We report on the optical storage of digital data in a semiconductor sample containing DX centers. The diffraction efficiency and the bit-error-rate performance of multiplexed data images are shown to agree well with a simple model of the material. Uniform storage without an exposure schedule is demonstrated. The volume sensitivity is found to be ~10^3 times that of LiNBO3:Fe. The importance of coherent addition of scattered light with diffracted light in holographic data storage is discussed

Chuang, E.

Linke, R. A.

Psaltis, D.

Redmond, I. R.

English

Caltech Authors - Main

August 1, 1997 / Vol. 22, No. 15 / OPTICS LETTERS 1189Holographic data storage in a DX-center material
I. R. Redmond and R. A. Linke
NEC Research Institute, 4 Independence Way, Princeton, New Jersey 08540
E. Chuang and D. Psaltis
Department of Electrical Engineering, Mail Stop 116-81, California Institute of Technology, Pasadena, California 91125Received March 27, 1997
We report on the optical storage of digital data in a semiconductor sample containing DX centers. The
diffraction efficiency and the bit-error-rate performance of multiplexed data images are shown to agree well
with a simple model of the material. Uniform storage without an exposure schedule is demonstrated. The
volume sensitivity is found to be ,103 times that of LiNBO3:Fe. The importance of coherent addition of
scattered light with diffracted light in holographic data storage is discussed.  1997 Optical Society of
AmericaThe use of bulk photoresponsive materials for the
storage of massive amounts of information by use of
holographic techniques was proposed as early as 1963
by van Heerden, who showed that, in principle, a
binary bit of information can be stored in a volume
approximately equal to a cubic wavelength in the
medium.1 Considerable interest has been shown in
photorefractive materials for this application; the most
widely studied of these materials is iron-doped lithium
niobate (LiNbO3:Fe).
2 Although this material has
permitted impressive demonstrations of holographic-
storage systems (e.g., Ref. 3), it has several severe
limitations, including a rather small refractive-index
change sDn , 1024d on exposure to light and a very
poor sensitivity, typically ,1025 cm3yJ.4
Semiconductor materials containing deep donor
states known as DX centers were recently shown to
exhibit strong optically induced changes in refractive
index, which are persistent at a sufficiently low tem-
perature.5– 7 The origin of this response involves the
optical release of electrons from the DX states and
the resulting change of the local free-carrier density.
The rate of electron recapture back into the DX state
is governed by thermal activation over an energy
barrier, and thus sample temperature determines the
persistence time of the index change, which in practice
is adjustable from years to less than a microsecond.
Recently, new DX materials with higher persistence
temperatures were identified,8 indicating promise for
the future application of these materials to storage
systems.
Here we report on a series of experiments aimed at
investigating the holographic-data-storage properties
of DX-center-based photoresponsive materials. The
sample used in these experiments was a crystal of
the compound III–V semiconductor AlGaAs doped with
tellurium at a concentration of 8 3 1017 cm23. The
sample was 345 mm thick and had an area of approxi-
mately 5 mm 3 7 mm. Both faces were antiref lection
coated, resulting in ,1% ref lection per surface. The
sample was held in a variable-temperature cryostat
and cooled in the dark to 40 K to sensitize it by forcing
the dopant atoms into the DX states. At this tempera-0146-9592/97/151189-03$10.00/0ture, photoinduced persistence time is projected to be
many years. The saturated change in refractive in-
dex between the dark-cooled and the fully ionized
samples was measured to be Dnsat ­ 0.0020, and the
exposure required for 90% of saturation is 3.0 mJycm2,
which corresponds to a volume exposure sensitivity S
of 1.1 3 1022 cm3yJ, or 1000 times that of LiNbO3:Fe.
