Beyond Magneto Optics: OSD And The Future Of Optical Storage.
For years, Magneto Optic (MO) technology has played an indispensable role in storing large quantities of data. A hybrid technology, MO makes it possible to read and write vast amounts of digital information by using a laser light source and a magnetic field to change the direction of magnetization, and therefore the contents of the storage disk. With its impressive capacity, high reliability, and exceptional performance MO is ideal for any document- or data-intensive organization that has substantial data warehousing needs, such as financial institutions, engineering firms, or Internet content providers. Its attractive features also inspire a flood of new applications. One of the most significant of these is the storage of e-mail and other electronic documents.
While MO technology has improved dramatically, recent attempts to enhance its read/write capability, capacity, durability, and performance have been hampered by a number of technical "roadblocks". These roadblocks have slowed efforts to improve the efficiency of the lasers used to record and read the data and their ability to read two sides of the medium simultaneously, factors that can have a substantial effect on the performance of an optical storage system. Progress has been slow, too, in reducing contamination of the medium on which data is written and read and in increasing the data transfer rate between medium and user.
Fortunately, though, some very recent developments in optical technology have opened up new possibilities for enhancing optical storage. Before considering these developments, it's helpful to briefly describe some of the key issues affecting the performance of an optical storage system, such as MO.
The Laser, Media Contamination And Data transfer Rate
The performance of the laser in an optical system is obviously of great importance. The amount of data a laser can read or write depends, in part, on the laser's wavelength, its ability to focus the smallest possible diameter of light on the storage medium, and the precision with which the light is focused. A laser's ability to read or write also depends on the thickness of the substrate--the protective surface on the recording medium--that the laser must penetrate.
Contamination of the recording medium is a major concern as well, since this can affect the laser's ability to read and write data. The degree of contamination is affected by a number of factors, including contact between the head and recording medium and the nature and durability of the substrate.
Factors affecting the data transfer rate are equally complex. For example, when a laser can read smaller marks on the medium, the medium can contain greater bit densities, and therefore more data, resulting in greater drive capacities and data transfer rates.
Technological limitations in these areas--efficiency of the laser, control of media contamination, and the data transfer rate have, until recently, slowed the development of new optical storage technologies. Now though, a new technology--Optical Super Density (OSD)--promises to revolutionize optical storage and boost performance by reducing or eliminating these problems.
Criteria For OSD
OSD, which results from a synthesis of new and preexisting technologies, grew out of an effort by Maxoptix and technology partners Mitsubishi Chemical, Asahi/Pentax, and the Lafe Group to develop a high capacity (40GB or more) removable, rewritable, high-reliability optical drive. This innovative drive had to meet a number of criteria. It had to be as rugged and reliable as today's ISO-standard optical solutions. It had to achieve data transfer rates competitive with hard disk and tape products (30MB/sec). It also had to provide the user with significantly lower cost per megabyte than current MO solutions, and be highly competitive with other optical and tape products.
800% Increase In Capacity Compared To Today's MO
One of the most impressive facts about the new OSD technology is that it increases storage capacity by almost 800% in comparison to the MO drives of today. It also offers MO-like reliability and ruggedness, making OSD suitable for the harshest environments. Removable OSD media--cartridges very similar to conventional MO media--can be overwritten 10 million times without data degradation, while providing a 50-year shelf life. OSD can function as a single drive or may be integrated into a library environment.
OSD will be compatible with existing automation in today's ISO-standard libraries. The products will be backward compatible with ISO-standard MO, and future generation drives will be able to read and write today's OSD media.
Improving Four Key Areas Of Optical Storage Technology
OSD achieves new levels of performance and durability by incorporating dramatic improvements in four key areas of optical storage technology. 1) It decreases the diameter of laser light focused on a spot in the storage medium. This increases the density and therefore the amount of data that can be written or read and increases the capacity and speed of access; 2) OSD incorporates a thick overcoat on top of the recording layer. The substrate is as thick as conventional ISO MO and the overcoat that protects the recording layer is thicker than conventional ISO MO, so the medium is rugged and less easily contaminated. At the same time the thickness of the overcoat makes it easily penetrated by the laser, so the laser can read and write data more efficiently. 3) OSD technology doubles the data rates and online capacity by making it possible to access both sides of the disk simultaneously. 4) OSD also enhances the head/disk interface's immunity to contamination and allows for continuous focus of the objective lens .
Improvements in these areas are a result of the application of five primary technologies and some additional enabling technologies. A brief discussion of these technologies will illustrate how they function in OSD and enhance optical storage technology.
High Numerical Aperture Lens (For Higher Data Density)
One of the primary goals of optical technology is to achieve (focus) the smallest diameter of light, incident or spot, on the storage disk. That's because the smaller the diameter of light, the higher the track density and the higher the bit density that can be achieved. With higher density more data can be read and written.
The diameter of a laser light depends on the Numerical Aperture (NA) of the lens. The higher the NA, the smaller the diameter of light that may be achieved. While the NA of the lens in current ISO-standard products is 0.55, in OSD it is .8. Once production of OSD begins, however, it is possible that an NA of .85 might he achieved. Not only does OSD technology make possible higher bit and track density increasing the amount of data that can be read and written, but this higher density can be accomplished using low-cost laser diodes that are currently employed in volume production of optical products.
