Objective lens and optical head device

Abstract

An objective lens ( 1 ) for an optical disk, which focuses a light beam from a light source, is designed so that a third-order coma aberration generated when the objective lens is inclined at a unit angle is larger than a third-order coma aberration generated when the optical disk ( 2 ) is inclined at the unit angle, mounted on an actuator for inclining the objective lens according to an inclination amount of the optical disk, and used. With this structure, it is possible to obtain an objective lens for an optical disk that has a large numerical aperture and is easy to manufacture and assemble, and in which the third-order coma aberration generated when the optical disk surface is inclined owing to a warp or the like can be corrected by small inclination of the objective lens, so as to reduce a residual astigmatism, which is generated according to the inclination amount, after the correction.

1. An objective lens for an optical disk, which focuses  for focusing a light beam from a light source on an optical medium,
 wherein DC/LC>−0.9 is satisfied, where DC (mλ) represents a third-order coma aberration generated when the optical disk  medium is inclined at 0.5° and LC (mλ) represents a third-order coma aberration generated when the objective lens is inclined at  0.5°, and    
 the objective lens is mounted on an actuator for inclining the objective lens according to an inclination amount of the optical disk .  
 
     
     
       2. The objective lens for the optical disk  according to  claim 1 , further satisfying DC/LC>−0.8. 
     
     
       3. The objective lens for the optical disk  according to  claim 1 , wherein DC+LC is at least 10 mλ and not larger than 40 mλ. 
     
     
       4. An objective lens for for the optical disk, which focuses  focusing a light beam from a light source and is used for a recordable and/or reproducible  on an optical medium for recording information on, or reproducing information from, the optical disk having a thickness of about 0.6 mm  medium, wherein
 a numerical aperture is at least 0.62 and not larger than 0.66,  
 DC+LC is at least 10 mλ, where DC (mλ) represents a third-order coma aberration generated when the optical disk  medium is inclined at 0.5° and LC (mλ) represents a third-order coma aberration generated when the objective lens is inclined at  0.5°, and    
 the objective lens is mounted on an actuator for inclining the objective lens according to an inclination amount of the optical disk,  
 the objective lens is used in an optical system in which the objective lens is inclined so as to generate the third-order coma aberration, which cancels out the third-order coma aberration generated when the optical disk is inclined, and  
 a coefficient a is at least 30 and not larger than 35 when a relationship between X and Y is approximated by Y=aX 2 +bX+c, where X (°) represents an angle of inclination of the optical disk  medium and Y (mλ) represents an astigmatism that remains after canceling out the third-order coma aberration generated when the optical disk  medium is inclined at X (°) by inclining the objective lens.  
 
     
     
       5. An optical head device comprising:
 a light source;  
 a collimating optical system for converting a light beam emitted from the light source into a substantially parallel light beam; 
 an objective lens for focusing the substantially parallel light beam  a light beam emitted by the light source on an information recording surface of an optical disk  medium;  
 a light beam splitting member  splitter for splitting the  a light beam that has been modulated by the information recording surface of the optical disk  medium; and  
 a photodetector receiving member  for receiving the light beam that has been modulated by the information recording surface of the optical disk;  medium, 
 wherein the objective lens is subjected to a tilt adjustment so as to reduce a third-order coma aberration, and  
 DC/LC>−0.9 is satisfied, where DC (mλ) represents a third-order coma aberration generated when the optical disk  medium is inclined at 0.5° and LC (mλ) represents a third-order coma aberration generated when the objective lens is inclined at 0.5°.  
 
     
     
       6. An optical head device comprising:
 a light source;  
 a collimating optical system for converting a light beam emitted from the light source into a substantially parallel light beam; 
 an objective lens for focusing the substantially parallel light beam  a light beam emitted by the light source on an information recording surface of an optical disk  medium having a thickness of about 0.6 mm;  
 a light beam splitting member  splitter for splitting the light beam that has been modulated by the information recording surface of the optical disk  medium; and  
 a photodetector receiving member  for receiving the light beam that has been modulated by the information recording surface of the optical disk  medium;  
 wherein the objective lens has a numerical aperture of at least 0.62 and not larger than 0.66 and is subjected to a tilt adjustment so as to reduce a third-order coma aberration,  
 DC+LC is at least 10 mλ, where DC (mλ) represents a third-order coma aberration generated when the optical disk  medium is inclined at 0.5° and LC (mλ) represents a third-order coma aberration generated when the objective lens is inclined at 0.5°, and  
 a coefficient a is at least 30 and not larger than 35 when a relationship between X and Y is approximated by Y=aX 2 +bX+c, where X (°) represents an angle of inclination of the optical disk  medium and Y (mλ) represents an astigmatism that remains after canceling out the third-order coma aberration generated when the optical disk  medium is inclined at X (°) by inclining the objective lens.  
 
     
     
       7. The objective lens for the optical disk  medium according to  claim 1 , wherein a numerical aperture is equal to or larger than 0.62. 
     
