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Optics Letters

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  • Vol. 21, Iss. 12 — Jun. 15, 1996
  • pp: 890–892
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Holographic digital data storage in a photorefractive polymer

P.M. Lundquist, C. Poga, R.G. DeVoe, Y. Jia, W.E. Moerner, M.-P. Bernal, H. Coufal, R.K. Grygier, J.A. Hoffnagle, C. M. Jefferson, R. M. Macfarlane, R. M. Shelby, and G.T. Sincerbox  »View Author Affiliations


Optics Letters, Vol. 21, Issue 12, pp. 890-892 (1996)
http://dx.doi.org/10.1364/OL.21.000890


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Abstract

We report high-contrast storage of 64-kbit digital data pages in a photorefractive polymer material. Single-page writing, reading, and erasing operations were demonstrated with a dual-function-dopant polymeric material having a dark lifetime of several days. Data were reconstructed without error by use of several simple readout algorithms.

© 1996 Optical Society of America

The polymeric samples were 130 μm thick and were composed of poly (methyl methacrylate) (PMMA), 33-wt. % 1,3-dimethyl-2,2-tetramethyl-5-nitrobenzimidazoline (DTNBI), and 0.5-wt. % C60. [12]

12. S. M. Silence, J. C. Scott, J. J. Stankus, W. E. Moerner, C. R. Moylan, G. C. Bjorklund, and R. J. Twieg, J. Phys. Chem. 99, 4096 (1995). [CrossRef]

This was the first PR polymer material shown to have a long dark lifetime, with a time constant >300 h, [12]

12. S. M. Silence, J. C. Scott, J. J. Stankus, W. E. Moerner, C. R. Moylan, G. C. Bjorklund, and R. J. Twieg, J. Phys. Chem. 99, 4096 (1995). [CrossRef]

, [13]

13. S. M. Silence, R. J. Twieg, G. C. Bjorklund, and W. E. Moerner, Phys. Rev. Lett. 73, 2047 (1994). [CrossRef] [PubMed]

and samples exhibit the excellent optical quality required for storage of holograms containing complex data pages. As is well known, successful holographic storage depends on maximizing diffraction efficiency and minimizing unwanted light scattering. The bidirectional transmissive distribution function, [14]

14. J. C. Stover, Optical Scattering: Measurement and Analysis, 2nd ed. (Society of Photo-Optical Instrumentation Engineers, Bellingham, Wash., 1995). [CrossRef]

a standard measure of unwanted scattering efficiency, of our PMMA:DTNBI:C60 samples was measured to be 3 × 10−3 per unit solid angle at a scattering angle of 60°, which is low in relation to achievable diffraction efficiencies in this material. The samples were sensitized with incoherent white light as described previously, [15]

15. S. M. Silence, G. C. Bjorklund, and W. E. Moerner, Opt. Lett. 19, 1822 (1994). [CrossRef] [PubMed]

and the (continuously) applied poling field was 76 V/μm.

We have demonstrated a variety of digital data page writing, reading, and erasing operations in our photorefractive polymer samples, using a precision test stand developed for high-resolution holographic optical storage experiments that was described previously. [16]

16. M.-P. Bernal, H. Coufal, R. K. Grygier, J. A. Hoffnagle, C. M. Jefferson, R. M. Macfarlane, R. M. Shelby, G. T. Sincerbox, P. Wimmer, and G. Wittmann, Appl. Opt. 35, 2360 (1996). [CrossRef] [PubMed]

A schematic diagram of the 4-f recording geometry (f = 10 cm) is displayed in Fig. 1. The output of a Kr+ laser (λ = 676 nm) was divided into object and reference beams and used to write and read holograms. The object beam was used to image a 64-kbit chromium-on-glass data mask through the PR polymer with unit magnification onto a CCD camera. The PR polymer sample was positioned several millimeters from the Fourier plane to reduce the intensity variation in the Fourier transform of the object beam. The incident angles of the p-polarized object and reference beams were 20° and 60°, respectively, relative to the surface normal. The object and reference beam powers were 1 and 10 mW at the sample, and the typical exposure time was 500 s, with reading times of the order of milliseconds. The diameter of the collimated reference beam was 4 mm, and the spatial extent of the Fourier transform of the image beam was similar to this size at the sample location. The average diffraction efficiency attained under these conditions was roughly 0.03%, or 4.6 × 10−9 for each of the 65, 536 data bits.

