Storage Technology
The future begins today
Layer for layer: It looks like a conventional disc, but is really the next generation of optical storage media. The actual storage layer of the holo-disc from InPhase is embedded between two protective layers, each one millimeter thick.
The first audio CDs hit the market in 1982. 25 years later there are discs with 80 times the storage capacity. And development continues.
25 years after the introduction of the audio CD made from Bayer’s Makrolon and countless refinements in the intervening period, physicists, chemists and engineers are still coming up with new ideas for packing the data more closely together with the constant goal of fitting more bits and bytes on a disc. They have accomplished quite a bit over the last quarter century.
The most recent milestone was the market introduction of the Blu-ray Disc and the High-Definition Digital Versatile Disc (HD DVD). Current disc formats support capacities up to 50 gigabytes, nearly 80 times the 650 megabytes of the original CD. This significant increase in the amount of data – even compared to “normal” DVDs – means that movies can now be sold in high-definition quality.
To get there from the original CD the developers had to make changes in a number of areas. For one, they reduced the wavelength of the laser light used to read the data – from infrared light (CD) to red (DVD) to blue light (Blu-ray Disc, HD DVD). This, together with other measures, enabled the data to be packed much more tightly. The dimensions of the pits used to store the data have become steadily smaller. Blu-ray discs have a minimum pit length of 160 nanometers (1 nanometer is one millionth of a millimeter), which is one-fifth that of a CD. Furthermore, the distance between the individual data tracks, which are roughly equivalent to the grooves in a record, was reduced by 80 percent to 320 nanometers. But researchers want to focus the laser beam even more tightly. For the time being, technicians have run up against a limit with respect to the wavelength of the laser, as laser diodes with even smaller wavelengths are not (yet) available.
They are therefore focusing their efforts elsewhere, such as on the objective lens. This is the optical element through which the reading laser beam is focused on a certain area of the disc. “To significantly reduce the size of this area even further, current research is concentrating on near-field optics,” explains Dr. Konstantinos Douzinas, Head of Polymer Physics in the Polycarbonates (PCS) Business Unit of Bayer MaterialScience. According to Douzinas, fundamental laws of physics stand in the way of attempts to further increase the optical resolution of existing systems. Bayer has been an important player in the development of optical data storage from the very beginning. When Polygram (now EDC) introduced the first CDs for the European market on August 17, 1982, these were made of Makrolon, a Bayer polycarbonate.
The most recent milestone was the market introduction of the Blu-ray Disc and the High-Definition Digital Versatile Disc (HD DVD). Current disc formats support capacities up to 50 gigabytes, nearly 80 times the 650 megabytes of the original CD. This significant increase in the amount of data – even compared to “normal” DVDs – means that movies can now be sold in high-definition quality.
To get there from the original CD the developers had to make changes in a number of areas. For one, they reduced the wavelength of the laser light used to read the data – from infrared light (CD) to red (DVD) to blue light (Blu-ray Disc, HD DVD). This, together with other measures, enabled the data to be packed much more tightly. The dimensions of the pits used to store the data have become steadily smaller. Blu-ray discs have a minimum pit length of 160 nanometers (1 nanometer is one millionth of a millimeter), which is one-fifth that of a CD. Furthermore, the distance between the individual data tracks, which are roughly equivalent to the grooves in a record, was reduced by 80 percent to 320 nanometers. But researchers want to focus the laser beam even more tightly. For the time being, technicians have run up against a limit with respect to the wavelength of the laser, as laser diodes with even smaller wavelengths are not (yet) available.
They are therefore focusing their efforts elsewhere, such as on the objective lens. This is the optical element through which the reading laser beam is focused on a certain area of the disc. “To significantly reduce the size of this area even further, current research is concentrating on near-field optics,” explains Dr. Konstantinos Douzinas, Head of Polymer Physics in the Polycarbonates (PCS) Business Unit of Bayer MaterialScience. According to Douzinas, fundamental laws of physics stand in the way of attempts to further increase the optical resolution of existing systems. Bayer has been an important player in the development of optical data storage from the very beginning. When Polygram (now EDC) introduced the first CDs for the European market on August 17, 1982, these were made of Makrolon, a Bayer polycarbonate.
