Objective lens element and optical head device including the same

Abstract

An objective lens element having improved diffraction efficiency at at least one of used wavelengths is provided. The objective lens element includes one surface including: a first region including an optical axis; and a second region surrounding the first region. A periodic first diffraction structure is formed on the first region, and a periodic second diffraction structure different from the first diffraction structure is formed on the second region. The objective lens element satisfies the following conditions. | A 1− B 1|<| A 2− B 2|  (1) | B 1|≧| B 2|  (2) Here, A1 and B1 are diffraction orders at the first region to converge light of a first wavelength and light of a second wavelength on a recording surface, respectively, and A2 and B2 are diffraction orders at the second region to converge the light of the first wavelength and the light of the second wavelength on a recording surface, respectively.

What is claimed is: 
     
         1 . An objective lens element configured for converging each of light of a first wavelength, light of a second wavelength longer than the first wavelength, and light of a third wavelength longer than the second wavelength on an information recording surface of an optical disc, the objective lens element being configured such that:
     L 1<0  (3),
       L 2>0  (4),
   
       where,
 L1 is a first distance from an incident surface of the objective lens element to an object point of a light source of the second wavelength, and 
 L2 is a second distance from the incident surface of the objective lens element to an object point of a light source of the third wavelength. 
 
     
     
         2 . An optical head device configured for converging a first incident light beam of a first wavelength through a base plate of a first thickness to form a spot, a second incident light beam of a second wavelength, longer than the first wavelength, through a base plate of a second thickness, larger than the first thickness, to form a spot, and a third incident light beam of a third wavelength, longer than the second wavelength, through a base plate of a third thickness, larger than the second thickness, to form a spot, the optical head device comprising:
 a first light source configured for emitting a light beam of the first wavelength;   a second light source configured for emitting a light beam of the second wavelength;   a third light source configured for emitting a light beam of the third wavelength;   an objective lens element according to  claim 1 ; and   a detection element configured for detecting light reflected by an information recording surface of an optical disc.