In holographic data storage, pages of information are overlapped in the volume of the recording medium. Due to destructive read-out of holograms in photorefractive crystals such as LiNbO_3:Fe, holograms are recorded with an exposure schedule in order to equalize diffraction efficiency. This leads to a final diffraction efficiency proportional to 1/M^2, where M is the number of exposures. Coherent erasure of a particular page also erases all the other pages stored in the same volume. We believe to have found a technique that does not require an exposure schedule and that can record M holograms with diffraction efficiency following a 1/M dependence. Our technique is based on non-destructive read-out in doubly-doped LiNbO_3. The technique is based on the recording of localized holograms in thin layers across the volume of the crystal

Maravic, Irena

Moser, Christophe

Psaltis, Demetri

Schupp, Benjamin

Caltech Authors - Main

PROCEEDINGS OF SPIE
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Localized holographic recording in
doubly doped lithium niobate
Christophe  Moser, Benjamin  Schupp, Irena  Maravic,
Demetri  Psaltis
Christophe  Moser, Benjamin  Schupp, Irena  Maravic, Demetri  Psaltis,
"Localized holographic recording in doubly doped lithium niobate," Proc. SPIE
4089, Optics in Computing 2000,  (24 May 2000); doi: 10.1117/12.386813
Event: 2000 International Topical Meeting on Optics in Computing (OC2000),
2000, Quebec City, Canada
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Localized Holographic Recording in doubly doped Lithium Niobate
Christophe Moser*, Benjamin Schupp, Irena Maravic,Demetri Psaltis
California Institute of Technology
Department ofElectrical Engineering,Pasadena, CA 91125
ABSTRACT
I_n holographic data storage, pages of information are overlapped in the volume of the recording medium. Due to destructive
read-out of holograms in photorefractive crystals such as LiNbO3:Fe, holograms are recorded with an exposure schedule in
order to equalize diffraction efficiency. This leads to a fmal diffraction efficiency proportional to 1/M2, where M is the
number of exposures. Coherent erasure of a particular page also erases all the other pages stored in the same volume. We
believe to have found a technique that does not require an exposure schedule and that can record M holograms with
diffraction efficiency following a l/M dependance. Our technique is based on non-destructive read-out in doubly-doped
LiNbO3 [1]. The technique is based on the recording oflocalized holograms in thin layers across the volume ofthe crystal.
Keywords: doubly-doped LiNbO3, localized holograms, data storage
1. ThTRODUCTION
In holographic data storage [1], pages of information are overlapped in the volume of the recording medium. Coherent
erasure of a particular page also erases all the other pages stored in the same volume, due to destructive read-out of
holograms in photorefractive crystals such as LiNbO3:Fe. Recently, non-destructive read-out has been demonstrated in
doubly-doped LiNbO3 [2]. When volume holograms are multiplexed in such crystals [3], the erasure of a single page of
information requires the sensitizing light to be present, causing the erasure of all the other pages of information stored in the
same volume. Here, we propose to holographically record localized holograms in doubly doped LiNbO3 [4] The method can
be applied to any other material that produces non-volatile read-out [5,6,7]. The method can be considered the holographic
analog of the three-dimensional optical two-photon memories architecture [8,9]. In the first portion of the paper, we
experimentally demonstrate the recording of 100 localized holograms in doubly-doped LiNbO3. We quantify the quality of
the reconstruction by measuring the signal to noise ratio (SNR) of the recorded holograms. These localized holograms can be
erased and refreshed selectively by illuminating the hologram with the sensitizing and reference beam. We present theoretical
results on the diffraction efficiency obtainable and the total recording time for M holograms and compare those results with
the conventional distributed volume recording. We show that, when the modulation depth of the holograms is maximized, the
final diffraction efficiency after M localized recordings varies as 1IM. This behavior represents a large improvement
compared to the 1i2 dependance obtained in distributed volume recording. An experiment is presented that confirms those
predictions.
2. LOCALIZED RECORDING
2.1 SNR of 100 localized holograms
The set-up for SNR measurement is shown in fig 1 . The reference beam is focused by a cylindrical lens of focal
length 4 cm. The blue beam from a laser diode (Nichia, 40mm) is co-propagating with the green (532 nm) reference beam. A
Spatial light Modulator (SLM) of 640 x 480 pixels is used to display random binary patterns. The 4-f imaging system is not
