Gus, INSIGHT is working again. Here is the recent article on NFR. It is really amazing.
Regards,
Mark
NEAR FIELD RECORDING
By Dr. Gordon R. Knight, TeraStor Corporation
By utilizing a number of new and existing technologies, TeraStor has been able to develop a new storage technology that is an order of magnitude higher in areal density compared to conventional magnetic and optical technologies that exist today. This storage, called near field recording (NFR), currently provides the capacity of high-end tape drives and libraries but has the seek time and transfer rate (performance) comparable to mid-range hard drives.
NFR technology combines design elements from a variety of data storage fields including magnetic recording (typical of HDDs), optical recording, consumer electronics, and microscopy. The key elements of NFR are flying optical head, near field recording, first surface recording, and crescent recording.
This combination of technology and techniques results in storage densities an order of magnitude higher than current conventional technologies. Furthermore, this areal density advantage is sustainable to capacities of hundreds of gigabytes per disk as the laser, lens shape and lens materials, read channels, as well as other elements of the system, improve over time. Equally important is that NFR technology draws heavily on existing MO and HDD materials and processes. This enables low manufacturing costs and rapid production ramps comparable to those currently available with high volume HDD technology.
The process of near field recording occurs when the tiny magnetic coil in the flying head writes the information to the heated spot. The ultra-small bit domains are written in overlapping sequences, creating a series of crescent-shaped bit domains. This crescent recording effectively doubles the bit linear density, thus allowing NFR technology to achieve an even higher areal density.
While each element will be discussed in detail, it is easy to see that each one provides a distinctive piece to the technology. The flying optical head allows the recording element to be placed close enough to the recording media so that the distance separating them is less than a wavelength of the laser light (near field). One of the key optical components inside the flying head is a solid immersion lens (SIL) which is used to tightly focus the laser beam to produce an ultra-small spot. The energy from this spot is then transferred or coupled onto the first surface of the disk. This effect is called evanescent coupling.
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Flying Optical Head
The flying optical head acts as an aerodynamic, mechanical platform for mounting the objective lens, the SIL lens, and the tiny write coil together. This assembly is then positioned at a close distance above the surface of the spinning disk. If the distance between the SIL lens and the recording disk is much less than a wavelength, the resolution of the spot within the SIL is maintained across the air gap. Using a standard red laser as a reference point, the distance between the bottom of the SIL lens and the recording surface would have to be a small fraction of the wavelength of a red laser, or much less than 0.685 microns. By using a flying head, achieving this close proximity is actually much easier than one might expect. Drawing from the flying heads used in HDD technology, we are able to produce a laser-based flying head that flies at a distance less than 6 microinches or 0.15 microns, well within the fraction of the 0.685 micron requirement (see Figure 1).
ÿSince the air-bearing slider accurately controls the fly height, there is no need for a servo control system to maintain the focus between the lens and the media. Because areal bit density resulting from NFR is not directly related to fly height, as it is in HDD systems, significant increases in areal density can be achieved without reducing flying height, unlike present HDD systems.
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Planar Magnetic Coil
The evanescent coupling or transfer of laser energy heats the spot on the recording surface to a temperature of approximately 300ø C in roughly nanoseconds. At this temperature, called the Curie Point, the irradiation heats the molecules in that spot to a finite depth, enabling magnetization when placed within a relatively small magnetic field. This magnetic field (positive or negative) is pulsed into the heated spot by a planar coil embedded within the head substrate.
The planar magnetic coil is, as the name suggests, a flat coil that rests near the same plane as the flying head surface. This extremely light, small coil resides inside the flying head assembly, rather than on the other side of the substrate of the recording media, as is the case with traditional magneto-optical technology (see Figure 2).
This orientation has two important advantages. The first is direct overwrite. Through high-speed switching of magnetic pulses, this small head-based coil is able to directly overwrite without going through a complete rotation of the disk. This produces an increase in the write performance vis-a-vis MO technology. The second advantage is two-sided recording. By embedding the magnetic coil in the head rather than the backside of the disk, this solution supports two-sided disks with two heads on line per disk.
