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Claims  |
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What is claimed is:
1. A method for optically fixing holograms in photorefractive crystalline
materials comprising the steps of:
providing a crystal of photorefractive material in which holograms can be
created by the redistribution of electrons and fixed by the redistribution
of positive ions bound within said crystal,
forming a hologram (light interference pattern) in said material in which
electrons are redistributed therein and cause variations of the electric
fields in said crystal defining said hologram,
illuminating the material with light of a frequency in an absorption band
of the ionic bond and having sufficient energy to cause the positive ions
to be redistributed into new locations to neutralize the electric field
variations caused by the electrons, whereat said positive ions remain
fixed, forming a protonic grating in response to the electric fields.
2. The method of claim 1 in which said positive ions are protons.
3. The method of claim 1 further in which said forming step includes
illuminating said crystal with a reference beam at a first angle, and
simultaneously illuminating said crystal with an object beam containing
information at a second angle, and
further including the step of subsequently illuminating the material with
said reference beam at said first angle and at the same frequency by which
said hologram was written to create a reconstructed beam containing said
information.
4. The method of claim 1 further in which said forming step includes
illuminating said crystal with a reference beam at a first angle, and
simultaneously illuminating said crystal with an object beam containing
information at a second angle, and
further including the step of subsequently illuminating the material with
said same reference beam after imposing a 180 degree phase change thereon
and at said first angle and at the same frequency by which said hologram
was written and simultaneously again illuminating said crystal with object
beam to selectively erase said hologram containing said first written
information.
5. A method for optically fixing holograms in photorefractive crystalline
materials comprising the steps of:
a. providing a crystal of photorefractive material in which holograms can
be created by the redistribution of electrons and fixed by the
redistribution of ions (protons) ionically bound within said crystal,
b. illuminating said crystal with a reference beam at a first angle, and
simultaneously illuminating said crystal with an object beam containing
information at a second angle, to form a first hologram (light
interference pattern) in said material in which electrons are
redistributed therein and cause variations of the electric fields in said
crystal defining said hologram,
c. illuminating the material with light of a frequency in an absorption
band of the ionic bond and having sufficient energy to cause the ions to
be redistributed into new locations to neutralize the electric field
variations caused by the electrons to fix thereby said first hologram in
the form of a first protonic grating,
d. repeating said illuminating steps of subparagraph b. with said reference
beam shifted to a new angle different from said first angle, and with an
object beam at the same angle and containing different information to form
a second hologram (light interference pattern) in said material in which
electrons are redistributed therein and cause variations of the electric
fields in said crystal representing said second hologram,
e. again illuminating the material with light of a frequency in an
absorption band of the ionic bond and having sufficient energy to cause
the ions to be redistributed into new locations to neutralize the electric
field variations caused by the electrons to thereby fix said second
hologram in the form of a second protonic grating, and
f. continuing the above steps to write and fix a plurality of angularly
multiplexed holograms into said crystal.
6. The method of claim 5 further including the step of
subsequently illuminating the material with said reference beam at said
first angle and at the same frequency and angle by which one of said
holograms was written to selectively reconstruct an object beam containing
the information in said one hologram.
7. In a method for storing phase holograms in a crystal of iron-doped
lithium niobate (Fe:LiNbO.sub.3) in which holograms are written by the
redistribution of electric charge and fixed by the redistribution of
protons, the steps of:
forming a hologram by exposing the crystal to a light interference patters
to record the same as an electronic space charge pattern or hologram in
the volume of the crystal by photo-excitation of electrons trapped at Fe+2
sites,
further exposing the crystal to light at the proton absorption band to
neutralize said electronic pattern or hologram space charge by optically
liberating the protons from OH bonds within the Fe:LiNbO.sub.3, which
allows the protons to migrate to create a protonic grating which mirrors
said electronic pattern or hologram and neutralizes the space charge
patterns of the electrons, said protonic grating thereafter remaining
fixed.