Holograms were completely erased by either of two procedures. Illumination by the reference beam alone caused erasure after roughly 4 times the original exposure time. Alternatively, an incoherent white-light source of intensity 5 mW/cm2 was found to cause complete erasure much more quickly, after roughly 1/20 of the initial exposure time. Holograms were typically read out within 5–10 min after the end of the exposure period, without noticeable degradation on this time scale.

The mask used contained a 64-kbit random data page. The spatial extent of each data bit was 18 μm × 18 μm, and each bit was surrounded by a 9-μm-thick, opaque border, with an overall pitch of 36 μm × 36 μm. This data pattern was imaged onto a CCD camera having 9 μm × 9 μm pixels, so each data bit was spread over 4 CCD pixels. This 4× oversampling permitted several methods of decoding the stored data (see below). Figure 2(a) shows the transmitted image of the mask with a blowup of a small region at the lower left corner, and a reconstructed hologram is shown similarly in Fig. 2(b). The quality of the transmitted image is a direct result of the optical quality of the sample in use.

We compared data in the reconstructed hologram and the original data mask, using the average, minimum, and maximum photon count found in each 4-pixel data location. The best separation between 1 and 0 data bits was observed when the average count number was used. Each of these measures was superior to schemes that included intensities from the dark border that surrounds each bit.

Figure 3 shows a histogram of the intensity distribution of the entire data page from a reconstructed hologram, along with a histogram of the original transmitted image. The inset shows a line section of 30 data bits located in one of the corners of a reconstructed hologram, where the alignment is poorest. The use of a single global threshold in distinguishing 1 and 0 bits is not practical, either in existing magnetic or optical storage schemes or in realistic holographic storage systems. However, this technique can be useful for purposes of comparison. Data read out from a typical PMMA:DTNBI:C60 hologram by use of a single threshold contained no errors in the transmitted image and 10 errors in the reconstructed hologram, for a bit error rate (BER) of 1.5 × 10−4.

Several simple data readout algorithms that account for nonuniform illumination did result in error-free readout of the entire 64-kbit random data page from the reconstructed hologram. The average intensity was roughly constant, but the corners of the 2.54 cm × 2.54 cm mask had weaker intensity than the central region, and the tilted geometry also resulted in weak vertical interference fringes, which were apparent in the reconstructed holograms. These fringes, which were not observed when the object beam was imaged through the sample, could be attributable to multiple reference beam reflections, either by the formation of multiple holograms during writing or by multiple readout of the same hologram during reading. In the hologram corresponding to Fig. 3, all the data 1 bits that were incorrectly read as data 0 were located near the middle of a dark vertical fringe centered near pixel column 85. All the data 0 bits that were incorrectly read as data 1 were located in the upper right corner of the hologram, where the transmitted image was somewhat distorted owing to limitations of the imaging optics.

Several implementations of an adaptive local threshold were found to correct successfully for such nonuniformities. One strategy is the use of a differential threshold, [5]

5. G. D. Bacher, M. P. Chiao, G. J. Dunning, M. B. Klein, C. C. Nelson, and B. A. Wechsler, Opt. Lett. 21, 18 (1996). [CrossRef] [PubMed]

in which two data locations (coded either dark–bright or bright–dark) are devoted to each data bit and the relative intensity of these locations is used to distinguish the two data states. In reconstructing our unencoded data we used several variations of this strategy. For example, the relative difference in intensity of adjacent bits is considered as follows: an increase by more than a factor of 3 corresponds to a change from 0 to 1, a decrease of more than a factor of 3 corresponds to a 1-to-0 transition, and any relative change less than this corresponds to the two bits’ being identical. Using this simple algorithm, we read out the reconstructed data without error. Similarly, a local threshold based on a comparison of the difference in intensity between adjacent bits applied in the same manner also resulted in error-free readout of the reconstructed hologram. For a 64-kbit page a single error would correspond to a BER of 1.5 × 10−5, which can be considered an upper limit for our data when these adaptive readout schemes are used. These techniques are superior because the data bits contained in the high end of the 0 distribution and the low end of the 1 distribution are located in different regions of the data page. These and other, more sophisticated thresholds analogous to a running average are much more practical than the use of a single absolute threshold, but an accurate BER requires a larger data sample or a proper statistical model of the histogram.

