20022013082649-optical-data-security[1]

8/20/2019 20022013082649-optical-data-security[1]

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SeminarsTopics.com Optical Data Security

ABSTRACT

Due to the development digital computer technologies anddigital television for the next generation, there is a growing demand tostore large sizes of data. Optical technology can provide a number of waysto solve the problem of large storage and fast transmission of data.Unlike bit-oriented optical memories such as DD and !D, in opticalstorage two-dimensional data is stored as a hologram on a photosensitivecrystal, by illuminating the interference pattern formed by an ob"ect beamand a reference beam. Using angle wavelength, and phase multiplexingtechni#ues, one can store multiple images at the same position, thusincreasing the storage capacity.

In practical systems, data security is an important issue. Optical encryption techniques

provide a high level of security because there are many degrees of freedom with which toencode the information, such as amplitude, phase, wavelength, and polarization. To protect thestored information it is required to encrypt the data. Here the encryption means that the originaldata is converted into stationary white-noise data by ey codes, and unauthorized users cannotobtain the original data without nowledge of the ey code.

Original data may be encoded optically by using various encryption techniques. !oublerandom phase encryption, three dimensional position encryption and wavelength-codeencryption are some of the ma"or techniques of encryption available at present.

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TABLE OF CONTENTS

$cnowledgement%%%%%%%%%%%%%%%%%%%%%%%%%%%%%&i'

$bstract%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% #Table of contents%%%%%%%%%%%%%%%%%%%%%%%%%%%%%. (#.Introduction%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% )(.Optial data storage principle. %%%%%%%%%%%%%%%%%%%%%%%% *).+asic components %%%%%%%%%%%%%%%%%%%%%%%%%%%%.

).#.$/0%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 1).(.ens and 2irrors%%%%%%%%%%%%%%%%%%%%%%%%% .1

).(.#.ens as 3ourier transform%%%%%%%%%%%%%%%%%% 4).).patial light modulators%%%%%%%%%%%%%%%%%%%%%%.. 5).6.7hotosensitive materials%%%%%%%%%%%%%%%%%%%%%%.#8

).6.#.7hotorefractive crystals%%%%%%%%%%%%%%%%%%...#8

).6.(.7hotopolymers%%%%%%%%%%%%%%%%%%%%%% ##).*.9harge coupled devices%%%%%%%%%%%%%%%%%%%%%% .#()..7hase mass for encryption%%%%%%%%%%%%%%%%%%%% #)

6.0ecording and reading of data %%%%%%%%%%%%%%%%%%%%%%% #*6.#.Optical recording of data %%%%%%%%%%%%%%%%%%%%% #*6.(.Optical reading of data %%%%%%%%%%%%%%%%%%%%%% #

*./ncryption techniques%%%%%%%%%%%%%%%%%%%%%%%%%%. #*.#./ncryption using double random phase mass%%%%%%%%%%%%... #1*.(./ncryption using )-d eys in the 3resnel domain%%%%%%%%%%% ()*.)./ncryption using wavelength code and random phase mass%%%%%%%. (*

.$pplications%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% (1

.#.$pplications in ultra short communications%%%%%%%%%%%%%% (1.(.$pplications in optical drive %%%%%%%%%%%%%%%%%%%% (41.9onclusions%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%... (54.0eferences%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% )8

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1. INTRODUCTION

Optical data storage is an alternative to magnetic dis data storage. 9urrentlydata access times are e:tremely slow for magnetic diss when compared to the speed of

e:ecution of 97;s so that any improvement in data access speeds will greatly increase thecapabilities of computers, especially with large data and multimedia files. Optical memory is atechnology that uses a three dimensional medium to store data and it can access such data a page at a time instead of sequentially, which leads to increases in storage density and accessspeed. Optical data storage systems are very close to becoming economically feasible. 7hoto-refractive crystals and photopolymers have been used successfully in e:perimental optical datastorage systems. uch systems e:ploit the optical properties of these photosensitive materialsalong with the behavior of laser light when it is used to record an image of an ob"ect. Opticalmemory lies between main memory magnetic dis in regards to data access times, data transfer rates, data storage density.