Like a jet plane flying over a lawn – at an altitude of four millimeters
And Bayer MaterialScience remains involved in new developments today, because the work on a new objective lens also has consequences for the disc material. It is therefore no surprise that these material requirements are the subject of current research being performed at Bayer’s PCS Global Innovation laboratory in Uerdingen – at the same historic site where the story of Makrolon began more than 50 years ago. The researchers there are working in close cooperation with Sony in Japan.
Sony has issued a clear specification: Because of the physics involved, the lens system for future near-field systems cannot be more than 20 to 25 nanometers from the surface of the rotating disc. By comparison, this distance in a conventional CD is more than a millimeter, or more than about 50,000 times greater. While the dimensions involved in near-field optics are immediately clear to physicists, they are much less so to the uninitiated. Douzinas has a ready analogy that illustrates the required distance between the reader optics and the disc: “Imagine a jet plane flying over a mown lawn - but just four millimeters above the tips of the blades of grass.”
A distance of only 20 to 25 nanometers is simply impossible with current discs. “When the disc begins to rotate, it also begins to vibrate vertically and could destroy a lens placed so close,” says Douzinas. Just as the lawn over which the jet is flying at an altitude of only a few millimeters would essentially have to be perfectly even.
Sony has issued a clear specification: Because of the physics involved, the lens system for future near-field systems cannot be more than 20 to 25 nanometers from the surface of the rotating disc. By comparison, this distance in a conventional CD is more than a millimeter, or more than about 50,000 times greater. While the dimensions involved in near-field optics are immediately clear to physicists, they are much less so to the uninitiated. Douzinas has a ready analogy that illustrates the required distance between the reader optics and the disc: “Imagine a jet plane flying over a mown lawn - but just four millimeters above the tips of the blades of grass.”
A distance of only 20 to 25 nanometers is simply impossible with current discs. “When the disc begins to rotate, it also begins to vibrate vertically and could destroy a lens placed so close,” says Douzinas. Just as the lawn over which the jet is flying at an altitude of only a few millimeters would essentially have to be perfectly even.
Why twelve centimeters?
Ludwig van Beethoven probably had a lot of things running through his head as he worked on his ninth symphony. But he certainly never suspected that the length of this composition would determine the diameter of a recording medium.
Legend has it that Sony boss Akio Morita wanted the compact disc developed by his company to have enough capacity to hold conductor Herbert von Karajan’s recording of the “Ninth.” Thus the audio CD became a disc twelve centimeters in diameter. This has remained unchanged through the later developments DVD, Blu-ray Disc and HD DVD. Of course, the first commercial CDs featured Abba and Chopin, not Beethoven.
Legend has it that Sony boss Akio Morita wanted the compact disc developed by his company to have enough capacity to hold conductor Herbert von Karajan’s recording of the “Ninth.” Thus the audio CD became a disc twelve centimeters in diameter. This has remained unchanged through the later developments DVD, Blu-ray Disc and HD DVD. Of course, the first commercial CDs featured Abba and Chopin, not Beethoven.
A partner from the start
Even the developers of the CD had stringent requirements for their basic material. It had to be heat-stable, transparent, metalizable and capable of being injection molded with high dimensional accuracy. The material of choice was the CD 2000 grade of the Bayer polycarbonate Makrolon. BMS still supplies a significant amount of the more than 900,000 tons of polycarbonate used each year to produce all types of discs. Over the last 25 years, BMS has continuously optimized its Makrolon for optical data storage media, e.g. with respect to purity, optical properties and flow behavior. With the current raw material Makrolon OD 2015, a CD can be produced and removed from the injection mold in under three seconds. In 1982 it took nearly half a minute. Makrolon OD 2015 is also suitable for the new-generation Blu-ray Disc and HD DVD, and is an established product in these applications.