8. The method of claim 7 further in which said forming step includes
illuminating said crystal with a reference beam at a first angle, and
simultaneously illuminating said crystal with an object beam containing
information at a second angle, and
further including the step of subsequently illuminating the material with
said reference beam at said first angle and at the same frequency by which
said hologram was written to create a re-constructed object beam
containing said information.
9. The method of claim 8 further in which said forming step includes
illuminating said crystal with a reference beam at a first angle, and
simultaneously illuminating said crystal with an object beam containing
information at a second angle and
further including the step of subsequently illuminating the material with
said same reference beam after imposing a 180.degree. phase change thereon
and at said first angle and at the same frequency by which said hologram
was written and simultaneously again illuminating said crystal with object
beam to selectively erase said hologram containing said first written
information.
10. A method for optically fixing holograms in photorefractive crystalline
materials comprising the steps of:
a. providing a crystal of photorefractive Fe:LiNbO.sub.3 in which holograms
can be created by the redistribution of electrons and fixed by the
redistribution of protons ionically bound within said crystal,
b. illuminating said crystal with a reference beam at a first angle, and
simultaneously illuminating said crystal with an object beam containing
information at a second angle, to form a first hologram (light
interference pattern) in said material in which electrons are
redistributed therein and cause variations of the electric fields in said
crystal representing said hologram,
c. illuminating the material with light of a frequency in an absorption
band of the ionic bond of said protons and having sufficient energy to
cause the protons to be redistributed into new locations to neutralize the
electric field variations caused by the electrons to fix thereby said
first hologram in the form of a first protonic grating,
d. repeating said illuminating steps of subparagraph b. with said reference
shifted to a new angle different from said first angle, and with an object
beam at the same or a different angle and containing different information
to form a second hologram (light interference pattern) in said material in
which electrons are redistributed therein and cause variations of the
electric fields in said crystal representing said second hologram in the
form of a second protonic grating,
e. again illuminating the material with light of a frequency in an
absorption band of the ionic bond of said protons and having sufficient
energy to cause the protons to be redistributed into new locations to
neutralize the electric field variations caused by the electrons to
thereby fix said second hologram, and
f. continuing the above steps to write and fix a plurality of angularly
multiplexed holograms into said crystal.
11. The method of claim 10 further including the step of
subsequently illuminating the material with said reference beam at the same
frequency and angle by which one of said holograms was written to
selectively create a re-constructed object beam containing the information
from said one hologram.
12. A holographic memory for optically storing holograms in photorefractive
crystalline materials comprising:
a crystal (Fe:LiNbO.sub.3) of photorefractive material in which holograms
can be created by the redistribution of electrons and fixed by the
redistribution of ions,
means for forming a hologram by interfering an input beam having
information impressed thereon with a plane-wave reference beam in said
material in order to redistribute the electrons therein and thereby form
spatial charge density variations of the electric fields in said crystal
representing said hologram,
means for illuminating the material with light of a frequency in an
absorption band of the ionic bond and having sufficient energy to cause
the ions to be redistributed to neutralize the electric field charge
density variations of the electrons, said ions thereafter remaining fixed
as an ionic grating,
means for illuminating said neutralized electronic/ionic hologram with a
light beam which is mutually incoherent with said information beam and
reference beams and which is of a frequency sufficient to liberate the
electrons from their redistributed sites in order to create a spatially
homogeneous electronic distribution within the crystal and thereby leave
said ionic grating accessible for reading, and
means for illuminating said memory with a read reference beam for
interacting with the charge field of said ions in order to recover said
information in said input beam stored in said ionic grating within said
hologram.