Practical storage schemes will require higher data densities than in our single-page experiments. The thin-film geometry of PR polymers limits the Bragg selectivity to the order of 1°, allowing only tens of holograms to be angularly multiplexed. The saturation diffraction efficiency of 130 μm of PMMA:DTNBI:C60 is roughly 7%, so its dynamic range allows many holograms of the strength described here to be stored simultaneously. The number of holograms stored at one location in a 100-μm film can be increased from tens to hundreds by peristrophic multiplexing techniques. [11]

11. K. Curtis, A. Pu, and D. Psaltis, Opt. Lett. 19, 993 (1994). [CrossRef] [PubMed]

In addition, stratified volume PR materials, [17]

17. J. J. Stankus, S. M. Silence, W. E. Moerner, and G. C. Bjorklund, Opt. Lett. 19, 1480 (1994). [CrossRef] [PubMed]

which comprise multiple layers of PR polymers, each of roughly 100-μm thickness, offer the potential for higher Bragg selectivity and larger numbers of angularly multiplexed holograms and thus higher storage densities. Detailed studies of multiplexing in our PR polymers and of lifetime issues such as hologram quality and BER over longer dark times are in progress.

In conclusion, we have demonstrated high-contrast digital optical storage of 64-kbit data pages in a photorefractive polymer film. A global absolute threshold resulted in a readout BER of 1.5 × 10−4, and the use of a relative threshold in which the intensities of adjacent bits are compared resulted in error-free readout, corresponding to an upper limit of ~1.5 × 10−5 for the associated BER. The high optical quality, convenient erasure, and long dark lifetime of the PMMA:DTNBI:C60 polymer studied suggest that its implementation in practical data storage schemes, including angular and other multiplexing strategies, may be possible. Clearly, however, higher diffraction efficiency and faster writing speed would be desirable. Further investigations of these and other polymeric photorefractive materials are in progress.

This research was supported in part by Advanced Research Projects Agency Defense Sciences Office contract DAAB07-91-C-K767 and by Photorefractive Information Storage Materials agreement MDA972-94-20008.

Figures

Fig. 1 Schematic diagram of the 4-f holographic recording geometry.
Fig. 2 64-kbit random data page and a blowup of the lower-left-corner region (inset) for (a) the image transmitted through the sample and (b) the reconstructed hologram.
Fig. 3 Intensity distribution of data bits contained in the original transmitted image (dashed curve) and the reconstructed hologram (solid curve) obtained by averaging the intensities for each 4-pixel data bit and ignoring the opaque boarder regions. The horizontal scales for the two histograms are not complete. Inset: Line sections of pixel counts in a transmitted image (black strips) and the subsequent reconstructed hologram (gray bars).

References

1.

P. van Heerden, Appl. Opt. 2, 393 (1963). [CrossRef]

2.

J. M. Hong, I. McMichael, T. Y. Chang, W. Christian, and E. G. Paek, Opt. Eng. 34, 2193 (1995). [CrossRef]

3.

L. Hesselink and M. C. Bashaw, Opt. Quantum Electron. 25, S611 (1993). [CrossRef]

4.

G. T. Sincerbox, Opt. Mater. 4, 370 (1995). [CrossRef]

5.

G. D. Bacher, M. P. Chiao, G. J. Dunning, M. B. Klein, C. C. Nelson, and B. A. Wechsler, Opt. Lett. 21, 18 (1996). [CrossRef] [PubMed]

6.

J. F. Heanue, M. C. Bashaw, and L. Hesselink, Science 265, 749 (1994). [CrossRef] [PubMed]

7.

F. H. Mok, Opt. Lett. 18, 915 (1993). [CrossRef] [PubMed]

8.

S. Ducharme, J. C. Scott, R. J. Twieg, and W. E. Moerner, Phys. Rev. Lett. 66, 1846 (1991). [CrossRef] [PubMed]

9.

W. E. Moerner and S. M. Silence, Chem. Rev. 94, 127 (1994). [CrossRef]

10.

B. L. Volodin, Sandalphon, K. Meerholz, B. Kippelen, N. V. Kukhtarev, and N. Peyghambarian, Opt. Eng. 34, 2213 (1995). [CrossRef]

11.

K. Curtis, A. Pu, and D. Psaltis, Opt. Lett. 19, 993 (1994). [CrossRef] [PubMed]

12.

S. M. Silence, J. C. Scott, J. J. Stankus, W. E. Moerner, C. R. Moylan, G. C. Bjorklund, and R. J. Twieg, J. Phys. Chem. 99, 4096 (1995). [CrossRef]

13.

S. M. Silence, R. J. Twieg, G. C. Bjorklund, and W. E. Moerner, Phys. Rev. Lett. 73, 2047 (1994). [CrossRef] [PubMed]

14.

