| 摘要 |
Light from a light source is applied to a diffraction grating via a collimator lens. The light diffracted by the diffraction grating is applied to a polarization beam splitter. The light polarized by the polarization beam splitter at a given polarizing plane is transmitted and is applied to a recording surface of an optical disc via a ¼-wave plate and an objective lens. The light reflected from the recording surface of the optical disc is applied to the polarization beam splitter via the objective lens and the ¼-wave plate and is reflected by the polarization beam splitter toward a focusing lens. The light incident to the focusing lens is thereby condensed and is applied to a holographic element which diffracts the light and disposes the focal points of the ±1 diffraction orders to be offset from each other along the optical axis of the zeroth diffraction order with the focal point of the zeroth diffraction order being between the focal points of the ±1 diffraction orders. The diffracted light is applied to a photo-detector via an optical element, for example, a flat plate disposed to be inclined with respect to the optical axis, for increasing the diameter of the diffracted light in a given direction.
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