Image transfer by mutually pumped phase conjugators

Image transfer by mutuallypumped phase conjugators

Richard J. Anderson, Edward J. Sharp, Gary L. Wood, and Gregory J. Salamo

Cross talk is observed during the transient time of the photorefractive grating formation in a mutuallypumped phase conjugator. We show that this feature can be used to transfer pictorial information fromone location to another. The transfer is instantaneous and is demonstrated at a resolution of 6lines@mm. r 1996 Optical Society of America

1. Introduction

Multiwave mixing in photorefractive crystals hasbeen used for a number of applications in opticalimage processing. The photorefractive propertiesof energy exchange and phase conjugation have ledto demonstrations of image amplification, edge en-hancement, addition and subtraction of images, res-toration of aberrated images, correlation and convo-lution of images, conversion of incoherent imagesinto coherent images, and image transfer from onelocation to another.1 Sending spatial informationfrom one location to another can be accomplished bymany different techniques. One technique is to usephotorefractive crystals as a medium for the transferof images between two laser beams or between anincoherent source and a laser beam.2–7 For ex-ample, in the case of the photorefractive incoherent-to-coherent converter described in Ref. 6, which isbased on a self-pumped phase conjugator, the imagetransfer was demonstrated for both low-power cw-laser beams and for white light. This particulardevice exhibited a spatial resolution of ,40 linepairs@mm, and the typical time to transfer a single,two-dimensional image was 140 ms at 1 [email protected] slow response time is typical for photorefractivematerials and corresponds to the time to write orerase a photorefractive grating.

R. J. Anderson is with the National Science Foundation, Wash-ington, D.C. 20550. E. J. Sharp and G. L. Wood is with the U.S.Army Research Laboratory, Fort Belvoir, Virginia 22060-5838.G. J. Salamo is with the Department of Physics, University ofArkansas, Fayetteville, Arkansas 72701.Received 21 February 1995; revised manuscript received 26

September 1995.0003-6935@96@050854-06$06.00@0r 1996 Optical Society of America

854 APPLIED OPTICS @ Vol. 35, No. 5 @ 10 February 1996

While the concept of image transfer between lightsources by use of photorefractive crystals can be usedto transfer information from one location to another,it is both alignment sensitive and slow. In thispaper we present a new technique to accomplish thistransfer that is based on a mutually pumped phaseconjugator 1MPPC2 that is self-aligning and instanta-neous. Previous studies8 have shown that temporalinformation at one location could instantaneously betransferred to another location by the MPPC. Inthis paper we show, for the first time to our knowl-edge, that pictorial information can also be trans-ferred. In addition we characterize the quality ofthe image transfer.

2. Double Phase Conjugation

Anew type of phase conjugator, unique to photorefrac-tive crystals, called the mutually pumped phaseconjugator 1MPPC2, has been demonstrated in avariety of geometries.8,9 In these devices, two beamsare incident 1usually on opposite faces of the crystal2and overlap in some region of the crystal. Thebeams may be derived from different lasers as longas the laser wavelength is nominally the same 1i.e.,two He–Ne lasers, for example2. These conjugatorscan be classified by the number of internal reflec-tions the beams experience before conjugation:none,8,10,11 one,12 two,13 or three.14 There have alsobeen MPPC’s demonstrated that are based on thesemilinear mirror design.15,16 In all these devices,two phase-conjugate outputs 1double phase conjuga-tion2 are produced simultaneously by the interactionof the two mutually incoherent beams of the samewavelength within the photorefractive crystal.Since MPPC’s differ in geometry rather than in theirphysical mechanism, they have been shown to sharemany common characteristics: 112 the conjugationof two beams occur simultaneously, 122 the conjugat-

ing beams may be mutually incoherent, 132 the lightfor the phase conjugate of one beam originates fromthe other beam, 142 no cross talk is observed betweenthe conjugating beams in the steady state, and 152 thereported resolution of conjugated images is typicallyapproximately 5 to 10 lines@mm. To date, MPPC’shave been used only when they operate in the steadystate. In this paper we show that they can also bestudied in the transient regime.A schematic of the bridge MPPC8 is shown in Fig.

