Holographic properties of doubly doped lithium niobate crystals with Indium

Transcription

1 Hologaphic popeties of doubly doped lithium niobate cystals with Indium Qingsheng He, Yunbo Guo, Yi Liao, Liangcai Cao, Guodong Liu, and Guofan Jin State Key Laboatoy of Pecision Measuement Technology and Instuments, Tsinghua Univesity, Beijing, China ABSTRACT The hologaphic popeties of doubly doped lithium niobate cystals with Indium have been investigated. It is found that doping slight Indium concentation (0.5mol%) in LiNbO 3 :In, Fe cystal can efficiently incease the sensitivity with small dynamic ange changing. To impove the whole hologaphic popeties of LiNbO 3 cystal, LiNbO 3 :In, Fe with ideal doping concentations and a new photoefactive mateial LiNbO 3 :In, Mn have been poposed and achieved lage dynamic ange, highe sensitivity and bette signal-to-noise atio than LiNbO 3 :Fe cystal (Fe:0.03wt.%) that is geneally consideed as a pefeable stoage medium in hologaphic memoies. In a common volume of LiNbO 3 :In, Fe cystal poposed, 2030 hologams have been successfully multiplexed and exactly identified in a compact volume hologaphic data stoage and coelation ecognition system. Keywods: Hologaphic popeties; LiNbO 3 :In, Fe cystal; LiNbO 3 :In, Mn cystal; Hologaphic memoies. 1.INTRODUCTION Volume hologaphic memoies have attacted intense inteest because of thei potential in high capacity stoage, fast paallel pocess and content addessability [1, 2]. To a lage extent, the system pefomances depend on the popeties of stoage media. Fo hologaphic stoage mateials, two of the most impotant paametes ae dynamic ange (M/#) and sensitivity (S). In addition, low scatteing will esult in high signal-to-noise atio (SNR) [1, 3, 4]. Photoefactive lithium niobate (LiNbO 3 ) cystal has been widely investigated fo applications in hologaphic data stoage. Usually tansition-metal dopants, such as Fe, Cu, Ce and Mn, ae added to the melt to impove the photoefactive effect. And Fe is most effective in poducing lage impovements in both dynamic ange and sensitivity [5]. Many successful demonstations of hologaphic stoage systems have been achieved in LiNbO 3 :Fe cystals [1, 6]. One appoach to boosting the M/# and sensitivity fo Fe-doped LiNbO 3 cystal is to incease the Fe doping level. Howeve, the highe the concentation of Fe, the lage the photoefaction effect will be. The chaacteistic will cause stong scatteing noise hamful fo hologaphic stoage. Thus a tade-off between M/#, S and SNR has to be consideed when we design LiNbO 3 :Fe cystals fo hologaphic applications, and 0.03wt.% Fe has been geneally consideed as a pefeable doping concentation fo high-capacity hologaphic data stoage. In addition, due to the dak-decay mechanism, thee is a limit on the highest pactical doping level 0.06 wt.% fo LiNbO 3 :Fe cystal and any futhe guoyb98@mails.tsinghua.edu.cn; Tel: ; fax: Hologaphy, Diffactive Optics, and Applications II, edited by Yunlong Sheng, Dahsiung Hsu, Chongxiu Yu, Byoungho Lee, Poceedings of SPIE Vol (SPIE, Bellingham, WA, 2005) X/05/$15 doi: /

2 incease of the doping level above this limit cannot esult in lage M/# and sensitivity [7]. Howeve, Manganese (Mn) cente has a deepe enegy level than Fe cente, and is expeimentally found that the highest pactical doping level in Mn-doped LiNbO 3 is ~0.5wt.% MnCO 3, which pomises lage M/# and sensitivity [4, 8]. It is well known that damage-esistant dopants (e.g., Mg, Zn, In, etc.) have been doped in LiNbO 3 cystal to educe optical damage [9~11]. Among them, In impuity is the most efficient due to the lowest theshold concentation which was epoted about 1.5~2.0mol.