$s processors and buses roughly double their data capacity every three years &2oore


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Optical memory uses the basic principles of holography for the recording purposes and hence itis also called as holographic memory system. Optical memory is a promising technology for data storage because it is true three dimensional storage system, data can be accessed an entire

page at a time instead of sequentially, and there are very few moving parts so that thelimitations of mechanical motion are minimized. Optical memory uses a photosensitive materialto record interference patterns of a reference beam and a signal beam of coherent light, wherethe signal beam is reflected off of an ob"ect or it contains data in the form of light and dar areas. The nature of the photosensitive material is such that the recorded interference patterncan be reproduced by applying a beam of light to the material that is identical to the reference beam. The resulting light that is transmitted through the medium will tae on the recordedinterference pattern and will be collected on a laser detector array that encompasses the entiresurface of the holographic medium. 2any holograms can be recorded in the same space bychanging the angle or the wavelength of the incident light. $n entire page of data is accessed inthis way.

9urrently, optical memory techniques are very close to becoming technologically andeconomically feasible. The ma"or obstacles to implementing optical data storage are recordingrate, pi:el sizes, laser output power, degradation of holograms during access, temporal decay of holograms, and sensitivity of recording materials. $t an estimated cost of between =##and=() for a complete optical memory system, this may become a feasible alternative to magneticdis in the near future.

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2. Optical Data Storage Principle

The basic principle of optical data storage is that of holography. Holography was invented by!eni >abour in #564.Holographic method was a two step coherent image forming process in

which a record is made of the interference pattern produced by the interaction of the wavesdiffracted by the ob"ect and a coherent bacground or a reference wave. ?hen this hologram isilluminated, the original wave front is reconstructed. Hence we get an image of the originaldiffracting ob"ect as a real ) dimensional ob"ect.

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?hen two light beams one from the ob"ect whose image is to be recorded and the other being a

reference beam, interference in space then form an interference pattern of alternate bright anddar fringes as shown in the diagram. @ow if a photosensitive material or medium is placed atthe position of the interference then these interference patterns are recorded on the material inthe form of change in refractive inde: or the absorption property. @ow, in order to regeneratethe original beam i.e. the source beam from the ob"ect, the reference beam alone is2ade to be incident on the photosensitive material. The material in turn diffracts this beaminside its structure so as to replicate the original beam. This is the basic principle used to recordand read data in the case of optical storage system.


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3.Baic Co!ponent

Optical data storage system requires certain important materials for its data storage and retrieval processes. The important components required for the optical data storage areA

#.$/0 (.ens and 2irrors).patial ight 2odulators &2'6.7hotosensitive materials

6.#.7hotorefractive crystals6.(.7hotopolymers

*.9harge 9oupled !evices &99!'.7hase mass for encryption

3.1. LASER

ight amplification by stimulated emission of radiation is abbreviated as $/0. aser is adevice for the generation of coherent, nearly monochromatic and highly directionalelectromagnetic radiation emitted, somewhere in the range from sub-millimeter throughultraviolet and B-ray wavelengths. 2ore than two hundred types of lasers have been fabricatedwhich range in power, size, performance, use and cost. 3undamental attributes of a laser aredirectionality, monochromaticity, coherence and brightness. These attributes mae it ideal for optical recording. To record holograms on the crystals usually argon ion lasers, rypton lasers

and diode lasers are used.

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3.2.Len an" #irror

2irrors are used to reflect laser beams to the desired direction. enses areusually used to converge the laser to a point. $ special type of lens is used in the case of opticalrecording called the 3ourier lens. The lens has the property of obtaining the 3ourier transformand the inverse transform system is described below.

3.2.1.T$e len a a Fo%rier tran&or! 'te!

