Focus detection mechanism and optical head and optical storage device that use it

An optical head for performing recording and/or playback on an optical medium disposed on a target plane, comprising:

• a light source for emitting a light beam onto a surface of said medium along an optical axis, the light beam being reflected off said medium as a return light;
• a focus detection mechanism for detecting focusing of the light beam on said surface, said mechanism comprising:
• converging means for focusing the light beam on said surface and for directing the return light reflected off said surface,
• holographic means including first and second areas and a boundary line in between the first and second areas, each area having a periodic light modulatory pattern for diffracting the return light directed by said converging means to produce two astigmatic diffracted beams, each periodic light modulatory pattern including a group of curves substantially following hyperbolic patterns oriented about an origin, wherein said holographic means is arranged with respect to said light source such that the optical axis of the light beam is offset from the origin,
• means for receiving the astigmatic diffracted beams, said receiving means including four photodetection means disposed on a substantially same detection plane for detecting the respective astigmatic diffracted beams to produce corresponding output signals, and
• means for processing the output signals to detect focusing of the light beam on said surface;
• and
• actuating means, responsive to said processing means, for adjusting the position of at least said converging means in accordance with the output of said processing means so that the light beam is continually focused on said surface.