Focus detection device for reading information from an optical recording medium

A focus detection device for reading an information from an optical recording medium having a recording track formed thereon, said device comprising:

• a first light emission means for emitting a first light beams in a first direction toward said optical disk;
• a Fresnel zone plate means located in a path of said emitted first light beam for splitting said emitted first light beam into said first light beam, a second light beam, and a third light beam, said second and third light beams acting as emitted from light sources located on opposite sides of said first light emission means aligned with said first direction, said Fresnel zone plate means comprising a first area and a second area in a round shape formed inside said first area, said first area being formed with no Fresnel pattern having a first diffraction axis formed therein for passing said first emitted light beam and a second area with Fresnel pattern formed therein for splitting said emitted first light into said first, second, and third light beam;
• a converging means provided between said optical recording medium and said first emission means for conversing said first beam to make a first spot focused on said recording track, said second light beam being focused beyond said optical recording medium to make a second spot having a second predetermined size around said recording track, said third light beam being focused above said optical recording medium to make a third spot having a second predetermined size around said recording track, said first and second sides are opposed to each other, said second and third spots are located on opposite sides of said first spot;
• a beam guide means for guiding said first, second, and third light beams reflected from said optical recording medium in a second direction;
• a first photodetection means inserted in a path of said guided first light beam for receiving said guided first light beam to produce an information signal representing information recorded on said recording track;
• a second photodetection means inserted in a path of said guided second light beam for receiving said guided second light beam and producing a first focus signal;
• and
• third photodetection means inserted in a path of said guided third light beam for receiving said guided third light beam and producing a second focus signal.

A focus detection device for reading and writing information on and from an optical recording medium having a recording track formed thereon, said device comprising:

• first light emission means for emitting a first light beam in a first direction toward said optical disk;
• converging means provided between said optical recording medium and said first emission means for converging said first beam to make a first spot focused on said recording track;
• second light emission means for emitting a second light beam such that said emitted second light beam is focused by said converging means beyond said optical recording medium to make a second spot around said recording track;
• first light restriction means provided in a path of said second emitted light beam adjacent said second light emission means for restricting the amount of said second emitted light beam passing there through;
• third light emission means for emitting a third light beam such that said emitted third light beam is focused by said converging means above said optical recording medium to make a third sport around said recording track;
• second light restriction means provided in a path of said third emitted light beam adjacent said third light emission means for restricting the amount of said second emitted light beam passing there through;
• beam guide means for guiding said first, second, and third light beams reflected from said optical recording medium in a second direction;
• first photodetection means inserted in a path of said guided first light beam for receiving said guided first light beam to produce an information signal representing information recorded on said recording track;
• second photodetection means inserted in a path of said guided second light beam for receiving said guided second light beam and producing a first focus signal;
• and
• third photodetection means inserted in a path of said guided third light beam for receiving said guided third light beam and producing a second focus signal.