Optical Disk Data Storage, Distribution and Retrieval

It is no coincidence that we interact with the world primarily through visual means—light has many ideal properties for communication of information. These same properties make light ideal for data storage. The wavelength of visible light is small (on the order of 0.5 microns or about 1/7000 the height of a letter written in 10-point font), which indicates it can be used to read or write a large amount of data per unit area. Unlike direct-contact methods, such as printing or phonographs, light can access the storage medium from a distance,andsoitdoesnotriskdamagetothedata.This factor is particularly important when the medium is being moved veryquickly,whichisrequiredbothforrapidaccessandhigh read/write speed. Thus, optics has fundamentally attractive qualities to enable high density, fast access, rapid-transfer rateand long-termreliability.These aspectsare theprimary user requirements for any data-storage system. These qualities were all exploited in microfiche, which was the first large-scale system for read-only optical data storage, distribution, and retrieval. The analogies with later optical disks are striking. Microfiche masters are first created by precision machines that photographically reduce the document to be distributed. These masters are then replicated by contact optical printing to make many inexpensive distribution copies. These distribution copies are shipped to end users such as libraries, where high information density and low cost make them much preferable to paper copies of the original text. Finally, individuals access the data rapidly by loading a fiche into a two-dimensional scan table and an optical projector. Many of these reading machines are equipped with photocopier to print an image back to paper, which completes the distribution loop. The need for a digital version of this system for distribution of audio and video data was financially motivated by the entertainment industry and made possible by the development of semiconductor electronics. The distribution needs are similar to the microfiche model—possibly highcost mastering equipment to enable high-volume, inexpensive reproduction of distribution media, which will be read by low-cost machines to reproduce the original content. The primary difference is that microfiche provides manual access to two-dimensional images, whereas the entertainment application needs fully automatic access to primarily serial data, such as audio. This need suggests the use of a single ‘‘track’’ wrapped in a spiral onto a two-dimensional disk spun by a motor, which of course describes mechanically fabricated and mechanically accessed phonograph records. An ‘‘optical phonograph’’ is not possible with conventional light sources, such as incandescent lamps, because of a version of the law of conservation of energy called the ‘‘conservation of radiance.’’ The radiance of a light source is given by its power in watts divided by its area and the solid angle into which it radiates. For example, a typical light bulb might radiate 10 watts from a mm filament into a 2 p hemisphere, which results in a radiance on the order of 10 W m 2 Sr . The conservation of radiance does not allow any optical system such as a lens to increase this quantity because as the area of a focus is reduced, the angular divergence is increased proportionally. Thus, as one attempts to focus a light bulb onto a small spot, power is inevitably lost because the maximum solid angle the lens can capture is limited. For the example lamp, the maximum power that could be delivered into a mm spot is roughly a microwatt, which is insufficient power to transfer data at audio rates. The situation changed in 1960 with Maiman’s demonstration of light amplification through the stimulated emission of radiation (‘‘laser’’). It is often reported in the popular literature that laser light differs from conventional light sources by being of a single frequency. This is not necessarily true—the lasers used in optical disk players oscillate simultaneously at a set of frequencies. The primary interesting property of lasers as a light source is their radiance. The semiconductor lasers used in consumer read/write disk players today have roughly 10 mW of optical power, which is a factor of 1000 lower than our hypothetical light bulb, but they radiate this light from an aperture of a few square microns with a divergence angle of tens of degrees. Their radiance is thus on the order of 10 W m 2 Sr , which is ten million times larger than the lamp. If expanded to mm area, the divergence can be very low, which is why laser pointers can deliver a bright spot across a large conference room. Similarly, when focused, virtually all laser power can be delivered to a spot size limited only by the physics of diffraction to be roughly the size of the wavelength. Although the few milliwatts of total optical power is low, the intensity (formally irradiance) in watts per square meter is a million times greater than the surface of the sun. This high intensity is used during writing to heat the surface of the disk by hundreds of degrees Celsius in just a few nanoseconds. OPTICAL DISK DATA STORAGE, DISTRIBUTION AND RETRIEVAL 2069