11a2. When the output of two independent laserswith incident optical fields E1 and E2 are directedinto a photorefractive medium, a phase-conjugatereplica of each beam appears under appropriateconditions. Both beams are required for either con-jugate 1E*1 or E*22 to exist, and the energy for theconjugate of one beam is supplied by the other beam.For example, if E1 is blocked, the conjugate E*2disappears instantly, whereas the conjugate E*1 dis-appears slowly with the characteristic erasure timeof the crystal. A similar behavior is observed ifE2 isblocked. In this conjugator, beam E1 writes thegratings responsible for the production of its conju-gate, and beam E2 reads these gratings and suppliesthe energy for the conjugate wave and vice versa.To understand the physical mechanism behind the

MPPC operator, consider that the extraordinarypolarized input beams E1 and E2, at frequencies v1and v2, enter the crystal on opposite faces and eachinterferes with its own scattered 1fanned2 light, de-noted as E1f and E2f, respectively. The input beamsand the corresponding fanned light then write a setof fan gratings by means of the photorefractiveprocess. There is a set of gratings that occurs ineach fan that is identical to a set of gratings in theother fan. These two sets reinforce each other anddominate all other fan gratings. As a result, thefans are coupled and bend into each other, resem-bling a bridge, hence the name bridge conjugator.Maximum spatial overlap occurs when the two beamsbecome counter propagating phase conjugates ofeach other, and this results in maximum gain for theprocess. The strong volume gratings shared byboth beams are written throughout the crystal, butfor convenience they are shown only in Fig. 11a2 atlocations A and B near the input faces of the crystal.The transmission grating written by E1 and E1f atposition A is read by the fanned beam E2f, whichsupplies the light for the phase conjugate of theincident beam E1 as E*1. There is a similar picturefor the grating written by E2 and E2f at position B sothat two conjugate signals, E*1 and E*2, are producedsimultaneously.The grating picture of Fig. 11a2 must be modified

depending on the coherence and the frequencies ofthe input beams. The three cases of importance forthe present discussion occur when 112 the beams areincoherent but the frequencies are the same, 122 thebeams are coherent and the frequencies of the beamsare equal, and 132 when the lasers have significantlydifferent wavelengths. In case 112 when the beams

are derived from different lasers but have the samewavelength 1two argon-ion lasers or two He–Nelasers, for example2, then the transmission-gratingpicture of Fig. 11a2 is appropriate and the phase

1a2

1b2

1c2

Fig. 1. Schematic diagrams of the grating locations in a bridgeMPPC resulting from different interference conditions deter-mined by the mutual coherence or incoherence of the input laserbeams: 1a2 Two mutually incoherent input beams of the samenominal wavelength. 1b2 Two input beams that are in coherencewith each other. 1c2 Two input beams of greatly differing wave-lengths. In these diagrams the transmission gratings 1T2 and thereflection gratings R are shown only in regions near the crystalsinput faces at A and B.

10 February 1996 @ Vol. 35, No. 5 @ APPLIED OPTICS 855

conjugate outputs of the MPPC’s are stable. In case122 the beams used to form the MPPC in the bridgeconjugator are mutually coherent, resulting in un-stable outputs caused by the presence of reflectiongratings10 written by each input beam and thefanned light from the other input beam. Thesereflection gratings are shown in Fig. 11b2 for writingbeams E1 and E2f, at the E1 input face, and for E2 andE1f, at the E2 input face.In case 132, when the wavelengths are different

1v1 fi v22, then the device is not strictly a MPPC andis called a double-color-pumped oscillator 1DCPO2.In this device the self-generated beams and theirgratings are self-organized to fulfill the Bragg condi-tion, and the self-generated beams propagate at anangular offset 1established by the Bragg condition2 tothe input beams, as shown in Fig. 11c2. The double-color-pumped oscillator has been used for self-Bragg-matched beam steering17 to transfer images frombeams E1 and E2 to E3 and E4, respectively, and tocolor convert the images.18