Digital information was stored in the crystal in
the form of holographic recordings of ‘‘pages’’ of two-
dimensional intensity-modulated data bits.9 We used
peristrophic multiplexing,10 in which the data beam is
normally incident upon the hologram and the possible
reference-beam directions form a cone around the
object beam (Fig. 1). In these experiments we stored
as many as 100 holograms, whereas fewer than 10
would have been possible with the conventional angle-
multiplexing technique.3
Exposures were made with a 5-mW, 847-nm laser
diode, which was expanded, filtered, and split at the
rotation arm (Fig. 1). The transmitted beam is modu-
lated by a fixed data mask and imaged onto the
sample. The mask was a conventional chrome pho-
tomask containing a regular array of 96-mm squares,
which were randomly transparent (binary 1) or opaque
(0), each with 50% probability. The reference beam
was directed along the rotation arm and toward the
sample, meeting it at 30– to the normal. To record
multiple pages, we rotated the arm about the sample
normal and moved the data mask so that different im-
ages were recorded. On readout, the diffracted light
from the sample was imaged by a 4-f lens system onto
Fig. 1. Optical setup for holographic recording and read-
ing by peristrophic multiplexing.3 FT, Fourier transform. 1997 Optical Society of America
1190 OPTICS LETTERS / Vol. 22, No. 15 / August 1, 1997a cooled CCD camera with 24-mm-square pixels. Any
unwanted diffracted pages were blocked by an iris in
the intermediate Fourier-transform plane. The holo-
graphic exposures were made and analyzed at 40 K,
and we erased and resensitized the sample by increas-
ing its temperature to 150 K and dark cooling it again
to 40 K.
The relation between h and exposure was found to
match accurately that expected from an exponentially
saturating response: An initial rise in h (proportional
to the square of the f luence) was followed by a drop
as the DX centers became depleted and the strength
of the fundamental grating fell. For this sample the
peak diffraction eff iciency of a single hologram was
45%, which corresponds to an My# (Ref. 11) of 0.67.
Here we use relative diffraction efficiency; we obtained
the absolute diffraction eff iciency by multiplying our
values by the transmission T of the sample, which was
in the range 0.3–0.9 after exposure.
We performed experiments to investigate the trade-
off between peak efficiency and exposure for several
series of multiplexed holograms, and the results are
summarized in Table 1 for various numbers of holo-
grams, M . Included in the table is the expected
efficiency, ht, derived from the transmission volume-
grating-efficiency relation12
ht ­ sin
•
pDnt
lscos ui cos uod1/2
‚2
, (1)
where t is the hologram thickness, l is the wavelength
in vacuum, and ui and uo are the incident and the dif-
fracted angles, respectively. We assume that the op-
timum Dn for each hologram in a multiplexed set is
simply the value for an optimum single-exposure holo-
gram divided by M . The dependence of efficiency on
number of exposures in these measurements is consis-
tent with the model of the holographic process in DX
materials as a local phenomenon.13 In this model the
index change at each point in the volume is considered
to depend only on the total exposure at that point and
to be independent of the exposure at neighboring points
and of the time or sequence involved in reaching that f i-
nal exposure. This in turn implies that a sequence of
multiplexed equal exposures will yield holograms with
the same diffraction eff iciency. When saturation of
the index change occurs, it is expected to reduce the
efficiency of all the holograms equally. This was con-
firmed experimentally, as illustrated by Fig. 2, which
shows the measured diffraction eff iciencies of a series
of 100 multiplexed holograms, each with an exposure of
26 mJycm2. The uniformity of the holograms is seen
to be 620%, roughly half of which is attributable to the
systematic variation in exposure with rotation-arm po-
sition. This uniformly saturating property of the DX
materials makes following an exposure schedule when
writing many holograms, as is done with photorefrac-
tives,14 unnecessary.
We investigated the bit-error-rate (BER) perfor-
mance of this sample by capturing images of the
diffracted pages on a cooled CCD camera, f inding
the optimum global threshold, and analyzing the
distributions of the powers in the reconstructed pixels.
Since there were typically no errors observed in the,1700 pixels within the sample area, we estimated
BER’s by fitting the data distribution to the Gauss-
ian function and extrapolating. First, we found the
baseline capability of the system by imaging the data
mask directly through the sample onto the camera.
