Holographic optical element, position shift detecting apparatus, optical pickup apparatus, optical recording medium drive and method of fabricating holographic optical element

A position shift detecting apparatus that irradiates an object to be detected with a light beam and detects a returned light beam from the object to be detected, comprising

• a light source that emits the light beam;
• a holographic optical element comprising a diffraction surface for diffracting the returned light beam from said object to be detected;
• and
• a photodetector that detects said returned light beam diffracted by said holographic optical element,
• the diffraction surface of said holographic optical element having in at least its part a holographic pattern having the function of twisting said diffracted light beam using the optical axis of the diffracted light beam as an axis of rotation, and
• a light spot based on said twisted diffracted light beam being formed on said photodetector.

An optical recording medium drive comprising:

• a rotation driving unit that rotates an optical recording medium;
• an optical pickup apparatus including a position shift detecting apparatus;
• an objective lens that condenses a light beam emitted from said optical pickup apparatus on said optical recording medium;
• a pickup driving unit that moves said optical pickup apparatus in the radial direction of said optical recording medium;
• a lens driving unit that moves said objective lens relative to an object to be detected;
• and
• a signal processing unit that processes an output signal from a photodetector in said optical pickup apparatus,
• said position shift detecting apparatus comprising
• a light source that emits a light beam,
• a holographic optical element comprising a diffraction surface for diffracting a returned light beam from said object to be detected, and
• a photodetector that detects said returned light beam diffracted by said holographic optical element,
• said holographic optical element having in at least its part a holographic pattern having the function of twisting said diffracted light beam using the optical axis of the diffracted light beam as an axis of rotation, and forming a light spot based on said twisted diffracted light beam on said photodetector.

A method of fabricating a holographic optical element comprising a diffraction surface having in at least its part a holographic pattern for diffracting an incident light beam, comprising:

• the step of representing, when said diffraction surface is defined by XY-coordinates comprising the X-axis and the Y-axis which are orthogonal to each other, and letting (X0, Y0) be a point to be a basis on said diffraction surface, (X, Y) be an arbitrary point on said diffraction surface, f (X, Y) be an X-direction component of a grating vector for performing predetermined diffraction at the point (X, Y) on said diffraction surface, and g (X, Y) be a Y-direction component of said grating vector, said holographic pattern by a set of points (X′, Y′) satisfying the following equation:
• ∫X0X′f(X,Y0)dX+∫Y0Y′g(X′,Y)dY=2πM+C (19)
• or
• ∫Y0Y′g(X0,Y)dY+∫X0X′f(X,Y′)dX=2πM+C (20)
• setting, when a Z-axis coordinate perpendicular to said diffraction surface is defined at the origin of said XY-coordinates, and letting (X, Y, 0) be an arbitrary point on said diffraction surface, (Xp(X, Y), Yp(X, Y), Zp) be a point on a predetermined detection surface on which a diffracted light beam diffracted by said diffraction surface impinges, (Xr, Yr, Zr) be the coordinates of a luminescent point of a light source for emitting a light beam to said diffraction surface, λ be the wavelength of the light beam, and n be the refractive index of a substrate including said holographic pattern, the X-direction component f (X, Y) and the Y-direction component g (X, Y) of the grating vector for performing predetermined diffraction at the point (X, Y, 0) on said diffraction surface, respectively, so as to satisfy the following equations:
• 5 f ⁡ ( X , Y ) = - ( 2 ⁢ π / λ ) · [AutoLeftMatch] [ ( X - Xp ⁡ ( X , Y ) ) · { ( X - Xp ⁡ ( X , Y ) ) 2 + ( Y - Yp ⁡ ( X , Y ) ) 2 + Zp 2 } - 1 / 2 - n ⁡ ( X - Xr ) · { ( X - Xr ) 2 + ( Y - Yr ) 2 + Zr 2 } - 1 / 2 ] ( 16 ) g ⁡ ( X , Y ) = - ( 2 ⁢ π / λ ) · [AutoLeftMatch] [ ( Y - Yp ⁡ ( X , Y ) ) · { ( X - Xp ⁡ ( X , Y ) ) 2 + ( Y - Yp ⁡ ( X , Y ) ) 2 + Zp 2 } - 1 / 2 - n ⁡ ( Y - Yr ) · { ( X - Xr ) 2 + ( Y - Yr ) 2 + Zr 2 } - 1 / 2 ] ;
• ( 17 )
• and
• the step of forming said holographic pattern set by said equations (16), (17), (19), and (20) on said diffraction surface by a photolithographic process and an etching process.

An optical pickup apparatus that irradiates an optical recording medium with a light beam and detects a returned light beam from the optical recording medium, comprising:

• a light source that emits the light beam;
• a holographic optical element comprising a diffraction surface for diffracting the returned light beam from said optical recording medium;
• and
• a photodetector that detects said returned light beam diffracted by said holographic optical element,
• the diffraction surface of said holographic optical element having in at least its part a holographic pattern having the function of twisting said diffracted light beam using the optical axis of the diffracted light beam as an axis of rotation, and forms in said photodetector a light spot based on said twisted diffracted light beam.