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3igure # show how a 3ourier transform and inverse transform can beachieved optically. $ collimated beam is pro"ected through a signal, f&:,y', contained on atransparency. The transform lens causes parallel bundles of rays to converge in the bac focal plane of the lens. This bac focal plane is nown as the 3ourier transform plane. In this plane

the spatial image is transformed into spatial frequency spectra. In affect the lens has carried outa two-dimensional 3ourier transform at the speed of light. $ far field diffraction pattern isobserved by placing a screen in the transform plane. The intensity of the pattern is related to thesquare of the amplitude of the 3ourier Transform of the input signal. +y placing a stop at a particular frequency lobe in this plane a spatial frequency can be removed from the image.Typically all but the zero order diffraction is removed, thus removing the noise from the image.This cleaning of the beam is achieved by using a spatial filter. The diameter and quality of thelens limit the upper frequency bandwidth. The lower bandwidth is limited by the ability of the user to discriminate all but the zero order diffraction information. This analog optical system

5

3igure (A Two spatial signals and their optical 3ourier Transforms


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is difficult to adapt into alternative inds of filters and so is not very versatile .The aperture of the lens limits the resolution of the 3ourier transform. The second lens forms the inversetransform and recovers the original signal.

If the input signal is a sinusoidal grating, figure(, there will be two spotseither side of the central !9 component. The two spots correspond to the spatial frequency

content of the input signal. The radial distance between these spots and the !9 term representsthe spatial frequency of the input signal. Ther)e will be a row of spots in the transform plane of the square wave bar grating, indicating the presence of harmonics of the fundamental frequency.

3.3.Spatial Lig$t #o"%lator (SL#)

2 is an optical device that is used to convert the real image or data into asingle beam of light that will intersect with the reference beam during recording. It basically

consists of an array of pi:els which are usually microscopic shutters or 9! displays. Thesecan be controlled by a computer. The computer sends binary data to the 2. /ach pi:el of the2 corresponds to bit of data. o depending on whether the bit is a # or a 8 the pi:el will godar or transparent in the case of a 9!, or will be open or shut in the case of microscopicshutters.

3igure ) show a )d model of a spatial light modulator. The white pi:elsrepresent a binary # while the blac pi:els denote a binary zero. The white or thetransparent pi:els allows the light beams incident on it to pass through it while the dar or opaque pi:els restrict the transmission of light through it. In effect the light beamcoming out of the 2 contains the binary information transmitted to the 2 by the

computer. $nother important point to be noted here is that a complete page of binarydate is converted to a single beam at a time. The access of a complete page at a timeaccounts for the increase in the access speeds of the optical storage system.

3.*.P$otoeniti+e !aterial

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3.*.1.P$otore&racti+e cr'tal

There are two main classes of materials used for the holographic storage medium. Theseare photo refractive crystals and photo polymers. The recording medium usually used is a photorefractive crystal such as i@bO) or +aTiO) that has certain optical characteristics.

These characteristics include high diffraction efficiency, high resolution, permanent storageuntil erasure, and fast erasure on the application of e:ternal stimulus such as ;C light. Onecharacteristic of the recording medium that limit the usefulness of holographic storage is the property that every time the crystal is read with the reference beam the stored hologram at thatlocation is disturbed by the reference beam and some of the data integrity is lost. ?ith currenttechnology, recorded holograms in 3e-and Tb-doped i@bO) that use ;C light to activate theTb atoms can be preserved without significant decay for two years.

The most commonly used photo refractive crystals used are i@bO) and+aTiO).!uring hologram recording, the refractive inde: of the crystal is changed by migration

of electron charge in responds to imprinted three-dimensional interference pattern of thereference and signal beams. In a photo refractive crystal, illumination with a sufficientwavelength content e:cites the electrons in the conduction band from the donor level betweenthe valence band and conduction band. The donor level is created by impurity ions or defects.The photo-e:cited electrons can move in the crystal by the diffusion, the drift, and the photovoltaic effect and then get trapped in the ionized donors. $t the steady state, the spacecharge density is proportional to the interference pattern in the diffusion-dominant region. Thisspace charge density creates the space charge field that can cause the refractive inde: changevia the electro optic effect. The created refractive inde: distribution is proportional to theinterference pattern and can be stored for along time &more than two months' in the dar ini@bO) crystal. $s more and more holograms are superimposed into the crystal, the more

decay of the holograms occurs due to the interference from the superimposed holograms. $lsoholograms are degraded every time they are read out because the reference beam used to readout the hologram alters the refractive nature of the crystal in that region. 7hoto refractivecrystals are suitable for random access memory with periodic refreshing of data, and can beerased and written to many times.