The resulting distribution of pixel values had a signal-
to-noise ratio (SNR) of ,200 and a corresponding
estimated BER of 10245. The SNR is defined here as15
SNR ­
sm1 2 m0d2
s12 1 s02
, (2)
where m1 and m0 are the mean 1 and 0 levels and s12
and s02 are the corresponding variances. The BER
can be estimated for two Gaussian distributions of
equal area15:
BER ø
1
2
erfc
µ
Qp
2
¶
, (3)
where
Q ­
m1 2 m0
s1 1 s0
. (4)
In practice m0 was not 0 but typically 2.5% of m1. This
result was due to the finite optical density of the mask
(1.6). The f inite width of the distributions (and the
SNR) was found to be predominantly the result of the
spatial variation of the irradiance of the beam.
Next a single page was recorded at 1 mJycm2 and
analyzed. A histogram of these data is shown in
Fig. 3. The SNR here is , 50, and the estimated
BER, 10221. This is poorer than the direct mask-
image result, primarily because of the holographic
addition of the spatial variations on the reference
beam. Thus we can consider 10221 to be the estimated
BER measurement limit of our holographic test system.
As exposures decreased in the single-hologram
case, the estimated BER increased, reaching 1025 at
Table 1. Peak Diffraction Efficiency
of Multiplexed Holograms
Number of
Holograms (M )
Exposure
smJycm2d h s31026d ht s31026d
20 180 950 990
50 100 113 163
100 72 27 40
Fig. 2. Diffraction eff iciency hologram number for 100-
hologram exposure in an AlGaAs:Te sample. All exposure
values were 26 mJycm2; i.e., no exposure schedule was used.
August 1, 1997 / Vol. 22, No. 15 / OPTICS LETTERS 1191Fig. 3. Frequency of occurrence of pixel values plotted
versus pixel values in arbitrary units for a strong hologram
(1 mJycm2). The projected BER derived from these data
is 10221.
Fig. 4. Theoretical BER’s for coherent (solid curves) and
incoherent (dashed curves) addition of scattered light.
Measurements (circles) agree reasonably well with the
coherent addition model for the observed scattering level
of 1026.
5 mJycm2 per exposure. The increasing noise is seen
as an increase in the spread of both the 0 and the
1 distributions. Even at this exposure level the dif-
fracted light power was sufficiently far above system
noise levels that we would not expect such an increase
in BER. The origin of this noise was found to be
coherent addition of the diffracted light and the light
scattered from imperfections on the sample surface.
Inclusion of the scattered light fraction hS, which we
assume to have random phase, leads to the following
modif ication to Q [Eq. (4)]:
Q 0 ­
h
p
2
shs2 1 2h2s02d1/2 1 fhshs 1 2hs12dg1/2
. (5)
Using values of s0 ­ 0.0086 and s1 ­ 0.098 experimen-
tally obtained from the strong-hologram case, in Fig. 4
we plot curves for BER versus diffraction efficiency for
two scattering levels, hS . Also shown in the f igure are
measured data points for our sample, which appear to
be in good agreement with the theory for hS ­ 1026,
which is also the typical value measured independently
from scatter-only images.Finally, we investigated the effect of multiple expo-
sures on error rate by measuring the BER of a single
hologram as additional holograms were superimposed
at different peristrophic angles. Each exposure was
18 mJycm2, and the diffraction eff iciency of the probed
page decreased from an initial value of 9.5 3 1025 to
1.3 3 1025 as the successive exposures pushed the ma-
terial into saturation (with a final total exposure of
4.4 mJycm2). The BER increased from 10210 to 1028
as the additional holograms were added. A degrada-
tion in BER similar to this is expected from the drop in
h alone. This suggests that interpage cross talk is not
significant in these experiments.