Overcoat Incident Recording (Providing A Tougher, Yet Transparent Medium)
The type and thickness of substrate present in a product and its location--above or below the medium--is a significant factor in the performance of the product, too. It can affect how close the laser can get to the recording medium and, therefore, the laser's ability to read/write data. The nature and location of the substrate can also affect the amount of contamination the medium is subject to, as well as the medium's durability. Differences in the type of substrate and the way it is applied have given rise to different recording methods.
Air Incident Recording.
In conventional magnetic products, such as hard disks and tape cartridges, the recording layer is on top of a substrate. This method is referred to as air incident recording. The recording surface is covered, in turn, by a very thin coating, approximately a few nanometers thick. This coating's purpose is to provide lubrication in case there is brief contact between the read/write head and the storage medium.
This method proves effective in a sealed, non-removable system, such as a hard disk. With this approach the head can fly very close to the recording surface, enhancing read/write accuracy. Unfortunately, air incident recording does not protect the recording surface from contamination or oxidation because the coating on the recording surface is so thin. This is a similar problem as with floppy drives and tape drives, as the overcoat is extremely thin--about 60nm or so.
Substrate Incident Recording. In ISO-standard MO media there is a transparent substrate on top of the recording layer. This is referred to as substrate incident recording. In this method the laser light must pass through the substrate to reach the recording layer. While the substrate protects the recording layer from contamination and oxidation, the thickness of the substrate also limits the numerical aperture (na) that can be used in the objective lens. A smaller na has been the primary limiting factor to higher capacity and performance of ISO-type MO drives.
Neither air incident recording nor substrate incident recording has the appropriate characteristics for the type of recording OSD requires. For example, it is not possible to achieve the OSD product's target of 20GB per surface and a 30MB/sec data transfer rate using the traditional substrate incident recording of ISO-type MO drives. This is due to the high NA of the objective lens and the size of the magnetic field in the flying head design. The Head must fly closer to the recording layer in order to change the magnetic state after the laser has heated the spot. And it is not possible to achieve the durability of ISO MO media using air incident recording. For these reasons it was necessary to develop Over Coat Incident Recording (OCIR).
With OCIR the recording layer is on top of the substrate, as in a hard disk. In addition there is a thick, transparent overcoat of acrylic on the recording surface that protects the recording layer. The acrylic coating helps achieve the reliability of the traditional ISO MO media. This coating, which is similar to the coating on the back of CD and DVD media, is more than 1,000 times thicker than that of hard disk and tape products, but is much thinner than the substrate used on today's ISO media.
Because the overcoat used in OCIR allows the lens to be positioned much closer to the recording surface, it is possible to use a higher numerical aperture lens and achieve much higher data densities, significantly increasing capacity and performance.
Surface Array Recording (For Nearly Twice The Data Rates)
One of the most significant ways to boost performance is to develop a process that enables both sides of the disk to be accessed simultaneously. Unlike traditional MO, OSD users will not need to flip the media in order to read data stored on the opposite side of the disk. OSD products will incorporate independent read/write heads on both sides of the media.
While hard disk products utilize multi read/write heads, because the heads are not independent, they must switch from one side (or head) to the other. As a result the data rates are no greater than the data rate of a single surface. Surface Array Recording (SAR), a critical component of OSD, provides simultaneous read or write to both sides of the media. This results in data rates that are nearly twice those of a single side, and very similar to the rates of hard disk products. Simultaneous read or write on both sides not only doubles the performance, but also the online capacity.
Recessed Objective Lens (Protects Against Contamination And Allows Continuous Focus)
Another important aspect of OSD technology is the use of a Recessed Objective Lens (ROL), a lens that is recessed above the magnetic head. Because of the objective len's position, the lens will not be subjected to particulate contamination that may be introduced into the drive during insertion of the media. With OSD the objective lens is decoupled from the magnetic head, is placed behind the magnetic head, and protected by the slider.
OSD also incorporates a number of enabling technologies that enhance performance. An innovative Magnetic Field Modulation (MFM) head design removes the limitations on bit density and data throughput imposed by standard MO designs. By utilizing a small magnetic head in close proximity to the disk, the polarity of the magnetic field on an OSD drive can be switched at a very high frequency. This allows for higher density, and makes it possible to read or write both sides of the disk simultaneously.
The MFM technology is combined with a Magnetic Super Resolution (MSR) masking technology that enables read back of very high densities. It does this by isolating the individual bit to be read, resulting in further improvements in capacity and performance. The readout layer magnifies the bit area, providing higher resolution for progressively smaller bits. This increases the capacity and the performance. The implementation of MSR in conjunction with MFM will enable future generations of OSD drives to exceed 40GB per disk.
There are two additional areas in which OSD performance is likely to be enhanced in the future.
In the future, improvements in MSR technology will provide amplification of the reflected (laser) signal in addition to simply isolating the bit. This will allow the read back of ever smaller marks. This type of media will allow far greater bit densities and will result in greater drive capacities and data transfer rates.
In addition, by the year 2003, it is anticipated that the first blue laser technology products will reach the market. Blue lasers (with a wavelength of approximately 410nm) will provide a spot incident 37 percent smaller than is possible with today's red lasers (650nm). This 37 percent smaller spot incident will more than double the aerial density and substantially increase the data transfer rate. Blue lasers will more than double capacity to between 80 and 100GB on a single 5.25-inch disk.
With these, and other innovations yet to come, OSD is certain to be a crucial component of optical storage.
Fred Bedard is the senior vice president of sales and marketing at Maxoptix Corp. (Fremont, CA).
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|Title Annotation:||Technology Information|
|Publication:||Computer Technology Review|
|Date:||Jan 1, 2001|
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