13. A holographic memory for storing phase holograms containing information
in photorefractive material which holograms are written by the
redistribution of electric charges, comprising:
a photorefractive crystal of Fe:LiNbO.sub.3 in which holograms can be
created by the redistribution of electrons and fixed by the redistribution
of protons,
means for exposing the crystal to light interference pattern to record the
hologram as a space charge pattern in the volume of the crystal by
photo-excitation of electrons trapped at Fe.sup.+2 sites,
means for exposing the crystal to light at the proton absorption band to
neutralize said space charge by optically liberating the protons from the
OH bond with LiNbO.sub.3 which allows the protons to migrate to create a
proton space charge pattern corresponding to said hologram and to
neutralize the space charge patterns of the electrons, whereat said proton
space charge patterns remain fixed, and
means for illuminating said memory with a read beam for interacting with
the protonic space charge patterns to recover the information in said
input beam by creating a reconstructed object beam containing said
information. |
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Claims |
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Description |
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FIELD OF THE INVENTION
This invention describes an optical method for fixing holograms in
photorefractive materials and a read/write memory based on this optical
method. The invention has particular application to the photorefractive
material lithium niobate.
BACKGROUND OF THE INVENTION AND PRIOR ART
Photorefractive crystalline materials such as iron-doped lithium niobate
(Fe:LiNbO.sub.3) respond to exposure to light in such a way as to cause a
charge redistribution which can later diffract incident light. Thus, if
the initial exposure creates a hologram, the latter can be reconstructed
and recovered.
To date, the utility of lithium niobate as a holographic data storage
medium has been severely limited by the lack of a rapid, selective, and
reversible storage (write) procedure which is compatible with
non-destructive data recovery (read).
D. L. Staebler and J. J. Amodei ("Thermally fixed holograms in
LiNbO.sub.3," Ferroelectrics, 3, p. 107, 1971) demonstrated that holograms
generated within Fe:LiNbO.sub.3 could be thermally fixed by heating the
Fe:LiNbO.sub.3 to roughly 100.degree. C. for at least 30 minutes after the
holograms had been generated. While this technique is reversible (by
heating the Fe:LiNbO.sub.3 to greater than 170.degree. C. for
approximately 30 minutes) and does permit non-destructive data read out,
it is slow, results in low holographic diffraction efficiency, and is
non-selective (i.e., all holograms multiplexed within a given volume of
Fe:LiNbO.sub.3 are fixed and erased, en mass).
Subsequently, D. L. Staebler, W. J. Burke, W. Phillips and J. J. Amodei
("Multiple Storage and Erasure of Fixed Holograms in Fe-doped
LiNbO.sub.3," Applied Physics Letters, 26, p. 182, 1975) demonstrated that
volume holograms generated within Fe: LiNbO.sub.3 could be thermally fixed
by heating the Fe:LiNbO.sub.3 to approximately 160.degree. C. while the
holograms were being generated. While this technique resulted in
high-diffraction efficiency holograms that could be non-destructively
read-out, it necessitated an inconvenient physical rotation of the
Fe:LiNbO.sub.3 crystal during read-out in order to compensate for hologram
read-out angles which changed slightly upon cooling the Fe: LiNbO.sub.3
crystal to room temperature.
D. von der Linde, A. M. Glass and K. F. Rogers ("Multiphoton
photorefractive processes for optical storage in LiNbO.sub.3," Applied
Physics Letters, 25, p. 155, 1974) describe a two-photon storage technique
which is rapid (picosecond time scale), but which requires high-intensity
laser beams (.about.10.sup.9 watts/cm.sup.2) for storage and relatively
low intensity laser beams for non-destructive read-out. Such picosecond,
gigawatt laser pulses are difficult to obtain at repetition rates
(10-1,000 Hz) necessitated by practical data storage systems and,
furthermore, may cause damage to critical system components (e.g.,
acousto-optic beam deflectors used for hologram multiplexing and spatial
light modulators used for entering holographic data). Moreover, the
requirement to use a low-intensity laser beam to avoid destructive
read-out at the second harmonic frequency inevitably leads to reduced
output data transfer rates.
