Ring-toric lens for focus-error sensing in optical data storage

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 poi nt source of light to a ring-shaped image. Focus-error sensing is accomplis hed by means of monitoring the change in ring radius: The ring expands in r esponse 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 g enerate 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 describ ed in this paper than for other techniques such as the astigmatic and the o bscuration methods. We measured an FES slope of 0.7 per micrometer of disk defocus (mu m(-l)) The corresponding theoretical FES slope is 0.96 mu m(-1) . (C) 1999 Optical Society of America. OCIS codes: 050.1220, 050.1970, 210. 0210, 220.2560, 220.3630.