%. Volk et al. [9, 10] detailedly analyzed the popeties of In-doped LiNbO 3 cystals. Qiao et al. [12] got a high sensitivity of photoefactive effect in LiNbO 3 :In cystal, and the significant suppession of the photoefactive light-induced scatteing has been obseved in LiNbO 3 :In, Fe cystal [11]. All these expeiments show that doping Indium is a vey efficient way to incease the photoefactive sensitivity and the damage-esistance ability of LiNbO 3 cystal. Howeve, the photoefaction is invesely popotional to the photoconductivity [9], so it is difficult to get high S and SNR simultaneously with lage M/# in singly doped LiNbO 3 :In cystal. Theefoe, it is expected that doubly doped LiNbO 3 :In, Fe (o LiNbO 3 :In, Mn) cystal combining with the advantages of In doping with Fe (o Mn) doping is a pomising photoefactive mateial fo hologaphic applications. In this pape, we will fistly investigate the popeties of LiNbO 3 :In, Fe cystals with a seies of In doping concentations. Next, based on synthesizing the effects of In doping and Fe (o Mn) doping, LiNbO 3 :In, Fe with ideal doping concentations and a new photoefactive mateial LiNbO 3 :In, Mn ae poposed to impove the whole hologaphic popeties of LiNbO 3 cystal. Moeove, two samples poposed have been gown and compaed with LiNbO 3 :Fe cystal (Fe:0.03wt.%) unde the same expeimental conditions. Finally, we applied this poposed LiNbO 3 :In, Fe cystal to a compact volume hologaphic data stoage and coelation ecognition system and got excellent pefomances. 2. EXPERIMENTS ON THE EFFECT OF INDIUM DOPING In the expeiments, we used fou conguently melting LiNbO 3 :In, Fe samples whose concentations of In wee 0.5, 1, 2 and 3mol% in the melt, which we efeed to as InFe1, InFe2, InFe3 and InFe4. Fo compaison, a sample of single doped LiNbO 3 :Fe cystal (efeed as Fe1) was also investigated. All samples have the same Fe concentation of 0.03wt%, the gown state and the dimensions of (thickness) 10mm 3. Additionally, these samples ae 45 o -cut and polished to optical quality. The expeimental setup is a typical 90 o -geomety system. A He-Ne lase beam at the wavelength of 632.8nm was used to ecod and ease hologams. To get the maximum dynamic ange and sensitivity, the cystals wee immesed in a sodium chloide solution which was used to minimize the influence of the photovoltaic field [13, 14], and then placed on a otation stage. The light beam with a diamete of 4mm was split by the PBS into two equal-intensity beams as a efeence beam and an object beam. The gating vecto was always aligned along the c axis. Duing ecoding, the object beam was blocked fom time to time to measue the hologaphic diffaction efficiency as the atio of diffacted and incident light intensities. Afte the gating eached the satuation, we used Bagg-mismatched easue [8], i.e., duing easue the sample was otated fa away fom the Bagg-matched position and illuminated by the same two beams to ecod hologams. In ode to avoid building anothe stong hologam, the sample was otated 0.02 o evey 10s duing easue. At the end of each peiod of easue, the diffaction efficiency was measued by scanning ove an adequate ange of angle and finding the maximum diffaction efficiency with the only efeence beam on. Figue 1 shows a typical ecoding and easing behavio of the single gating in LiNbO 3 :In, Fe cystal. The tempoal tace of η duing ecoding and easing could be well descibed by functions given Poc. of SPIE Vol

