Confocal Head for Digital Optical Data Disk Drive
Original Publication Date: 1988-Jan-01
Included in the Prior Art Database: 2005-Feb-15
The theoretical minimum focused spot size for a conventional R/W head is determined by the laser wavelength and the numerical aperture (NA) of the objective lens. The full width half maximum (FWHM) spot diameter is ***** SEE ORIGINAL FOR MATHEMATICAL EQUATIONS IN DOCUMENT ***** 0.53g approximately FWHM = NA . This fundamental value limits the maximum areal density of data which can be written and reliably read. Adding a pinhole aperture to the detector branch of an otherwise typical optical system converts it to a confocal system as shown in Fig. 1. This reduces the effective spot size for reading by about !2. This can be thought of as using the point spread function (PSF) twice; i.e., the read spot size becomes proportional to PSF2 .