Optical disk apparatus

Abstract

It is intended to provide an optical disk apparatus which detects a light amount greater than zero even when used in conjunction with an optical disk substrate having a large birefringence, so that it is possible to properly read a signal without errors and properly perform optical disk controls. The optical disk apparatus includes: a light source for emitting light; an objective lens for converging the light onto a signal surface of an optical disk; a polarized beam diffraction element for diffracting the light reflected from the optical disk; a photodetector for detecting the light diffracted from the polarized beam diffraction element; and a wavelength plate disposed between the optical disk and the polarized beam diffraction element. The wavelength plate has a two-dimensional array of a plurality of birefringent regions including first and second regions, the first and second regions differing in birefringent phase difference and/or optic axes from each other, and the plurality of birefringent regions including the first and second regions cause the light to have different polarization states.

1 - 7 . (canceled) 
     
     
         8 . An optical disk apparatus comprising:
 a light source for emitting light of a wavelength λ 1  and light of a wavelength λ 2  (where λ 1 ≠λ 2 );   an objective lens for converging the light onto a signal surface of an optical disk;   a polarized beam diffraction element for diffracting the light reflected from the optical disk;   a photodetector for detecting the light diffracted from the polarized beam diffraction element; and   a wavelength plate disposed between the optical disk and the polarized beam diffraction element, wherein,   an imaginary line L on the beam splitter is defined, the line L being perpendicular to a radial direction of the optical disk, and intersecting an optical axis of the objective lens;   the beam splitter at least has a regional, a region a 2 , a region a 3 , a region A 1 , a region A 2 , and a region A 3 , such that
 the regional, the region a 2 , and the region a 3  are on a same side of the line L on the beam splitter, and 
 the region A 1 , the region A 2 , and the region A 3  are substantially symmetrical regions to the regional, the region a 2 , and the region a 3 , respectively, with respect to the line L; 
   the photodetector at least has two regions b and B;   light of the wavelength λ 1  entering the region a 3 , the regional, and the region A 2  of the beam splitter produces 1 st  order diffracted light which is projected onto the region b of the photodetector, and light of the wavelength λ 1  entering the region A 3 , the region A 1 , and the region a 2  of the beam splitter produces 1 st  order diffracted light which is projected onto the region B of the photodetector;   light of the wavelength λ 2  entering the region a 3  of the beam splitter produces 1 st  order diffracted light which is projected onto the region B of the photodetector, and light of the wavelength λ 2  entering the region A 3  of the beam splitter produces 1 st  order diffracted light which is projected onto the region b of the photodetector; and   based on a difference between a detection signal from the region b and a detection signal from the region B, the optical disk apparatus generates a tracking error signal for the optical disk or a correction signal for correcting the tracking error signal.   
     
     
         9 . The optical disk apparatus according to  claim 8 , wherein,
 the photodetector further has at least two regions b′ and B′;   light from a first light source or a second light source entering the region a 3 , the regional, and the region a 2  of the beam splitter produces −1 st  order diffracted light which is projected onto the region b′ of the photodetector, and light from the first light source or the second light source entering the region A 3 , the region A 1 , and the region A 2  of the beam splitter produces −1 st  order diffracted light which is projected onto the region B′ of the photodetector; and   the optical disk apparatus generates a difference signal based on a difference between a detection signal from the region b′ and a detection signal from the region B′, and generates a tracking error signal for the optical disk by adding to the difference signal a value obtained by multiplying the correction signal by an arbitrary coefficient.   
     
     
         10 . An optical disk apparatus comprising:
 a light source for emitting light of a wavelength λ 1  and light of a wavelength λ 2  (where λ 1 ≠Δ 2 );   an objective lens for converging the light onto a signal surface of an optical disk;   a polarized beam diffraction element for diffracting the light reflected from the optical disk;   a photodetector for detecting the light diffracted from the polarized beam diffraction element; and   a wavelength plate disposed between the optical disk and the polarized beam diffraction element, wherein,   an imaginary line L on the beam splitter is defined, the line L being perpendicular to a radial direction of the optical disk, and intersecting an optical axis of the objective lens;   the beam splitter at least has eight regions a 1 , a 2 , a 3 , A 1 , A 2 , A 3 , and A 4  such that
 the regional a 1 , the region a 2 , the region a 3 , and the region a 4  are on a same side of the line L on the beam splitter, and 
 the region A 1 , the region A 2 , the region A 3 , and the region A 4  are substantially symmetrical regions to the region a 1 , the region a 2 , the region a 3 , and the region a 4 , respectively, with respect to the line L; 
   the photodetector at least has six regions b, B, b′, B′, b′, and B″;   light of the wavelength λ 1  entering the region A 2  and the regional of the beam splitter produces −1 st  order diffracted light which is projected onto the region b of the photodetector, and light of the wavelength λ 1  entering the region a 2  and the region A 1  of the beam splitter produces −1 st  order diffracted light which is projected onto the region B of the photodetector, the optical disk apparatus generating a tracking error signal for the optical disk based on a difference between a detection signal from the region b and a detection signal from the region B;   light of the wavelength λ 2  entering the region a 3  and the region a 4  of the beam splitter produces −1 st  order diffracted light which is projected onto the region b′ of the photodetector, and light of the wavelength λ 2  entering the region A 3  and the region A 4  of the beam splitter produces −1 st  order diffracted light which is projected onto the region B′ of the photodetector, the optical disk apparatus generating a difference signal based on a difference between a detection signal from the region b′ and a detection signal from the region B′; and light of the wavelength λ 2  entering the region a 3  further produces 1 st  order diffracted light which is projected onto the region b″ of the photodetector, and light of the wavelength λ 2  entering the region A 3  further produces 1 st  order diffracted light which is projected onto the region B″ of the photodetector, the optical disk apparatus generating a correction signal based on a difference between a detection signal from the region b″ and a detection signal from the region B″;   the optical disk apparatus generates a tracking error signal for the optical disk by adding to the difference signal a value obtained by multiplying the correction signal by an arbitrary coefficient.   
     