3 2 2 by η( t) = sin {( π n L/ λcos θ)[1 exp( t/ τ )]} and η( t) = sin {( π n L/ λcos θ)exp( t/ τ )]}, espectively. Hee θ sat is the incident angle inside the cystal, τ, τ e ae the ecoding and easing time constants, λ is the wavelength outside the cystal, n is the amplitude of efactive-index change, and L is the effective inteaction length. In addition, M/#, S can be defined as M /# = ( A0 / τ) τe and S = ( A0 / τ )/( IL), whee A 0 = π nsat L/( λcos θ), and I is the total incident ecoding intensity [3, 4]. Theefoe, a good fit between expeimental data and functions of the diffaction efficiency indicates that the values of M/#, S can be well obtained by the expeimental cuves. 0 e Fig.1 Expeimentally measued and fitting cuves of LiNbO 3 :In,Fe(In:2mol%,Fe:0.03wt.%) cystal. Fig2. Dynamic ange M/# and sensitivity S of LiNbO 3 :In, Fe cystals vesus In doping concentations. Figue2 illustates the dependences of measued M/# and S of LiNbO 3 :In, Fe cystal on In doping concentations. Diffeent In concentations coespond to diffeent samples. The asteisk signs descibe the sensitivity and show that S inceases similaly linealy with the incease of In concentation unde 2mol% i.e., the theshold concentation of In-doped LiNbO 3 cystal. Howeve, when In concentation inceases to 3mol%, the sensitivity shaply impoves with the inceasing atio twice as lage as that of the theshold concentation below. In addition, the dot signs epesent dynamic ange, and M/# deceases with In concentation inceasing, but it is supising that InFe1 has an identical value of M/# with Fe1. And M/# changes shaply when In concentation is impoved fom 0.5mol% to 1mol% and fom 2mol% to 3mol%. Compaing the popeties of InFe1 with InFe4, we can find that S has a shap incease by a facto of 2.3, but M/# stongly deceases by a facto of 3.5. Hologaphic popeties of LiNbO 3 cystal mainly depend on photoefactive centes. In doubly doped LiNbO 3 :In, Fe cystal, In ions do not paticipate in the chage tanspot and simply affect concentations and incopoation of othe ions which contibute to the photoconductivity [5]. When In concentation is below the theshold, In ions incopoate onto Li-sites, emoves Nb Li and also substitute both fo Fe 2+ and Fe 3+. Theefoe the photoconductivityσ inceases due to a eduction of Nb Li and a vaiation of the atio Fe 2+ /Fe 3+. The incease of σ will cause time constants τ, τ e to educe. Though A 0 educes too, A / 0 τ is still impoved due to the shap incease of σ. So the sensitivity inceases with In concentations unde 2.0mol% shown as Fig.2. Geneally τ, τ e change at the same magnitude fo LiNbO 3 :Fe cystal [15]. Howeve, the asymmety in the eduction of time constants exists fo LiNbO 3 :In, Fe cystal, which can be explained in tems of a multiple-defect-cente model [16]. When In concentation is above 0.5mol%, a complete disappeaance of Nb Li fom LiNbO 3 :In, Fe cystal pehaps happens [10], and the asymmety disappeas. With a decease of A 0, M/# will decease damatically. When In concentation is beyond the theshold, In ions incopoate both onto Li 314 Poc. of SPIE Vol. 5636

4 (In Li ) and Nb (In Nb ) sites, and Fe 3+ incopoates mainly onto the Nb-sites, loosing its accepto popeties. It will esult in a shap incease of photoconductivity, and M/#, S change geatly. Theefoe, slightly In doping (such as 0.5mol%) unde the theshold concentation in LiNbO 3 :In, Fe cystal could be a good advice to impove the sensitivity with small M/# changing. 3. IMPROVEMENT OF HOLOGRAPHIC PROPERTIES OF LINBO 3 :IN, FE CRYSTAL Due to the tade-off between M/#, S and SNR, LiNbO 3 :Fe cystal with 0.03wt.% Fe has been geneally consideed as a pefeable stoage medium fo volume hologaphic data stoage. Nevetheless, fo pactical hologaphic applications, lage dynamic ange and highe sensitivity ae equied, and it is always attacting to dope the highest possible Fe concentation in LiNbO 3 cystal. Since thee is a limit on the highest pactical doping concentation (0.06wt.%) of Fe:LiNbO 3 fo electon tunneling effect [7], and any futhe incease of the doping concentation beyond the limit cannot esult in lage M/# and sensitivity, 0.06wt% Fe is an ideal doping concentation fo the impovement of the hologaphic popeties of LiNbO 3 cystal [7, 15]. Obviously, if Fe concentation diectly inceases to 0.06wt.