Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
1999-10-05
2003-09-09
Huber, Paul W. (Department: 2653)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
C369S124040
Reexamination Certificate
active
06618342
ABSTRACT:
TECHNICAL FIELD
The invention pertains to methods and apparatus for optical data storage.
BACKGROUND
Optical data storage systems use changes in absorption, reflection, and/or refractive index of a storage material to store and retrieve data. In conventional optical data storage systems, individual spatial locations (“cells”) on a substrate are allocated to store individual bits. A sequence of data bits is recorded in such memory systems by mapping each bit onto a different storage cell, and changing a material parameter at each cell to represent the data bit. Readout of the memory is accomplished by illuminating each cell with a light source in conventional implementations of such memories (CD-ROM, magneto-optic disks, etc.), each cell records one data bit.
Rather than directly allocating each cell to an individual data bit, frequency-selective data storage (“FSDS”) memories have been demonstrated in which multiple data bits are recorded in each storage cell. Volatile frequency-selective memories are disclosed in, for example, U.S. Pat. No. 3,896,420, and non-volatile memories are disclosed in, for example, U.S. Pat. No. 4,101,976. FSDS memories record multiple bits in each cell using materials that allow spectral addressing of individual atoms molecules. FSDS memories thus use both spectral and spatial addressing to access different portions of the storage material's absorption spectrum as well as different locations in the storage material.
FSDS systems use storage materials that have inhomogeneously broadened absorption profiles such as the absorption profile
101
of FIG.
1
. An absorption profile of an inhomogeneously broadened material (such as the absorption profile
101
) is characterized by an inhomogeneous linewidth &Dgr;&ngr;
i
that is a measure of the spectral width of the absorption profile (typically the full width of the absorption profile at one-half of the maximum value of absorbance). The absorption profile results from a summation of absorption profiles from individual absorbers (atoms, molecules or other active absorber centers), each having a spectral absorption profile and a spectral width referred to as a homogeneous absorption profile and a homogeneous linewidth &Dgr;&ngr;
h
, respectively.
FIG. 2
shows an example of a homogeneous absorption profile
201
of an individual absorber. Inhomogeneous broadening arises from the differing microenvironments for individual absorbers shifting the optical frequencies at which absorption occurs. Thus, the inhomogeneous absorption profile represents a combination of narrower, homogeneous absorption profiles centered at different frequencies. FSDS systems use materials in which the inhomogeneous linewidth is larger than the homogeneous linewidth.
When an inhomogeneously broadened material is illuminated with a single frequency light source, only the absorbers resonant with this single frequency interact with the light, resulting in optical excitation of these absorbers. Illuminating such a material with light having a bandwidth less than the material's inhomogeneous absorption linewidth produces a dip, or “spectral hole” in the absorption profile. The minimum width of a spectral hole is approximately equal to the homogeneous absorption linewidth.
FIG. 3
illustrates a spectral hole
301
of width &Dgr;&ngr;
h
in an inhomogeneous absorption profile
303
. FSDS systems use multiple spectral holes to record multiple bits in a single cell. The number of spectral storage channels available in a single cell of an inhomogeneously broadened material is determined by the ratio &Dgr;&ngr;
i
/&Dgr;&ngr;
h
of the inhomogeneous linewidth &Dgr;&ngr;
i
to the homogeneous linewidth &Dgr;&ngr;
h
. The number of spectral channels used is referred to as “spectral multiplicity.” For additional discussion of spectral hole-burning, see, for example, W. E. Moerner, ed,
Persistent Spectral Hole Buring: Science and Applications
(Springer Verlag, New York, 1988).
Two types of FSDS systems have been demonstrated and both can achieve the same spectral multiplicity. The first type is referred to as “frequency-domain” FSDS, and the second class is referred to as “time-domain FSDS.” These two types are discussed briefly below. In addition to these two types of FSDS systems, a “swept-carrier” system is disclosed in Mossberg, U.S. Pat. No. 5,276,637, incorporated herein by reference.
Frequency-domain FSDS systems directly address individual spectral channels in an inhomogeneously broadened material. In such systems, a narrowband light source having a spectral width less than the inhomogeneous linewidth &Dgr;&ngr;
i
illuminates a storage material. A continuous wave (“CW”) laser is typically used as the narrow-band source. Absorbers which the narrow-band light source fulfills the resonant condition are excited, recording data. Photo-induced absorption or refractive index changes produced by this excitation are probed to retrieve recorded data. If the linewidth &Dgr;&ngr;
l
of the narrow-band light source is less than the homogeneous linewidth &Dgr;&ngr;
h
, the achievable storage capacity in each cell is &Dgr;&ngr;
i
/&Dgr;&ngr;
h
. If the source linewidth &Dgr;&ngr;
l
is larger than the homogeneous linewidth &Dgr;&ngr;
h
, then the storage capacity is instead &Dgr;&ngr;
i
/&Dgr;&ngr;
l
and is said to be “laser linewidth limited.”
Frequency-domain FSDS imposes data-rate limitations on single bit recording. A spectral channel width &Dgr;&ngr;
ch
must be addressed with illumination having a pulse duration greater than 1/&Dgr;&ngr;
ch
because of a Fourier-transform relationship between pulse duration and linewidth. Thus, to access the kHz-scale linewidths available in some rare-earth-doped crystals, recording and readout pulses of approximately millisecond durations are required. The spectral holes produced in such FSDS systems can be either transient or permanent, as disclosed in U.S. Pat. No. 3,896,420, incorporated herein by reference.
Rather than allocating individual frequency channels to individual bits, time-domain FSDS systems use pulses with spectral widths larger than the homogeneous linewidth &Dgr;&ngr;
h
and therefore can use pulses with durations less than 1/&Dgr;&ngr;
h
. Time-domain FSDS systems can record data streams containing pulses that are as short as 1/&Dgr;&ngr;
i
. In time-domain FSDS systems, a storage material is exposed to a brief reference pulse and a data pulse corresponding to a data-bit stream. These pulses illuminate the storage material sequentially to record an interference between the frequency spectra of the reference pulse and the data pulse, resulting in the direct recording of the spectrum of the data-bit stream. If the reference pulse precedes the data pulse, subsequent illumination of the storage material with a replica of the reference pulse produces a reconstruction of the data pulse. Such time-domain FSDS systems are described in, for example, U.S. Pat. No. 4,459,682, incorporated herein by reference.
Time-domain FSDS systems use temporally distinct reference pulses to record the spectrum of a data-bit stream, while swept-carrier FSDS systems record the spectrum of a data-bit stream using frequency-swept (chirped) reference and data beams. The reference and data beams simultaneously illuminate the storage material, and subsequent illumination with the frequency swept reference beam reproduces the data beam. Such systems are disclosed in, for example, Mossberg, U.S. Pat. No. 5,276,637 and Mossberg et al.,
Opt. Lett
. 17, 535 (1992).
In conventional FSDS systems, a positioning system directs a laser beam to a particular cell, and data is recorded in, or read from, the entire spectral capacity at the cell. Thus, the laser is stationary in two spatial dimensions while the data is stored or retrieved using a third dimension (frequency).
An important limitation of both time-domain and swept-carrier data storage is excitation-induced frequency shifts, also referred to as excitation-induced dephasing or instantaneous dephasing, as described in, for example, Huang et al.,
Phys. Rev. Lett
.
Johnson Alan E.
Maniloff Eric S.
Mossberg Thomas W.
Huber Paul W.
Intel Corporation
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