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Optical data storage

01 Oct 1998

Companies are continuing to build on the tremendous success of the compact disc by developing systems that could eventually replace the traditional video recorder

The compact disc has revolutionized the way information is stored. Audio CDs have all but wiped out the vinyl record, while CD-ROMs are now replacing more traditional forms of information such as encyclopaedias and reference books. CDs have become one of the cheapest and most convenient ways to distribute digital information.

Compact discs are also an example of the much heralded convergence of computers, communication systems and consumer electronics in an era of digital information. The latest versions allow us to record and edit audio fragments with a personal computer or home CD recorder, and then listen to the results on a normal CD player. Digital video discs (DVD) are now being introduced for distributing prerecorded movies, and an optical disc system for recording digital video will soon emerge from research laboratories.

These more futuristic systems will require higher storage densities than are currently available. A single-sided compact disc now stores 650 Mbytes of information, corresponding to a data density of 1 bit per square micron. In the early 1980s this storage density was 100 times greater than could be achieved with conventional magnetic hard disks – at the time floppy disks with a storage capacity of only 1.4 Mbytes could store the entire operating system of a personal computer. Since then, however, increasing storage capacities and processing power have led to a software explosion, with many of today’s computer games easily filling an entire CD.

The recordable and rewritable versions of the compact disc, which became available only a few years ago, have already met with tremendous commercial success. Although the market for read-only CD-ROMs is still growing, it is expected that recordable or rewritable optical storage will represent up to 40% of the market for optical disc drives – estimated at $10bn – by the year 2000.

But the holy grail for optical storage is to replace the traditional videocassette recorder with a system based on optical discs. Initial attempts to introduce a read-only optical video disc met with limited success. An early attempt, an analogue video disc player called Laser Vision, flopped because the picture quality was not good enough and customers did not want a system that could not record. The CD-Video failed because it could only store 6 minutes of analogue video and 20 minutes of digital audio on a standard-sized disc. The Video-CD – which uses data compression techniques to store 74 minutes of digital audio and video – is quite successful in China where video broadcasting is not yet widespread and where the demands on picture quality are less severe.

The latest attempt, DVD-Video, is another read-only system now being launched on the European market. A storage capacity of 4.7 Gbytes and the use of data compression techniques allows DVD-Video to offer high-quality prerecorded video, 8 channels of audio and up to 32 channels of subtitles.

But a worthy successor to present-day video recorders should incorporate a rewritable optical-disc system together with 4 hours of superior picture and sound quality. It would also provide the rapid random access of a disc-based system, eliminating the need for tedious rewinding and complicated searching inherent with cassette-based systems. A video recorder based on optical discs would also allow digital video fragments to be recorded and edited, making the personal computer a true multimedia platform. Together with on-line services and distribution of digital video by cable or satellite, many new applications are expected to emerge.

Such applications would require a disc capacity of about 10 Gbytes, with data being written to disc and accessed at rates of at least 5-10 Mbit s-1. The race is now on to develop such a high-powered digital video recorder.

Replicable and removable

Any new form of optical data storage must retain the technology’s many intrinsic benefits. Compact discs, for example, have become popular because they offer a cheap and robust way to distribute information, can be easily removed, will reliably retain data for over 30 years, and can rapidly access and retrieve information. Their success can also be attributed to the fact that discs and players made by different manufacturers are entirely compatible. Worldwide standardization – with its associated company politics – is therefore a crucial step in the development of any new technology for optical storage.

The digital information on a CD is encoded in a pattern of pits with varying lengths (figure 1). This pattern can be replicated at low cost and in high volumes with a process known as injection moulding, which uses a stamper containing the desired pattern to mould the surface layer of a 1.2 mm thick plastic disc. This “information layer” is then coated with a metallic mirror and covered by a protective lacquer.

To read the data, a laser beam is focused through the substrate onto the information layer, and a photodiode detector collects the light reflected back through the lens (figure 2). The pits reflect back less light than the regions between them.

The focused spot produced by the objective lens has a finite diameter because of diffraction and is limited to about l/2NA, where l is the wavelength of light and NA is the numerical aperture of the lens. Compact-disc players use an infrared laser with a wavelength of 780 nm, and the numerical aperture of a lens is given by NA = nsinqmax, where n is the refractive index of the medium (n = 1 in air) and qmax is the outer angle of the cone formed by the converging beam of light. Because of Snell’s law of refraction, which says that nsinq is a constant, the numerical aperture does not change at the transition from air to the plastic substrate. In compact disc players, NA is typically 0.40-0.52, which gives a spot diameter of about 1 µm.