3.*.2. P$otopol'!er

7hotopolymers have been developed that can also be used as a holographic storagemedium. Typically the thicness of the photopolymers is much less than the thicness of photo

refractive crystals because the photopolymers are limited by mechanical stability and opticalquality. $n e:ample of a photopolymer is !u7ont


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development, large dynamic range, good optical properties, format fle:ibility, good imagestability and relatively low cost, mae photopolymers promising materials for write once, readmany times &?O02' optical data storage applications. Cinyl monomers that polymerizethrough a free radical mechanism, such as acrylate esters, are used in most photopolymer systems. ;nfortunately volume shrinage during optical recording is a series problem for manyfree-radical-based photopolymer systems. /ach time a monomer adds to a growing polymer

chain, the volume of the system decreases as a covalent chemical bond replaces a non-bondedcontact. In severe cases, volume shrinage distorts the recorded interference pattern and prevents accurate recovery of the data.

7olaroid 9orporation recently developed a holographic recording system that e:hibitssignificantly less shrinage than conventional photopolymers. The 7olaroid polymers usesmonomers that polymerize using a cationic ring-opening &90O7' mechanism to replace moreconventional free-radical monomers. hrinage during hologram recording for 90O7monomers is partially compensated by a volume increase produced by the ring-opening polymerization mechanism.

3.,. C$arge Co%ple" De+ice (CCD)

The charge-coupled device is, by far, the most common mechanism for converting optical images to electrical signals. 99!


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?hen requested the elements form a bucet brigadeG each row of charges is passedfrom element to element, a process which is nown as clocing, down the columns andhorizontally along the final row. The value in each pi:el is measured in turn and recordeddigitally, to ensure that only positive numbers result from this analog to digital conversion process a fi:ed offset nown as the bias level is introduced, the charge transfer process is

essentially noise-free and all most all of the noise contributed to the signal by the 99! is fromthe out put stage.

3.-. P$ae #a

There is wide spread interest in the development of encryption systems, which operatein the optical domain. The advantages inherent in the optical approach to encryption, such as ahigh space-bandwidth product, the difficulty of accessing, copying or falsification and the possibility of including biometrics are widely recognized. In an encryption system, we wish toencode information in such a fashion that even if it is viewed or copied only the application of

the correct ey will reveal the original information. 7resently encryption approach is based onthe direct mapping of an encrypted phase-mas and a decrypting phase ey, resulting in thedecryption of information completely within a phase-only domain.

In the case of encryption, a plane polarized monochromatic wave front illuminatesthe encrypted phase mas, which consists of a random array of phase-shifting pi:els. These phase-mass are produced by electronically scrambling the original information, to beencrypted, with a random pattern and using this to generate an encrypted phase mas. Thedecrypting ey effectively reverses the scrambling operation in the optical domain and results inthe production of a wave front in which the information of interest is encoded as a relative phase shift between different sections of the wave front, in this case corresponding to the pi:els.

The mas and ey can be placed, either directly in contact with one another so that

the decryption taes place in the same image plane, or alternatively they can be imaged on toone another with an optical system. +y using a spatial light modulator the phase ey can bescrolled electronically until it overlies the phase pattern of the encrypted mas removing thenecessity for precise mechanical positioning in the optical system.

The encryption technique could equally well be applied to systems in whichmultiple phase levels are used for the mass and eys. However, the fabrication issues involvedin the production of a multiple phase level fi:ed phase mas are more complicated than for the production of a binary mas, so for the purposes of the e:perimental demonstration of decryption binary phase mass and phase eys have been used.