Finally, D. von der Linde, A. M. Glass and K. F. Rogers ("Optical storage
using refractive index changes induced by two-step excitation," Journal of
Applied Physics, 47, p. 217, 1976) describe a two-step holographic storage
process involving chromium-doped LiNbO.sub.3 which is, in principle,
capable of non-destructive read (at the write wavelength) and selective
optical erasure, and which requires peak storage laser beam intensities of
only about 10.sup.7 watts/cm.sup.2. This approach, however, yields
relatively small holographic diffraction efficiencies (compared with
Fe:LiNbO.sub.3) which are not linear with storage exposure energy
(important for retaining holographic dynamic range) and, most importantly,
yields short storage times of only about 20 hours. Furthermore, this
approach requires the use of specialized (Q-switched ruby and tunable dye)
lasers which are complicated, unreliable, and expensive.
There is, therefore, a need for improved methods of fixing holograms in
photorefractive storage media which will overcome the above limitations
and disadvantages and, thereby, make possible practical read/write
memories based thereon.
SUMMARY OF THE INVENTION
The present invention is predicated on a finding that there exists in
Fe:LiNbO.sub.3 a broad, polarization-sensitive optical absorption band,
centered at about 1 .mu.m, which corresponds to the liberation of protons
from their oxygen bonds. In accordance with the present invention it is
found that optically generated, electronic holograms can be optically
fixed in Fe:LiNbO.sub.3 by using radiation (not necessarily coherent)
within this polarization-sensitive absorption band (roughly 0.85 .mu.m to
1.6 .mu.m) to optically excite and liberate protons from potential well
traps, which protons then drift within the field generated by the
electronic hologram to compensate and fix the latter.
In accordance with this invention, a method and read/write memory based
thereon is disclosed by which phase holograms are stored and fixed in a
photorefractive crystalline material, preferably Fe:LiNbO.sub.3, having
protons bound to oxygen atoms within the LiNbO.sub.3 lattice. Holograms
are stored (and data written) by exposing the Fe:LiNbO.sub.3 to a light
interference pattern formed by interfering a plane-wave reference beam and
a data-bearing object beam, both beams of which have a wavelength suitable
for photo-exciting electrons trapped at Fe.sup.+2 sites within the
Fe:LiNbO.sub.3. The photo-excited electrons then become trapped at
Fe.sup.+3 sites within the Fe:LiNbO.sub.3 and form a space-charge pattern,
or hologram, which is identical spatially to the original interference
pattern. Afterwards, or simultaneously, the crystal is exposed to light
having a wavelength within the broad, proton absorption band. Optically
excited protons then drift within the electric field of the electronic
space-charge pattern and compensate the latter. The resulting compensated
pattern is then exposed to light having a wavelength equal to or nearly
equal to the wavelength of the light used to generate the original
interference pattern, thereby causing the trapped electrons to become
spatially homogeneous and the latent protonic grating to be developed or
revealed. The latter represents a "fixed" grating which can be
non-destructively read at the same optical wavelength used to write the
original hologram.
The holographic writing technique described above may be extended to
include angle-multiplexed holograms, wherein multiple holograms are
written sequentially by incrementing the angle with which the reference
beam interferes with the object beam within the Fe:LiNbO.sub.3 crystal. As
in the case of a single hologram, angle-multiplexed holograms may be
compensated, developed, and, hence, fixed via protonic excitation either
after all holograms have been recorded electronically or simultaneously
with their recording. Moreover, having once stored a sequence of
angle-multiplexed holograms, individual holograms may be completely or
partially erased by phase-shifting the reference beam associated with the
hologram in question by .pi. radians and simultaneously illuminating the
Fe:LiNbO.sub.3 with that part of the data-bearing object beam to be erased
.
These and other features of the invention will become apparent from the
following descriptions and claims when taken with the accompanying
drawing, of which:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows transmittance spectra taken of Fe:LiNbO.sub.3 (0.1 mole
percent iron) containing interstitial hydrogen bound to oxygen atoms. In
the graph labeled (a) the light is polarized parallel to the crystalline
c-axis and in the graph labeled (b) the light is polarized perpendicular
to the crystalline c-axis.
