Optical system for detecting data signal and tracking error signal

Abstract

In an optical system, reflected light from the disk is diffracted by a hologram optical element, and received by an optical detector. A focusing error signal is detected from −1st-order diffracted light of the hologram optical element. A tracking error signal by the differential phase method, a tracking error signal by a push-pull method and the data signal recorded on the disk are detected from +1st-order diffracted light of the hologram optical element. A diffraction efficiency of the +1st-order diffracted light is higher than a diffraction efficiency of the −1st-order diffracted light.

1 . An optical system comprising: 
 a light source;    an object lens for focusing emitted light from the light source onto an optical recording medium;    first optical separating means which is provided between the light source and the object lens and which separates an optical path of reflected light from the optical recording medium, from an optical path of the emitted light from the light source;    second optical separating means which separates the reflected light from the optical recording medium via the first optical separating means into a first group of light and a second group of light; and    an optical detector for receiving the first group of light and the second group of light;    wherein an optical signal strength of the first group of light is larger than an optical signal strength of the second group of light.    
   
   
       2 . An optical system as claimed in  claim 1 , wherein: 
 wherein the optical system is constituted such that a tracking error signal by a differential phase method, a tracking error signal by a push-pull method and a data signal recorded on the optical recording medium are detected from the first group of light while a focusing error signal is detected from the second group of light.    
   
   
       3 . An optical system as claimed in  claim 1 , wherein: 
 the first optical separating means and the second optical separating means are an integrated polarizing hologram optical element,    the polarizing hologram optical element transmits the emitted light from the light source and diffracts the reflected light from the optical recording medium, and    the first group of light is +1st-order diffracted light of the polarizing hologram optical element while the second group of light is −1st-order diffracted light of the polarizing hologram optical element.    
   
   
       4 . An optical system as claimed in  claim 3 , wherein: 
 the polarizing hologram optical element is divided into four regions by two divided lines respectively in parallel with a radial direction and a tangential direction of the optical recording medium, and    directions of lattices or pitches of the lattices of the four regions are different from each other.    
   
   
       5 . An optical system as claimed in  claim 3 , wherein: 
 a phase distribution of lattices in the polarizing hologram optical element is formed in a step-like shape of four levels,    when phase differences of light transmitting through the two contiguous levels for ordinary light and extraordinary light are designated respectively by notation φo and φe and widths of the lattices of a 1-st stage through a 4-th stage are respectively designated by notations p/2-w, w, p/2-w and w, φo is substantially equal to 0, φe is substantially equal to    π/2 and w/p falls within the range of 0<w/p<0.25 or 0.25<w/p<0.5, and    the emitted light from the light source is incident on the polarizing hologram optical element as the ordinary light while the reflected light from the optical recording medium is incident on the polarizing hologram optical element as the extraordinary light.    
   
   
       6 . An optical system as claimed in  claim 3 , wherein: 
 a phase distribution of lattices in the polarizing hologram optical element is formed in a step-like shape of four levels,    when phase differences of light transmitting through the two contiguous levels for ordinary light and extraordinary light are designated respectively by notations φo and φe and widths of the lattices of a 1-st stage through a 4-th stage are respectively designated by notations p/2-w, w, p/2-w and w, φo is substantially equal to π/2, φe is substantially equal to 0 and w/p falls within the range of 0<w/p<0.25 or 0.25<w/p<0.5, and    the emitted light from the light source is incident on the polarizing hologram optical element as the extraordinary light while the reflected light from the optical recording medium is incident on the polarizing hologram optical element as the ordinary light.    
   
   
       7 . An optical system as claimed in  claim 1 , wherein: 
 the second optical separating means comprises a Wollaston prism,    the first group of light is one of two refracted lights of the Wollaston prism, and    the second group of light is the other of two refracted lights of the Wollaston prism.    
   
   
       8 . An optical system as claimed in  claim 7 , wherein: 
 the Wollaston prism includes a first prism disposed on an incident side of the reflected light from the optical recording medium and a second prism disposed on an emitting side of the reflected light from the optical recording medium,    an optical axis of the first prism is inclined by an angle θ to a direction in parallel with a polarizing direction of the reflected light from the optical recording medium,    an optical axis of the second prism is inclined by the angle θ to a direction orthogonal to the polarizing direction of the reflected light from the optical recording medium,    the first group of light is refracted light constituting extraordinary light in the first prism and constituting ordinary light in the second prism of the reflected lights from the optical recording medium,    the second group of light is refracted light constituting the ordinary light in the first prism and constituting the extraordinary light in the second prism in the reflected light from the optical recording medium, and    θ falls within the range of −45°<θ<0° or 0°<θ<θ<45°.    
   
   
       9 . An optical system as claimed in  claim 7 , wherein: 
 the Wollaston prism includes a first prism disposed on an incident side of the reflected light from the optical recording medium and a second prism disposed on an emitting side of the reflected light from the optical recording medium,    an optical axis of the first prism is inclined by an angle θ to a direction in parallel with a polarizing direction of the reflected light from the optical recording medium,    an optical axis of the second prism is inclined by the angle θ to a direction orthogonal to the polarizing direction of the reflected light from the optical recording medium,    the first group of light is refracted light constituting ordinary light in the first prism and constituting extraordinary light in the second prism in the reflected light from the optical recording medium,    the second group of light is refracted light constituting the extraordinary light in the first prism and constituting the ordinary light in the second prism of the reflected lights from the optical recording medium, and    θ falls within the range of −90°<θ<−45° or 45°<θ<90°.    
   
   
       10 . An optical system as claimed in  claim 7 , wherein: 
 a four division prism for refracting the reflected light from the optical recording medium is provided between the Wollaston prism and the optical detector or between the first optical separating means and the Wollaston prism,    the four division prism is divided into four regions by two dividing lines respectively in parallel with a radial direction and a tangential direction of the optical recording medium, and    directions of inclination of the emitting faces in respect of the incident faces or angles made by the emitting faces and the incident faces of the four regions are different from each other.    
   
   
       11 . An optical system as claimed in  claim 7 , wherein: 
 a hologram optical element for diffracting the reflected light from the optical recording medium as +1st-order diffracted light is provided between the Wollaston prism and the optical detector or between the first optical separating means and the Wollaston prism,    the hologram optical element is divided into four regions by two dividing lines respectively in parallel with a radial direction and a tangential direction of the optical recording medium, and    directions of lattices, pitches of the lattices or phase distributions of the lattices are different from each other.    
   
   
       12 . An optical system as claimed in  claim 11 , wherein: 
 the phase distribution of the lattices in the hologram optical element is formed in a step-like shape of N levels (N is an integer equal to or larger than 3), and    when a phase difference of light transmitting through the two contiguous levels is designated by a notation φ and all of widths of the lattices of a 1-st stage through an N-th stage are designated by a notation p/N, φ is substantially equal to 2π/N.