*. Recor"ing An" Rea"ing O& Data

*.1. Optical recor"ing o& "ata

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In holographic data storage, light from a coherent laser source is split into two beams, signal &data-carrying' and reference beams. !igital data to be stored are encodedJ ontothe signal beam via a spatial light modulator. The data or strings of bits are first arranged into pages or large arrays. The o


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The interference pattern induces modulations in the refractive inde: of the recordingmaterial yielding diffractive volume gratings. The reference beam is used during readout todiffract off of the recorded gratings, reconstructing the stored array of bits. The reconstructedarray is pro"ected onto a pi:ilated detector that reads the data in parallel. This parallel readout of data provides holography with its fast transfer rates.

The readout of data depends sensitively upon the characteristics of the reference beam .+y varying the reference beam, for e:ample by changing its angle of incidence orwavelength, 2any different data pages can be recorded in the same volume of material andread out by applying a reference beam identical to that used during writing. This process of

multiple:ing data yields the enormous storage capacity of holography.

,. Encr'ption Tec$ni/%e

There are many different types of encryption techniques available. Here three main techniquesused for encryption has been described. They are A

#. /ncrypted 2emory ;sing !ouble 0andom 7hase /ncryption

(. /ncrypted 2emory ;sing Three-!imensional eys in the fresnel !omain

). /ncrypted 2emory ;sing ?avelength-9ode and 0andom 7hase 2ass

,. Encr'pte" #e!or' Uing Do%0le Ran"o! P$ae Encr'ption

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3igure 6 shows an illustration of the encrypted optical memory system.?ecan do/ncryption and decryption of optical memory using double random phase encryptionet gi&:,y' denote ith positive real-valued image to be encrypted. Here, : anddenote the spatial Kdomain coordinates. The original data is converted into a white-noise-lie image by using two random phase mass, e:pL-"ni&:,y'M and e:pL-"hi&n,h'M,

located at the input and 3ourier planes. Here, ni&:,y' and hi&v,η' are two independentwhite sequences that are uniformly distributed on the intervel N8,(π. @ote that v and η

denote 3ourier domain coordinates.The original data is illuminated by a collimated light beam and multiplied by a random phase function e:pL-"ni&:,y'M. The 3ourier transform of

the input data is multiplied by another random phase function H&v,η'Pe:pL-"hi&v,η'M and isgiven by

i&v,η'P>i&v,η'Hi&v,η'%%%%%%%%%%%%%%%%%%%%%%%%%'

?here

>i&v,η 'P3Ngi&:,y'e:pL-"n&:,y'M

P∫∫ gi&:,y'e:pL-"n&:,y'Me:p{− "n&:,y'Me:p K"&( QDRf'&:SyU' d:.dy%%%..&('

In /q . &(',3N• denotes the 3ourier transform operation ,λis the wavelength of the light,$nd ƒ is the focal length of the 3ourier-transform lens. /ach encrypted data frame isobtained by taing another 3ourier transformA

si&:,y'PNgi&:,y'e:pL− "ni&:,y' ⊗ 3Ne:pL− "h&v,η 'M %%%%%%%%%%%%%..&)'

?here ⊗ denotes the convolution ./quation &)' shows that the two phase functions,I&:,y' $nd hi& ν,η',convert the original data into stationary-white-noise-lie data.

The 3ourier-Transformed pattern of the encrypted data that is described in /q-'

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Is stored hollographically together with a reference beam in a photo refractive material.To store

many frames of data,angular multiple:ing is employed. The interference pattern φ& ν,η' to bestored in a photo refractive material is written as

φ& ν,η'PΣiP# to mVi& ν,η'0i & ν,η'V(%%%%.&6'

where 2 is the total number of stored images and is a reference at specific angle used to

record the ith encrypted data .In a photorefractive crystal illumination with a sufficientwavelength content e:cites the electrons in the conduction band in the donor level between thevalence band and the conduction bands. $t the steady states, the space charge density is proportional to the interference pattern in the diffusion-dominant region.This space chargedensity creates the space charged field that can cause the refractive- inde: change via theelectro-optic effect. The created refractive inde: distribution is proportional to the interference pattern and can be stored for a long time&more than ( months' in the dar in i@bO) crystal.