%, the scatteing noise will incease and can not be suitable fo hologaphic applications. As discussed in section 2, slightly In doping (0.5mol% In) in LiNbO 3 :Fe cystal is an efficient way to solve the poblem. On one hand, In doping can geatly educe the scatteing noise caused by the incease of Fe doping, that is to say, In doping makes it possible to dope high Fe concentation in LiNbO 3 cystal with the assuance of a suitable SNR. On the othe hand, In doping also impoves the sensitivity of LiNbO 3 cystal. Meantime, the value M/# is pehaps educed by slightly In doping at a cetain degee compaed with LiNbO 3 :Fe cystal (Fe:0.06wt.%), but it still inceases compaed with LiNbO 3 :Fe cystal (Fe:0.03wt.%). Theefoe, the whole hologaphic popeties of LiNbO 3 cystal will be impoved. In addition, M/# and S in Fe-doped LiNbO 3 cystals ae stong functions of the oxidation state [15, 17, 18]. Typically, the moe the cystal is educed, the lage the sensitivity will be. Howeve, too lage absoption coefficient α which is elated to the oxidation state [15], will geatly shoten the stoage time. Moeove, thee exists an optimal value α fo maximum M/#. A theoetical model has been developed and shown that the optimum α fo the cystal with thickness d is α d = 1 [17]. So a suitable oxidation state is needed to get excellent hologaphic popeties. Accoding to the guide line above, LiNbO 3 :In, Fe cystal with 0.5mol% In, 0.06wt.% Fe is gown (efeed as InFe5). The dimension is mm 3 1. Afte themal annealing it is slightly oxidized with α 0.25cm at 1 λ = 632.8nm and α 0.5cm at λ = 532nm. Figue 3 shows InFe5 measued and fitting cuves unde the same expeimental conditions with section 2. Fo compaison, Fe1 is also descibed. Fom the single-hologam ecoding and easue cuve, we can calculate M/#, S and photoefactive paametes of Fe1&InFe5 cystals. nsat. Table1 descibes these Table1 Photoefactive popeties of Fe1&InFe5 cystals Sample η m(%) M /# S 2 5 (10 cm/ J) n sat Fe InFe (10 ) Poc. of SPIE Vol

5 Fig.3 Expeimentally measued and fitting cuves of LiNbO 3 :In, Fe(InFe5) and LiNbO 3 :Fe (Fe1) cystals. Note that M/# is popotional to the effective inteaction length L [1, 3]. The values of L fo Fe1 and InFe5 ae both 4mm, so the value M/# can be diectly compaable. The esults show that InFe5 has lage diffaction efficiency. A lage nsat esults fom the incease of Fe doping, and slightly In doping causes that the incease of nsat is less than twice. All that makes InFe5 cystal achieve the values M /# = 7.47 and S 0.02 cm/ J, twice as lage as those of Fe1. Futhemoe, we pefomed a hologaphic image stoage expeiment to evaluate the scatteing chaacteistics of InFe5 cystal and Fe1 cystal. An agon-ion lase beam with the wavelength of 514nm was used to stoe and etieve hologams in a 90 o geomety system. The econstucted hologams ae shown in Figue 4, fom which we can see that the econstucted hologam fom InFe5 has a quite high SNR and even bette than that fom Fe1. (a) (b) Fig.4. The econstucted hologam fom the sample of: (a) LiNbO 3 :Fe cystal; (b) LiNbO 3 :In, Fe cystal. In summay, we have gown a sample of LiNbO 3 :In, Fe cystal with 0.5mol%In, 0.06wt.%Fe and slightly oxidized state, and achieved lage M/#, highe S and bette SNR than LiNbO 3 :Fe cystal with 0.03wt.%Fe unde the same expeimental conditions. 4. HOLOGRAPHIC PROPERTIES OF LINBO 3 :IN, MN CRYSTAL It is well known that Mn cente is deepe than Fe cente, thus the electon tunneling effect in Mn-doped LiNbO 3 cystal is smalle [4], and it is possible to use LiNbO 3 :Mn with highe doping levels fo hologaphic stoage to get lage M/# 316 Poc. of SPIE Vol. 5636

