Optical system for optical pickup device, coupling lens and optical information recording/reproducing apparatus

Abstract

An optical system includes an objective lens; and a plastic coupling lens to change a divergent angle of a light flux emitted from a light source and is structured such that the light flux emitted from the light source passes through both of the coupling lens and the objective lens when the light flux is converged onto an optical information recording medium and the reflected light flux reflected from the optical information recording medium passes through the objective lens without passing through the coupling lens when the reflected light flux is detected by a photo-detector. A ring-shaped diffractive structure is formed on at least one surface of the coupling lens and the diffractive structure generates an under-spherical aberration when a light flux having a longer wavelength comes in, and the coupling lens has a positive refractive power as a whole.

What is claimed is:  
     
         1 . An optical system for use in an optical pickup apparatus which comprises a light source to emit a light flux having a wavelength λ, the optical system to converge the light flux onto an optical information recording medium, a focusing device to focus the converged light flux on the optical information recording medium, and a photo-detector to detect a light flux reflected from the optical information recording medium; the optical system comprising: 
 an objective lens; and  
 a plastic coupling lens to change a divergent angle of the light flux emitted from the light source; wherein the optical system is structured such that the light flux emitted from the light source passes through both of the coupling lens and the objective lens when the light flux is converged onto the optical information recording medium and the reflected light flux reflected from the optical information recording medium passes through the objective lens without passing through the coupling lens when the reflected light flux is detected by the photo-detector,  
 wherein a ring-shaped diffractive structure is formed on at least one surface of the coupling lens and the diffractive structure generates an under-spherical aberration when a light flux having a longer wavelength comes in, and  
 wherein the coupling lens has a positive refractive power as a whole.  
 
     
     
         2 . The optical system of  claim 1 , wherein the image forming magnification of the coupling lens as a single optical element is almost constant.  
     
     
         3 . The optical system of  claim 1 , wherein the following conditional formula is satisfied: 
       0.0001 <|δL/f|< 0.002 where δL is a shifting distance in a direction along the optical axis to shift the objective lens by the focusing mechanism in order to conform the optimum image forming position with a recording surface of the optical information recording medium when temperature of the optical pickup device is raised by 30° C., and    f is a focal length of the objective lens.    
     
     
         4 . The optical system of  claim 1 , wherein the ring-shaped diffractive structure formed on the coupling lens satisfies the following formula: 
       10λ< pmin/n< 60λ where pmin represents a minimum value of a diffractive pitch within an effective diameter, and    n is a utilized diffraction order.    
     
     
         5 . The optical system of  claim 1 , wherein the following conditional formulas are satisfied: 
       2.0 <f< 5.0 0 . 45   ≦NA≦ 0.65−⅓ <m≦ 0600 nm<λ<800 nm where NA is a numerical aperture, m is an image forming magnification, and f is a focal length.    
     
     
         6 . The optical system of  claim 1 , wherein on one surface of the coupling lens is formed the ring-shaped diffractive structure and on the other surface of the coupling lens is formed a lattice-shaped diffractive structure to split the light flux emitted from the light source into a plurality of light fluxes.  
     
     
         7 . The optical system of  claim 1 , wherein the surface on which the lattice-shaped diffractive structure is formed is almost a flat surface.  
     
     
         8 . The optical system of  claim 1 , wherein the objective lens is made of a plastic.  
     
     
         9 . A coupling lens for use in an optical system comprising the coupling lens and an objective lens, wherein the optical system is used in an optical pickup apparatus which comprises a light source to emit a light flux having a wavelength λ, the optical system to converge the light flux onto an optical information recording medium, a focusing device to focus the converged light flux on the optical information recording medium, and a photo-detector to detect a light flux reflected from the optical information recording medium; the coupling lens comprising: 
 a ring-shaped diffractive structure formed on at least one surface of the coupling lens and to generate under-spherical aberration when a light flux having a longer wavelength comes in,  
 wherein the coupling lens has a positive refractive power to change a divergent angle of a light flux emitted from the light source and is located outside of an optical path when a reflected light flux from the optical information recording medium is detected by the photo-detector.  
 
     
     
         10 . The coupling lens of  claim 9 , wherein the image forming magnification of the coupling lens as a single optical element is almost constant even when temperature of the optical pickup apparatus is changed.  
     