     
       8. An optical head device comprising:
 a light source;  
 a collimating optical system for converting a light beam emitted from the light source into a substantially parallel light beam; 
 an objective lens for focusing the substantially parallel light beam  a light beam emitted by the light source on an information recording surface of an optical disk  medium;  
 a light beam splitting member  splitter for splitting the light beam that has been modulated by the information recording surface of the optical disk  medium; and  
 a photodetector receiving member  for receiving the light beam that has been modulated by the information recording surface of the optical disk  medium;  
 wherein the objective lens is the objective lens according to  claim 1 .  
 
     
     
       9. An optical head device comprising:
 a light source;  
 a collimating optical source for converting a light beam emitted from the light source into a substantially parallel light beam; 
 an objective lens for focusing the substantially parallel light beam on an information recording surface of an optical disk  medium;  
 a light beam splitting member  splitter for splitting the light beam that has been modulated by the information recording surface of the optical disk  medium; and  
 a photodetector receiving member  for receiving the light beam that has been modulated by the information recording surface of the optical disk  medium;  
 wherein the objective lens is the objective lens according to claim  6    4 .  
 
     
     
       10. A method for manufacturing an optical head device, the optical head device  comprising the steps of:
 positioning a light source;  
 a collimating optical system for converting a light beam emitted from the light source into a substantially parallel light beam; 
 positioning an objective lens according to  claim 1  for focusing the substantially parallel light beama light beam emitted by the light source on an information recording surface of an optical diskmedium while being inclined such that a coma aberration of a wavefront of the light beam from the objective lens decreases;  
 positioning a light beam splitting membersplitter for splitting thea light beam that has been modulated by the information recording surface of the optical diskmedium; and  
 positioning a photodetector receiving member  for receiving the light beam that has been modulated by the information recording surface of the optical disk;  
 wherein, after the light source and the collimating optical system are placed, the objective lens is disposed while being inclined such that a coma aberration of a wavefront of the light beam from the objective lens decreases  medium.  
 
     
     
       11. A method for manufacturing an optical head device, the optical head device  comprising the steps of:
 positioning a light source;  
 a collimating optical system for converting a light beam emitted from the light source into a substantially parallel light beam; 
 positioning an objective lens according to  claim 4  for focusing the substantially parallel light beama light beam emitted by the light source on an information recording surface of an optical diskmedium while being inclined such that a coma aberration of a wavefront of the light beam from the objective lens decreases;  
 positioning a light beam splitting membersplitter for splitting thea light beam that has been modulated by the information recording surface of the optical diskmedium; and  
 positioning a photodetector receiving member  for receiving the light beam that has been modulated by the information recording surface of the optical disk;  
 wherein, after the light source and the collimating optical system are placed, the objective lens is disposed while being inclined such that a coma aberration of a wavefront of the light beam from the objective lens decreases  medium.  
 
     
     
       12. The optical head device according to  claim 8 , wherein the objective lens is subjected to a tilt adjustment so as to reduce a third-order coma aberration. 
     
     
       13. The optical head device according to  claim 9 , wherein the objective lens is subjected to a tilt adjustment so as to reduce a third-order coma aberration. 
     
     
       14. An optical head device comprising:
   an objective lens for focusing a light beam onto a surface of an optical medium;        a detector for detecting either an inclination of the optical medium or the third - order coma aberration generated based on the inclination; and        an actuator for inclining the objective lens based on either the inclination amount or the third - order coma aberration detected by the detector,        wherein DC/LC>− 0 . 9  is satisfied, where DC  ( mλ )  represents a third - order coma aberration generated when the optical medium is inclined at  0 . 5 ° and LC  ( mλ )  represents a third - order coma aberration generated when the objective lens is inclined at  0 . 5 °.     
     
     
       15. The optical head device according to  claim 14 , wherein the objective lens is subjected to a tilt adjustment so as to reduce the third- order coma aberration.   
     
     
       16. The optical head device according to  claim 14 , wherein DC/LC>− 0 . 8  is satisfied. 
     
     
       17. The optical head device according to  claim 14 , wherein DC+LC is at least  10  mλ and not larger than  40  mλ. 
     
     
       18. An apparatus for recording information and/or reproducing recorded information on the surface of an optical medium, comprising:
   a light source;        an objective lens for focusing a light beam from the light source onto the surface of the optical medium;        a detector for detecting either an inclination of the optical medium or the third - order coma aberration generated based on the inclination;        a light beam splitter for splitting a light beam that has been modulated by the information recording surface of the optical medium; and        a photodetector for receiving the light beam that has been modulated by the surface of the optical medium, and        an actuator for inclining the objective lens based on either the inclination amount or the third - order coma aberration detected by the detector,        wherein DC/LC>− 0 . 9  is satisfied, where DC  ( mλ )  represents a third - order coma aberration generated when the optical medium is inclined at  0 . 5 ° and LC  ( mλ )  represents a third - order coma aberration generated when the objective lens is inclined at  0 . 5 °.     
     
     
       19. The apparatus according to  claim 18 , wherein the objective lens is subjected to a tilt adjustment so as to reduce the third- order coma aberration.   
     
     
       20. The apparatus according to  claim 18 , wherein DC/LC>− 0 . 8  is satisfied. 
     
     
       21. The apparatus according to  claim 18 , wherein DC+LC is at least  10  mλ and not larger than  40  mλ.