FIGS. 2A-2D describe a read/write memory using Fe: LiNbO.sub.3 as the
storage medium in accordance with the present invention, in which: FIG. 2A
shows simultaneous hologram (data) writing and fixing step; FIG. 2B shows
the hologram development step; FIG. 2C shows the hologram (data) read
step; and FIG. 2D shows the hologram (data) erasure step. .lambda..sub.1
.lambda..sub.2, and .lambda..sub.3 are the wavelengths of the write,
fixing, and development beams, respectively. Since neither the fixing nor
the developing beams need be coherent or narrow band, .lambda..sub.2 and
.lambda..sub.3 can represent center frequencies.
FIG. 3 shows how N holograms can be sequentially written and fixed using an
angle-multiplexing technique.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
This invention describes a process for optically fixing holograms stored
within iron-doped lithium niobate (Fe:LiNbO.sub.3) which contains hydrogen
ions (protons), and also describes an apparatus, based on this process, by
which data may be holographically written, read, and erased within
Fe:LiNbO.sub.3.
Iron is introduced into the LiNbO.sub.3 host during growth of the latter
using a conventional Czochralski pulling technique. Subsequently, protons
can be introduced in the Fe:LiNbO.sub.3 using (as taught in S. Klauer, M.
Wohlecke, and S. Kapphan, "Influence of H-D isotopic substitution on the
protonic conductivity of LiNbO.sub.3," Physical Review B, 45, p. 2786,
1992) electric field or wet annealing techniques. Protons are believed to
be the mobile species which compensate (and fix) optically generated,
electronic holograms within Fe:LiNbO.sub.3 (S. Klauer, et al). These
protons are normally bound to oxygen atoms within the LiNbO.sub.3, as
revealed by a highly polarization-sensitive absorption band at 2.87 .mu.m
which corresponds to the fundamental vibration resonance of an OH.sup.-
molecule (as modified slightly by crystal field effects within the
LiNbO.sub.3).
The energy required to free a proton (i.e., break the OH.sup.- bond) is
inferred to be approximately 1.17 eV within Fe:LiNbO.sub.3 (compared with
a dissociation energy of approximately 4.2 eV for a free OH.sup.- ion).
This activation energy was measured using thermal conductivity techniques
by both Staebler et al. in 1975 and by Klauer et al. in 1992. The
literature does not report any attempt to optically liberate the proton
from its bound state within either LiNbO.sub.3 or Fe:LiNbO.sub.3.
FIG. 1 shows polarized transmittance spectra of Fe: LiNbO.sub.3 taken from
0.5 .mu.m to 3.1 .mu.m. Graph (a) in FIG. 1 plots the transmittance for
light polarized parallel to the Fe:LiNbO.sub.3 c-axis and graph (b) in
FIG. 1 plots the transmittance for light polarized perpendicular to the
Fe:LiNbO.sub.3 c-axis. The characteristic (and highly
polarization-sensitive) OH.sup.- absorption band centered at 2.87 .mu.m is
clearly present in graph (b) and absent in graph (a). Also present in
graph (b) is a broad absorption band (c) centered at approximately 1.15
.mu.m, which appears to overlay a weaker background absorption band (d)
evident in graph (a). This broad polarization-sensitive absorption band
(the center of which corresponds closely to the thermally measured proton
activation energy of 1.17 eV) is attributed to optical activation (or
breaking) of the OH.sup.- bond within the Fe:LiNbO.sub.3. Optically
generated, electronic holograms can be optically fixed in Fe:LiNbO.sub.3
by using radiation (not necessarily coherent) within this broad,
polarization-sensitive absorption band (roughly 0.85 .mu.m to 1.6 .mu.m)
to liberate protons, which then drift within the field generated by the
electronic hologram to compensate the latter.
Optical fixing, as described here, of holograms stored within
Fe:LiNbO.sub.3 overcomes all of the shortcomings of the fixing techniques
cited in the prior art and permits, for the first time, the building of a
true, high-data-transfer-rate read/write memory in Fe:LiNbO.sub.3.