     
         11 . The coupling lens of  claim 9 , wherein the ring-shaped diffractive structure formed on the coupling lens satisfies the following formula: 
       10λ< pmin/n< 60λ where pmin represents a minimum value of a diffractive pitch within an effective diameter, and    n is a utilized diffraction order.    
     
     
         12 . The coupling lens of  claim 9 , wherein the following conditional formulas are satisfied: 
         2.0   <f< 5.00.45 ≦NA≦ 0.65−⅓ <m≦ 0600 nm<λ<800 nm where NA is a numerical aperture, m is an image forming magnification, and f is a focal length.    
     
     
         13 . The coupling lens of  claim 9 , wherein on one surface of the coupling lens is formed the ring-shaped diffractive structure and on the other surface of the coupling lens is formed a lattice-shaped diffractive structure to split the light flux emitted from the light source into a plurality of light fluxes.  
     
     
         14 . The coupling lens of  claim 9 , wherein the surface on which the lattice-shaped diffractive structure is formed is almost a flat surface.  
     
     
         15 . An optical information recording and/or reproducing apparatus, comprising 
 a light source to emit a light flux having a wavelength λ,    an optical system to converge the light flux onto an optical information recording medium,    a focusing device to focus the converged light flux on the optical information recording medium, and    a photo-detector to detect a light flux reflected from the optical information recording medium;    wherein the optical system comprises an objective lens and a plastic coupling lens having a positive refractive power to change a divergent angle of the light flux emitted from the light source;    wherein the optical system is structured such that the light flux emitted from the light source passes through both of the coupling lens and the objective lens when the light flux is converged onto the optical information recording medium and the reflected light flux reflected from the optical information recording medium passes through the objective lens when the reflected light flux is detected by the photo-detector,    wherein a ring-shaped diffractive structure is formed on at least one surface of the coupling lens and the diffractive structure generates under-spherical aberration when a light flux having a longer wavelength comes in.    
     
     
         16 . The optical information recording and/or reproducing apparatus of  claim 15 , wherein the image forming magnification of the coupling lens as a single optical element is almost constant even when temperature of the optical pickup apparatus is changed.  
     
     
         17 . The optical information recording and/or reproducing apparatus of  claim 15 , wherein the following conditional formula is satisfied: 
       0.0001 <|δL/f|< 0.002 where δL is a driving amount to shift the objective lens by the focusing mechanism so as to conform the optimum image forming position with a recording surface of the optical information recording medium when temperature of the optical pickup device is raised by 30° C., and    f is a focal length of the objective lens.    
     
     
         18 . The optical information recording and/or reproducing apparatus of  claim 15 , wherein the following conditional formula is satisfied: 
       0.0001 <|δL/f|< 0.002 where δL is a shifting distance to shift the objective lens by the focusing mechanism so as to conform the optimum image forming position with a recording surface of the optical information recording medium when temperature of the optical pickup device is raised by 30° C., and    f is a focal length of the objective lens.    
     
     
         19 . The optical information recording and/or reproducing apparatus of  claim 15 , wherein the ring-shaped diffractive structure formed on the coupling lens satisfies the following formula: 
       10λ< pmin/n< 60λ where pmin represents a minimum value of a diffractive pitch within an effective diameter, and    n is a utilized diffraction order.    
     
     
         20 . The optical information recording and/or reproducing apparatus of  claim 15 , wherein the following conditional formulas are satisfied: 
       2.0 <f< 5.00.45 ≦NA≦ 0.65−⅓ <m≦ 0 600  nm<λ<800 nm where NA is a numerical aperture, m is an image forming magnification, and f is a focal length.    
     
     
         21 . The optical information recording and/or reproducing apparatus of  claim 15 , wherein on one surface of the coupling lens is formed the ring-shaped diffractive structure and on the other surface of the coupling lens is formed a lattice-shaped diffractive structure to split the light flux emitted from the light source into a plurality of light fluxes.  
     
     
         22 . The optical information recording and/or reproducing apparatus of  claim 15 , wherein the surface on which the lattice-shaped diffractive structure is formed is almost a flat surface.  
     
     
         23 . The optical information recording and/or reproducing apparatus of  claim 15 , wherein the objective lens is made of a plastic.