6 and sensitivity. Yang et al has investigated LiNbO 3 :Mn cystals with the doping levels of 0.2at.%Mn and 0.5wt%MnCO 3, and found that these cystals have vey lage M/# and sensitivity with little hologaphic scatteing [4, 7]. If we dope slight In concentation into LiNbO 3 :Mn cystal as section 3, the sensitivity will futhe incease and the scatteing will be less, with small M/# changing. The new photoefactive mateial LiNbO 3 :In, Mn pomises to have lage M/#, S and less scatteing than LiNbO 3 :Fe cystal. A sample of LiNbO 3 :In, Mn cystal with In:0.5mol%, Mn:0.2at.% and as-gown state (efeed as InMn1) was used to investigate the hologaphic popeties of this new mateial. The dimension is mm 3. Fo deep cente Mn, the cut-off wavelength of D-band is about 520 nm [19], so an agon-ion lase beam with the wavelength of 514nm was used to pefom hologaphic gating expeiments in the same 90 o geomety system as section 2. LiNbO 3 :Fe cystal (Fe1) is also tested fo compaison unde the same expeimental conditions. Fig.5 descibes the hologaphic gating ecoding and easing behavios of InMn1 and Fe1 cystals espectively. Fig.5 Expeimental measued and fitting cuves of LiNbO 3 :In, Mn and LiNbO 3 :Fe cystals. Fig.6 The scatteing behavio measuements of LiNbO 3 :In, Mn and LiNbO 3 :Fe cystals. The calculated values fom the cuves ae listed in Table2, fom which we can see that InMn1 has the diffaction efficiency thee times as that of Fe1, lage M/#, nsat and highe S. Though the bulk photovoltaic coefficient of Mn is smalle than that of Fe in LiNbO 3 cystals, the highe doping level makes LiNbO 3 :In, Mn cystal get bette hologaphic pefomances than LiNbO 3 :Fe cystal. Table2 Hologaphic popeties of Fe1&InMn1 cystals Sample η m(%) M /# S 2 5 (10 cm/ J) n sat (10 ) Fe InMn In addition, we also pefomed an expeiment to quantitatively assess the scatteing behavio of InMn1 and Fe1 with the same thickness [4]. We used the A + lase beams with the wavelength of 514nm and the aveage intensities in the cases wee 180mW/cm 2. The tansmission light spot of InMn1 didn t change, while that of Fe1 cystal dispesed out. And thei nomalized tansmitted powes ae plotted as a function of time in Fig.6. The esults show that InMn1 has a vey high damage-esistant ability and the hologaphic scatteing in InMn1 builds up fa slowe than in Fe1. The expeimental esults above pove that InMn1 has bette hologaphic popeties than Fe1. We expect that the pefomance of LiNbO 3 :In, Mn cystal fo hologaphic applications can be impoved futhe by use of the optimal Poc. of SPIE Vol

7 ecoding wavelength 458nm fo Mn-doped LiNbO 3 cystal [4], optimizing the mateial with lage Mn concentation (0.5wt.% MnCO 3 ) and highly oxidized state, tansmission ecoding geomety, o a combination of these pocedues. 5. APPLICATIONS IN HOLOGRAPHIC MEMORIES To futhe investigate hologaphic popeties of doubly doped LiNbO 3 with Indium, we applied InFe5 cystal (In:0.5mol%, Fe:0.06wt.%, slightly oxidized state) to a compact volume hologaphic data stoage and coelation ecognition system. This system is a typical 90 o -geomety ecoding setup. A diode-pumped solid-state lase ( λ = 532nm ) is used as the light souce, and all of the hologams ae angula-factal multiplexed in a coheent volume of the cystal which is immesed in a sodium chloide (NaCl) solution to minimize the influence of the photovoltaic field on the multiplexed hologams. Inputting an oiginal image to illuminate the coheent volume, we can get the coelation esults of the image with all stoed hologams with a chage coupled device (CCD). A hologaphic diffuse is put just befoe the spatial light modulation (SLM, pixels, 26 26µm 2 ) to suppess the sidelobes of coelation pattens and the coss-talk noise caused by them [20]. Recently we have achieved good expeimental esults in LiNbO 3 :Fe cystal (Fe:0.03wt%, gown state, mm 3, efeed as Fe2) in this system [21]. Expeiments show that InFe5 cystal has a maximum toleable exposue time of 15s lage than 10s in Fe2, and a minimum toleable