Three-dimensional color holographic display

Brian P. Ketchel, Christy A. Heid, Gary L. Wood, Mary J. Miller, Andrew G. Mott,Richard J. Anderson, and Gregory J. Salamo

Three-dimensional ~3D! color holograms are recorded in a cerium-doped, strontium barium niobate~SBN:60! photorefractive crystal. These holograms are shown to reconstruct true color reproductions ofthe original object with an observable field of view of 37°. Angle multiplexing of two or more 3D colorholograms is also demonstrated with angle tuning of the reference beam corresponding to a separationangle between stored images of 0.082°. Each of these results is compared with corresponding theoreticalpredictions. © 1999 Optical Society of America

OCIS codes: 090.2870, 100.6890, 190.5330, 090.4220.

1. Introduction

The possibility of storing a three-dimensional ~3D!image library holographically in a small crystal haslong attracted the attention of researchers. This at-traction is driven by the fact that, among the many3D image display techniques, holography providesthe most pleasing 3D images for the human eye–brain system. Applications of holography to cinema-tography, artificial intelligence, and security havelong been a goal of scientists and engineers. Al-though progress toward this goal has been slow, re-cent experiments have clearly demonstrated thepotential of photorefractive crystals for real-timestorage and retrieval of 3D images.1 Moreover, thepotential for storage and reconstruction of 3D imageshas been demonstrated with freedom from distor-tion,2,3 high resolution,4 large depth of field,5–7 andwide field of view ~FOV!.8 In this paper we reportthe demonstration of the corresponding storage andretrieval of multiple 3D color images. These imagesare shown to have a wide FOV as demonstrated bymovement of one’s head back and forth during view-ing of the hologram or by use of an imaging lens witha color CCD camera to record the 3D image from

B. P. Ketchel, C. A. Heid, G. L. Wood, M. J. Miller~mmiller@arl.mil!, and A. G. Mott are with the U.S. Army ResearchLaboratory, 2800 Powder Mill Road, Adelphi, Maryland 20783-1197. R. J. Anderson is with the National Science Foundation,4201 Wilson Boulevard, Arlington, Virginia 22230. G. J. Salamois with the Department of Physics, University of Arkansas, Fay-etteville, Arkansas 72701.

Received 23 February 1999; revised manuscript received 18June 1999.

0003-6935y99y296159-08$15.00y0© 1999 Optical Society of America

different perspective views. The observed FOV, ac-curacy of color, resolution of multiplexed images, andstorage time are found to be in excellent agreementwith theory.

Our approach to color 3D holographic storage isbased on four-wave mixing in photorefractive crys-tals.9 The term photorefractive is used to describe aspecial kind of optically induced refractive-indexchange that can occur in electro-optic materials.The microscopic details of the photorefractive mech-anism are normally described by use of a band trans-port model,10–12which assumes the existence of a poolof charges residing in low-lying traps. When a spa-tially varying intensity pattern is produced at a pho-torefractive medium, photoexcitation of the trappedcharges occurs at the maxima of the spatially varyingintensity pattern. The photoexcited charges mi-grate by drift or diffusion out of the illuminated re-gions and are eventually retrapped in the darkregions of the crystal. The charge transport thenresults in a spatially varying charge distribution thatis balanced by a strong space-charge field accordingto Poisson’s equation. This strong electrostatic field~E0 ; 104 Vycm! then produces a change in the re-fractive index ~Dn > 0.0001! through the electro-opticeffect, and a phase hologram is written.

In the case of holography the spatially varying in-tensity pattern is produced when a reference beam,Eref, is interfered with light scattered off of the object,Eobj, which itself may be thought of as a summationof plane waves. As long as these plane waves andthe reference beam are mutually coherent and thephotorefractive storage crystal has sufficient photore-fractive response at the laser wavelength along withlow dark current, interference of Eref and Eobj in thecrystal will yield a refractive-index grating propor-

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S

d

o

iolc

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6

tional to ErefEobj* .9 The object is thus recorded as an

array of index gratings within the crystal volume andis referred to as a volume hologram. To reconstructthe object, a third beam, Eread, incoherent to both thereference and the object beams and counterpropagat-ing to the reference beam, essentially reads the trans-mission phase gratings. This read beam may begenerated with a third collimated beam or with thephase conjugate of the reference beam produced by adouble phase conjugator.13 The advantage of thedouble phase conjugator is that it produces an exactcounterpropagating wave that results in optimiza-tion of the FOV.1

When we consider a single plane-wave componentof the object beam, the resulting diffracted field, Ediff,is then given by

Ediff } Eread~ErefE*obj 1 EobjE*ref!