In conclusion, we have demonstrated holographic
storage of digital data in a semiconductor sample con-
taining DX centers. The diffraction eff iciency and
the BER performance of multiplexed data images are
shown to agree well with a simple model of the mate-
rial in which the index change at each point in the vol-
ume depends only on the total exposure at that point
and is independent of that at neighboring points and
of the time or sequence involved in reaching the fi-
nal exposure. This implies that a sequence of multi-
plexed equal exposures will yield holograms with the
same diffraction efficiency, as we have confirmed ex-
perimentally. When saturation of the index change is
present it appears as a reduction of the eff iciency of
all the holograms equally. Thus no exposure sched-
ule is required with these materials. The recording
sensitivity of our AlGaAs sample was shown to be 103
times better than that of LiNbO3:Fe. BER estimates
revealed no error-rate f loor intrinsic to the storage ma-
terial but showed that the coherent addition of stray
scattered light with the diffracted signal (a potential
problem for any holographic storage system) can in-
crease error rates well beyond those expected for in-
coherent background light at the same level.
References
1. P. J. Van Heerden, Appl. Opt. 2, 393 (1963).
2. D. Psaltis and F. Mok, Sci. Am. 273(5), 70 (1995).
3. F. Mok, Opt. Lett. 18, 915 (1993).
4. C. Chen, D. Kim, and D. von der Linde, Appl. Phys.
Lett. 34, 321 (1979).
5. R. A. Linke, T. Thio, J. D. Chadi, and G. E. Devlin,
Appl. Phys. Lett. 65, 16 (1994).
6. R. L. MacDonald, R. A. Linke, J. D. Chadi, T. Thio,
G. E. Devlin, and P. Becla, Opt. Lett. 19, 2131 (1994).
7. D. D. Nolte, J. Appl. Phys. 79, 7514 (1996).
8. A. I. Ryskin, A. S. Shecheulin, B. Koziarska, J. M.
Langer, A. Suchocki, I. I. Buczinskaya, P. P. Fedorov,
and B. P. Sobolev, Appl. Phys. Lett. 67, 31 (1995).
9. L. Hesselink and M. Bashaw, Opt. Quantum Electron.
25, 611 (1993).
10. K. Curtis, A. Pu, and D. Psaltis, Opt. Lett. 19, 993
(1994).
11. F. H. Mok, G. W. Burr, and D. Psaltis, Opt. Lett. 21,
896 (1996).
12. H. Kogelnik, Bell Syst. Tech. J. 48, 2909 (1969).
13. R. L. MacDonald, R. A. Linke, G. E. Devlin, and M.
Mizuta, Opt. Lett. 20, 1322 (1995).
14. K. Bløtekjaer, Appl. Opt. 18, 57 (1979).
15. B. E. Saleh and M. C. Teich, Fundamentals of Photonics
(Wiley, New York, 1991), p. 905.


Holographic data storage in a DX-center material

August 1, 1997 / Vol. 22, No. 15 / OPTICS LETTERS 1189Holographic data storage in a DX-center material
I. R. Redmond and R. A. Linke
NEC Research Institute, 4 Independence Way, Princeton, New Jersey 08540
E. Chuang and D. Psaltis
Department of Electrical Engineering, Mail Stop 116-81, California Institute of Technology, Pasadena, California 91125Received March 27, 1997
We report on the optical storage of digital data in a semiconductor sample containing DX centers. The
diffraction efficiency and the bit-error-rate performance of multiplexed data images are shown to agree well
with a simple model of the material. Uniform storage without an exposure schedule is demonstrated. The
volume sensitivity is found to be ,103 times that of LiNBO3:Fe. The importance of coherent addition of
scattered light with diffracted light in holographic data storage is discussed.  1997 Optical Society of
AmericaThe use of bulk photoresponsive materials for the
storage of massive amounts of information by use of
holographic techniques was proposed as early as 1963
by van Heerden, who showed that, in principle, a
binary bit of information can be stored in a volume
approximately equal to a cubic wavelength in the
medium.1 Considerable interest has been shown in
photorefractive materials for this application; the most
widely studied of these materials is iron-doped lithium
niobate (LiNbO3:Fe).