pixel matched, so that 4 x4 SLM pixels are used to represent 1 data pixel. The imaging system consists of an achromat lens
with N.A 0.4 placed in front of the SLM and an objective Nikon lens with N.A=l.4. The imaging system's magnification of
_ 0.3 allows to image the entire SLM surface onto the camera. A removable mirror can be placed after the second imaging
lens to measure the diffracted power while leaving the camera in place. The holograms are recorded in the Fresnel region.
The focal plane of the first lens is located 3mm before the crystal's surface. An iris is placed at the focal plane of the first
imaging lens to filter out the high orders generated by the SLM. The signal beam propagates through the crystal close to one
of its edges. This is to ensure that the violet light sensitizes efficiently the area to be recorded from the side as shown in the
inset of fig 1. The holograms are recorded from position 1 (exit crystal facet) to M (entrance crystal facet).
* Correspondance: Email: moser@sunoptics.caltech.edu;Telephone: 626 395 3159; Fax: 626 568 8437
In Optics in Computing 2000, Roger A. Lessard, Tigran Galstian, Editors,
118 SPIE Vol. 4089 (2000) 0277-786X/O0/$15.O0
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Sgnat
M
Re &
Sens
I
This way, the reference beam interferes with the signal beam before it propagates through the previously recorded holograms,
thus reducing distortion of the signal. At the overlap between the focused reference and the signal beam, the signal diameter
is —1.5mm. The experiment is performed with a congruent 90 degree-cut LiNbO3 crystal doped with 0.15 wt.% Fe203 and
0.015 wt.% MnO. The crystal dimensions are 1 x lx 1 cm3. The recording curve using two extended plane waves is shown in
fig.2.
>10
a)0
'4-
a)
C0
0
'4-
Figure 2: Recording and read-out curve for a plane wave hologram in a 90 degree-cut
LiNbO3 crystal doped with 0.15 wt.% Fe203 and 0..015 wt.% MnO. The recording beams
are from a solid state green laser (wavelength 532 nm, ordinary polarization, l/e2 beam
diameter 2.0 mm and intensity 160 mW/cm). The sensitizing light is from a laser diode
from Nichia, wavelength 401 nm, linearly polarized, intensity 25 mW/c m2.
119
Green Iaser(532nm)
— --pIate
ii. fpiatePB m SLM
LD
CL
Translation-stage
ñth crystal
Camera
Fig 1: Recording set-up for the SNR measurement
tei=1083 [S]
7.5
5.0
2.5
0.0
te2=30403 [s]
=216 [s]
0.0 0.5 1.0
I I
Time [h]
1.5 2.0 2.5 3.0 3.5 4.0
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The erasing curve is fitted by a hi-exponential function giving two time constants. 'ri=l.0t3Is] and 't2=30,403 [sj
corresponding respectively to the fast erasure of the grating in Fe and a slower erasure of the grating in Mn. The crystal was
oxidized for ( hours at 900°C in 02 atmosphere. The oxidation state could not he further reduced by longer oxidation. The
slow erasure time constant Te2 is about 15 times shorter than that obtained in a similar crystal with half the Concentration of
Fe0. 1-lowever. since the recording is performed with equal power in reference and signal beam, the signal intensity is much
less than the reference intensity. With the intensities used in the experiment, we obtain a ratioT ' =140. large enough to
record hundreds of localized holograms. This ratio is measured by recording the localized hologram to saturation and by
letting the signal beam erase the hologram.
In the experiment of fig. I. the spot size 2W0 has lateral extension 25m (intensity I/c2 value). The waist W(z) in the crystal
increases with distance z as:
W (z) = W0 + m W0,i
where z0 is the depth of focus. Fora propagation distance ofz 3mm inside the crystal. the waist increases to 45tim.
The signal. reference and violet beam intensities are 20 mW/cm'-. 200 mWicm2, 40 mWicin2 respectively. This choice of
intensities results in equal power for reference and signal beam, which yields fast recording as will he shown later. in this
experiment, we are interestmg in obtaining just enough diffraction efficiency to compute the SNR. A diffraction efficiency of
5l0' gives a good image on a video camera sampling at 30Hz. During recording of subsequent holograms, the previous
holograms are partially erased by the green signal beam, but not by the reference beam unlike distributed recording. The
spacing between each localized holograms is chosen to be twice the lateral spot size i.e 50 im to avoid erasure of the
previous hologram while sensitizing the next hologram. Each hologram is sensitized for 5 mm. and recorded for 3 mm and
recorded with a different random binary bit pattern. 4 x 4 camera pixels represent I bit of information. Fig. 3 shows the
diffraction efficiency versus position for the 100 holograms. Each peak corresponds to the diffraction efficiency of a
hologram stored at a different spatial location as the crystal is translated in the direction perpendicular to the reference beam.
The recording did not follow an exposure schedule.
