Diffractive optical element, objective optical system including the same, and optical pickup including the same

An objective optical system for focusing each of a plurality of kinds of laser beams of different wavelengths on an information recording surface of an optical information recording medium, comprising:

• a diffractive optical element including a first optical part and a second optical part bonded to each other with a bonded surface therebetween formed as a diffraction surface, for diffracting said plurality of kinds of laser beams on said diffraction surface;
• and
• an objective lens for focusing each of said plurality of kinds of laser beams having passed through said diffractive optical element on said information recording surface,
• wherein diffracted light with the largest quantity of light out of diffracted light for at least one of said plurality of kinds of laser beams obtained on said diffraction surface of said diffractive optical element has a second or higher diffraction order,
• said plurality of kinds of laser beams include a first laser beam of a first wavelength and a second laser beam of a second wavelength different from said first wavelength, and
• the following conditional formula (1) is satisfied in a region of said diffractive optical element where at least said first laser beam and said second laser beam enter:
• 0.7<(m1·λ1(n1(λ2)−n2(λ2))/(m2·λ2(n1(λ1)−n2(λ1))<1.3 Formula (1)
• wherein m1 is a diffraction order of diffracted light with the largest quantity of light out of diffracted light for said first laser beam obtained on said diffraction surface;
• m2 is a diffraction order of diffracted light with the largest quantity of light out of diffracted light for said second laser beam obtained on said diffraction surface;
• λ1 is said first wavelength;
• λ2 is said second wavelength;
• n1(λ1) is a refractive index of said first optical part to said first laser beam;
• n1(λ2) is a refractive index of said first optical part to said second laser beam;
• n2(λ1) is a refractive index of said second optical part to said first laser beam;
• and n2(λ2) is a refractive index of said second optical part to said second laser beam.

An optical pickup for focusing a laser beam on an information recording surface of an optical information recording medium, comprising:

• a light source for emitting a plurality of kinds of laser beams of different wavelengths;
• and
• an objective optical system for focusing each of said plurality of kinds of laser beams emitted from said light source onto said information recording surface,
• wherein said objective optical system contains:
• a diffractive optical element including a first optical part and a second optical part bonded to each other with a bonded surface therebetween formed as a diffraction surface for diffracting each of said plurality of kinds of laser beams emitted from said light source on said diffraction surface;
• and
• an objective lens for focusing each of said plurality of kinds of laser beams having passed through said diffractive optical element on said information recording surface,
• diffracted light with the largest quantity of light out of diffracted light for at least one of said plurality of kinds of laser beams obtained on said diffraction surface of said diffractive optical element has a second or higher diffraction order,
• said light source at least emits, as said plurality of kinds of laser beams, a first laser beam of a first wavelength and a second laser beam of a second wavelength different from said first wavelength, and
• the following conditional formula (1) is satisfied in a region of said diffractive optical element where at least said first laser beam and said second laser beam enter:
• 0.7<(m1·λ1(n1(λ2)−n2(λ2))/(m2·λ2(n1(λ1)−n2(λ1))<1.3 Formula (1)
• wherein m1 is a diffraction order of diffracted light with the largest quantity of light out of diffracted light for said first laser beam obtained on said diffraction surface;
• m2 is a diffraction order of diffracted light with the largest quantity of light out of diffracted light for said second laser beam obtained on said diffraction surface;
• λ1 is said first wavelength;
• λ2 is said second wavelength;
• n1(λ1) is a refractive index of said first optical part to said first laser beam;
• n1(λ2) is a refractive index of said first optical part to said second laser beam;
• n2(λ1) is a refractive index of said second optical part to said first laser beam;
• and n2(λ2) is a refractive index of said second optical part to said second laser beam.