The Development of a Large Beam Defect Scanner
Document ID: 339
Senior Honors Thesis
The Pennsylvania State University, Department of Electrical Engineering, University Park, PA, U.S.A.
Abstract
This paper describes an alternative to defect scanning erasable magneto-optic (MO) disks. Magneto-optic disks require that their writing surface be totally reflective, like a mirror. The information is read from the disk by detecting a change in the polarization of the light, rather than by a phase change such as that used by the popular compact disc. Therefore, a good way of checking for defects would be to shine light on the disk's surface and to detect where the light does not reflect uniformly off the disk. At these points we would expect to find reflectivity defects. Alternatively, one could look for very small holes in the disk's magnetic coating, by shining light on the disk and placing a detector behind the disk. Where too much light shines through to the detector, these points would be locations of transmission defects. Equipment is now available that will focus a 1 µm laser an the disk's surface and scan each track for reflectivity defects. Since there are eighteen thousand tracks per disk, one scan takes approximately twelve minutes.
This paper discusses the use of a large laser beam scanner (about 10 X 30 μm). This type of scanner does not follow each track and will therefore scan an entire disk's surface in about 30 seconds, rather then 12 minutes. Because this type of tester does not follow each individual track, no focusing or tracking servo will be required. In an industrial environment this tester's simplicity as well as rapid scan time would provide an excellent alternative to the current method of sorting out defective disks.
| Citation: | T. D. Stevens, "The Development of a Large Beam Defect Scanner", The Pennsylvania State University, Senior Honors Thesis, 1990, 81 pages |