Optical waveguides – Planar optical waveguide
Reexamination Certificate
2003-03-19
2004-09-21
Lee, John D. (Department: 2874)
Optical waveguides
Planar optical waveguide
C385S146000, C369S013320, C369S112270
Reexamination Certificate
active
06795630
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to optical waveguides, and more particularly to optical waveguides that can be used in heat assisted magnetic recording.
BACKGROUND OF THE INVENTION
Magnetic recording heads have utility in magnetic disc drive storage systems. Most magnetic recording heads used in such systems today are “longitudinal” magnetic recording heads. Longitudinal magnetic recording in its conventional form has been projected to suffer from superparamagnetic instabilities at high bit densities.
Superparamagnetic instabilities become an issue as the grain volume is reduced in order to control media noise for high areal density recording. The superparamagnetic effect is most evident when the grain volume V is sufficiently small that the inequality K
u
V/k
B
T>70 can no longer be maintained. K
u
is the material's magnetic crystalline anisotropy energy density, k
B
is Boltzmann's constant, and T is absolute temperature. When this inequality is not satisfied, thermal energy demagnetizes the stored bits. Therefore, as the grain size is decreased in order to increase the areal density, a threshold is reached for a given material K
u
and temperature T such that stable data storage is no longer feasible.
The thermal stability can be improved by employing a recording medium formed of a material with a very high K
u
. However, with the available materials the recording heads are not able to provide a sufficient or high enough magnetic writing field to write on such a medium. Accordingly, it has been proposed to overcome the recording head field limitations by employing thermal energy to heat a local area on the recording medium before or at about the time of applying the magnetic write field to the medium. Heat assisted magnetic recording generally refers to the concept of locally heating a recording medium to reduce the coercivity of the recording medium so that the applied magnetic writing field can more easily direct the magnetization of the recording medium during the temporary magnetic softening of the recording medium caused by the heat source. Heat assisted magnetic recording allows for the use of small grain media, which is desirable for recording at increased areal densities, with a larger magnetic anisotropy at room temperature to assure sufficient thermal stability. Heat assisted magnetic recording can be applied to any type of magnetic storage media, including tilted media, longitudinal media, perpendicular media and patterned media. By heating the medium, the K
u
or the coercivity is reduced such that the magnetic write field is sufficient to write to the medium. Once the medium cools to ambient temperature, the medium has a sufficiently high value of coercivity to assure thermal stability of the recorded information.
It is believed that reducing or changing the bit cell aspect ratio will extend the bit density limit. However, different approaches will likely be necessary to overcome the limitations of longitudinal magnetic recording.
An alternative to longitudinal recording that overcomes at least some of the problems associated with the superparamagnetic effect is “perpendicular” magnetic recording. Perpendicular magnetic recording is believed to have the capability of extending recording densities well beyond the limits of longitudinal magnetic recording. Perpendicular magnetic recording heads for use with a perpendicular magnetic storage medium may include a pair of magnetically coupled poles, including a main write pole having a relatively small bottom surface area and a flux return pole having a larger bottom surface area. A coil having a plurality of turns is located adjacent to the main write pole for inducing a magnetic field between the pole and a soft underlayer of the storage media. The soft underlayer is located below the hard magnetic recording layer of the storage media and enhances the amplitude of the field produced by the main pole. This, in turn, allows the use of storage media with higher coercive force, consequently, more stable bits can be stored in the media. In the recording process, an electrical current in the coil energizes the main pole, which produces a magnetic field. The image of this field is produced in the soft underlayer to enhance the field strength produced in the magnetic media. The flux density that diverges from the tip into the soft underlayer returns through the return flux pole. The return pole is located sufficiently far apart from the main write pole such that the material of the return pole does not affect the magnetic flux of the main write pole, which is directed vertically into the hard layer and the soft underlayer of the storage media.
When applying a heat or light source to the medium, it is desirable to confine the heat or light to the track where writing is taking place and to generate the write field in close proximity to where the medium is heated to accomplish high areal density recording. In addition, for heat assisted magnetic recording (HAMR) one of the technological hurdles to overcome is to provide an efficient technique for delivering large amounts of light power to the recording medium confined to spots of, for example, 50 nm or less. A variety of transducer designs have been proposed and some have been experimentally tested. Among these are metal coated glass fibers and hollow pyramidal structures with metal walls. For all these approaches, confinement of the light depends on an aperture which is to be fabricated into the end of the structure and gives this kind of transducer the name “aperture probes.” Generally these devices suffer from very low light transmission rendering the devices useless for HAMR recording. For example, tapered and metallized optical fibers have demonstrated light confinement down to approximately 50 nm with a throughput efficiency of 10
−6
. Pyramidal probes made from anisotropic etching of Si wafers have been designed with throughput efficiencies of 10
−4
for similar spot sizes. Although this is the state of the art, it is still about two orders of magnitude too small for HAMR.
Improvements in throughput efficiency have been achieved for these transducers by changing the taper angles, filling the hollow structures with high index materials, and by trying to launch surface plasmons (SP) on integrated edges and corners of these tip-like structures. Although doing so does increase the throughput to some extent, the most promising SP approach is still very inefficient due to a lack of an efficient SP launching technique. In addition, all aperture probes suffer from a lower limit on spot size which is twice the skin depth of the metal film used to form the aperture. Even for aluminum, the metal with the smallest skin depth for visible light, this corresponds to a spot size of ~20 nm.
Solid immersion lenses (SILs) and solid immersion mirrors (SIMs) have also been proposed for concentrating far field optical energy into small spots. The optical intensity is very high at the focus but the spot size is still determined by the diffraction limit which in turn depends on the refractive index of the material from which the SIL or SIM is made. The smallest spot size which can be achieved with all currently known transparent materials is ~60 nm, which is too large for HAMR.
A metallic pin can be used as a transducer to concentrate optical energy into arbitrarily small areal dimensions. The metallic pin supports a surface plasmon mode which propagates along the pin, and the width of the external electric field generated by the surface plasmon mode is proportional to the diameter of the pin. Smaller pin diameters result in smaller spots, and in principle the spot size can be made arbitrarily small. However, smaller pin diameters also result in much shorter propagation lengths for energy transport. In fact, for a 50 nm spot size the 1/e propagation length of the surface plasmon is typically substantially less than a micron. Therefore, a metallic pin by itself is not useful as a near field transducer.
There is a need for transducers that can provide a
Challener William Albert
Mihalcea Christophe Daniel
Rausch Tim
Lee John D.
Lenart, Esq. Robert P.
Pietragallo Bosick & Gordon
Seagate Technology LLC
Stahl Mike
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