Sadik C. Esener


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.

Fig. 1.1. Key components of an optical disk system.

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.

The access time is a critical parameter in computing applications such as transaction processing and represents how fast a data location can be accessed on the disk. It is mostly governed by the latency of the head movements and is proportional to the weight of the pick-up head and the rotation speed of the disk.

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.

Consequently optical storage has remained limited to market segments requiring removability and reliability that are not well served by magnetic hard disks. Typical applications involve archival storage, including software distribution, storing digital photographs and medical imaging, information appliances including recording movies, other video materials, and multimedia presentations at home and business, and online databases including video servers. More recently the magnetic tape market for video camcorders and VCRs has also been targeted.

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. Approximately a decade later, the DVD format offers 4.7 GB capacity, 10 Mb/s data rate and 100 ms access time by using a smaller spot size (from a shorter wavelength laser and a higher numerical aperture lens), faster rotation speeds, higher power lasers, more powerful error correction codes, and faster servo systems. Because the head is not flying on the media, one head is also capable of recording and reading multiple storage layers, thus increasing the capacity to 9.4 GB in a two-layer DVD-ROM disk.

Magneto-optic (MO) storage systems record data by thermally heating (with the laser spot) the media under the influence of a magnetic field. Data are recorded by re-orienting magnetic domains within the heated spot. During readout the polarization of the laser beam is modulated by the orientation state of the magnetic domains. Until recently, all systems using magneto optic media used a standard format to shorten the access times (at the expense of capacity) and approach hard disk like speed performance with a removable media. With standard formatted systems the disk turns at a constant angular velocity and data are recorded on concentric tracks as in magnetic hard drives. While reading the inner or outer tracks the speed of rotation remains constant, allowing for faster access times. However, this format results in constant number of bits per track, limited by the number of bits that can be supported by the most inner track, and wastes valuable real estate on the outer tracks. To eliminate this waste, a "banded" format is now used where tracks of similar length are grouped in bands, allowing the outer bands to support much larger number of bits than the inner bands. This, however, requires different channel codes for the different bands to achieve similar bit error rates over the bands. Today, removable MO systems provide 640 MB capacity with 3.5 inch diameter disks with speed performance comparable to hard drives. The WTEC team members learned during the visits that in the near future certain MO disk drive products might adopt the DVD format and become contenders for the video market as a VCR replacement.

In contrast to non-removable systems, for removable storage, yearly increases in performance are not necessarily desirable. This is because removable storage systems and media are tightly constrained by standards that are established for compatibility purposes. Removable storage manufacturers instead introduce products at an entry capacity and performance point that is desirable for a particular data type. Thus, the optical storage market is essentially driven by applications rather than by progress made in technology. Therefore, projections made on optical storage critically depend on application roadmaps such as the one shown in Fig. 1.2.

Fig. 1.2. Potential evolution of application requirements for removable storage (information gathered from OITDA and surveys performed by Call/Recall, Inc. and USC).

Over the next decade with the emergence of the Internet, a new an important class of applications referred to as server-based applications will emerge. While personal applications will define future removable storage standards, server-based applications will significantly boost the size of the optical and magnetic disk storage market.

Many server-based applications, such as electronic commerce, medicine, and libraries, among others, require modest access times (<10 ms) but very large storage capacities and appreciable data transfer rates, as shown in Fig. 1.3. These applications, because of their very large capacity requirements, will initially be constructed as RAID or disk library systems based on commodity personal computer drives. Drive and database maintenance costs will be among deciding factors for various technology solutions.

With the DVD ROM standard it is possible to distribute one two-hour video movie per disk. Soon the DVD RAM standard will enable consumers to record a two-hour long video per disk. An important jump in performance will be needed slightly after the turn of the millennium to address HDTV applications requiring 15 GB capacities per movie. Most of WTEC's hosts indicated that the technology was in already place to address HDTV quality video. There are also serious considerations by both the phase change and MO manufacturers to use 30-40 GB disks as VCR replacements, perhaps shortly before the year 2005. Guessing the type of applications that may drive optical storage technologies beyond 2005 is certainly speculative at this point in time. It is, however, plausible that 3D interactive video in some form of virtual reality application might become a driver for higher capacities within the next decade, pushing capacity requirements beyond 0.1 TB to 1 TB per disk. Several MO disk drive manufacturers also point out that the performance gap between magnetic hard drives and MO drives has shrunk to a point where MO drives might be considered for general computing applications. This will depend on the capacity and cost performance that hard disk drives offer in the future.

Fig. 1.3. (a) Capacity and (b) data rate requirements imposed by various applications by the years 2005 and 2010 (information gathered from OITDA and surveys performed by Call/Recall, Inc. and USC).

A different, yet critical, category of applications concerns portable and handheld devices that place more importance on system volume and power dissipation considerations. Typical applications here include compact storage systems for camcorders, personal digital assistants and communicators. Within the next decade, these applications will require about 50 GB capacity and reasonably fast access times (microseconds) and transfer rates (100 Mb/s) within very small volumes and with power dissipations of less than a few milliwatts. Miniaturized hard disk drives, solid state disks, probe storage, and even single electron DRAM chips might serve this category of applications in the future. At this point it is doubtful that conventional optical storage technologies can address the needs of this market segment.

It was the strong belief of our Japanese hosts in general that the performance of data storage systems during the next decade will not be limited by a lack of applications pull but will rather be constrained by the capabilities of technologies in hand.

Published: June 1999; WTEC Hyper-Librarian