3. Transient Regime

In our discussion so far we have talked only aboutthe properties and the behavioral characteristics ofMPPC’s when they operate in the steady state.MPPC’s operating in the steady state exhibit stableconjugate output signals after some characteristicbuild-up time. The build-up time 1grating forma-tion time2 can vary from microseconds to minutes.The exact value depends on the particular crystalbeing used and also on several other factors associ-ated with the geometry and the pumping conditions,such as beam intensities, angles of incidence, therelative coherence of the pumping beams, and thespot size of the pumping beams.12,15,19A number of optical processing demonstrations

have been conducted with MPPC’s operating in thesteady state. For example, it has been pointed outthat once steady-state operation is established, theconjugate signals can be temporally modulatedmerely by modulating either or both of the inputbeams so that digital information can be exchangedinstantaneously from one beam to the other. Thisfeature has been proposed as a way to implementtwo-way communication systems through distortingmedia.20 MPPC’s have also been used to demon-strate important image-processing applications suchas addition and subtraction,8 color imaging,16 thresh-olding,21,22 edge enhancement,21 and image trans-fer.18Recently, MPPC’s operated in the transient regime

have been used in optical processing applications.23This paper demonstrates that MPPC’s operated inthe transient regime 1i.e., during grating build up2can also effectively transfer pictorial informationfrom one location to another. In particular, usingsubmilliwatt pumping beams, we demonstrate theinstantaneous transfer of pictorial information in abridge MPPC operating in the transient regime.The no-cross-talk criterion established for MPPC’s


refers to steady-state conjugate signals 1a time thatis long compared with the photorefractive responsetime of the particular crystal being used2.24 Forexample, suppose the double-phase-conjugate mir-ror is formed by a plane wave and an image-bearingbeam. After steady state is reached, there is noobserved cross talk between the two beams, i.e.,there is no evidence of the image present in thephase-conjugate signal of the plane wave.25 If how-ever, while the MPPC is operating in the steadystate, the amplitudes of the input beams are spa-tially modulated in a time that is short comparedwith the photorefractive response time, the crosstalk between the input beams is observed. Forexample, suppose the MPPC is formed by two planewaves originating from two separate locations.When spatial information is suddenly added at onelocation, the spatial information is instantaneouslytransferred to the other location, i.e., cross talk isobserved.

4. Experiment

The experimental apparatus is shown in Fig. 2.The beam from an argon-ion laser, oscillating at 514nm, was polarized extraordinary with respect to thec axis and expanded to ,2.5 mm. The expandedbeam was divided into two arms whose optical pathlengths were adjusted to make the laser light in thetwo armsmutually incoherent. The bridge conjuga-tor formed in the MPPC, therefore, satisfied theconditions described in case 112 of Section 2. Nega-tive or positive binary transparencies 1placed atlocations O1 and O2, respectively2 were used tospatially modulate the laser beams. The modu-lated beams, E1 and E2, were subsequently imagedinto the MPPC crystal by lenses L1 and L2 1each of

Fig. 2. Experimental setup that uses a bridge MPPC to demon-strate the transfer of spatial information between two differentlocations: PR, polarization rotator; BX, beam expander; BS’s,beam splitters; S’s, shutters; negative or positive binary transpar-encies placed at locations O1 and O2; L1 and L2, 2f@2f imaginglenses, each of 150-mm focal length; ND’s, neutral-density filters;VC1 and VC2, CCD cameras.

150-mm focal length2 placed in a 2f@2f arrangement.This imaging system provided beam diameters atthe crystal surface of sufficient size 1,2.5 mm2 toachieve significant beam fanning and subsequentbeam overlap in the crystal.26,27 Spatially modu-lated input beams E1 and E2 were incident uponopposite sides of a cerium-doped SBN:60 crystal 16mm 3 12.2 mm 3 13.4 mm2. The crystal was ori-ented as shown in Fig. 2 such that the beamsoverlapped across its 12.2 mm face. The angles ofincidence for the two beams were u1 5 60° and u2 540°. Laser beam powers at the crystal were typi-cally a few microwatts to several milliwatts with anE1@E2 beam ratio of approximately one. The devicewas allowed to come to steady state, and the outputbeams E*1 and E*2, which were the phase conjugatesofE1 andE2 at locations O1 andO2, respectively, wereobserved as follows. The phase conjugates wereredirected from their original paths by beam split-ters, imaged with slight magnification by separatelenses 1not shown2, and recorded at their respectiveimage planes with CCD cameras VC1 and VC2.1Using this arrangement, we recently demonstratedan order of magnitude improvement in the spatialresolution of phase-conjugated images by usingMPPC’s28,292.This experimental arrangement allowed observa-