exposue time of 0.8s smalle than 1.3s in Fe2, which is due to its lage dynamic ange and highe sensitivity than Fe2 cystal. It is expected that bette pefomances will be achieved in InFe5. In ode to ealize high capacity hologaphic stoage in InFe5 cystal, an appopiate exposue time schedule must be designed accoding to the chaacteistics of the cystal and the system. The often adopted oiginal time schedule put foth by Psaltis et al [22], is based on the invaiability of witing- and easue-time constants. In ou system, the hologaphic diffuse is vey effective to impove the ecognition accuacy, but it bings a seious poblem that the object beam becomes quite weak afte passing though the diffuse, which causes that ecoding a hologam will need longe exposue time, and the whole ecoding time will geatly incease. What s wose, long exposue time will cause the dynamic incease of photoconductivity esulting fom the incease of the electons in the conduction band with the illuminating time and the gadual incease of the photovoltaic electic cuent which foms a cicuit in NaCl solution. The change of the photoconductivity will cause the dynamic eduction of time constants. And the peviously stoed hologams will be badly eased unde the oiginal time schedule. Deived fom the oiginal time schedule model, the exposue time of each hologam in the same hoizontal diection (called a ow) vaies appoximately accoding to the exponential function with a decay constant that inceases by the numbe of hologams in the ow. We amended the time schedule by multiplying the decay constant of each ow with a estaint facto that changes efeencing to the ecoded esult of the last ow. The adaptable estaint facto compensates fo the dynamic change of time constants, and the new estictedly gadually-educed exposue time schedule pomises to equalize the diffaction efficiency of all stoed hologams. 3 With the new exposue time schedule, in a coheent volume of 0.074cm in InFe5 cystal, we have successfully multiplexed 2030 images using 70 angles in the hoizontal diection and 29 lines in the vetical diection. The longest exposue time is 10s, while the shotest is 0.9s, and the whole ecoding time is 91min, 13.3% shote than that in Fe2 cystal which has achieved the same stoage capacity. Figue 5 shows the coelation spots aay of the 2030 hologams ead by a white blank. Thee is an appoximately unifom diffaction efficiency except a few images which ae not well 318 Poc. of SPIE Vol. 5636

8 ecoded because of the inaccuacy of the shuttes. Inputting the oiginal image one by one, we can get the coelation ecognition esults of all 2030 hologams. The coelation accuacy is about 98.4%, which also indicates that almost of 2030 hologams have been ecoded well and the stoage density of cystal. 21 / 3 Gbits cm has been achieved in LiNbO 3 :In, Fe Fig.5 The coelation spots aay of the 2030 hologams ead by a white blank in LiNbO 3 :In, Fe cystal (70 29). 6. CONCLUSIONS In conclusions, we have shown that doubly doped LiNbO 3 with Indium is vey pomising fo hologaphic ecoding. Slightly In doping concentation (such as 0.5mol%) unde the theshold concentation can efficiently impove the sensitivity and SNR with M/# changed little. Two ways have been poposed to impove the hologaphic popeties of LiNbO 3 cystals, and LiNbO 3 :In, Fe cystal (In:0.5mol%, Fe:0.06wt.%) and LiNbO 3 :In, Mn (In:0.5mol%, Mn: 0.2at.%) have got lage dynamic ange, highe sensitivity and bette signal-to-noise atio than Fe:LiNbO 3 cystal (Fe:0.03wt.