By reading the information through the substrate, dust particles and small scratches on the disc surface do not affect the read-out signal (figure 3). The diameter of the laser beam is typically about 0.7 mm as it enters the substrate, which ensures that any blemishes on the disc are out of focus.

Since the substrate is part of the optical path, it is important to control properties such as its thickness, angular orientation, optical homogeneity, birefringence and surface flatness. Moreover, the laser beam can easily become distorted as it passes through the substrate – spherical aberration causes concentric rings to form around the focused spot, which makes it difficult to read the information pattern. Although an aspherical objective lens can compensate for spherical aberration, it is still vital to maintain tight control over variations in disc thickness. Any tilting of the disc during read-out creates other aberrations that are not easily compensated for.

Commercial discs are often slightly warped, or have central holes that are not exactly in the middle. To keep the laser beam focused and centred on a track, the objective lens is mounted on an electromechanical actuator to adjust its vertical and radial position. The electronic signals needed to control these positions are derived from optical error signals.

The entire process from creating the disc to playing it back can be regarded as a problem in data transmission. Concepts from information theory are widely exploited to optimize the transmission capability of the optical recording “channel”. First of all, extra bits are added to the data to allow faulty bits to be detected and corrected during read-out. Particular attention must also be paid to the creation of the data pattern on the disc, since the transmission characteristics of the optical channel are determined by the physics of reading the data with a focused laser beam.

Because this spot has a finite size, marks on the disc are only detected if they have a certain minimum length. A so-called channel modulation code is applied to the binary data before they are recorded on the disc, resulting in an encoded pattern of marks on the disc that is optimally adapted to the characteristics of the optical channel. When reading the data, the modulation code is decoded before errors are detected and corrected. Extra steps are needed to ensure the proper timing of the bit detector, to compensate for the non-ideal frequency response of the channel, and to determine the physical location of the data.

Many different schemes have been considered for modifying conventional CDs so that they can record information once or many times. An early magneto-optic version failed because it was incompatible with conventional CDs. A disc that could be written to once and then read back many times was introduced as long ago as 1984. Intended as a replacement for magnetic hard disks in computers, its success was again rather limited.

A new format for recordable compact discs with a capacity of 650 Mbytes -the same as conventional CDs – was standardized in 1989. The tremendous success of these “CD-Recordables” is due to their compatibility with the read-only counterpart. This is achieved by recording the data using a laser that creates pits in a disc substrate covered with a carefully selected film of organic dyes. Data can be recorded only once and read back many times, which makes the discs ideal for making a copy of a CD, or for photo archiving, data back-up and file distribution.

In 1996 a rewritable compact disc became available that can record data many times, and is now considered to be a possible successor to the floppy disk. Instead of storing information as pits on the disc surface, this technology uses laser pulses to change the structural phase of the surface material. The recording marks consist of a series of overlapping amorphous dots that reflect less light than the surrounding crystalline material. The amorphous areas are created by first melting the material with the laser and then rapidly cooling the molten material below its crystallization temperature (figure 4). Data can be erased by heating the material above the crystallization temperature, but below the melting point, until the material regains its polycrystalline state. This is known as phase-change recording.

Rewritable CDs can also be played back on modern CD-ROMs or CD players, even though the data are stored in a quite different way.

Video goes digital

The latest advance in optical data storage has been the recent launch of read-only digital video discs (DVDs). This new generation of DVDs can store 4.7 Gbytes of data, seven times more than a conventional compact disc. Such an increase in capacity is the result of several key developments. The basic improvement was to make the spot size used for reading the data smaller by using shorter wavelength light (650 nm instead of 780 nm) and a lens with a larger numerical aperture (NA = 0.6). This made it possible to reduce the distance between pits and the length of the smallest pit, but other advances were needed to create a practical system.

The greater storage density of DVDs means that they are more susceptible to defects. The manufacturing tolerance of both the discs and the drives had to be tightened. Changes were also made to the data format and the channel modulation code, and a more powerful error-correction scheme was introduced. To keep operating parameters such as disc tilt and focusing errors within acceptable levels, the substrate thickness had to be halved to 0.6 mm. But to maintain mechanical stability and the same physical appearance as a compact disc, a DVD consists of two 0.6 mm substrates bonded back-to-back.