5 ErefE*objEread 1 EobjE*refEread. (1)

ince the fourth wave, Ediff, can be viewed as beingproduced by the mixing of three other waves, Eref,Eobj, and Eread, the technique can be considered to befour-wave mixing. In the experiments describedhere the photorefractive crystal stores a thick holo-gram that requires Bragg matching, and only thefirst term on the right-hand side of relation ~1! pro-

uces a diffracted beam, Ediff.14 The diffracted beamthen travels in a direction exactly opposite to Eobj and

Fig. 1. Gain coefficient for the SBN photorefractive crystal measgeometry used to obtain the measurements.

160 APPLIED OPTICS y Vol. 38, No. 29 y 10 October 1999

re-creates a 3D image of the object at its originalposition. In this way 3D holographic storage in aphotorefractive crystal by use of four-wave mixing isachieved.

The photorefractive medium used in this studyis a cerium-doped, strontium barium niobate@Sr0.6Ba0.4Nb2O6 ~SBN:60!# crystal with dimensionsf 20 mm 3 20 mm 3 1.3 mm. Beam-fanning ef-

fects, which tend to degrade stored information, arereduced by use of a thin crystal.15 To take advan-tage of the largest electro-optic coefficient, r33 forSBN, the crystal is oriented such that its c axis is bothn the plane of incidence and along the entrance facef the crystal, while the polarization of the incidentight is also in the plane of incidence with a largeomponent along the c axis.

To obtain the maximum modulation of the storedndex grating, the intensities of Eref and Eobj are ad-

justed to be nearly equal. In addition, the optimumangle between Eobj and Eref is determined by mea-surement of the two-beam coupling response of thecrystal at various angles. For example, a strongpump and a weak probe beam operating at an inten-sity ratio of approximately 100:1 are interfered in thephotorefractive crystal ~insert in Fig. 1!, and theransmitted probe intensity is measured with ~Ic, cou-

pled probe intensity! and without ~Iuc, uncoupledprobe intensity! the pump beam incident on the crys-tal. Assuming exponential gain in the undepleted

as a function of angle for l 5 488 nm and l 5 647 nm. Inset,

ured

w

tc

Tpbetp

pump beam approximation,16 the gain G can be esti-mated with Eq. ~2!:

G 51Lc

lnS Ic

IucD 5

reff

cos u

kg

1 1 ~kgyko!2 , (2)

here Lc is the length of the crystal along which theprobe beam propagates; reff is the effective electro-optic coefficient; u is the full external crossing angle ofhe reference beam and the line drawn between theenter of the object and the center of the crystal; kg is

the grating wave number, kg 5 2k sin uy2; k is thevacuum wave number; and ko is the inverse Debyescreening length. The gain for the photorefractivecrystal as a function of angle is shown in Fig. 1 forwavelengths of 488 and 647 nm. In each case thesolid curve is a fit to the right-hand side of Eq. ~2!.

hese measurements indicate that the strongest cou-ling occurs between 20° and 50° for 488 nm andetween 30° and 60° for 647 nm. Therefore, in thisxperiment, the full external crossing angle is main-ained at approximately 40° to obtain efficient cou-ling for both wavelengths.

2. Three-Dimensional Color Holographic Display

A. Hologram Production

Figure 2 is a schematic diagram of the experimentalapparatus used to study color holographic reproduc-tion within a photorefractive crystal. The laserbeams used to write the index gratings consist ofcopropagating 488- and 647-nm beams from argon-and krypton-ion lasers, respectively, operating in asingle transverse mode. The object consists of a pairof sterling silver dice, 2 mm in length on each side.

Fig. 2. Experimental apparatus employed to study color holographused to write the color hologram consists of copropagating l 5respectively. A, aperture; BS, beam splitter; BX, beam expander

One die is painted with blue enamel paint, and theother is painted with dark-red enamel paint. Thelight scattered off of the dice is incident on the pho-torefractive crystal. In practice one obtains themaximum allowable FOV for the stored hologram byhaving both the reference and the read beams com-pletely fill the photorefractive crystal and by ensur-ing that they counterpropagate exactly. Thereforebeam expanders are used to increase the diameters ofthe reference and the read beams and to collimateeach beam before it enters the recording medium.The object beam is also expanded to match the size ofthe object and uniformly illuminate the object. In-cident angles are chosen such that the majority of thelight scattered from the object is directed toward thephotorefractive recording material where it inter-feres with the reference beam. Moreover, care istaken to ensure coherence between the object and thereference beams by maximization of the object beampower incident on the crystal while the path differ-ence between both beams is adjusted to zero.