%). Futhemoe, in this poposed LiNbO 3 :In, Fe cystal, a high-density hologaphic data stoage has been achieved with the high ecognition accuacy. We believe that doubly doped LiNbO 3 cystal tailoed by slightly damage-esistant dopant Indium doping, highly effective photoefactive centes Fe o Mn doping and suitably themal annealing, will be a vey pomising photoefactive mateial fo lage density, fast access and high SNR hologaphic memoies. ACKNOWLEDGMENT This wok was suppoted by National Natual Science Foundation of China (No ) and National Reseach Fund fo Fundamental Key Pojects NO.973 (G ). REFERENCES 1. H. J. Coufal, D. Psaltis, and G. T. Sincebox, Hologaphic Data Stoage (Spinge, New Yok, 2000). 2. G. W. Bu, S. Kobas, H. Hanssen, and H. Coufal, Content-addessable data stoage by use of volume hologams, Appl. Opt. 38, (1999). 3. S. Tao, M. Lee, K. Kitamua, H.Hatano, L. Galambos, L. Hesselink, Hologaphic popeties of doped stoichiometic LiNbO 3 cystals, in Fifth Intenational Symposium on Optical Stoage, F. Gan and L. Hou, eds., Poc. SPIE, 4085, (2001). Poc. of SPIE Vol

9 4. Y. Yang, I. Nee, D. Psaltis, M. Luennemann, D. Beben, U. Hatwig and K. Buse, Photoefactive popeties of lithium niobate cystals doped with manganese, J. Opt. Soc. Am. B. 20, (2003). 5. D. McMillen, T. D. Hudson, J. Wagne, and J. Singleton, "Hologaphic ecoding in specially doped lithium niobate cystals," Opt. Expess 2, (1998), 6. G. W. Bu, C. M. Jeffeson, H. Coufal, M. Juich, J. A. Honagle, R. M. Macfalane, and R. M. Shelby, Volume hologaphic data stoage at aeal density of 250 gigapixels/in 2, Opt. Lett. 26, (2001). 7. Y. Yang, I. Nee, K. Buse, and D. Psaltis, Ionic and electonic dak decay of hologams in LiNbO 3 :Fe cystals, Appl. Phys. Lett. 78, (2001). 8. Y. Yang, K. Buse, and D. Psaltis, Photoefactive ecoding in LiNbO 3 :Mn, Opt. Lett. 27, (2002). 9. T. Volk, N. Rubinina, and M. Wöhlecke, Optical-damage-esistant impuities in lithium niobate, J. Opt. Soc. Am. B. 11, (1994). 10. T. Volk, M. Wöhlecke, N. Rubinina, N. V. Razumovski, F. Jemann, C. Fische, and R. Bowe, LiNbO 3 with the damage -esistant impuity indium, Appl. Phys. A. 60, (1995). 11. N.Y. Kambe, J. Xu, S.M. Mikha, G. Zhang, S. Liu, G. Zhang, Theshold effect of incident light intensity fo the esistance against the photoefactive light-induced scatteing in doped lithium niobate cystals", Opt.Commun.176, (2000). 12. H. Qiao, J.Xu, Q. Wu, X. Yu, Q. Sun, X. Zhang, G. Zhang, and T. Volk, An incease of photoefactive sensitivity in In: LiNbO 3 cystal, Opt. Mate. 23, (2003). 13. C. Gu, J. Hong, H. Li, D. Psaltis and P. Yeh, Dynamics of gating fomation in photovoltaic media, J. Appl. Phys. 69, (1991). 14. Q. He, G. Liu, X. Li, J. Wang, M. Wu, and G. Jin, Suppession of the influence of a photovoltaic dc field on volume hologams in Fe:LiNbO 3, Appl. Opt. 41, (2002). 15. K. Peithmann, A. Wiebock, and K. Buse, Photoefactive popeties of highly doped lithium niobate cystals in the visible and nea-infaed, Appl. Phys. B. 68, (1999). 16. G. Zhang, Y. Tomita, X. Zhang, and J.Xu, Nea-infaed hologaphic ecoding with quasi-nonvolatile eadout in LiNbO 3 : In, Fe, Appl. Phys. Lett. 81, (2002). 17. G. W. Bu, D. Psaltis, Effect of the oxidation state of LiNbO 3 :Fe on the diffaction efficiency of multiple hologams, Opt. Lett. 21, (1996). 18. J. Wang, Q. He, G. Jin, and M. Wu, Hologaphic stoage paametic optimization of the cystal LiNbO 3 :Fe based on one-cente model, Chinese Jounal of Lases, B11, (2002). 19. Y. Liu, L. Liu, L. Xu, and C. Zhou, Expeimental study of non-volatile hologaphic stoage in doubly- and tiply-doped lithium niobate cystals, Opt. Commun. 81, (2000). 20. C. Ouyang, L. Cao, Q. He, Y. Liao, M. Wu, and G. Jin, Sidelobe suppession in volume hologaphic optical coelatos by use of speckle modulation, Opt. Lett. 28, (2003). 21. Y. Liao, Y.Guo, L.Cao, X.Ma, Q.He, and G.Jin, Expeiment on paallel coelated ecognition of 2030 human faces based on speckle modulation, Opt.Expess 12, (2004), D. Psaltis, D. Bady, and K. Wagne, Adaptive optical netwoks using photoefactive cystals, Appl. Opt. 27, (1988). 320 Poc. of SPIE Vol. 5636