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Solid Immersion Lens (SIL)
In current diffraction-limited optical systems, spot size can be reduced in one of two ways. This is achieved by either decreasing the wavelength of the laser or increasing the numerical aperture (NA) of the objective lens. This approach has clear theoretical limits. For example, in a standard diffraction-limited optical system, the numerical aperture of a lens in air can never be greater than one.
However, during the early 1990s, Stanford University scientists overcame the previous limits on spot size through the use of a new optical system that provides a completely different approach to reducing spot size. This approach increases the effective NA above the theoretical limit of one by adapting an old, but relatively obscure, technique known as liquid immersion microscopy.
During liquid immersion microscopy, the lens and the object to be studied are placed in a medium such as oil. This action increases the magnification of the object beyond what is possible with the microscope alone. The Stanford scientists, most notably Dr. Gordon Kino, simply inverted this effect through the use of a solid lens shaped like a truncated sphere (thus the term "solid immersion lens"). When placed between the standard objective lens and the writing surface, the SIL focuses the incident rays of the laser from the objective lens to a single spot at the base of the partial sphere, as shown in Figure 3. The resulting spot is approximately half the size of the spot achieved by conventional means that use only an objective lens.
In technical terms, the SIL lens slows down the laser light beam to a fraction of its normal speed in air, thus shortening the beam's wavelength and creating a very fine spot size. Notably, the SIL lens of the refractive index plays an important role in this process. In particular, materials with a high index of refraction will produce a higher effective NA than those with a lower index of refraction. For example, air has an index of refraction of 1.0, while common glass has an index of refraction of 1.5, and a diamond's index of refraction is 2.4. The diameter of the focused spot can be calculated using the following equation:
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Diameter of Spot (in microns) = Wavelength of Light
2 x NA of Objective Lens x n
(where n = refractive index of SIL)
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For example, with a common red laser of 0.685 micron wavelength, and a numerical aperture of 0.65, the focused spot diameter would equal 0.53 microns. If this same focused beam is put into a SIL with an index of refraction of 2.0, the spot diameter is reduced by a factor of two to 0.26 microns.
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Evanescent Coupling
Stanford researchers took advantage of another little known technique known as evanescent coupling to make the small spot at the bottom of the SIL lens appear on the surface. Without this, the objective lens focusing the laser's rays would result in a spot that exists only within the SIL lens. Given this, if the lens were placed near a recording surface, only a small fraction of the light from the laser would appear on that surface.
Evanescent coupling was once thought to be impossible, since a sudden discontinuity, such as a wave jumping across barriers, was believed to actually violate the laws of physics. However, equations developed in the last century by Scottish physicist James Maxwell indicate that any form of electromagnetic radiation enclosed within a boundary will produce waves outside that boundary that decay at an exponential rate. If an object is close enough to that boundary (within a fraction of a wavelength), it will receive some of the evanescent radiation. In some cases, evanescent coupling can produce a very high-quality effect, inducing more than 50% of the energy in the source of radiation to couple with the material in the near field. This is precisely what happens in NFR.
In this case, a recording surface is placed within the near field of a SIL lens and a laser spot is focused on the bottom of the lens. The close proximity evokes an evanescent coupling of laser energy, resulting in a "transfer" of the spot from inside the SIL to the surface of the disk, creating a small but well-defined spot on the recording disk.
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The Areal Density Advantage
Increased areal density is achieved by the combination of the small spot size produced by the SIL and evanescent coupling combined with the overlapping pattern of the crescent recording. This results in comparable linear density or BPI to traditional HDDs but with significantly higher TPI (see Figure 4). Continued areal density increases in NFR technology will come from different types of SIL lenses, the higher index SIL lens materials, and shorter wavelength (green and blue) lasers (see Figure 5).
Because of these attributes, this new mass storage technology is sure to redefine the boundaries in the storage hierarchy. It should also change user expectations of the capacity, cost, and performance features available for any given application (see Figure 6).
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Dr. Gordon Knight is the chief technical officer for TeraStor Corporation located in San Jose, California. |