2 Although this material has
permitted impressive demonstrations of holographic-
storage systems (e.g., Ref. 3), it has several severe
limitations, including a rather small refractive-index
change sDn , 1024d on exposure to light and a very
poor sensitivity, typically ,1025 cm3yJ.4
Semiconductor materials containing deep donor
states known as DX centers were recently shown to
exhibit strong optically induced changes in refractive
index, which are persistent at a sufficiently low tem-
perature.5– 7 The origin of this response involves the
optical release of electrons from the DX states and
the resulting change of the local free-carrier density.
The rate of electron recapture back into the DX state
is governed by thermal activation over an energy
barrier, and thus sample temperature determines the
persistence time of the index change, which in practice
is adjustable from years to less than a microsecond.
Recently, new DX materials with higher persistence
temperatures were identified,8 indicating promise for
the future application of these materials to storage
systems.
Here we report on a series of experiments aimed at
investigating the holographic-data-storage properties
of DX-center-based photoresponsive materials. The
sample used in these experiments was a crystal of
the compound III–V semiconductor AlGaAs doped with
tellurium at a concentration of 8 3 1017 cm23. The
sample was 345 mm thick and had an area of approxi-
mately 5 mm 3 7 mm. Both faces were antiref lection
coated, resulting in ,1% ref lection per surface. The
sample was held in a variable-temperature cryostat
and cooled in the dark to 40 K to sensitize it by forcing
the dopant atoms into the DX states. At this tempera-0146-9592/97/151189-03$10.00/0ture, photoinduced persistence time is projected to be
many years. The saturated change in refractive in-
dex between the dark-cooled and the fully ionized
samples was measured to be Dnsat ­ 0.0020, and the
exposure required for 90% of saturation is 3.0 mJycm2,
which corresponds to a volume exposure sensitivity S
of 1.1 3 1022 cm3yJ, or 1000 times that of LiNbO3:Fe.
Digital information was stored in the crystal in
the form of holographic recordings of ‘‘pages’’ of two-
dimensional intensity-modulated data bits.9 We used
peristrophic multiplexing,10 in which the data beam is
normally incident upon the hologram and the possible
reference-beam directions form a cone around the
object beam (Fig. 1). In these experiments we stored
as many as 100 holograms, whereas fewer than 10
would have been possible with the conventional angle-
multiplexing technique.3
Exposures were made with a 5-mW, 847-nm laser
diode, which was expanded, filtered, and split at the
rotation arm (Fig. 1). The transmitted beam is modu-
lated by a fixed data mask and imaged onto the
sample. The mask was a conventional chrome pho-
tomask containing a regular array of 96-mm squares,
which were randomly transparent (binary 1) or opaque
(0), each with 50% probability. The reference beam
was directed along the rotation arm and toward the
sample, meeting it at 30± to the normal. To record
multiple pages, we rotated the arm about the sample
normal and moved the data mask so that different im-
ages were recorded. On readout, the diffracted light
from the sample was imaged by a 4-f lens system onto
Fig. 1. Optical setup for holographic recording and read-
ing by peristrophic multiplexing.3 FT, Fourier transform. 1997 Optical Society of America
1190 OPTICS LETTERS / Vol. 22, No. 15 / August 1, 1997a cooled CCD camera with 24-mm-square pixels. Any
unwanted diffracted pages were blocked by an iris in
the intermediate Fourier-transform plane. The holo-
graphic exposures were made and analyzed at 40 K,
and we erased and resensitized the sample by increas-
ing its temperature to 150 K and dark cooling it again
to 40 K.
The relation between h and exposure was found to
match accurately that expected from an exponentially
saturating response: An initial rise in h (proportional
to the square of the f luence) was followed by a drop
as the DX centers became depleted and the strength
of the fundamental grating fell. For this sample the
peak diffraction eff iciency of a single hologram was
45%, which corresponds to an My# (Ref. 11) of 0.67.
Here we use relative diffraction efficiency; we obtained
the absolute diffraction eff iciency by multiplying our
values by the transmission T of the sample, which was
in the range 0.3–0.9 after exposure.