tion of a number of interesting effects related to the

transfer of pictorial information from one location toanother. Initial observations indicated the follow-ing: 112 If either input beam was blocked, the phaseconjugate of the other beam disappeared instantly,whereas the phase conjugate of the blocked beamdisappeared slowly with the characteristic photore-fractive response time. 122 When a neutral-densityfilter was used to suddenly reduce the intensity ofone of the input beams, a corresponding decrease insignal strength occurred instantly in the conjugatesignal of the other beam, whereas the conjugatesignal of the reduced-intensity beam slowly in-creased in strength. Most important to the effec-tive transfer of pictorial information between loca-tions was that, when the intensity distribution of oneof the incident beams was modulated by the suddenintroduction of a spatially complex pattern, thepattern was instantly observed on the phase-conjugate signal of the other beam. Furthermore, ifthe spatial distribution of either of the incidentbeams was altered while the MPPC was operating insteady state, the MPPC shifted back into its tran-sient regime and the spatial distribution was trans-ferred between locations 1e.g., from O1 to O2 or fromO2 to O12.Figure 3 displays the results of using the MPPC

operating in the transient regime to transfer spatialinformation from one location to another. Initially

Fig. 3. Photographs of the steady-state and the transient phase-conjugate outputs recorded by VC1 and VC2 for different inputconditions: 1a2 Near-uniform spatial distributions at steady state. 1b2 Spatial distributions in the transient regime when a binarytransparency is placed in beam E1 at location O1. 1c2 Spatial distribution at steady state with a transparency located at O1 in beam E1.1d2 Spatial distributions in the transient regime when the transparency located at O1 in beamE1 is moved. 1e2 Spatial distributions in thetransient regime when the positive binary transparency of the letter C at location O1 in beam E1 is replaced with a negative binarytransparency of a portion of the U.S. Air Force resolution chart.


the MPPC is operating in the steady state, and bothinput beams E1 and E2 are near-uniform spatialdistributions. Figure 31a2 shows the steady-statespatial distribution of the input beams and theirrespective phase-conjugate outputs. Images wereencoded on input beam E1 by passing the beamthrough a binary transparency located at O1. Figure31b2 displays the input and the output phase-conjugate spatial distributions of E1 and E2 immedi-ately after a positive binary transparency of theletter C is inserted in E1 at position O1. During thetime the MPPC is forced to operate in the transientregime by the insertion of the object, the spatialinformation of location O1 appears on the conjugateof E2 and is effectively transferred to location O2.After the MPPC returns to steady-state operation,the input and the output beams have the spatialdistributions shown in Fig. 31c2. If the binary trans-parency in beam E1 is suddenly moved, the MPPCreturns to the transient mode, and the spatial infor-mation at location O1 is once again transferred tolocation O2 3i.e., the resulting input and outputspatial distributions shown in Fig. 31d2 are similar tothose of Fig. 31b24. In the latter case, the position ofthe transparency at location O1 was moved ,0.5 mmand the photographs were taken ,1 s after move-ment occurred, a time significantly less than thegrating formation time of the MPPC for the ,1-mWbeam powers used. Figure 31e2 shows a result simi-lar to that of Figs. 31b2 and 31d2 except that a negativetransparency 1a portion of the U.S. Air Force resolu-tion chart2was placed at position O1.Physically, the transient image transfer can be

understood as follows: The transmission gratingsTA and TB are established by the input beams oneach side of the crystal and the respective input fanbeams. In particular, these gratings represent thecooperativemutually shared gratings found in steadystate and together allow Braggmatching throughoutthe crystal. In our experiment, beamsE1 andE2 arethe Fourier transforms of objects placed at locationsO1 and O2.In steady state, the transmission function TA at

grating locationA, written by E1 and E1f, is

TA , 1E1f 1 E121E1f 1 E12*, 112

which, if both beams are initially spatially uniform,gives the uniform function E*1E1f. When an objectwith spatial information is suddenly introduced atO1, the uniform gratingE*1E1f is instantly read byE81,the Fourier transform of the spatial object, givingthe diffracted beam