J. C. Stover, Optical Scattering: Measurement and Analysis, 2nd ed. (Society of Photo-Optical Instrumentation Engineers, Bellingham, Wash., 1995). [CrossRef]

15.

S. M. Silence, G. C. Bjorklund, and W. E. Moerner, Opt. Lett. 19, 1822 (1994). [CrossRef] [PubMed]

16.

M.-P. Bernal, H. Coufal, R. K. Grygier, J. A. Hoffnagle, C. M. Jefferson, R. M. Macfarlane, R. M. Shelby, G. T. Sincerbox, P. Wimmer, and G. Wittmann, Appl. Opt. 35, 2360 (1996). [CrossRef] [PubMed]

17.

J. J. Stankus, S. M. Silence, W. E. Moerner, and G. C. Bjorklund, Opt. Lett. 19, 1480 (1994). [CrossRef] [PubMed]

History
Original Manuscript: December 6, 1995
Published: June 15, 1996

Citation
M.-P. Bernal, P.M. Lundquist, C. Poga, H. Coufal, R.K. Grygier, R.G. DeVoe, Y. Jia, J.A. Hoffnagle, C. M. Jefferson, W.E. Moerner, R. M. Macfarlane, R. M. Shelby, and G.T. Sincerbox, "Holographic digital data storage in a photorefractive polymer," Opt. Lett. 21, 890-892 (1996)
http://www.opticsinfobase.org/ol/abstract.cfm?URI=ol-21-12-890


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References

  1. P. van Heerden, Appl. Opt. 2, 393 (1963). [CrossRef]
  2. J. M. Hong, I. McMichael, T. Y. Chang, W. Christian, E. G. Paek, Opt. Eng. 34, 2193 (1995). [CrossRef]
  3. L. Hesselink, M. C. Bashaw, Opt. Quantum Electron. 25, S611 (1993). [CrossRef]
  4. G. T. Sincerbox, Opt. Mater. 4, 370 (1995). [CrossRef]
  5. G. D. Bacher, M. P. Chiao, G. J. Dunning, M. B. Klein, C. C. Nelson, B. A. Wechsler, Opt. Lett. 21, 18 (1996). [CrossRef] [PubMed]
  6. J. F. Heanue, M. C. Bashaw, L. Hesselink, Science 265, 749 (1994). [CrossRef] [PubMed]
  7. F. H. Mok, Opt. Lett. 18, 915 (1993). [CrossRef] [PubMed]
  8. S. Ducharme, J. C. Scott, R. J. Twieg, W. E. Moerner, Phys. Rev. Lett. 66, 1846 (1991). [CrossRef] [PubMed]
  9. W. E. Moerner, S. M. Silence, Chem. Rev. 94, 127 (1994). [CrossRef]
  10. B. L. Volodin, Sandalphon, K. Meerholz, B. Kippelen, N. V. Kukhtarev, N. Peyghambarian, Opt. Eng. 34, 2213 (1995). [CrossRef]
  11. K. Curtis, A. Pu, D. Psaltis, Opt. Lett. 19, 993 (1994). [CrossRef] [PubMed]
  12. S. M. Silence, J. C. Scott, J. J. Stankus, W. E. Moerner, C. R. Moylan, G. C. Bjorklund, R. J. Twieg, J. Phys. Chem. 99, 4096 (1995). [CrossRef]
  13. S. M. Silence, R. J. Twieg, G. C. Bjorklund, W. E. Moerner, Phys. Rev. Lett. 73, 2047 (1994). [CrossRef] [PubMed]
  14. J. C. Stover, Optical Scattering: Measurement and Analysis, 2nd ed. (Society of Photo-Optical Instrumentation Engineers, Bellingham, Wash., 1995). [CrossRef]
  15. S. M. Silence, G. C. Bjorklund, W. E. Moerner, Opt. Lett. 19, 1822 (1994). [CrossRef] [PubMed]
  16. M.-P. Bernal, H. Coufal, R. K. Grygier, J. A. Hoffnagle, C. M. Jefferson, R. M. Macfarlane, R. M. Shelby, G. T. Sincerbox, P. Wimmer, G. Wittmann, Appl. Opt. 35, 2360 (1996). [CrossRef] [PubMed]
  17. J. J. Stankus, S. M. Silence, W. E. Moerner, G. C. Bjorklund, Opt. Lett. 19, 1480 (1994). [CrossRef] [PubMed]

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