Three-beam CD optical pickup using a holographic optical element

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Abstract
A three beam CD optical pickup has been developed with a holographic optical element (HOE). The HOE has a hologram for beam splitting and servo signal generation on one side of a glass substrate, and a grating for tracking-beam generation on the other side. The hologram pattern is divided into two regions. The grating periods of these regions are different from each other, so that the focus error signal is detected by Foucault knife-edge method. And also the radial error signal is detected by the twin-spot method. The HOE is cemented at the top of a package in which a laser diode and a photodiode for signal detection are mounted. The HOE offers a improvement of environmental stability of the pickup too. In spite of the usage of a plastic housing, the optical pickup shows superior characteristics of environmental stability.

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