Amplitude detection for the focus error in optical disks using a birefringent lens

A lens made of a birefringent material shows two focal points. If the polarization of incident light is switched periodically and a detector is placed between the two focal points, this property can be utilized to detect a focus error signal equivalent to that of a dual detection optics. The proposed system offers the possibility to achieve amplitude detection for the focus error. It results in a compact optical pickup with reduced adjustment requirements. A second possible pickup uses a birefringent lens and a Wollaston prism. In this case, a compact dual detection method is achieved.