Scanning Techniques For Optical Data Storage

Scanning techniques for optical data storageDavid K. TownerDisc Memory Division, Hewlett- Packard Co.Mail Stop 342, P.O. Box 39, Boise, Idaho 83707IntroductionIt seems almost paradoxical that beams of light can be moved and steered at very highspeeds using a variety of scanning methods, yet the optical disk drives now being designedand marketed for data storage applications have comparatively long access times. Knowingthat optical data storage has unrealized potential is of interest, but of more immediateconcern is the recognition that poor access performance is a serious design issue. Magneticdisk drives offer average seek times in the 15 -25ms range, compared to about 80 -500ms (ormore, for CD ROMS) for current optical drives. This performance disparity exists, in part,because the relatively massive "optical heads" in use today cannot be transported across theradius of a disk as quickly as a stack of much lighter magnetic heads. Any potentialdistance advantage that the optical drive might have, due to its substantially higher trackdensity, is offset by the magnetic drive's use of a multi -disk stack. As a result, thedrive must achieve similar radial accelerations during seeks if it is to have similar averageaccess times. The inability of current optical drives to approach the access speeds ofcomparable magnetic drives significantly reduces the competitiveness of optical products inmajor segments of the very large data storage market. This shortcoming is especially dis-turbing when we know that opto- mechanical scanners typically operate in the 1 -10ms range andthat non -mechanical scanning techniques can be substantially faster than that.This design study resulted from an attempt to find out if scanning techniques could beapplied to the design of optical data storage systems. An obvious goal was to identifygeometries which offer the possibility of very rapid access to large amounts of data. Asecondary goal was to identify alternatives to well known approaches, in the hope of findingsystems with different, and perhaps useful, sets of properties. Other than the expectationthat a rotating recording medium would be used, no initial restrictions were placed on thegeometry of the optics or mechanics. Instead, fundamental design issues were addresseddirectly, and the various system geometries resulted. A goal of this paper is to stimulatethinking and discussion and to invite consideration of non -conventional approaches to thedesign of optical data storage systems.A basic requirementDiffraction effects control the dimensions of the focused recording spot. A convergingbeam numerical aperture (NA) in the range of .47 to .55 is needed at the recording mediumA choice in this range will yield micrometer sized spots and sufficient irradiance forreading and writing with a GaAlAs laser diode source.Scanners for optical recordingGiven that an objective lens, in some form, is needed in order to focus a read /writebeam onto the surface of the storage medium at about .5 NA, there are two fundamentallydifferent ways of scanning the focused spot. A scanning element located in the convergingbeam between the lens and focus, as shown in Figure 1, is referred to as a post- objectivescanner. If the scanning element is located in the collimated or low NA space on the otherside of the objective, it is appropriately known as a pre- objective scanner; illustrated inFigure 2. As we will see, scanners of these two types have very different properties.Post -objective scanning is very attractive, where it can be used, because the objectivelens is only required to provide good image quality near the optical axis. As a result,the objective can often be compact, simple to design, and relatively inexpensive. Unfortun-ately, it is impractical to consider post- objective scanning at these numerical aperturesfor several reasons. First, the marginal rays at one side of the ray cone are incident onthe scanning mirror at very large angles and the beam focus is only barely beyond themarginal rays at the other side. As a result, the mirror must be quite large compared tothe scanned field and the possibility of scanning large fields at high speed is lost.