Focus error detecting method and optical head using the same

A focus error detecting method in which an information recording surface is used on which a large number of marks for modulating the amount of the reflection light of projected beams are intermittently located in a track traveling direction and these marks are aligned in the direction within a range of at least several tracks, comprising the steps of:

• in a focusing optical system including a diffraction element used to permit a part of the beam emitted from a light source as a main beam to pass and generate a pair of sub-beams having positive astigmatism with two focal lines and negative astigmatism with two focal lines in the direction different from the main beam at a minute angle, through the diffraction element, focusing the main beam on the information recording surface as a main spot whereas focusing the pair of sub-beams at the positions different from the main spot in the focal point direction and the track traveling direction;
• taking out the beams reflected from the information recording surface separately from the optical path of the focusing optical system so that the reflection beams of the pair of sub-beams are received by two photo-detecting elements;
• and
• detecting the amplitudes of the reflection beams of the pair of sub-beams which are modulated by the marks in terms of the outputs from the two photo-detecting elements, thereby providing a focus error detecting signal based on a difference therebetween.

An optical head for projecting a light beam on an information recording surface on which a large number of marks for modulating the amount of the reflection light of projected beams are intermittently located in a track traveling direction and these marks are aligned in the direction nearly perpendicular to the track traveling direction within a range of at least several tracks, comprising:

• a light source;
• a focusing optical system including a diffraction grating located on the optical path of the beam for permitting a part of the beam emitted from the light source as a main beam to pass and generate a pair of sub-beams having positive astigmatism with two focal lines and negative astigmatism with two focal lines in the direction different from the main beam at a minute angle, the focusing optical system for focusing the main beam on the information recording surface as a main spot while focusing the pair of sub-beams at the positions different from the main spot in the focal point direction and the track traveling direction;
• separated optics for taking out the beams reflected from the information recording surface separately from the optical path of the focusing optical system;
• photo-detecting means for receiving the reflection beams, including at least two photo-detecting elements for receiving the reflection beams of the pair of sub-beams, respectively;
• and
• a focus error detecting circuit for detecting the amplitudes of the reflection beams of the pair of sub-beams which are modulated by the marks in terms of the output from the two photo-detecting elements, thereby providing a focus error detecting signal based on a difference therebetween.

A focus error detecting method comprising the steps of:

• emitting a light beam from a light source;
• providing positive astigmatism having two focal lines and negative astigmatism having two focal lines to the beam using an acoustooptic element by varying a driving frequency therefor;
• focusing the beam provided with the astigmatism on an information recording surface;
• receiving the beam reflected from the information recording surface;
• and
• comparing the reflected beams with the positive astigmatism and the negative astigmatism to calculate a difference therebetween and providing a focus error detecting signal on the basis of the difference.

A focus error detecting method comprising the steps of:

• emitting a light beam from a light source;
• providing positive astigmatism and negative astigmatism to the beam using an acoustooptic element by varying a driving frequency therefor;
• focusing the beam provided with the astigmatism on an information recording surface;
• receiving the beam reflected from the information recording surface;
• and
• comparing the reflected beams with the positive astigmatism and the negative astigmatism to calculate a difference therebetween and providing a focus error detecting signal on the basis of the difference;
• wherein the position of a circle of least confusion of a light spot due to positive and negative astigmatism is changed in a time-divisional manner.

An optical head comprising:

• a light source for emitting a light beam;
• a focusing optical system including an acoustooptic element, located on the optical path of the beams emitted from the light source, for supplying positive astigmatism having two focal lines and negative astigmatism having two focal lines to the beams by varying the frequency of the acoustooptic element so that the beams are focused as spots on the information recording surface;
• separated optics for taking out the beams reflected from the information recording surface separately from the optical path of the focusing optical system;
• a photo-detecting element for receiving the reflected beams thus separated;
• an acoustooptic element driving circuit for varying the frequency of the acoustooptic element;
• a focus error detecting circuit for comparing the reflected beams with positive astigmatism and negative astigmatism received by the photo-detecting means to calculate a difference therebetween and producing a focus error detecting signal on the basis of this difference;
• and
• a control circuit for controlling the focusing optical system on the basis of the focus error detecting signal and focusing the beams at the focusing position on the information recording surface.