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
Lateral shift [mm]
Figure 3: Spatial multiplexing of 100 localized holograms
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122
To measure the SNR, we use 5 windows of 30 x 30 superpixels (1 superpixel is composed of 4 x 4 camera pixels) located
on the 4 corners and at the center of the image. For each superpixel, the edges are left out, so effectively 3 x3 pixels form a
superpixel. SNR is computed using the formula:
SNR=
Jc?+c522
Where ,o, i 1 ,O are the mean and variance respectively of the fitted Gaussian distribution of the 1 s and zero's.
3 sample holograms respectively located at the front, middle, rear of the crystal are shown in fig 4. We fmd that the average
SNR values measured is 3.7 and constant throughout the crystal. This value is less than the SNRvalue of 4.2 measured with a
focused reference of 1 OOjnn. Because the SNR is constant for holograms along the crystal, we attribute this loss to the longer
exposure of the crystal to the signal beam. The remnant electrons in Fe act as absorbtion centers for the green (532nm) light
and produces fanning. The direct imaging of the SLM through the crystal after the storage of 100 hologram shows a
degradation of the SNR by 1 compared to the SNIR measured before storage. The signal beam intensity varies along the
crystal. The intensity is higher at the entrance face of the crystal than the exit face by a factor 15. Before the memory is
written, the first hologram recorded has SNR4.7 as illustrated in fig. 5.
z
Cl)
8
7
6
5
4
3
2
I
0
Figure 5: SNR comparison: (a) direct imaging (b) imaging through crystal(c) first hologram (d) after 100 holograms
direct
imaging
imaging
through crystal
1st hologram
holograms
after 100
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2.1 Diffraction efficiency oflocalized holograms
The recording dynamics of localized holograms depends on the relative power distribution between the reference and the
signal beam since the recording beam areas are very different. The recording slope S in photorefractives is given by:
SPrPs (1)
Tw
where Pr and P are the power in the reference and signal beam respectively. Eq. 1 is subject to
Pr + P PtotaicoI5tant. Thus the recording slope is maximized for equalized power P, Ps. In this case the recording slope is
optimized but the saturation diffraction efficiency is low because ofthe low resulting modulation depth. The saturation
diffraction efficiency A0 is proportional to the modulation depth m:
'fIrA0ocm= rs (2)1r
This quantity is maximized for equal intensity Ir= 1s independant ofthe total power.
The following experiment illustrates this dynamic. We used a 90 degree--cut LiNbO3 crystal doped with 0.01 wt.% Fe203.
We used a Coherent solid state laser, wavelength 532 urn, linearly polarized, for the recording beams. A cylindrical lens
focuses the reference beam (6 mm diameter) in the crystal. The reference beam interferes with a plane wave signal beam (6
mm diameter) in the similar set-up used for the SNRmeasurement (fig.!). The total power is distributed into the reference
and signal beam using a halfwave-plate and a polarizing beam splitter. In the first experiment, the reference and signal beam
have equal power.
Figure 6: Recording curves of a localized hologram. The plain curve is
recorded with equal intensity in the reference and signal beam. The dashed
curve is recorded with equal power.
In the second experiment, intensity between the reference and signal beam is equalized. The result is shown in
fig.6 for the case of a reference beam focused to a lateral extension equal to l2Opm(!/e2 value). As predicted, the curve
recorded with equal power has a larger recording slope than the curve recorded with equal intensity and saturates to a much
lower diffraction efficiency (dashed curve). For the case of equal intensity in the reference and signal beam, the localized
hologram growth can be written as [10]:
123
time fminj
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(3)
Where M is defined as the ratio of the crystal length L and U which is defined as the I/c2 lateral extension of the facused
reference beam. A,, is the saturation diffraction efficiency ofa hologram with extension L (a distributed volume hologram).
is the recording tune constant ofa distributed volume hologram of extension L. From Eq. 3, the maxiinuni diffraction
etliciency of a single localized hologram is i1'=AM. If we assume that subsequent recording do not erase previous
hologram, the final diffraction efficiency after M exposure varies as l/M. Localized hologram can thus he written with strong
diffraction efficiency. This is achieved at the expense of a much longer recording time. The total recording time is equal to
M•T which is a factor M/In(M) longer than when recorded using au exposure schedule using volume holograms.
We experimentally recorded 50 localized holograms with equal writing intensity in the retrence and signal beam. Localized