The researchers at PCS Global Innovation began by modifying the Makrolon to dampen the natural frequency of the disc to the point where the vibration amplitude was no longer a risk. But that’s not all. Another requirement pertains to the coating of the disc: Again because of the physics involved, this layer will have to have a relatively high index of refraction so that the laser beam exiting the near-field lens can even enter the disc at all and not be completely reflected off the surface.
BMS has succeeded here as well. “We collaborated with Sony to develop a coating with an index of refraction of more than 1.84 – a world record,” says Douzinas with pride. It goes without saying that this layer must also be scratch-resistant and satisfy several other requirements. All of these measures are intended to increase storage capacity according to the classical principle to well over 100 gigabytes in just a few years. Because it is now standard to use several storage layers in one and the same (multi-layer) disc, the total capacity of a disc would in fact be several times greater.
BMS has succeeded here as well. “We collaborated with Sony to develop a coating with an index of refraction of more than 1.84 – a world record,” says Douzinas with pride. It goes without saying that this layer must also be scratch-resistant and satisfy several other requirements. All of these measures are intended to increase storage capacity according to the classical principle to well over 100 gigabytes in just a few years. Because it is now standard to use several storage layers in one and the same (multi-layer) disc, the total capacity of a disc would in fact be several times greater.
The future: 300 gigabyte disc using the holographic data storage principle - Substrate developed by Bayer MaterialScience
Triple-digit gigabyte numbers are also taken for granted at InPhase Technologies in Colorado, U.S.A. The young startup’s first product, a 300 gigabyte disc called tapestry 300r, is currently in use by test customers. InPhase uses the holographic data storage principle, in which information is no longer written and read sequentially bit for bit, but rather all at once as whole data pages. This not only increases the storage density and thus the capacity, it also vastly accelerates the read and write process. And there is another difference from the classical disc: Whereas there the digital information is stored in the form of pits in the storage layer, the “holo-disc” uses local polymerization of a light-sensitive substance to store data. Polymerization also changes the index of refraction, and this is what is recognized when it is read subsequently.
This chemical storage principle enables not just the surface but also the third dimension to be used for data storage. With tapestry 300r, the light-sensitive substance is distributed in a 1.5 mm thick layer that is penetrated completely by the laser beam. It is this three-dimensional aspect that makes the high storage capacity possible. The substrate in which the optically active substance is embedded is made from a material developed by Bayer MaterialScience specifically for this application.
At present, holographic data storage is primarily of interest to institutional archivists. “Our beta users are therefore television broadcasters, publishers, libraries and hospitals – institutions that produce large volumes of data and therefore have significant archiving needs,” explains Liz Murphy, Vice President for Marketing at InPhase.
One thing is clear: Mankind will continue to generate new data, creating the need for more storage. It remains to be seen how the various optical storage media of the future will be accepted by users. From a strictly technical standpoint, possibilities abound. The 300 gigabytes currently available from InPhase are by no means the end of the line. The company is already working on the development of discs with capacities of 800 and even 1,600 gigabytes, the latter corresponding to roughly 2,500 audio CDs.
This chemical storage principle enables not just the surface but also the third dimension to be used for data storage. With tapestry 300r, the light-sensitive substance is distributed in a 1.5 mm thick layer that is penetrated completely by the laser beam. It is this three-dimensional aspect that makes the high storage capacity possible. The substrate in which the optically active substance is embedded is made from a material developed by Bayer MaterialScience specifically for this application.
At present, holographic data storage is primarily of interest to institutional archivists. “Our beta users are therefore television broadcasters, publishers, libraries and hospitals – institutions that produce large volumes of data and therefore have significant archiving needs,” explains Liz Murphy, Vice President for Marketing at InPhase.
One thing is clear: Mankind will continue to generate new data, creating the need for more storage. It remains to be seen how the various optical storage media of the future will be accepted by users. From a strictly technical standpoint, possibilities abound. The 300 gigabytes currently available from InPhase are by no means the end of the line. The company is already working on the development of discs with capacities of 800 and even 1,600 gigabytes, the latter corresponding to roughly 2,500 audio CDs.