Advanced Imaging February 1998
Storage Systems Want the
By Sheldon Liebman
As research in commercializing blue lasers continues, one of the areas receiving a lot of attention is the use of this technology for digital storage technology. Initial research suggested that red laser technology with a wavelength of 800 nanometers (nm) could be replaced by blue or violet lasers with wavelengths of 450 nm or 400 nm, respectively. If this could be achieved, the result would be an increase of approximately 4X over the storage capacity of existing devices.
Today, the numbers being used are significantly different and the expected jump in capacity is not as optimistic. For example, Sony demonstrated a prototype last May for a high capacity optical disc recorder. With red laser technology, this device has a capacity of 8.0 GigaBytes (GB) per disc. By using a blue-green laser, the capacity increases to 12 GB. In the case of a blue laser, the disc can hold 18 GB. This represents a storage jump of 225% over current technology, a little more than half of the original projections.
The main reason for this is that red laser technology continues to develop. Currently, red laser diodes are available with wavelengths under 700 nm. In the Sony prototype and a more recently introduced product (October 1997), the wavelength of the red laser is listed as 635 nm. This type of "extended" red laser already offers an increase in storage capacity over the 800 nm lasers assumed before. The numbers for the blue and blue-green lasers in the Sony prototype are also different. According to the companys press release on the research, the blue-green laser has a wavelength of 515 nm. The number quoted for the blue laser is 410 nm, slightly higher than the 400 nm number quoted in initial research.
Although 18 GB on a single CD-size disc sounds impressive, its important to remember that were talking about optical disc, which has a higher density than standard CD-ROM or DVD discs. According to Sony, the current recording density of a CD is 730 Megabits (Mb) per square inch. For DVD, the figure jumps to 3.2 Gigabits (Gb) per square inch. Their high capacity product introduced last year achieves a density of 6.1 Gb/square inch with the extended red laser, almost 10 times the density of current CD technology.
With read/write capabilities, Sony is speculating that these technologies can eventually be used to create digital disk recorders and players for professional and consumer use. Through the use of MPEG2 compression technology, Sony estimates that 3 hours of recording time can be achieved with the red laser (635 nm) and 4.5 hours with the 515 nm blue-green laser. The higher capacity also opens the possibility of using this technology to record HDTV format video signals.
Philips LMS is also looking carefully at the impact of blue lasers on optical storage. Today, Philips delivers 12 inch discs that hold 12 GB of information using both sides of the disc. In the Sony examples above, all of the numbers are for single sided applications.
At Philips, they see blue laser technology as increasing capacity not just by enabling higher density recording, but by allowing new methods of reading and writing data. One example they mention is multi-layer recording. Steve Maccaux, Director of Sales at Philips LMS, says to "Think of it as a two story house or a house with a basement. We need to offset and record down to the next layer. With precise and finite recording, we can record two spots in the same way we record one." Blue lasers, adds Maccaux, help achieve this precision.
Philips uses numbers that are slightly different from Sony, but in the same general area. To them, the target for blue-green lasers is in the 520-460 nm range. In the lab, Maccaux says they have actually created a "violet-ultraviolet gas laser at 370-360." Of course, he quickly adds, "this is not technology that can be commercialized."
Commercializing this technology is the big question. From Maccauxs perspective, a commercial laser needs to be priced under $400, last for over 30,000 hours and use very low voltage. In addition, and perhaps more importantly, the laser must be available from multiple suppliers. The technology demonstrated so far doesnt come close to meeting these requirements. For example, the life of the lasers has only been 100-1000 hours. From a power perspective, they use up to 60 milliwatts compared to 20-30 for current red lasers. And the price, even with these limitations is over $1000 per laser.
At Sony, they are targeting the year 2000 to introduce products using blue-green or blue lasers. At this point, they cant commit to which one will actually be used. Maccaux thinks the timeframe is somewhat longer. Although he cant provide an official Philips projection, he personally feels that commercial products wont be available until 2002 or 2003. Its interesting to note that when Nichia Chemical first demonstrated blue laser technology, their estimate was that it would be commercially available 1997-1998. Obviously, the process is taking longer than initially anticipated.
For storage applications, at least, the timeframe for commercialization of this technology may not be critical. This is due to other advances that are allowing optical discs to increase their storage capacity using current laser technology.
At Philips, for example, they are delivering their 12 GB disc using a 750 nm red laser. In early 1999, they plan to shift to a 658 nm laser that will increase capacity to 30 GB per disc. To achieve this, the company is using advanced techniques to adjust the laser beam for increased clarity and a smaller spot size. At the same time, they can increase the number of zones and tracks on a disk so that more data can be stored and accessed.
Packing the data tighter also increases the speed at which you can read it back. Today, Philips LMS systems transfer data at 2.7 MegaBytes per second (MBps). In 1999, with the smaller wavelength red laser, they hope to achieve a throughput of 6.2 MBps. This represents a 225% increase in throughput to go along with the 250% increase in capacity. According to Maccaux, this isnt the result of spinning the disc much faster, just flying the data over the beam faster due to the increased density.
At Fujitsu, they are also looking at other technologies to use in addition to or instead of blue lasers. Dan Dalton is Technical Marketing Manager for Fujitsu Computer Products of America and acts as a liaison to the companys optical group in Japan. Today, he says, "we are at about 1.2 Gb per square inch on magneto optical and we are looking at effectively doubling that density."
To achieve this increase, Dalton describes a number of new technologies that are in various states of development. The first is Magnetically Induced SuperResolution (MSR), which Dalton says can effectively double the density from 1.2 to 2.4 Gb per square inch. With this technology, which is specific to magneto-optical devices, "we make a small mark look larger by adding an intermediate layer to the disc," explains Dalton. "This allows you to put more bits on the substrate."
Another technology that can be used is Land/Groove recording, which can also result in a 2X density increase. Typically, information is only recorded on the area referred to as the "Land," while the "Groove" is only used for tracking purposes. "The laser beam can focus on both," says Dalton, resulting in the ability to double the density.
Finally, there is a technology called Mammos developed by Hitachi Maxell. Mammos, which stands for (M)agnetic (AM)plification (M)agneto (O)ptical (S)ystem, is similar to MSR but at a much higher resolution. Mammos operates at approximately twice the density as MSR by using a magnetic field to play back smaller bits on the disk. "The key to recording technology," comments Dalton, "isnt so much writing the bits as it is reading them. Mammos effectively takes the very small recorded bit and can blossom it into a larger looking bit that can be read by a 650 nm laser beam."
To Dalton, the first step is to exhaust the possibilities with red laser technology. Then we can go back to the traditional models with blue and see how these new technologies can be applied to it as well. With MSR and Land/Groove, which are very close, we can hope to achieve a 4X increase over todays capacities. Mammos and Land/Groove may be able to take us to 8X todays resolution. If these technologies can be successfully married to blue lasers, we may even be able to achieve 20X todays density, although certainly not for a number of years.
However the technology advances, its clear that storage users wont have to sing the blues waiting for blue lasers to arrive.
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