FIGS. 2A-2D illustrate how the optical processes described herein can be
used to realize a read/write memory in Fe:LiNbO.sub.3. FIG. 2A illustrates
hologram storage (data writing); FIG. 2B hologram (data) development; FIG.
2C non-destructive hologram recovery (data reading); and FIG. 2D selective
hologram (data) erasure.
Referring to FIG. 2A, mutually coherent object and reference laser beams 10
and 12, interfere within a crystal 13 of Fe:LiNbO.sub.3 to form an
electronic phase hologram. Simultaneously, a fixing beam 14 (of wavelength
.lambda..sub.2) propagates (preferentially) along the Fe:LiNbO.sub.3
crystal's c-axis and excites protons quiescently bound to oxygen atoms
within the Fe:LiNbO.sub.3. Excited protons drift within the electric field
generated by the electronic phase hologram and continuously and
simultaneously compensate the electronic hologram. The result is a
compensated (charge-wise) hologram which temporarily has no diffracting
power. Wavelength .lambda..sub.1 may lie anywhere within the Fe.sup.2+
absorption band of Fe:LiNbO.sub.3 (approximately 0.40 .mu.m to 0.55 .mu.m)
and .lambda..sub.2 may lie anywhere within the OH.sup.- absorption band
(approximately 0.85 .mu.m to 1.6 .mu.m).
The source (not shown) of optical radiation (for beam 14) at wavelength
.lambda..sub.2 is preferably a Nd:YAG laser operated at 1.06 .mu.m. A
portion of the 1.06 .mu.m output of the Nd:YAG laser is doubled to 532 nm
and, preferably, serves as the source of optical radiation at
.lambda..sub.1 (for beams 10 and 12).
Also referring to FIG. 2A, the presence of an optical phase shifter 16 is
noted in the reference beam path. During the storage or writing of a
hologram, this phase shifter introduces zero phase shift into the
reference beam; during complete or partial erasure of a hologram (as will
be described below), it introduces a pi (.pi.) phase shift into the
reference beam.
The phase shifter is preferably an electro-optic device, incorporating, for
example undoped lithium niobate or potassium di-hydrogen phosphate (KD*P)
as the electro-optic medium. The spatial light modulator (SLM) 18 shown in
the object beam path in FIG. 2A is used to encode information (data)
within the stored hologram. Typically, the SLM comprises a two-dimensional
array of optical switches, which, when illuminated by a plane wave,
converts information read into these switches into amplitude modulations
of the plane wave. SLM switches (array elements) can be liquid crystals,
metallic membranes, magneto-optic modulators, or any similar devices
capable of modulating optical radiation.
Optimal hologram pagewriting (i.e., maximum hologram diffraction
efficiency) occurs when the dielectric relaxation time, .tau..sub.p, of
the protonic charge generated by the fixing beam equals the writing time
constant of the electronic phase hologram. The protonic dielectric
relaxation time is given by:
##EQU1##
where .epsilon..sub.0 =8.85.times.10.sup.-12 farads per volt per meter,
.epsilon..sub.c is the static dielectric constant of LiNbO.sub.3 along the
latter's c-axis (=28.5), and
.sigma..sub.p is the protonic conductivity induced by the fixing beam:
##EQU2##
where n.sub.p is the concentration of protons within the Fe:LiNbO.sub.3
crystal, e (=1.60219.times.10.sup.-19 coulombs) is the protonic charge,
F is the mean jumping distance for bound protons,
k.sub.B (=1.38062.times.10.sup.-23 joules per degree Kelvin) is Boltzmann's
constant,
T is the crystal temperature in degrees Kelvin,
I.sub.p0 is the intensity (watts/cm.sup.2) of the fixing beam,
.sigma..sub.H is the protonic absorption cross section,
h (=6.626.times.10.sup.-34) is Planck's constant (.div.2.pi.), and
v.sub.f is the optical frequency (in Hertz) of the fixing beam.
