Optical: systems and elements – Holographic system or element – Having particular recording medium
Reexamination Certificate
2001-11-02
2004-03-23
Juba, Jr., John (Department: 2872)
Optical: systems and elements
Holographic system or element
Having particular recording medium
C359S003000, C359S021000, C359S024000, C359S029000, C359S035000, C359S900000, C365S125000, C365S216000
Reexamination Certificate
active
06710901
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for providing volumetric, spatially resolved, high-density holographic data storage in photosensitive glass. More particularly, the present invention accomplishes laser storage by two-photon writing and one-photon reading of information stored in a volume of multi-chrome photosensitive glass, under the action of laser pulses having a duration of 100 femtoseconds (fs) or less.
BACKGROUND OF THE INVENTION
Rapid development of the Internet has led to an explosion of information available to users, and to the appearance of new information technologies. This development demands an increase of storage capacity. Conventional magnetic and optical data storage devices and technologies, where individual bits are stored as a dots with changed magnetic or optical properties on the surface of a storage device are approaching the absolute physical limits of capacity. One-photon holographic methods and devices provide for the storage of information in the volume of a medium, not only on the surface, but this conventional holographic method does not allow to the writing of different spatially resolved holograms in the same volume of recording media. That is a limiting factor of conventional one-photon holographic storage devices.
Prior art volumetric storage devices and methods for the writing of information in optically transparent materials, such as glass, crystals and polymers, can be divided in two approaches. The first approach involves the non-linear action of focused electromagnetic radiation on transparent material, leading to changes of the optical properties at a chosen point inside the material. This approach allows the writing of bits of information as dots placed in a volume of optically transparent material. Excessive time is required therein to write and read significant amounts of information.
A method and apparatus for providing a body of material with sub-surface marking in the form of an area of increased opacity to electromagnetic radiation is disclosed in U.S. Pat. No. 5,206,496 issued to Robert M. Clement on Apr. 27, 1993. The method includes directing at a surface of the body a high energy-density beam, to which the material is transparent, and bringing the beam to a focus at a location spaced from the surface, and within the body, so as to cause localized ionization of the material. The main disadvantage in the application to optical storage devices is in being time-consuming for useful quantities of information.
A method of forming images in optically transparent solids under action of focused laser radiation with power density exceeding the optical breakdown threshold is disclosed in Russian Patent RU2008288 issued to S. V. Oshemkov on Feb. 28, 1994. Pulsed or continues-wave laser radiation is focused on the chosen point of optically transparent solid samples to induce breakdown of the material at the focal point. The main disadvantage of this method when applied to optical storage is the relatively large size of the optical breakdown area. The typical area herein, of optical breakdown spots in glass and crystals, is 10 micron or more. This limiting factor reduces the maximal density of written information, and limits the capacity of storage devices based on this method.
A method of stable hole burning in crystals, containing Sm
2+
as active ions with the purpose of creating narrow holes by laser irradiation, is disclosed in U.S. Pat. No. 5,478,498 issued to N. Kodama, K. Hirao, S. Hara and Y. Inoue on Dec. 26, 1995. Pulsed laser radiation, with a power density insufficient for optical breakdown, is focused in a transparent crystal containing Sm
2+
as the active ions, which leads to a disordered fluorite-type photochemical hole burning. The main disadvantage of this method is the same as in the preceding methods—excessive time is required to write the information by the forming of spots in the volume.
Methods of using lasers to form small holes or spots with changed optical properties in a bulk of transparent solid dielectric are described in the following two references. J. Qui, et al, in an article entitled Permanent Photo Reduction Of Sm
3+
To Sm
2+
Inside A Sodium Aluminoborate Glass By An Infrared Femtosecond Pulsed Laser, //Appl. Phys., Lett. V.74 (1999) pp.10-12, describes the results of experimental observation of permanent photo reduction of Sm
3+
to Sm
2+
inside a sodium aluminoborate glass by an infrared femtosecond pulsed laser. After irradiation by an 800 nm-focused femtosecond (fs) pulsed laser, the focused part of the laser in the glass became orange. Absorption, luminescence and electron spin resonance spectra showed the permanent photo reduction of samarium ions after the laser irradiation. The authors indicate that the observed phenomenon is inferred to be useful for the fabrication of optical memory devices with an ultra-high storage density.
Y. Kondo, et al, in an article entitled Three-Dimensional Microscopic Crystallization In Photosensitive Glass By Femtosecond Laser Pulses At No Resonant Wavelength, //Jpn. J. Appl. Phys., V.37, (1998) pp. L94-L96, reported an observation of the three-dimensional microscopic crystallization in photosensitive glass by femtosecond laser pulses at no resonant wavelength. The glass used in the experiments was an aluminosilicate glass containing Ag
+
ions and Ce
3+
ions, in which NaF crystallites could be precipitated using the conventional process. The glass specimens were irradiated with a femtosecond laser of a 630 nm wavelength, not resonant with the Ce
3+
absorption. The irradiated specimens were heated at 540° C. for 30 minutes, held at 100° C. for 3 hours, and then heated again at 580° C. for 30 minutes to precipitate NaF crystallites. Whether the crystallization occurred or not was evaluated by using an optical microscope and by X-ray diffraction. Due to the use of transparent light and the presence of a threshold for crystallization, it is possible to precipitate micro-crystallites three-dimensionally within the photosensitive glass. The authors indicate that this technique can be applied to create three-dimensionally structured materials such as photonic band gap crystals, which offer unique ways to control the propagation of light. However, both methods of making spots with changed optical properties, by the change of absorption described by Kondo, and the change of refraction herein described, are insufficient to solve the problem of construction of high-density optical storage devices, because these devices may use the holographic methods of writing and reading of information.
The second approach of prior art optical storage devices is the conventional one-photon holographic devices, described in many articles, such as by J. Ashley et al, in Holographic Data Storage, //IBM J. Res. Develop., V.44 (2000), pp.341-366. The main problem of such conventional holographic methods is the lacking of good optical materials, allowing the long-lived writing of information with high-density volume of written information. Y. Kondo, et al, op. cit., indicate that in the case of holographic storage, the response of the recording medium, which converts the optical interference pattern to a refractive index pattern, i.e., a hologram, is generally linear in light intensity and lacks the response threshold found in bistable storage media such as magnetic films. Also, because the standard holographic mediums are linear and reversible, they are subject to erasure during readout or in darkness by thermal processes.
FIG. 1
is a prior art schematic illustration of the method of using holograms to write and read data
100
. A hologram is a recording of the optical interference pattern that forms at the intersection of two coherent optical beams. Typically, light from a laser is split into two paths called the object beam
110
and reference beam
130
(
FIG. 1
a
) . The beam that propagates along the object path
110
carries the information, while reference beam
130
is used to record and read o
Canadian Production Promotion Group Inc.
Jr. John Juba
Langer, Pat. Atty. Edward
Shiboleth Yisraeli Roberts Zisman & Co.
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