In the descryption process,the read out beam is the con"ugate of the reference beam.The read out using the con"ugate of the reference beam offers advantages.It is able to use thesame random phase mas in the encryption and decryption process, and it eleiminates

aberration of the optical system.The data of the ith stored image can be reconstructed and theread out beam is incident at the correct angle. The reconstructed data in the 3ourier-

7lane&37',!i & ν,η' is written as!i & ν,η'PL>i& ν,η' H i & ν,η'MF Wi & ν,η'%%%%%&*'

?here

W i & ν,η'Pe:pL- jk i & ν,η'M%%%%%%.&'The asteris in /X-&*' denotes the comple: con"ugate and W i & ν,η' is a phase ey used in thedecryption process. ?e can reconstruct the image by 3ourier transforming /q-&*'.The

reconstructed ith image !i&:,y' is written as di&:,y'PNgF&:,y'e:pL"ni&:,y'M⊗9i&:,y'

%%%%%&1' ?here

9i&:,y'P3Ne:pL-"hi& ν,η'M⊗ 3Ne:pL-"i& ν,η'M%%%&4' In /q-&4' ⊗ denotes correlation.?hen phase ey Wi& ν,η'PHi& ν,η', the con"ugation of theoriginal data is successfully recovered because /q-&4' becomes a delta function. The random phase function in the input plane,e:pL-"ni&:,y'M,may be removed by an intensity sensitivedevice such as charge coupled device &99!' camera.In a practical system the oprating at a highspeed detection of the reconstructed data, the parallel detection at each pi:el of two-dimensional

data is desirable.?hen one uses an incorrect phase ey,ey i& ν,η' ≠ hi& ν,η', the original datacannot be recovered.

Eperi!ental Set%p

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3igure #. /:perimental setup. 072 denotes random phase masG + denotes beam splitterG denotes lensG 2 denotes mirrorG +/ denotes beam e:panderG H denotes shutterG 99! denotes99! camera.

?e denote an encrypted memory system based on double random phaseencryption. 3igure * shows the e:perimental setup. $ #8 Χ#8 Χ#8mm) i@bO) crystal dopedwith 8.88)mol.Y 3e is used as the recording medium. The c a:is is on the paper and is at 6* °with respect to the crystal faces. The crystal is mounted on a rotary stage and a three-dimensionally movable stage. $n $r laser beam of wave length *#6.*nm is used as a coherentlight source. The light beam is divided into an ob"ect and a reference beam by abeam splitter &+I' for holographic recording. The reference beam is again divided into two reference beams.One of the beam is used for the con"ugate readout by another beam splitter &+('.$n inputimage is displayed on a liquid-crystal display that is controlled by a computer. The input imageis multiplied by an input random phase mas &072I' and is then 3ourier transformed by lens#. The 3ourier-transformed input image is multiplied by another random phase mas &072('

at the 3ourier plane. The 3ourier transformed image is imaged at the reduced scale in thei@bO) crystal by the lens (. The encrypted image is observed by a 99! camera &99!#'after the 3ourier transform is produced by the lens ). The focal length of #, ( and ) are688mm, *4mm, and *8mm respectively. 3or holographic recording the ob"ect and reference beams interfere at an angle of 58i in the i@bO) crystal. $ll of the beams are ordinarily polarized due to the creation of an interference fringe pattern. hutters H# and H( are open,while h) is closed.

In the decryption process, the readout beam is the con"ugate of the reference beamused for recording. hutters H# and H( are closed, while H) is open. If the same mas islocated at the same place as the one used to write the hologram, the original image is

reconstructed at 99!(.This is because the ideal reconstructed beam read out by using thecon"ugate of the reference beam eliminates the phase modulation caused by the random phasemas. Otherwise, the original data may not be recovered. In the e:periments we use a pair of counter propagating plane waves as the reference and the con"ugate beams.