As discussed above, the centers of the two beamsintersect at an angle of ;40° and are of equal inten-sity to maximize the magnitude of the correspondingindex grating modulation.9 In addition, we choosethe relative intensities of the red and the blue beamsby determining the sensitivity of the photorefractivecrystal to each wavelength. Since the photorefrac-tive crystal used in this study, SBN:60, is more sen-sitive in the blue wavelength regime than in the red,it was determined experimentally that a 15:1 powerratio between the red and the blue beams, respec-tively, produces an equivalent holographic responsetime within the photorefractive crystal. The powersof the red and the blue writing beams used in this

production with a photorefractive storage crystal. The laser lightm and l 5 647 nm beams from argon- and krypton-ion lasers,

mirror; ND, neutral-density filter; ly2, half-wave plate.

ic re488 n; M,

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bro

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6

experiment are approximately 200 and 13 mW, re-spectively.

B. Hologram Readout

After a color hologram is written in the photorefrac-tive crystal, the hologram is retrieved with copropa-gating red and blue read beams, Eread, which arecounterpropagating to the reference beam, as shownin Fig. 2. The generated signal beam, Ediff, is pro-duced by diffraction off of the index gratings stored inthe crystal. Maximum efficiency of the hologramreadout is obtained by use of the same 15:1 powerratio of the red and the blue read beams that wasused in writing the hologram and by expansion of thebeams to read the entire grating recorded in the stor-age crystal. In addition, the power levels of the readbeams are at least a factor of 10 lower than the writ-ing beams to preclude premature erasure of thehologram~s!.

In this experimental setup the object is placed atdistances varying from 20 to 40 mm from the storagecrystal to ~1! avoid blocking the reference beam, ~2!collect the maximum amount of scattered light, and~3! maximize the potential FOV. Since the readeam is counterpropagating to the reference beam, aeal image is formed at the same position as thebject. Therefore, when the writing beams, Eobj and

Eref, are blocked and the object is removed, it is pos-sible to view the hologram through an imaging lenseither directly with the eye or with a camera. Sincethe hologram persists for some time, it is also possibleto document it by use of an imaging lens with a colorCCD camera mounted on a goniometerlike arrange-ment to obtain various perspectives of the 3D truecolor hologram.

3. Results and Comparison with Theory

A. Field of View

The expected FOV of the holographic image can becalculated by use of the diagram shown in Fig. 3.The photorefractive recording crystal of length LC isoriented such that the normal to the crystal’s largestface bisects the angle between the reference and theobject beams, f. The object of width s is located a

Fig. 3. Calculation of expected FOV.

162 APPLIED OPTICS y Vol. 38, No. 29 y 10 October 1999

distance d from the projection of the recording crys-tal, where the projection of the crystal is in the planeperpendicular to d. The effective length of the re-cording material is

Leff 5 Lc cos~fy2! 2 s, (3)

where LC cos~fy2! is the projection of the crystal tothe plane normal to d and the object size is subtractedo that the entire object is observed throughout theOV. The angular range over which the object cane viewed through the crystal limits the maximumOV of the hologram. The FOV is calculated withhe expression

FOV 5 2 arctan~Leffy2d!, (4)

where Leff is defined in Eq. ~3! and d is the distancerom the object to the projection of the recording crys-al as shown in Fig. 3.

Figure 4~a! shows different perspectives of trueolor holographic images of dice placed 20 mm fromhe crystal. The parallax between the red and thelue dice is evident when we compare the relativeosition of parts of each die ~e.g., the lower right-handorner of the blue die with the upper left-hand cornerf the red die!. The FOV of this arrangement iseasured as 37° 6 3°, whereas the maximum FOV is

alculated with Eqs. ~2! and ~3! to be ;45°, when LC5 20 mm, d 5 20 mm, f 5 20°, and s 5 3 mm.Although the measured FOV of 37° is somewhat lessthan that predicted, the difference may be attribut-able to the fact that, at one extreme of the FOV,scattered light from the read beam makes observa-tion of the image difficult.

