Optical disk technology has played a key role in the data storage industry for more than three decades. The robust nature and stability of the media have established it as a very popular storage media for both consumer and professional applications. The well-established use of optical products and the roadmap for new technologies will ensure that it continues to be used for many years in the future.

In the late 1970’s the first Video Disks were developed using media tracked mechanically with a stylus. Products from Teldec in Germany and JVC in Japan found small niche markets before the use of Helium Neon lasers led to the first true Optical Video Disks. These disks used a 1.2 mm thick optically clear substrate and had embossed data pits whose variable lengths were used to generate an FM data stream. The non-contact reading allowed by the use of a laser was the key to practical success and the technology was widely adopted, especially in Japan. In parallel with this development, many companies were researching recordable optical disks and it was the commercialisation of directly modulated solid-state lasers that enabled practical systems to be shipped. Initially these products used 12-inch and 8-inch diameter formats and both analogue video recorders and digital data disks were shipped – mainly in Japan.

The development by Philips and Sony of the first replicated digital optical disc in the form of the Compact Disc led to the mass-market CD and DVD technologies that we know today. The widespread adoption of this format and the resulting volumes enabled the production of true low cost optical readers.

Around the same time various companies were developing 130 mm writable disks and drives aimed at the exploding professional data storage market. These drives have been designed for library applications with more robust mechanics allowing over 750,000 media swaps compared with consumer products with lifetimes of the order of 20,000 loads. Many US start up companies were founded in the mid 1980’s but none of these survived and Magneto Optic technology pioneered by Sony became the “de-facto” standard for professional optical data storage. At launch the media had a capacity of 650MB, well ahead of the storage capacity of the hard drives being shipped at the time.

In parallel, Matsushita, Ricoh and others were pioneering Phase Change technology and in the longer term this approach took the lead in the consumer market place with the successful development of CD-RW and then DVD-RW. The widespread adoption of Phase Change technology now being used for the next generation high definition video disks have been driven by the simpler drive and optical designs.

More recently Plasmon has developed and successfully marketed UDO (Ultra Density Optical) as the next generation of professional optical disk systems using high density blue lasers and Phase Change media to provide true Write Once and Rewritable solutions.

Blu-ray v HD DVD

There has been much discussion of the relative merits of HD DVD and Blu-ray as the next consumer video format. Both formats use blue/violet lasers operating at 405nm wavelength and advanced encoding and compression technology to deliver high definition video for the consumer market.

HD DVD pushes the theoretical optical limits further to achieve its capacity with a 0.6 mm thick substrate. For this reason, while HD DVD uses the “same manufacturing processes” as DVD, the tolerances are tighter and not all current DVD lines will be capable of meeting these higher quality standards. In reality most replicators are likely to install new manufacturing lines just as they did with the advent of DVD despite manufacturing equipment suppliers offering dual function lines.

By contrast, Blu-ray does not push the theoretical optical limits as hard but requires the development of new manufacturing processes to produce the 0.1 mm optical cover layer. Initial production used a very expensive cast polycarbonate laminate bonded using a pressure sensitive adhesive. This was always seen as an interim solution and today the manufacturing technology is available to make Blu-ray using modified spin coating processes familiar to the current CD and DVD industries. As these processes mature, the costs of the two products will reach parity.

Because of the higher density of both formats, mastering requires new equipment and processing. This is in limited supply today resulting in very high costs compared with CD and DVD masters. As the new technologies developed to meet these mastering requirements become widespread prices will fall. While Blu-ray will always be more difficult to master, the new generation equipment being developed is capable of producing both formats.

At the end of the day commercial issues will determine the winner, not the technology. Either format is capable of reproducing high definition video at the limits of domestic display technology.

Evolution to higher capacities

The basics of Optical Storage have remained the same since the 1970’s in that a focussed laser spot writes and reads serial bit streams. The early products used 835 nm infrared lasers with a low numerical aperture focusing lens (typically around 0.5). Initially, data bits were encoded by the position of identical marks recorded on the media surface. In time better media technologies allowed data to be encoded by writing variable length pits.

With the evolution from CD to DVD and now to high definition videodisc products, wavelengths have moved from the infrared to the violet end of the spectrum at 405 nm and numerical apertures have increased to 0.85 NA. Taken alone these two changes would equate to a 15 times increase in capacity.

For example, second generation UDO will have a capacity of 60GB on a double-sided cartridge when it ships in 2007. This is 92 times the capacity of the first MO disks.

UDO road map

First generation UDO uses a conservative 0.7 NA optical design and relatively straightforward read channel to deliver 30GB per cartridge. This approach led to the delivery of the world’s first blue laser recordable disk system for reliable professional data storage. For 60GB second generation UDO Plasmon has adopted the industry standard 0.85 NA from Blu-ray and a PRML read channel similar to HD DVD to achieve an areal density higher than either product.

A further doubling of capacity to 120GB per cartridge in third generation is straightforward through the use of dual layer media – i.e. two recording layers on each side of the disk. The disk manufacturing technologies required are now becoming established in the production of dual layer DVD recordable and more recently in early production of 50GB Blu-ray disks.