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$ngularly multiplied recording of four digital images is demonstrated. One of the original digital images is shown in 3ig. &a'.This image consists of )( )( randomlygenerated pi:els. The size of the liquid crystal display that shows the input image is (4.*mm(8mm. Two diffusers are used as the random phase mass, 072# and 072(. fig. &b' showsthe intensity distribution of the encrypted image. 0andom noise lie images were observed. Inthe recording process, the optical intensities of the ob"ect and the reference beams were14m?Dcm( and #.6m?Dcm(, respectively.The e:posure time of each image was 8s.$ngular

multiple:ing was achieved by rotating the i@bO) crystal in the plane of figure&*'.The angular separation between ad"ascent stored images was 8.(".This angular separation is enough to avoidthe crosstal between the reconstructed images.3ig 1&a' shows the reconstructed imagesobtained using the correct ey.The resolution of the reconstructed image is determined by thecrystal size and the space-bandwidth product of the optical system.This ey is the same as the phase mas in the 3ourier plane used to record the hologram.This result shows that the storedimages were reconstructed successfully.@o noise due to crosstal between the reconstructedimages was observed.$fter the binarisation of the reconstructed images,we confirmed that thereis no bit error in the four output digital data.3ig 1&b' shows the reconstructed images when

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incorrect eys were used.@o part of the original image can be seen.The average bit error rateobtained using incorrect eys was 8.)46.

Encr'pte" #e!or' Uing T$ree"i!enional e' in t$e Frenel Do!ain.

One can mae the memory system more secure by using random phase mass in the 3resnel

!omain.In addition to the phase information,the positions of two phase mass are used asencryption eys./ven if the phase mass are stolen,the unnown positions of the mass can protect the data.The positions of the mass have as many as three degrees of freedom.?e havedemonstrated encryption and decryption of three binary images by using anglemultiple:ing.The e:perimental setup is the same as that shown in fig&*'.3ig&4a' shows one of the three original images.072#Z072( were located at a distance of #88mm from #Z at thecenter of # Z37,respectievely,as shown in fig&*'.3ig&4b'shows an encrypted image of fig&4a'.0andom Knoise- lie images were observed.In the recording process the optical powersof the ob"ect and the reference beams were 6m?Dcm( and *88m?Dcm( respectievely.

/:perimental results &a' Original image ,&b' encrypted image,&c' and &d' are reconstructedimages when positions of the phase mass are correct and incorrect respectively.

The e:posure time was ##8s.3ig&4c' shows one of the reconstructed imagesobtained by using the same mass located at the same positions used in the recording.This resultshows that the original image was successfully reconstructed.3ig&4d' shows the reconstructedimage when the two phase mass were incorrectly located.?e can see that the reconstructedimage is still a white- noise Klie image.

?e estimate the available number of three dimensional positions of two random phase mass.et the dimensions of random phase mass be : : y.and [: and [y be thecorrelation lengths of the random phase mass along the B Z \ a:es,respectievely.TheB,yZz a:es are defined as shown in fig&*'.?hen a number of @z resolvable positions along theoptical a:is can be used for the encryption ey,the total number of three dimensional positionsto be e:amined in a three dimensional ey,7, is written by

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?here] is the focal length of # in fig&*' Z [z is computed according to the sensitivity of thedecryption to the shifts of the eys along the ^ a:is.ince two three dimensional eys are usedin this system,the total number of three dimensional positions to be e:amined is given by

@P7(%%%%%%%%%%%%%..#'

In the memory system shown in fig&*' ,@P):#8#4 when:PyP(*mm,P688mm,[:P[yP[m Z [zP6mm.@ote that [:Z[y were calculated from themeasurement of an auto correlation fuction of the phase mass used in the e:periments.?henone searches #8 positionDs,it taes 5* years to finish the whole search.It is practicallyimpossible to decrypt without the nowing position of two three dimensional eys.