     
         11 . An optical element comprising a two-dimensional array of a plurality of birefringent regions including first and second regions, the first and second regions differing in birefringent phase difference and/or optic axes from each other,
 wherein the plurality of birefringent regions including the first and second regions cause the light to have different polarization states.   
     
     
         12 . The optical element according to  claim 11 , wherein optic axes of the first and second regions are parallel to each other, and the first and second regions have different retardations from each other. 
     
     
         13 . The optical element according to  claim 11 , wherein optic axes of the first and second regions are oriented in different directions from each other. 
     
     
         14 . The optical element according to  claim 11 , wherein a plurality of said first regions and a plurality of said second regions alternate within a plane perpendicular to an optical axis. 
     
     
         15 . The optical element according to  claim 14 , wherein each of the first and second regions has a shape selected from the group consisting of: a strip shape, a checker shape, and an annular shape. 
     
     
         16 . The optical element according to  claim 11 , further comprising a polarization filter. 
     
     
         17 . The optical element according to  claim 16 , wherein the polarization filter is a polarization hologram. 
     
     
         18 . The optical element according to  claim 13 , wherein, the optic axis of the first region is at 45°+δ±α(−10°<δ<10°, 0°<α≦15°) with respect to a polarization direction of incident light; and
 the optic axis of the second region is at 45°+δ−α with respect to a polarization direction of incident light.   
     
     
         19 . The optical element according to  claim 18 , wherein, with respect to light of at least one wavelength among light of a plurality of wavelengths traveling back and forth through the optical element, an average retardation Δ of the plurality of birefringent regions is set equal to (2m+1)π/2 (where m is an integer). 
     
     
         20 . The optical element according to  claim 19  which is a broadband wavelength plate having a same retardation Δ for light of different wavelengths. 
     
     
         21 . The optical element according to  claim 13 , wherein optic axes of some of the plurality of birefringent regions are at 45° with respect to a polarization direction of incident light. 
     
     
         22 . An optical pickup comprising:
 a light source for emitting two or more kinds of laser light of different wavelengths;   a lens for converging the light emitted from the light source onto an optical information medium; and   a photodetector for receiving light reflected from the optical information medium,   wherein the optical pickup further comprises the optical element according to  claim 11 , the optical element being disposed in a region common to an optical path from the light source to the optical information medium and an optical path from the optical information medium to the photodetector.   
     
     
         23 . The optical pickup according to  claim 21 , wherein the light source and the photodetector are integrally formed. 
     
     
         24 . A method for producing an optical element having a two-dimensional array of a plurality of birefringent regions including first and second regions, the first and second regions differing in birefringent phase difference and/or optic axes from each other, the plurality of birefringent regions including the first and second regions causing the light to have different polarization states, the method comprising the steps of:
 (a) forming on a substrate an alignment film including a plurality of regions having different alignment directions from one another; and   (b) forming a liquid crystal layer on the alignment film and controlling the alignment direction of each region of the liquid crystal layer.   
     
     
         25 . The method according to  claim 24 , wherein the step (a) comprises the substeps of:
 (a1) depositing a photo-alignable film on the substrate;   (a2) subjecting a portion of the photo-alignable film to an exposure with ultraviolet light to form a first aligning region having a first alignment direction; and   (a3) subjecting another portion of the alignment film to an exposure with ultraviolet light to form a second aligning region having a second alignment direction, the second alignment direction being different from the first alignment direction.   
     
     
         26 . The method according to  claim 24 , wherein,
 the step (b) comprises the substeps of:   (b1) forming on the alignment film a liquid crystal layer containing a UV-curing material, and controlling the alignment directions within the liquid crystal layer in accordance with first and second alignment directions; and   (b2) curing the liquid crystal layer by irradiating the liquid crystal layer with ultraviolet light.