Storage media for optical storage systems

Details Storage media for optical storage systems Storage media for optical storage systems

2000-02-09

2002-09-10

Young, W. R. (Department: 2653)

Dynamic information storage or retrieval

Storage or retrieval by simultaneous application of diverse...

Magnetic field and light beam

C369S013400, C369S283000, C369S286000, C428S690000, C428S064200

Reexamination Certificate

active

06449221

ABSTRACT:

FIELD OF THE INVENTION
The present specification generally relates to optical data storage. More particularly, the present specification describes an electro-optical system for data storage and retrieval in a near-field recording configuration.
BACKGROUND OF THE INVENTION
Data storage is an important aspect of today's information technology. A great deal of effort has been made by the storage industry to increase the areal data density of a storage medium in order to meet the ever increasing demand for higher capacity storage devices.
Magnetic storage devices such as fixed or removable magnetic disks and tapes are widely-used conventional storage devices. The state-of-art conventional magnetic hard drive systems can achieve extremely high linear bit densities, especially with the new MR and GMR magnetic heads. For example, the areal density of many hard disk drives is on the order of magnitude of about one gigabit per square inch. One limitation in increasing areal data density in a magnetic device is the particle size or the characteristic dimension of a typical magnetic domain of the magnetic recording materials. Other limitations include the width of the magnetic read/write head and the limitations of mechanical tracking. Therefore, these hard drives are typically limited to less than 10,000 tracks per inch.
Optical storage devices are emerging as an alternative technology to the conventional magnetic technology because of their potential for high density data storage. The areal density of an optical storage device, in principle, is only limited by the diffraction limit of an illuminating optical beam for reading or writing. One type of commercial optical storage technology is based on magneto-optical materials. These materials can currently produce an areal data density of about one giga bit per square inch.
One well-known approach to increase the areal data density in an optical storage system is using smaller beam size. Due to the diffraction limit, this may be achieved by using a light source with shorter wavelengths such as those toward the blue end of the spectrum. For example, one application for the industrial development of compact blue lasers is aimed at the optical storage. Alternatively, one may increase the numerical aperture of the objective lens in the system to focus a beam at a given wavelength to a smaller spot within the diffraction limit.
FIG. 1
shows a block diagram of a typical rewritable optical storage system or drive
100
based on magneto-optic recording in the prior art. Several servo mechanisms are needed to keep the laser beam in focus and tracking on the disk, for example, an objective lens actuator
114
, a master-slave tracking servo control
130
, and a focusing servo control
120
. In particular, the objective lens in the prior-art system
100
is servo controlled for focusing and tracking the beam onto the storage medium layer(s) at a desired location. This type of conventional optics system is usually limited to numerical apertures of the objective lens of less than 1.0, and typically in a range about 0.55 to 0.60. Since the areal density of the data stored on the medium is directly proportional to the square of the numerical aperture, the limited numerical apertures of a conventional optical drive can significantly restrict a substantial increase in the data density.
SUMMARY OF THE INVENTION
The present disclosure includes an electro-optical storage system with an areal data density that is higher than that of the prior-art storage systems such as state-of-art magnetic hard disk drives and various optical drives. One embodiment of the systems of the present invention comprises a read/write head and a head positioning system, an optics module including beam relay optics and signal detectors, an optical medium and a corresponding medium driving unit, and an electronic control system.
The read/write head is preferably a “flying” head which is suspended over the optical medium by an air-bearing surface in a near-field recording configuration wherein the spacing between an exit facet of the flying head and a recording layer in the medium is a fraction of one wavelength of the radiation. An optical read/write beam exiting the near-field lens is then coupled to the optical medium by evanescent waves. The flying head includes a near-field lens with a high index of refraction and usually has a numerical aperture greater than unity under the preferred. near-field condition. A focused beam with a spot size smaller than that obtainable from a conventional optical system is thus achieved at least in part due to the use of a high index solid immersion lens (“SIL”) as the near-field lens.
One aspect of the invention is the automatic optimization and maintenance of focus under the preferred near-field condition. This is accomplished, at least in part, by the use of the air-bearing surface to suspend the flying head over the surface of the optical medium by a fraction of a wavelength at a prespecified height. Therefore, a conventional focusing servo system may not be required.
According to one embodiment, a solid immersion lens is used as the near-field lens with respect to an objective lens at a desired distance. A SIL cap lens that is part of a sphere may be laminated to a transparent base plate with an optical UV epoxy layer. A spacer having a void area that is larger than the SIL cap lens may be adhered to the base plate with the optical UV epoxy layer in a way so that the SIL cap lens is enclosed in the void area of the spacer. The thickness of the spacer is preferably at least the height of the SIL cap lens. The objective lens is then fixed to the spacer with an epoxy. The desired distance between the objective lens and the SIL cap lens may be determined in an alignment process by maximizing an optical feedback signal from an exit facet of the SIL cap lens. A transparent mesa may be formed on the base plate as a part of the near-field lens for coupling light between the flying head and the optical medium. The SIL cap lens and the base are preferably made of materials that have a similar index of refraction, including but not limited to cubic Zirconia, Schott glass (LaSF35), Hoya glass (TaFd43), Cleartran, Zinc Selenide, Gallium Phosphide and others. In one implementation, the index mismatch at the operating wavelength should be less than about 2% for optimal performance.
The optics module may be a fixed optics module, i.e., the relative positions of different optical elements within are fixed at predetermined distances. In one embodiment, the fixed optics module includes a light source, a collimator lens, an anamorphic prism, a front facet monitor, a polarization rotator, a data/servo detector, a relay lens, a galvanometer (“galvo”) mirror, and a folding mirror for guiding a read/write beam to the flying head. The orientation of the galvo mirror is controlled to provide a fine positioning mechanism for precisely positioning the read/write beam to a desired point on the optical medium.
In accordance with one embodiment, the galvanometer may have a compact and improved Winchester flexure with two load points on a rigid stiffener to define a single axis of rotation that is close to the reflecting surface of the galvo mirror. One or more capacitive position sensors may be implemented in the galvanometer for position monitoring and controlling.
A passive thermal compensation scheme may be implemented in the fixed optics module to maintain an optimal focus. The thermal and mechanical properties of optics mounting devices supporting the optical train of the disk drive are carefully chosen with respect to one another to minimize the overall thermal variation of the optical train over a certain temperature range. In addition, various mounting techniques can be used so that thermal expansion of different parts of a device may cancel one another. Furthermore, optical component materials can be selected to minimize the overall thermal effect.
A rotary actuator may be used as a coarse positioning means for the optical disk drive although other positionin

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