*.) ENCRPTED #E#OR USIN4 5A6ELEN4T7 CODE AND RANDO#P7ASE #AS8S

The wavelength of recording beams can be used as a ey for security in a

holographic memory system.The wave length code increases the ey space by onedimension.ince an optical storage medium such as photo refractive material dopped withimpurities has broad spectrum sensititvity we can use any wavelengths of light emitted fromtunable laser sources such as a !ye-aser.In this mrmory system shown in fig&*' one originaldata frame is stored by using a set of two random phase mass at the input and 3ourier planes aswell as a wavelength ey.The wave length ey can be protect the decrypted data evev if the phase mass have been illegally obtained.?hen the wavelength of the read out beam is differentfrom that of the recording beam the wavelength mismatch modifies the scale of thecoordinates at the 3ourier plane in the read out process.!ue to the incorrect wavelengh the phase modulation at the 3ourier plane is not completely cancelled because of a scalemismatch.If substantial part of the phase modulation at the 3ourier plane is not cancelled , the

original image cannot be recovered.?e note that the wavelength mismatch results in decreaseddiffraction efficiency due to the +ragg condition , because the volume grating structure of thehologram is comple:.

(#


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3ig 5&a' Z fig 5&b' show an original and an encrypted image.The encrypted image isstored hollographically using recording beams at a wavelength of *#6.* nm. In the dcryption process,we use two read out beams at wave length of *#6.*nm and )(.4 nm. @ote that in bothcases th +ragg conditions are satisfied.3ig 5&c' Z &d' shows the reconstructed images whenwavelengths of *#6.* nm Z)(.4 nm are used recpectievely.?hen the wavelength of the readout beam is same as that of the recording beam and when the same mass is located at the sae place as that used to record the hologram , we can obtain the rconstructed original image.Thewave length selectivity depends on the pi:el size of the random phase mass at the 3ourier

plane .One can use many wavelengths by utilizing the small pi:el size of the random phasemass.

-.Application

-.1 Application in %ltra $ort co!!%nication

/ncrypted memory used double random phase encryption cann be used insecure communication networ using ultra short pulses, as shown in fig'. 3ig #8 &a' Z &b'shows bloc and schematic diagrams of the secure communication systems using the encryptedmemory and spatial temporal converters.In this system the original data is stored in an

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encrypted memory system. The encrypted data read out from the memory is converted into aone dimensional temporal pulse using the space to time converter and then is transmitted tousers via optical fibers. $t the receivers the temporal signal is converted again into a spatialsignal by the time to space converter. The authorized users can decrypt the data using thecorrect ey. This system can be e:pected to communicate at an ultra high speed of more than #TbDs.

Concl%ion

Three encrypted optical memory systems have been discussed here. Thesesystems are secure because the total number of mathematical possibilities of themultidimensional eys, which consists of two dimensional phase mass, their three dimensional

positions, and wavelengths of light, is e:tremely large. The e:perimental results are veryencouraging. It is e:pected that the encrypted memory system is to play an important role inultra-fast secure communication systems using the spatial temporal converters with ultra short pulse that enable communication at ultra high speed of more than TbDs.

It is believed that the substantial advances in recording media, recording methods andthe demonstrated densities of _ )8 channel >bitsDin( coupled with the recent commercialavailability of system components remove many of the obstacles that previously prevented the

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practical consideration of optical data storage and greatly enhance the prospects for Hollography to become a ne:t generation storage technology.

9. Re&erence

#. www.bell-labs.com(. www.ieee.org

). www.laser(888.co.u 6. www.ing.iac.es*. www.src.le.ac.u

(6

http://www.bell-labs.com/http://www.ieee.org/http://www.laser2000.co.uk/http://www.ing.iac.es/http://www.src.le.ac.uk/http://www.bell-labs.com/http://www.ieee.org/http://www.laser2000.co.uk/http://www.ing.iac.es/http://www.src.le.ac.uk/


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:o%rnal

#. O.2atoba Z+.`avidi , ENCRPTED OPTICAL STORA4E 5IT7AN4ULAR #ULTIPLE;IN4< = Applications of Optics 6ol.39

(. 3.H 2o , AN4LE #ULTIPLE;ED STORA4E OF ,>>>7OLLO4RA#S IN Lit$i%! Nio0ate< = Optics letters 6ol.11

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