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

An optical pickup apparatus for recording and/or reproducing information on an optical information recording medium, comprising:

• a light source for emitting a light flux having a wavelength λ,
• an optical system for converging the light flux onto an optical information recording medium,
• a focusing device for focusing the converged light flux on the optical information recording medium, and
• a photo-detector for detecting a light flux reflected from the optical information recording medium;
• wherein the optical system comprises an objective lens, a plastic coupling lens having a positive refractive power as a whole to change a divergent angle of the light flux emitted from the light source and an optical element, provided between the objective lens and the plastic coupling lens, to pass light flux coming from the plastic coupling lens and to reflect light flux coming from the objective lens to the photo-detector;
• wherein the optical system is structured such that the light flux emitted from the light source passes through the coupling lens, the optical element and the objective lens in this order 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 and is reflected by the optical element to the photo-detector 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
• wherein a change of an image forming magnification of the coupling lens as a single optical elements negligible even when the temperature of the optical pickup apparatus is changed.

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 for changing 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,
• wherein the coupling lens has a positive refractive power as a whole, and
• wherein a change of an image forming magnification of the coupling lens as a single optical element is negligible even when temperature of the optical pickup apparatus is changed.

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,
• 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, and
• wherein a change of an image forming magnification of the coupling lens as a single optical element is negligible even when temperature of the optical pickup apparatus is changed.

An optical information recording and/or reproducing apparatus, comprising:

• a light source for emitting a light flux having a wavelength λ,
• an optical system for converging the light flux onto an optical information recording medium,
• a focusing device for focusing the converged light flux on the optical information recording medium, and
• a photo-detector for detecting 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 as a whole 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
• wherein a change of an image forming magnification of the coupling lens as a single optical element is negligible even when temperature of the optical pickup apparatus is changed.