The coexistence of DVD and CD calls for the development of players that can accommodate both types of disc. This is not a trivial task, however, due to the differences in physical parameters. For example, the optical system must be able to read an information layer through a substrate that may be either 1.2 or 0.6 mm thick, while the objective lens cannot correct for the different amounts of spherical aberration simultaneously. One solution is to create a light path with a single red laser and two different objectives that can be switched into the laser beam. Another solution is to keep the single objective, but to use an infrared laser for compact discs and a red laser for DVDs. In this case a dichroic aperture is placed in front of the objective so that its numerical aperture adapts automatically to the laser being used.

However, a rewritable counterpart of the DVD with a full 4.7 Gbyte capacity presents significant technical challenges. As stepping stones towards this goal, two industry consortia have developed lower-capacity systems that are optimized for real-time random access. A working group within the DVD Forum – an industry platform steered by 16 companies, including our company Philips of the Netherlands – has proposed a 2.6 Gbyte DVD-RAM, while Hewlett-Packard of the US, Sony of Japan and Philips have submitted a 3.0 Gbyte DVD+RW for standardization by ECMA, a body active in the worldwide standardization of computer-related technologies.

To understand the differences between these two systems, it is important to know that the substrate of a rewritable disc contains embossed grooves that provide tracking when the disc is blank. The DVD-RAM format records data both within the grooves and on the “lands” between the grooves. By optimizing the groove shape and geometry, it is possible to suppress interference between adjacent lands and grooves. In contrast, the DVD+RW only records inside the grooves, just as in a rewritable CD.

The difference in storage capacities is caused by the way in which each format provides the address information on the disc. DVD-RAM uses a header format, which means that the data are interrupted by regions of embossed data pits that contain the physical address information. In contrast, a DVD+RW disc encodes the address information in a “wobble” superimposed on the groove pattern. Although the tracking mechanism is too slow to follow such a high-frequency wobble, it can easily be detected by the track-following electronics. The addressing method used in the DVD+RW format is more efficient because no disc space is used for headers.

Current R&D effort is focused on developing full-capacity rewritable or recordable DVD systems. We believe that such a capacity can be attained without major changes to existing formats, but it will be important to improve the recording materials, to change the design of the stack of materials surrounding the information layer, and to optimize strategies for writing the data.

Towards digital video recording

The development of a robust and rewritable DVD system would be quite an achievement. But we have to aim beyond this goal, and work towards a digital video recorder. Using state-of-the-art computer chips for real-time video compression, such a machine would require a storage capacity of at least 9 Gbytes and a data transfer rate of 5 Mbit s-1 to provide high-quality video over a playing time of 4 hours. An alternative scenario might be to design a system that provides 2 hours of very high-quality video recorded at 10 Mbit s-1.

There are two basic options available to achieve a capacity of 9 Gbytes, which requires about double the data density of current DVDs. The first is to replace the red laser in a DVD with a blue laser. Blue lasers with a wavelength of 410 nm were first demonstrated in 1995 by Shuji Nakamura of Nichia Chemical Industries in Japan, and Nichia has announced that lasers with sufficient power, beam quality and lifetimes will come on the market by the end of this year. However, it could still take several years before optical recording systems based on blue lasers are mass-produced.

The other option is to increase the numerical aperture of the objective lens to 0.85, compared with 0.60 in DVDs. Such a high numerical aperture would place stringent constraints on disc tilt and variations in disc thickness, since the required tolerances vary inversely with higher powers of NA, and will also result in a shorter depth of focus. The same holds true for shorter wavelengths, but in this case the required tolerances scale linearly with wavelength.

The first problem in designing a lens with such a high numerical aperture is to provide enough space between the lens and the disc to prevent collisions. This requires a doublet lens – in other words, a combination of two lenses with at least two aspherical surfaces (figure 5). Second, a system must be found to allow for some variation in disc tilt. Philips Research Laboratories in the Netherlands has studied a system that uses an actuator to keep the second lens parallel to the disc. This active-tilt correction has the advantage that it achieves compatibility with read-only DVDs, since it allows recording and read-out through a 0.6 mm substrate. However, it requires a complex actuator that relies on a number of independent error signals to feed the electronic circuits controlling it.