hologram are recorded in a 0.85 mm thick ,transmission geometry. LiNbO crystal doped with 0.075 wt.% FeO and 0.015
wt.% MnO. We use green recording wavelength at 532 nm with a reference beam focused by a cylindrical lens yielding a
lateral extension of 3Olim (l!e value). The signal has a diameter of 3mm at the crystal. Each hologram is recorded up to
saturation for a recording time equal to two writing time constants. Center to center spacing of each hologram is ôOt.im. The
total spatial multiplexing is done over 3mm. Fig.7 shows the diffraction efficiency of each recorded localized hologram
immediately after each recording. An average value of 2% diffraction efficiency is achieved. Fig.8 shows the final diffraction
efficiency after the recording of 50 holograms. The average diffraction efficiency is about 1 10 .
We can explain the loss of diffraction efficiency measured from fig.7 to fig.8 by three factors. The first factor is a drop
caused by the erasure of the grating in Fe2O accounted by a factor 2.5 from experiments. The second factor is due to the
finite erasure time constant of the grating in Manganese due to slow erasure by green light. Experimentally, this is accounted
by a loss factor equal to 4. A third factor is the erasure caused by the sensitizing wavelength to adjacent holograms. This
factor varies with the center to center spacing of localized holograms. Figure 9 shows an example of the eftct of the
sensitizing beam on adjacent holograms. Although the adjacent holograms are located a distance equal to twice the spot size
of the reference beam, the adjacent holograms suffer from erasure due to the sensitzing beam. In this experiment, the loss due
to this effect is accounted by a factor approximately 2. Overall, the saturation diffraction efficiency of each localized
holograms is decreased by a factor 20 and explain the values obtained experimentally. The M number far this experiment is
given by M#M•l03 2=1.6. This result has to be compared with the Mfl obtained by distributed recording which far extra-
ordinary polanzation in the same crystal is 1 [31. The main problem to solve in order to obtain larger M# using localized
recording is related to the sensitizing light. Since the focused beam has a gaussian shape, the localized holograms are spaced
by twice the waist of the focused beam in order to avoid erasure of adjacent holograms while recording the next hologram.
Even though. with a center to center spacing of twice the waist, the erasure of adjacent holograms caused by the sensitizing
beam still have strong effect.
124
hologram #
Figure 7: Diffraction efficiency of each
localized hologram measured after each
recording
1 .6x10
1 .4x10
1.2x10
C
.2 l.OxlO
c 8.OxlO0
6.0x10
4.0x10
2.Oxl 0
0.0
Figure 8: t)iffraction efficiency after
recording Sf) holograms
transmission geometry
extra-ordinary polarization
crystal 778-12-0.8mm thick
0.0 0.5 1.0 1.5 2.0 2.5 3.0
shift tmm]
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likclO4 eda1
6ibclO4 Ifj
4O,c1O
3JdO
tmclo4
0.0
.1:ie iA11At
i_
. •••.••••55rfre
I'—
-itm.& . 4O J 0 40 80 100Sftn
Figure 9: Erasure effect of the sensitizing beam on adjacent holograms
REFERENCES
[1] G.T Sincerbox, Selectedpapers on Holographic storage, SPIE Milestone Series, vol. MS 1995.
[2] K. Buse, A. Adibi, D. Psaltis, "Non-volatile holographic storage in doubly doped lithium
niobate crystals" Nature. 393, 665, 1998.
[3] A. Adibi and K. Buse and D. Psaltis, "Multiplexing holograms in LiNbO3 : Fe : Mn crystals"
opt. Left. 24, 652,1999.
[4] C. Moser, B. Schupp, D. Psaltis, "Localized holographic recording in doubly doped lithium niobate", Opt Lett. 25, 162-
164, 2000
[5] H. Guenther and G. Wittmann and R.M. Macfarlane and R.R Neurgaonkar, "Intensity dependence and white-light gating
oftwo-color photorefractive gratings in LiNbO3 " Opt. Lett. 22, 1305, 1997.
[6] K. Buse and L. Holtmann and E. Kratzig, "Activation ofBaTiO3 for infrared holographic recording"
Opt. Commun. 85, 183, 1991.
[7] L. Hesselink and S. Orlov and A. Liu and A. Akella and D. Lande and R.R. Neurgaonkar, "Photorefractive materials for
nonvolatile volume holographic data storage" Science. 282, 1089, 1998.
[8] D. A. Parthenopoulos and P. M. Rentzepis, "3-Dimensional optical storage" Science. 245, 843, 1989.
[9] M. M. Wang and S. C. Esener and F. B. McCormick and I. Okgr and A. S. Dvornikov ,P. M. Rentzepis" Experimental
characterization of a two-photon memory", Opt. Left. 22, 558, 1997.
[10] F.H. Mok and G. Burr and D. Psaitis, "System metric for holographic memory systems"
Opt. Left. {\bf2l,} 896 (1996).
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Localized holographic recording in doubly doped lithium niobate

https://authors.library.caltech.edu/92270/1/118.pdf

Localized holographic recording in doubly doped lithium niobate

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