We discuss the design and performance of diffractive ring-toric lenses for focus-error sensing in optical data storage. A ring-toric lens images a point source of light to a ring-shaped image. Focus-error sensing is accomplished by means of monitoring the change in ring radius: The ring expands in response to a diverging wave front, and the ring contracts in response to a converging wave front. We describe the use of a segmented phi detector to generate a focus-error signal (FES). We found that the FES slope, a measure of sensitivity to disk defocus, is higher for the ring-toric lenses described in this paper than for other techniques such as the astigmatic and the obscuration methods. We measured an FES slope of 0.7 per micrometer of disk defocus (μm<sup>−1</sup>). The corresponding theoretical FES slope is 0.96 μm<sup>−1</sup>.
© 1999 Optical Society of America
(050.1220) Diffraction and gratings : Apertures
(050.1970) Diffraction and gratings : Diffractive optics
(210.0210) Optical data storage : Optical data storage
(220.2560) Optical design and fabrication : Propagating methods
(220.3630) Optical design and fabrication : Lenses
Michael R. Descour, Daniel I. Simon, and Wei-Hung Yeh, "Ring-Toric Lens for Focus-Error Sensing in Optical Data Storage," Appl. Opt. 38, 1388-1392 (1999)