Compound objective lens having two focal points

A focusing method for focusing light on a first information medium having a first thickness or a second information medium having a second thickness to record or reproduce a piece of information on or from the first information medium or the second information medium, comprising the steps of:

• moving an optical head apparatus in a direction to decrease or increase the distance between the optical head apparatus and the first or second information medium, the optical head apparatus comprising
• a light source for radiating a beam of incident light,
• hologram means for transmitting a part of the incident light radiated from the light source without any diffraction on an outgoing path to form a beam of transmitted light and diffracting a remaining part of the incident light radiated from the light source on the outgoing path to form a beam of diffracted light, the hologram means functioning as a lens for the diffracted light to diverge the diffracted light from the hologram means or converge the diffracted light,
• lens means for converging the transmitted light formed in the hologram means at a first focal length on the outgoing path to form a first converging spot at a front surface of the first information medium or converging the diffracted light formed in the hologram means at a second focal length on the outgoing path to form a second converging spot at a front surface of the second information medium, the transmitted light being incident on a rear surface of the first information medium and being converged at the front surface of the first information medium, the transmitted light being reflected at the rear surface of the first information medium and again passing through the lens means and the hologram means on an incoming path, the diffracted light being incident on a rear surface of the second information medium and being converged at the front surface of the second information medium, and the diffracted light being reflected at the rear surface of the first information medium and again passing through the lens means and the hologram means on the incoming path,
• wavefront changing means for changing a wavefront of the transmitted light or the diffracted light passing through the lens means and the hologram means on the incoming path to form one or more beams of information light, and
• detecting means for detecting intensities of the information light formed by the wavefront changing means and generating an information signal and a focus error signal according to the intensities of the information light, the information signal expressing a piece of information recorded on the first information medium or the second information medium;
• judging whether or not an intensity of the focus error signal generated in the detecting means is larger than a threshold value;
• and
• adjusting the position of the optical head apparatus to decrease the intensity of the focus error signal to zero when the intensity of the focus error signal becomes larger than the threshold value.

An information reproducing method for reproducing a piece of recording information from an optical disk in which the recording information are recorded at a high density on a first substrate having a first thickness and a piece of distinguishing information informing that the recording information are recorded on the first substrate is recorded at an ordinary density on a second substrate having a second thickness larger than the first thickness, comprising the step of:

• moving an optical disk apparatus under the second substrate of the optical disk, the optical disk comprising
• rotating means for rotating the optical disk at a regular speed,
• a light source for radiating a beam of incident light;
• hologram means for transmitting a part of the incident light radiated from the light source without any diffraction on an outgoing path to form a beam of transmitted light and diffracting a remaining part of the incident light radiated from the light source on the outgoing path to form a beam of diffracted light, the hologram means functioning as a lens for the diffracted light to diverge the diffracted light from the hologram means,
• lens means for converging the transmitted light formed in the hologram means on the first substrate of the optical disk rotated by the rotating means to record or reproduce a piece of recording information on or from the optical disk and converging the diffracted light formed in the hologram means on the second substrate of the optical disk rotated by the rotating means to reproduce the distinguishing information from the optical disk, the transmitted light being reflected by the first substrate of the optical disk and again passing through the lens means and the hologram means on an incoming path, and the diffracted light being reflected by the second substrate of the optical disk and again passing through the lens means and the hologram means on the incoming path,
• wavefront changing means for changing a wavefront of the transmitted light passing through the lens means and the hologram means on the incoming path to form one or more beams of recording information light and changing a wavefront of the diffracted light passing through the lens means and the hologram means on the incoming path to form one or more beams of distinguishing information light, and
• detecting means for detecting intensities of the recording information light formed by the wavefront changing means to generate a recording information signal according to the intensities of the recording information light and detecting intensities of the distinguishing information light formed by the wavefront changing means to generate a distinguishing information signal according to the intensities of the distinguishing information light, the distinguishing information signal expressing the distinguishing information recorded on the second substrate of the optical disk, and the recording information signal expressing the recording information recorded on the first substrate of the optical disk;
• and
• converging the diffracted light on the second substrate of the optical disk to reproduce the distinguishing information;
• moving the optical disk apparatus to a position under the first substrate of the optical disk to converge the transmitted light on the first substrate of the optical disk when the distinguishing information is detected in the detecting means;
• and
• reproducing the recording information by generating the recording information signal in the detecting means.