The possibility of increasing the FOV by addition ofa second identical storage crystal with similar dimen-sions ~20 mm 3 20 mm 3 1.3 mm! is also examined.Figure 4~b! shows the hologram obtained when twostorage crystals are placed side by side such that thewidth of the photorefractive storage area is increasedto 40 mm. A combination of spherical and cylindri-cal lenses is used to produce an elliptically shapedreference beam that filled the width of the storagecrystal. Although this arrangement produces a ho-logram that is clearly visible with twice the FOV @anobserved 74° compared with 86° predicted by use ofrelation ~1!#, a bright strip of light appears in thecenter of the image, owing to scattering from theinterface between the two crystals. In this case, forsimplicity, only blue laser light on silvered dice wasused.

B. Hologram Color

A hologram is said to have true color if it re-creates animage that has the same combination of wavelengthsand their relative intensities detected from the objectduring recording.17 To determine whether the holo-grams produced in this study are true color holo-grams, the color balance of the hologram is comparedwith that of the object illuminated by laser light. Acomputer image of the dice is produced with a colorCCD camera, and the color balance of the image is

pid6gooAr

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aser

then analyzed18 to determine the percentages of therimary colors—red, green, and blue—present in themages. Table 1 displays the color balance for theice when illuminated with laser light ~both 488 and47 nm! as compared with their respective holo-raphic images. For each die the color compositionf the hologram closely matches the color compositionf the object when illuminated by the laser light.lthough we have demonstrated true 3D color holog-aphy for only two colors, the data presented are

Fig. 4. ~a! Various perspectives of the 3D color hologram observvident by comparison of the relative position of parts of each die ~eorner of the red die!. ~b! Two perspectives of silver dice illumina

SBN crystals are placed side by side to double the effective image

Table 1. Color Balance of Dice Illuminated with L

Red Die ~%!

Red Green

Laser light 50 20Hologram 49 19

roof of the principle for storage and recall of trueolor 3D images in a photorefractive crystal.

C. Angle Multiplexing

Since the bulk crystal volume offers a larger storagecapacity than do conventional recording materialssuch as films, much research is currently focused onstudying holographic storage in photorefractive crys-tal.19 To take advantage of this greater capacity,holograms are stored with various multiplexing tech-

ring readout. The parallax between the red and the blue die ise lower right-hand corner of the blue die with the upper left-hand

y l 5 488 nm. The observed FOV is increased twofold when twoage area.

Light versus Color Balance of Holographic Image

Blue Die ~%!

ue Red Green Blue

0 17 41 412 14 39 47

ed du.g., thted bstor

Bl

33

10 October 1999 y Vol. 38, No. 29 y APPLIED OPTICS 6163

fdteigd

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6

niques.20 In the study reported here we demon-strate that multiple 3D color holograms may bestored by means of angle multiplexing in the photore-fractive crystal.

The same basic experimental setup shown in Fig. 2is used to study angle multiplexing of color 3D holo-grams in the photorefractive crystals. Two holo-grams of the dice are recorded by use of two angles ofthe reference beam that differ by only 0.1°. To re-duce the possibility of grating erasure during record-ing, the first hologram is recorded for 20 min whereasthe second hologram is recorded for only 10 min.The holograms are then read with a low-power readbeam to prevent their further erasure during recon-struction. Because of the low intensity of the holo-gram output, a color CCD camera is used to record adigital image of the hologram at various angles.

The holographic images are analyzed to measurethe luminosity of a particular area of the hologram.16

By averaging over approximately the same area onseveral successive holograms, we obtain the angulardependence of the luminosity of the dice as shown inFig. 5. The holograms are clearly distinguishablewhen recorded at an angular separation of 0.1°. Thesmallest angular separation at which two hologramscan be written within the crystal and still be distin-guished ~i.e., not degraded by grating cross talk! isdetermined by measurement of the FWHM luminos-ity. The FWHM of the luminosity curve for the dicerecorded in Fig. 5 yields a minimum resolution angleof 0.082°.

The expected angle bandwidth can be calculatedwith the following expression:

Du < Lyl, (5)

where L is the grating spacing and l is the width ofthe medium.21 The grating spacing is calculatedfrom the Bragg equation, 2Ln sin u 5 l, where n isthe refractive index of the material ~n 5 2.2 for SBN:60!, l is the laser wavelength in free space, and u isthe angle of incidence measured in the medium. Forour study the grating spacing L is calculated to be943 nm. When Eq. ~4! is used with l 5 488 nm and

Fig. 5. Intensity on readout of two angle-multiplexed hologramsdemonstrates a resolution of 0.1°.