We performed experiments to investigate the trade-
off between peak efficiency and exposure for several
series of multiplexed holograms, and the results are
summarized in Table 1 for various numbers of holo-
grams, M . Included in the table is the expected
efficiency, ht, derived from the transmission volume-
grating-efficiency relation12
ht ­ sin
∑
pDnt
lscos ui cos uod1/2
∏2
, (1)
where t is the hologram thickness, l is the wavelength
in vacuum, and ui and uo are the incident and the dif-
fracted angles, respectively. We assume that the op-
timum Dn for each hologram in a multiplexed set is
simply the value for an optimum single-exposure holo-
gram divided by M . The dependence of efficiency on
number of exposures in these measurements is consis-
tent with the model of the holographic process in DX
materials as a local phenomenon.13 In this model the
index change at each point in the volume is considered
to depend only on the total exposure at that point and
to be independent of the exposure at neighboring points
and of the time or sequence involved in reaching that f i-
nal exposure. This in turn implies that a sequence of
multiplexed equal exposures will yield holograms with
the same diffraction eff iciency. When saturation of
the index change occurs, it is expected to reduce the
efficiency of all the holograms equally. This was con-
firmed experimentally, as illustrated by Fig. 2, which
shows the measured diffraction eff iciencies of a series
of 100 multiplexed holograms, each with an exposure of
26 mJycm2. The uniformity of the holograms is seen
to be 620%, roughly half of which is attributable to the
systematic variation in exposure with rotation-arm po-
sition. This uniformly saturating property of the DX
materials makes following an exposure schedule when
writing many holograms, as is done with photorefrac-
tives,14 unnecessary.
We investigated the bit-error-rate (BER) perfor-
mance of this sample by capturing images of the
diffracted pages on a cooled CCD camera, f inding
the optimum global threshold, and analyzing the
distributions of the powers in the reconstructed pixels.
Since there were typically no errors observed in the,1700 pixels within the sample area, we estimated
BER’s by fitting the data distribution to the Gauss-
ian function and extrapolating. First, we found the
baseline capability of the system by imaging the data
mask directly through the sample onto the camera.
The resulting distribution of pixel values had a signal-
to-noise ratio (SNR) of ,200 and a corresponding
estimated BER of 10245. The SNR is defined here as15
SNR ­
sm1 2 m0d2
s12 1 s02
, (2)
where m1 and m0 are the mean 1 and 0 levels and s12
and s02 are the corresponding variances. The BER
can be estimated for two Gaussian distributions of
equal area15:
BER ø
1
2
erfc
µ
Q
p
2
∂
, (3)
where
Q ­
m1 2 m0
s1 1 s0
. (4)
In practice m0 was not 0 but typically 2.5% of m1. This
result was due to the finite optical density of the mask
(1.6). The f inite width of the distributions (and the
SNR) was found to be predominantly the result of the
spatial variation of the irradiance of the beam.
Next a single page was recorded at 1 mJycm2 and
analyzed. A histogram of these data is shown in
Fig. 3. The SNR here is , 50, and the estimated
BER, 10221. This is poorer than the direct mask-
image result, primarily because of the holographic
addition of the spatial variations on the reference
beam. Thus we can consider 10221 to be the estimated
BER measurement limit of our holographic test system.
As exposures decreased in the single-hologram
case, the estimated BER increased, reaching 1025 at
Table 1. Peak Diffraction Efficiency
of Multiplexed Holograms
Number of
Holograms (M )
Exposure
smJycm2d h s31026d ht s31026d
20 180 950 990
50 100 113 163
100 72 27 40
Fig. 2. Diffraction eff iciency hologram number for 100-
hologram exposure in an AlGaAs:Te sample. All exposure
values were 26 mJycm2; i.e., no exposure schedule was used.
August 1, 1997 / Vol. 22, No. 15 / OPTICS LETTERS 1191Fig. 3. Frequency of occurrence of pixel values plotted
versus pixel values in arbitrary units for a strong hologram
(1 mJycm2). The projected BER derived from these data
is 10221.