Generation four with a cartridge capacity of 240GB is feasible based on the current level of development of read channels and media advances in recent years. Beyond UDO 4 we will almost certainly move out of the “flat world” of the last 30 years of optical data storage to 3D recording technologies.

The future of optical storage

Many different technologies are being reported in conference proceedings and some are in active product development. The key areas are discussed as follows…

Holographic storage

The most advanced of the newer technologies is holographic storage with In Phase in the USA and Optware in Japan promising products with capacities of 200 to 300GB in the next few years. Several major corporations are also working in the area, but are less public in their announcements.

The basic principles of holographic storage have been well known for decades but practical development has been held back by the availability of manufacturable media and key components for the optical system. The latter constraints have been overcome with the development of spatial modulators and high-speed detector arrays for other applications. Write Once media is available today and while it has many complexities in the way it has to be written and read, these issues are slowly being overcome.

Holographic data storage uses two laser beams that interfere within the volume of the recording media – the signal beam contains the data in the form of a checkerboard pattern while the reference beam may contain special modulation to add additional security encryption. This checkerboard pattern is recreated when an identical reference beam illuminates the written media. In order to achieve the high capacities, many holograms are superimposed in the same volume of material. As the recording media is sensitive to the reading beam, a large block of overlapping holograms has to be written and “fixed” before the end user can access the data. This will impose constraints on the usage of the device in a data storage environment.

There are many other system issues as tolerances on media mechanical properties, operating temperature and laser wavelength are much tighter than conventional optical storage designs. Achieving reliable data interchange represents a significant technical challenge and it is not certain whether either of the start-up companies developing products will have the financial resources to see product development through to profitability. Holographic technology will become a reality, but it will require a number of years of development until commercial products are available.

Near Field Recording

In the mid 1990’s Terrastor tried to develop an optical storage drive and media system based on near field recording. By flying the optical head a few tens of nm above the media it is possible to achieve effective Numerical Apertures much greater than the practical limitations of conventional systems which have reached 0.85 in the case of UDO and Blu-ray. Terrastor was unsuccessful due to the many practical difficulties of maintaining the very small flying height just as the same issues have limited removable magnetic media areal densities. More recently the technical challenges of ‘flying’ heads are claimed to have been solved through the use of high performance servo systems. Philips has published work on a system where a very thin spin coated protective layer protects the media and the lens is servoed to fly a few nm above the surface. It is claimed that this system is robust against dust, which is swept aside, but there must be doubts about the ability to live with other contamination such as fingerprints.

Currently it is claimed that the capacity of a multi-layer system could reach 500GB using four recording layers of 125GB. At this time the technology is still in the research phase and there are no public plans to commercialise a product based on near field recording.

Super Resolution

The thermally initiated writing mechanism of optical recording has always allowed the writing of marks much smaller that the writing spot. This is achieved because the temperature contour needed to transform the material into the written state can be made much smaller than the beam size. The ultimate limit is determined by how accurately the laser power can be controlled during the write process. During the reading process these very small marks cannot be read back when they become smaller than approximately half the beam diameter.

Super resolution is a technique whereby the thermal energy from the reading laser beam opens a small “window” in a layer immediately above the recording film. In effect a moving window, which is at the centre of the read beam, is created so only the mark that is being read back is seen through this window. Again, the limits are determined by how accurately the read energy can be controlled. The first commercial products using this approach used Magneto Optic materials where a “magnetic window” was created. More recently there has been significant progress on materials, which create an optical window allowing the same principles to be applied to Phase Change recording films.

The advantage of this approach is that the drive uses conventional optics and is therefore backward compatible with current designs. While the technology to achieve super resolution for rewritable materials is some way off, it may be feasible to develop write once solutions in the mid to near term.

Two-Photon materials

Fluorescent multi-layer techniques have been around for many years. The basic principle uses a non-linear material that can be switched between states at very high energy densities. When illuminated at lower energy densities, the switched material re-emits light at a different wavelength by fluorescence. As the fluorescence is non-coherent and at a different wavelength, multiple layer disks are much easier to design from an optical perspective. Due to the low absorption of the materials necessary for multi layer disk designs, very high laser powers are required for writing and reading. The available laser power for reading and the low collection efficiency of the fluorescent signal currently limits write and read speeds. Work is ongoing to increase data rates by parallel readout but this will increase system complexity.

Several groups are working on this approach and are claiming up to 1TB capacity on a single disk and companies working on the technology are claiming that prototypes will be available before 2010.

Summary

Now is a very exciting time in the optical storage industry. Available today are high-density new generation consumer products, for example Blu-ray and HD DVD, and well-established and successful professional products such as UDO. In addition, there are many interesting developments happening in labs all over the world. While some of these new technologies may prove to be technical or commercial dead ends, others will certainly form the foundation for products that we will be using in 10 years. Optical storage is a very popular and strongly rooted storage technology and there is every reason to believe that it will continue to play a key role for many years to come.