optical recording[′äp·tə·kəl ri′kȯrd·iŋ]
The process of recording signals on a medium through the use of light, so that the signals may be reproduced at a subsequent time. Photographic film has been widely used as the medium, but in the late 1970s development of another medium, the so-called optical disk, was undertaken. The introduction of the laser as a light source greatly improves the quality of reproduced signals. The pulse-code modulation (PCM) techniques make it possible to obtain extremely high-fidelity reproduction of sound signals in optical disk recording systems.
Optical film recording
Optical film recording is also termed motion picture recording or photographic recording. A sound motion picture recording system consists basically of a modulator for producing a modulated light beam and a mechanism for moving a light-sensitive photographic film relative to the light beam and thereby recording signals on the film corresponding to the electrical signals. A sound motion picture reproducing system is basically a combination of a light source, an optical system, a photoelectric cell, and a mechanism for moving a film carrying an optical record by means of which the recorded photographic variations are converted into electrical signals of approximately similar form.
In laser-beam film recording, an optical film system utilizes a laser as a light source, a combination of an acoustooptical modulator (AOM) and an acoustooptical deflector (AOD) instead of a galvanometer. A 100-kHz pulse-width modulation (PWM) circuit converts the audio input signal into a PWM signal. The laser beam is made to continuously scan the sound track area at right angles to the direction of the film transport. This is done by means of the acoustooptical deflector, which in turn is driven by a 100-kHz sawtooth signal. Simultaneously, the laser beam is pulse-width-modulated by means of the acoustooptical modulator, which is driven by a 100-kHz PWM signal. The scanning signal and the pulse-width-modulated signal combine and generate the variable-area sound track exposure on the film. The traces of successive scans are fused into a pattern of variable-area recording.
Optical data storage
Optical data storage involves placing information in a medium so that, when a light beam scans the medium, the reflected light can be used to recover the information. There are many forms of storage media, and many types of systems are used to scan data.
In the recording process (Fig. 1), an input stream of digital information is converted with an encoder and modulator into a drive signal for a laser source. The laser source emits an intense light beam that is directed and focused into the storage medium with illumination optics. As the medium moves under the scanning spot, energy from the intense scan spot is absorbed, and a small localized region heats up. The storage medium, under the influence of the heat, changes its reflective properties. Since the light beam is modulated in correspondence to the input data stream, a circular track of data marks is formed as the medium rotates. After every revolution, the path of the scan spot is changed slightly in radius to allow another track to be written.
In readout of the medium (Fig. 2), the laser is used at a constant output power level that will not heat the medium beyond its thermal writing threshold. The laser beam is directed through a beam splitter into the illumination optics, where the beam is focused into the medium. As the data to be read pass under the scan spot, the reflected light is modulated. The modulated light is collected by the illumination optics and directed by the beam splitter to the servo and data optics, which converge the light onto detectors. The detectors change the light modulation into current modulation that is amplified and decoded to produce the output data stream.
Optical media can be produced in several different configurations. The most common configuration is the single-layer disk, such as the compact disk (CD), where data are recorded in a single storage layer. A substrate provides mechanical support for the storage layer. The substrate also provides a measure of contamination protection, because light is focused through the substrate and into the recording layer. Dust particles on the surface of the substrate only partially obscure the focused beam, so enough light can penetrate for adequate signal recovery.
In order to increase data capacity of the disk, several layers can be used. Each layer is partially transmitting, which allows a portion of the light to penetrate throughout the thickness of the layers. The scan spot is adjusted by refocusing the illumination optics so that only one layer is read out at a time.
Data can also be recorded in volumetric configurations. As with the multiple-layer disk, the scan spot can be refocused throughout the volume of material to access information. Volumetric configurations offer the highest efficiency for data capacity, but they are not easily paired with simple illumination optics.
The final configuration is to place the information on a flexible surface, such as ribbon or tape. As with magnetic tape, the ribbon is pulled under the scan spot and data are recorded or retrieved. Flexible media have about the same capacity efficiency as volumetric storage. The advantage of a flexible medium over a volumetric medium is that no refocusing is necessary. The disadvantage is that a moderately complicated mechanical system must be used to move the ribbon.
There are several types of optical storage media. The most popular media are based on pit-type, magnetooptic, phase-change, and dye-polymer technologies. CD and digital versatile disc (DVD) products use pit-type technology. Erasable disks using magnetooptic (MO) technology are popular for workstation environments. Compact-disk-rewritable (CD-RW) products [also known as compact-disk-erasable (CD-E)] use phase-change technology, and compact-disk-recordable (CD-R) products use dye-polymer technology. CD and DVD products are read-only memories (ROMs); that is, they are used for software distribution and cannot be used for recording information. CD-R products can be used for recording information, but once the information is recorded, they cannot be erased and reused. Both CD-RW and MO products can be erased and reused.