The document discusses the history and technology of optical storage. It describes three generations: First generation used infrared lasers, storing up to 700MB on CDs. Second generation used visible red lasers, allowing DVDs to store up to 4.7GB. Third generation uses blue-violet lasers and greater precision, enabling discs like Blu-ray to store over 5TB of data.

1. V. GANUPRIYA Y0803A08 II – B.SC ELECTRONICS SRI ARAVINDAR ARTS & SCIENCE COLLEGE

2. OPTICAL RECORDING AND TECHNOLOGY Optical storage systems consist of a drive unit and a storage medium in a rotating disk form. In general the disks are pre-formatted using grooves and lands (tracks) to enable the positioning of an optical pick-up and recording head to access the information on the disk. Under the influence of a focused laser beam emanating from the optical head, information is recorded on the media as a change in the material characteristics, often using a thermally induced effect. To record a bit, a small spot is generated on the media modulating the phase, intensity, polarization, or reflectivity of a readout optical beam which is subsequently detected by a detector in the optical head. The disk media and the pick-up head are rotated and positioned through drive motors and servo systems controlling the position of the head with respect to data tracks on the disk. Additional peripheral electronics are used for control and data acquisition and encoding/decoding. Such a system is illustrated in Fig. 1.1 .

3. As for all storage systems, the storage capacity, data transfer rate, access time, and cost characterize optical disks systems. The storage capacity is a direct function of the spot size (the minimum dimensions of a stored bit) and the geometrical dimensions of the media. A good metric that measures the efficiency in using the storage area is the areal density (Gb/in2). The areal density is governed by the resolution of the media and by the numerical aperture of the optics and the wavelength of the laser in the optical head used for recording and read-out. The areal density can be limited by how well one can position the head over the tracks. The track density (tracks/in) is used as a metric for this characteristic. In addition the areal density can be limited by how close the optical transitions can be spaced. This is measured by the linear bit density (bits/in). The data transfer rate is critical in applications where long data streams must be stored or retrieved such as in image storage or back-up applications. The linear density, the rotational speed of the drive, and the number of pickup heads determine data rate. It is often limited by the optical power available, the speed of the pick-up head servo controllers, and the tolerance of the media to high centrifugal forces

4. Finally, the cost of a drive, consisting of the drive cost and the media cost, strongly depends on the number of units produced, the automation techniques used during assembly, and component and overall system yields. Optical storage offers a reliable and removable storage medium with excellent robustness and archival lifetime and with very low cost. A key difference between optical recording and magnetic recording is the ease with which the optical media can be made removable. Both optical recording and readout can be performed with a head positioned relatively far away from the storage medium, unlike magnetic hard drive heads. This allows the medium to be removable and effectively eliminates head crashes, increasing reliability. In addition, during recording, optical radiation is used as a focused thermal source allowing the use of more stable materials suitable for archival lifetimes. On the other hand, the remote optical head is heavier and leads to slower access times when compared to hard disk drives. These types of applications, while benefiting from random access capabilities of disk systems, are less sensitive to access time requirements but require low cost and high capacity removable storage. The compact disk (CD) format and more recently the digital video disk (DVD) format based on phase change media are designed to best satisfy these requirements. By thermally heating at different rates, a laser beam can record bits of information by locally changing the reflectivity of the medium. With the CD and DVD formats, the information is recorded in a spiral while the disk turns at a constant linear velocity, thus maximizing data capacity at the expense of transfer rate. The original CD format used a 12 cm standard disk which offers a typical capacity of 650 MB with a seek time (access time) in the order of 300 ms and data rate of about 100 kbps.

5. FIRST GENERATION Initially, optical discs were used to store music and computer software. The laser disc format stored analog video signals, but, commercially, lost to the VHS videotape cassette, due mainly to its high cost and non-re-recordability; other first-generation disc formats were designed only to store digital data and were not initially capable of use as a video medium. Most first-generation disc devices had an infrared laser reading head. The minimum size of the laser spot is proportional to its wavelength, thus wavelength is a limiting factor against great information density, too little data can be stored so. The infrared range is beyond the long-wavelength end of the visible light spectrum, so, supports less density than any visible light colour. One example of high-density data storage capacity, achieved with an infrared laser, is 700MB of net user data for a 12 cm compact disc. NOTE: other factors affecting data storage density are, for example, a multi-layered infrared disc would hold more data than an identical single-layer disc; whether CAV, CLV, or zoned-CAV; how the data are encoded; how much clear margin at the center and the edge Compact Disc (CD) Laser disc Magneto-optical disc mini disc DVD