The writing time constant of the electronic phase grating is approximately
given by (see, for example, G. C. Valley and M. B. Klein, "Optimal
properties of photorefractive materials for optical data processing,"
Optical Engineering, 22, p. 704, 1983):
##EQU3##
where n.sub.e is the concentration of electrons liberated from Fe.sup.+2
traps within the Fe:LiNbO.sub.3,
e is the electronic charge, and
.mu..sub.e is the electronic mobility.
n.sub.c is given by:
##EQU4##
where .sigma..sub.e is the electronic absorption cross section,
I.sub.c0 is the peak intensity (watts/cm.sup.2) of the interference pattern
generated by the object and reference beams,
.gamma..sub.R is the recombination rate coefficient of the electrons, and
R is the ratio of Fe.sup.+2 to Fe.sup.+3 ions within the LiNbO.sub.3.
Referring to FIG. 2B, the compensated (and, hence, non-diffracting)
hologram generated during the writing step is revealed or developed by
illuminating the crystal by a third beam 20 of wavelength .lambda..sub.3.
The function of this beam is to liberate electrons from Fe.sup.3+ traps
(to which the electrons have diffused during generation of the electronic
phase grating) and to homogeneously redistribute them throughout the
Fe:LiNbO.sub.3 crystal, thereby revealing the protonic grating which
remains stationary under illumination by the developing beam. The
wavelength, .lambda..sub.3, of the developing beam is typically
approximately equal to .lambda..sub.1. The developing beam need not be a
laser, but can be some broad-band source of radiation centered at
.lambda..sub.3. This step is included because it is usually mentioned and
implied as necessary in the literature. In fact, it is believed that it
may not be necessary, since the act of reading a hologram. (see
description of FIG. 2c, below), will, in and of itself, redistribute the
electrons so that the hologram is developed and can be read.
Referring to FIG. 2C, the protonic grating (revealed by the developing beam
of wavelength .lambda..sub.3) is read by illuminating the Fe:LiNbO.sub.3
crystal 13 at angle .phi. by the reference beam and observing the
reconstructed object beam 22 which emerges from the crystal at angle
.theta.. During this step phase shifter 16 introduces zero phase shift
into the reference beam.
Referring to FIG. 2D, an entire hologram (page of data) can be erased by
exposing the Fe:LiNbO.sub.3 crystal 13 to object and reference laser beams
10 and 12, incident at the same hologram write angles, but with a pi
(.pi.) phase shift impressed on reference beam 12. Similarly, a portion of
a hologram can be erased by exposing the crystal to the reference beam
(incident at angle .phi. and shifted by .pi. radians), and that potion of
the original page of data (incident on the crystal at angle .theta.) to be
erased. Such complete or selective erasure of a hologram is possible
because the .pi. phase shift introduced into the reference beam
effectively creates both protonic and electronic phase holograms which are
180.degree. out of phase with the originally formed holograms, and which,
thereby, through superposition, exactly cancel that portion of the
hologram to be erased.
Referring to FIG. 3, multiple, angle-multiplexed page holograms can be
written, read, and erased in any order by sequentially interfering object
beams (spatially modulated by different data records) incident on the
Fe:LiNbO.sub.3 crystal 13 at angle .theta. with plane-wave reference beams
12.sub.1, . . . 12.sub.N incident on the crystal at angles .phi..sub.1,
.phi..sub.2, .phi..sub.3, . . . .phi..sub.N, where N is the total number
of holographic pages written.
Finally, it is noted that the proposed optical fixing process can be made
to occur on microsecond time scales by using short (approximately 5
nanoseconds long), high-peak-power (approximately 10.sup.6 watts/cm.sup.2)
laser pulses of the type which can be routinely obtained from a Q-switched
Nd:YAG laser. As an example, for n.sub.P =10.sup.24 m.sup.-3,
F=2.times.10.sup.-9 m, T=300.degree. K., .sigma..sub.H
=1.51.times.10.sup.-19 cm.sup.2, and I.sub.po =5.times.10.sup.6
watts/cm.sup.2, Equations (1) and (2) yield .pi..sub.p =2.5 .mu.sec.
* * * * *
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Description |
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