side with respect to the direction the light passes through a prepit P successively). Signals from photo-detector cells 17a and 17b are fed through a subtracter 18 and a low-pass filter 19 to out-of-focus detection corrector section 20. Thus out-of-focus detection in the knife edge method is performed. |