Another option is to use adaptive optics to cancel the aberrations induced in the wavefront. An interesting idea, recently demonstrated by Pioneer of Japan, is to achieve this with segmented liquid-crystal cells.

But Sony has suggested a simpler and more elegant solution: to access the information layer through a 0.1 mm cover layer on top of a 1.1 mm plastic substrate (figure 5). The cover layer can be manufactured by a spin coating process, or by bonding a thin plastic sheet onto the disc. This solution is more tolerant of disc tilt, and thickness variations in the cover layer can be made small enough to minimize spherical aberration. The only problem is that the disc is more sensitive to dust and fingerprints due to the small spot size at the surface of the cover layer, but this problem could be overcome by putting the disc inside a cartridge.

Taking all considerations into account, the most attractive option for a digital video recorder is to use a lens with NA = 0.85 combined with read-out through a thin cover layer. A storage capacity of 9 Gbytes should be achievable with a red laser, while a blue laser could probably increase this to 18 Gbytes or more. Such a capacity would meet future demands for storage of very high bit rate digital video at home, in a way compatible with standards for broadcasted high-definition digital television. Long recording times, which are needed to store, edit and retrieve several movies on a single disc, would also be possible.

What about near-field recording?

Several start-up companies in the US are developing disc-based storage systems that combine the technologies used in the optical recording and hard disk industries. For example, the California-based company TeraStor is developing an optical disc drive based on an objective lens that has a numerical aperture greater than one. The use of such lenses, known as solid immersion lenses (SILs), was pioneered by Gordon Kino and colleagues at Stanford University in the US.

The key concept here is that the numerical aperture of an objective lens can be larger than 1 if the light is focused within the glass of the lens, since glass has a refractive index greater than one. The simplest SIL comprises a hemispherical lens that focuses a converging beam of light to a very small spot close to the flat exit surface of the lens (figure 6). Unfortunately, all the rays for which nsinq > 1 are rendered useless due to total internal reflection at the bottom surface of the lens, so it becomes difficult to address the information layer on an optical disc.

This problem can be solved by exploiting a phenomenon known as frustrated total internal reflection. The trick is to keep the separation between the lens and disc much smaller than the wavelength of light used (a gap of about 50 nm is typical). In this case photons that would normally experience total internal reflection at the glass-air interface can “tunnel” through the gap between two materials with high refractive index, in much the same way as electrons can tunnel through a very thin insulating layer. This tunnelling of photons is usually referred to as evanescent wave or near-field coupling, and the coupling efficiency can be as high as 50%.

In this scheme it is important to have precise control over the size of the gap between the lens and the disc. This can be achieved with an air-bearing slider, which is commonly used in magnetic storage to hold a read/write head close to the surface of a magnetic hard disk. Moreover, a second lens attached to the slider could eliminate the need for active-focus control (figure 6).

This idea can also be extended to magneto-optical recording by mounting a miniature magnetic coil on the slider. The coil can be used in combination with a pulsed laser beam to create very small magnetic domains on a disc, a technique known as magnetic field modulation. This technique is attractive for ultrahigh storage densities because bits can be written at a spacing smaller than the optical spot diameter. Many people believe that magneto-optical recording will outperform phase-change recording at extremely high bit densities, mainly because of the noise associated with the finite size of grains in the polycrystalline phase.

It is expected that data-storage systems based on solid immersion lenses will initially target specialized markets, such as peripheral storage for PCs, because it will be difficult to achieve compatibility with other members of the CD family. In addition, conventional magnetic hard disks could find an insurmountable obstacle on their way to further evolutionary progress: the super paramagnetic limit. This limit, which marks the point where the magnetic domains in conventional recording media are so small that they become unstable, is predicted to occur at storage densities of about 5 Gbit cm-2. There is some hope that a combination of near-field optics, magnetic field modulation and advanced magneto-optical media will come to the rescue, allowing the rapid increases in storage capacity that have been achieved over the last two decades to be continued into the future.

Since the CD was introduced in 1982, it has made a major impact on the way we live and work. With continued development of the underlying technologies and the emergence of new applications such as digital video recording, it seems likely that optical data storage will have an equally profound influence on our lives in the years to come.

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