E81TA , E811E*1E1f2. 122

The diffracted beam then propagates to position B inthe crystal, where it is again Bragg diffracted by theuniform grating TB , E*2E2f or

E811E*1E1f2TB , E811E*1E1f21E*2E2f2. 132

As a result, the Bragg-matched signal term that


makes it back to the detector VC2 is given by

ES , F3E811E*1E1f21E*2E2f24, 142

where F is the inverse-Fourier-transform operation.Since E*1, E1f, E*2, and E2f are uniform functions,

ES , F3E814, 152

and the spatial information is therefore transferredto locationO2. As time progresses from the introduc-tion of E81, E81 itself begins to fan, E81 = E81f, so thatthe image transferred washes out with the photore-fractive time response.

5. Discussion

Although there is a finite time necessary to bring thebridge MPPC to steady state 3e.g., for 1 W@cm2 inputintensities this response time is ,3 s 1Ref. 824, thisdoes not affect the response time of the actualinformation transfer process. In principle, oncesteady state is established, information transfer canoccur instantaneously. In practice, instantaneoustransfer is not possible since even if the fastest SLMavailable were used to introduce spatial informationon the read beam, the response time of the SLMwould be the limiting factor determining the overalltime response of the transfer technique.Although the beam intensities did not affect the

image-transfer time, they did affect the length oftime over which the image transfer was maintained;that is, with high-intensity beams the transientwindow of time was short and equilibrium wasreached quickly. In addition, when low intensitieswere used, the intensity of the transferred imagewas correspondingly low.If large numbers of images are to be transferred,

this all-optical image-transfer device would need tobe refreshed periodically between images to main-tain the quality and the uniformity of the steady-state, plane-wave signal beam. This would ensurethat the contrast ratio of the image would be main-tained and close to constant, and it would preventghosting of the transferred images 1superposition ofan image with the preceding or the following imagein a sequential transfer process2. Another approachwould be to have the image-bearing beam reduced inintensity, thereby reducing the effect of gratingerasure even for the transfer of many images.The resolution of this device 1presently at 4 to 6

lines@mm2 depends on the crystal quality, the crystalsize, and the volume grating selectivity. The thick-ness of the crystal used for these image-transferdemonstrations was 12.2 mm, and, in the bridgeMPPC, the shared gratings that couple the twopump beams extend throughout the crystal thickness.However, we have recently shown that the portionsof the gratings that give rise to high-resolutionphase-conjugate images in the bridge MPPC arelocated near the input faces and are 91 mm inthickness.29 The remainder of the shared gratingsthroughout the bulk of the crystal carries out the

selective filtering of the input beams such that thespatial information on these beams is completelyfiltered out 1fanned2 when they read gratings on theopposite side 1output faces2 of the crystal. Thissame selective filtering by the strong volume grat-ings is responsible for the relatively low resolutionobtained in image transfer that occurs during thetransient regime. Nonetheless, we have recentlydemonstrated that these image-transfer techniquescan be used to detect and identify 1correlate2 movingobjects.23

6. Conclusions

Cross talk in the photorefractive MPPC during thetransient time of photorefractive grating formationhas allowed pictorial information to be instanta-neously transferred from one location to another.The advantage of this image-transfer technique overelectronic techniques, which transfer an image pixelby pixel, is that it is optical to optical, and the entireimage is transferred simultaneously. It also has adistinct advantage over photorefractive spatial lightmodulators, which require a grating formation pro-cess and therefore transfer images within the pho-torefractive response time. The technique demon-strated here, while photorefractive, does not involvegrating formation. It occurs instantaneously.Moreover, the technique is a self-aligning dynamiclink between locations that allows for multiplexing.

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Image transfer by mutually pumped phase conjugators