164 APPLIED OPTICS y Vol. 38, No. 29 y 10 October 1999

l 5 1.3 mm, the corresponding FWHM resolutionangle is calculated to be 0.042° in the medium. Us-ing Snell’s law, one finds that this resolution angletranslates to a separation of 0.084° in air, which is inclose agreement with the resolution angle of 0.082°measured for the data presented in Fig. 5.

D. Holographic Image Storage Time

Permanent storage of holographic images in photore-fractive crystals is often obtained by electrical fix-ing,22 thermal fixing,23 or periodic refreshing24 of theimages. However, the holograms produced in thecurrent study persist for significant duration withoutapplication of any of these methods. For example,the read beam first is blocked while the hologram isrecorded for approximately 5 min. The power of therecording beams is ;5 mW. The recording beamsare then blocked, and the object is removed so thatthe holographic image can be viewed. The hologramis reconstructed with a weaker read beam of ;0.8mW. The resulting hologram is bright with a dif-fraction efficiency of ;3% and persists during readingor approximately 30 min without any measurableegradation. For further exploration of the persis-ence of the stored hologram the crystal is then cov-red with a dark box for three days before the images again reconstructed. Even in this case the holo-raphic image appeared bright without any perceivedegradation.Subsequent examination of the crystal orientation

ssociated with the phenomena of the observed longtorage times with this SBN crystal indicated thathe direction of the c axis played a significant role.

To study this effect, we interfered two equally intense~10 mWycm2!, mutually coherent, 488-nm argon laserbeams in the photorefractive crystal. The recordingtime is 30 min, and the intensity of the beam used toread the resultant grating is 10% of that of the writ-ing beams ~1.0 mWycm2!.

After a grating is recorded, one of the writingbeams is blocked while the other beam is allowed toremain incident on the material as the read beam.The decay of the grating is recorded as a function oftime. Figure 6 shows the grating decay ~a! wheneam 2 is used as the read beam and beam 1 islocked and ~b! when beam 1 is used as the read beamnd beam 2 is blocked. The 1ye grating decay timeor the first case is measured to be 3 min, whereas inhe second case the 1ye decay time is measured to beuch longer, 20 min. The reason for the variation

n decay time may be explained by self-enhanced dif-raction.25 In this case erasure of the initial grating

competes with growth of the grating generated by thetwo-beam coupling between the read beam and thebeam diffracted from the original grating. The grat-ing decay-time data measured for the 488- and the514-nm wavelengths are presented in Table 2. Ineach case a significant enhancement of nearly 1 orderof magnitude is observed.

Table 2. Comparison of Holographic Grating Decay Time for Gratings

4. Conclusions

A simple method for recording real-time 3D color ho-lograms by use of a photorefractive crystal as thestorage medium has been demonstrated. The 3Dhologram produces a realistic image, which can beviewed over a large field of view ~FOV! of 37°. Weincreased the FOV of the hologram to 70° by storingthe hologram in a mosaic of two photorefractive stor-age crystals. Both of these observations are found tobe in excellent agreement with theoretical predic-tions. In addition, the color in the 3D image hasbeen analyzed and is found to be a faithful reproduc-tion of the color in the original object. Moreover,multiple 3D images were stored and retrieved in thecrystal with angle multiplexing implemented by po-sitioning of the reference beam at angles differing by0.01°. Here, again, the measured resolution wasfound to be in excellent agreement with theoretical

Written at l 5 488 and l 5 514 nm

Wavelength~nm!

1ye Decay Time ~min!

Beam 1 ~Read!yBeam 2 ~Blocked!

Beam 2 ~Read!yBeam 1 ~Blocked!

488 22 3514 44 6

prediction. Because of self-enhancement the holo-grams could be read for hours without fixing. Theseobservations demonstrate the feasibility of storing3D holographic libraries in small crystals.

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Fig. 6. Data show the holographic grating decay written at a wavelength of l 5 488 nm ~a! when beam 2 is used as the read beam andbeam 1 is blocked and ~b! when beam 1 is used as the read beam and beam 2 is blocked. Beam fanning occurs opposite the c-axis direction.BS, beam splitter; M, mirror.

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