Fig. 4. Theoretical BER’s for coherent (solid curves) and
incoherent (dashed curves) addition of scattered light.
Measurements (circles) agree reasonably well with the
coherent addition model for the observed scattering level
of 1026.
5 mJycm2 per exposure. The increasing noise is seen
as an increase in the spread of both the 0 and the
1 distributions. Even at this exposure level the dif-
fracted light power was sufficiently far above system
noise levels that we would not expect such an increase
in BER. The origin of this noise was found to be
coherent addition of the diffracted light and the light
scattered from imperfections on the sample surface.
Inclusion of the scattered light fraction hS, which we
assume to have random phase, leads to the following
modif ication to Q [Eq. (4)]:
Q 0 ­
h
p
2
shs2 1 2h2s02d1/2 1 fhshs 1 2hs12dg1/2
. (5)
Using values of s0 ­ 0.0086 and s1 ­ 0.098 experimen-
tally obtained from the strong-hologram case, in Fig. 4
we plot curves for BER versus diffraction efficiency for
two scattering levels, hS . Also shown in the f igure are
measured data points for our sample, which appear to
be in good agreement with the theory for hS ­ 1026,
which is also the typical value measured independently
from scatter-only images.Finally, we investigated the effect of multiple expo-
sures on error rate by measuring the BER of a single
hologram as additional holograms were superimposed
at different peristrophic angles. Each exposure was
18 mJycm2, and the diffraction eff iciency of the probed
page decreased from an initial value of 9.5 3 1025 to
1.3 3 1025 as the successive exposures pushed the ma-
terial into saturation (with a final total exposure of
4.4 mJycm2). The BER increased from 10210 to 1028
as the additional holograms were added. A degrada-
tion in BER similar to this is expected from the drop in
h alone. This suggests that interpage cross talk is not
significant in these experiments.
In conclusion, we have demonstrated holographic
storage of digital data in a semiconductor sample con-
taining DX centers. The diffraction eff iciency and
the BER performance of multiplexed data images are
shown to agree well with a simple model of the mate-
rial in which the index change at each point in the vol-
ume depends only on the total exposure at that point
and is independent of that at neighboring points and
of the time or sequence involved in reaching the fi-
nal exposure. This implies that a sequence of multi-
plexed equal exposures will yield holograms with the
same diffraction efficiency, as we have confirmed ex-
perimentally. When saturation of the index change is
present it appears as a reduction of the eff iciency of
all the holograms equally. Thus no exposure sched-
ule is required with these materials. The recording
sensitivity of our AlGaAs sample was shown to be 103
times better than that of LiNbO3:Fe. BER estimates
revealed no error-rate f loor intrinsic to the storage ma-
terial but showed that the coherent addition of stray
scattered light with the diffracted signal (a potential
problem for any holographic storage system) can in-
crease error rates well beyond those expected for in-
coherent background light at the same level.
References
1. P. J. Van Heerden, Appl. Opt. 2, 393 (1963).
2. D. Psaltis and F. Mok, Sci. Am. 273(5), 70 (1995).
3. F. Mok, Opt. Lett. 18, 915 (1993).
4. C. Chen, D. Kim, and D. von der Linde, Appl. Phys.
Lett. 34, 321 (1979).
5. R. A. Linke, T. Thio, J. D. Chadi, and G. E. Devlin,
Appl. Phys. Lett. 65, 16 (1994).
6. R. L. MacDonald, R. A. Linke, J. D. Chadi, T. Thio,
G. E. Devlin, and P. Becla, Opt. Lett. 19, 2131 (1994).
7. D. D. Nolte, J. Appl. Phys. 79, 7514 (1996).
8. A. I. Ryskin, A. S. Shecheulin, B. Koziarska, J. M.
Langer, A. Suchocki, I. I. Buczinskaya, P. P. Fedorov,
and B. P. Sobolev, Appl. Phys. Lett. 67, 31 (1995).
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Fundamentals of Photonics

https://authors.library.caltech.edu/2900/1/REDol97.pdf

Holographic data storage in a DX-center material

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