1550 nm WDM read-out of volume holographic memory

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1 55 nm WDM ead-out of volume hologaphic memoy Maia Chiaa Ubaldi *, Piepaolo Boffi, Davide Piccinin *, Elisabetta Flavia Rondinella and Maio Matinelli * CoeCom, Via Ampee 3, 23 Milano-Italy * Also with Dept. of Electonics and Infomation-Politecnico di Milano, P.zza Leonado da Vinci 32, 233 Milano-Italy ubaldi@coecom.polimi.it Abstact: The implementation of an optical memoy fo 55 nm communication signals is hee demonstated. In ode to pefom such device, supeimposed volume hologams have been stoed in LiNbO 3 :Fe exploiting angle-multiplexing ecoding in visible light and wavelength-multiplexing ead-out in nea infaed. Expeimental esults of econstuction of fou-bit digital wods by means of WDM optical beams ae pesented. OCIS codes: (5.733) Volume hologaphic gatings; (2.286) Hologaphic and volume memoies; (6.45) Optical communications Wavelength Division Multiplexing (WDM) is a well known λ-multiplexing technique developed afte the intoduction of the Ebium Doped Fibe Amplifie (EDFA) into the.55 µm optical communication window. WDM netwok systems equie optical devices in ode to pocess high bit ate digital signals; thus it seems desiable to optically implement fundamental pocessing opeations such as multiplexing and demultiplexing, outing, ecognition and memoization. Volume Hologaphy (VH) has been investigated as a useful suppot to build up optical memoies []. Thanks to lage stoage capacity togethe with expected fast esponse, VH epesents an extemely pomising technology fo all-optical devices implementation: multiple infomation can be ecoded within the same medium, pefoming an optical database accessible eithe by angle [2] o wavelength [3] multiplexing. Unfotunately photoefactive mateials don t stand out as demonstating high sensitivity in the nea infaed egion [4]; hence it s had to diectly ecod hologams by using.55 µm signals, in ode to obtain efficient devices opeating at communication wavelengths. A possible solution is epesented by the oppotunity of ecoding volume hologams in classical photoefactive mateials by means of visible light (blue-geen spectal ange) and eading such hologams at a diffeent wavelength, employing the so called two-colo technique. As in this case, the ead-out wavelength can belong to the EDFA s gain ange; in such way a VH memoy in a photoefactive mateial may successfully opeate upon typical optical communication signals. Thee ae no meaningful examples in liteatue about this topic, although the expeimental implementation of naow bandwidth tunable filtes at 55 nm in lithium niobate has been pesented [5]. The aim of this pape is to pesent a VH optical memoy fo signals in the thid window of optical communication systems, i.e. at 55 nm. Such memoy stoes multiple digital wods ecoded in a single LiNbO 3 :Fe cystal; these wods ae then ead out by means of WDM signals. Among photoefactive mateials, lithium niobate has been investigated fo use as a volume hologaphic data stoage mateial fo nealy two decades [6]. Its advantages ae the available lage cystal sizes, which allow fo inceased capacity and high angula selectivity, long stoage lifetime and high diffaction efficiencies. Ion doped lithium niobate shows a peak sensitivity fo witing hologams in the blue-geen

2 potion of the visible light spectum. Faily good diffaction efficiencies ae achievable also by eading ecoded hologams in the nea infaed egion. The behavio of the whole witing-eading pocess is well descibed by means of the equations deived fom Kukhtaev band tanspot model [7]. Fo witing volume hologams in a taditional LiNbO 3 :Fe cystal we used the expeimental setup shown in Fig.. Light fom an A + lase of wavelength 488 nm is split into two beams: the efeence one is a plane wave, the signal one caies out data in fom of digital wods of fou bits, obtained by inteposition of adequate spatial binay masks along its path. The incident light intensity is about 2 mw/cm 2 fo each witing beam and these beams ae simmetically incident with espect to the nomal to the cystal incidence face. The intefeence patten is ecoded into a.5% LiNbO 3 :Fe cystal (by Deltonics), size xx2 mm 3, unde a 48 seconds exposition. Mio Rotating mio READ-OUT WDM SIGNAL λ ' + λ '' Beam splitte WRITING BEAMS λ ' Spatial filte + binay mask λ ' Diffacted beams Agon lase λ w = 488 nm -cut LiNbO3:Fe Fig. Expeimental setup fo witing hologams at 488 nm and ead-out in the nea infaed. The two-colo technique has been widely studied in liteatue in the visible ange [8], with a witing wavelength of 488 nm and a ead-out wavelength of 633 nm in ode to obtain non volatile memoies. It is possible to choose the ead-out signal in the infaed egion by means of geometical consideations. In the k-space epesentation illustated in Fig.2(a), k w is the wave vecto associated to witing beams, k and k d ae espectively the ead-out beam and the diffacted beam wave vectos, K is the gating vecto. The wave vectos of the incident and diffacted beam must match the Bagg vectoial condition: k k d = K () Moeove, in ode to ead a hologam witten at wavelength λ w with a diffeent wavelength λ, the following elation must be satisfied: λ sin(ϑ ) = λ w sin(ϑ w ) (2)

3 whee θ and θ w ae espectively the intenal angles between ead-out and witing beams and the nomal to the cystal incidence face. k w k w k k K k d K K k k d k d a) b) Fig.2 K-space diagams: a) the two-colo technique in tansmission geomety; b) the ead-out stage of two supeimposed hologams with fixed incidence angle and vaiable wavelength. By using an infaed ead-out souce and satisfying elation (2), a nealy 4% diffaction efficiency at 55.5 nm has been expeimentally achieved (Fig.3) Nomalized diffaction efficiency Read-out wavelength (nm) Fig.3 Expeimental diffaction efficiency vs. ead-out wavelength. Multiple supeimposed hologam can be ecoded in the same volume thanks to cystal thickness. Because of Bagg conditions, each incident ead-out wave (chaacteized by the same incidence angle but diffeent wavelength, i.e. k and k in Fig.2(b)) is diffacted fom only one of the hologams (gating vecto K o K ): hence the collinea multi-wavelength beams ae demultiplexed into angulaly sepaated diections, identified by vectos k d and k d in Fig.2(b).

4 In optical netwok communications, WDM signals ae composed by multiple wavelength data channels; thus, by exploiting the capabilities of VH selectivity and two-colo technique, it is possible to ealize an optical memoy which associates a digital infomation wod to a specific wavelength. In this expeiment thee diffeent digital wods of fou bits have been ecoded, supeimposing the espective hologams by means of angle multiplexing technique in the lithium niobate cystal; aftewads such hologams have been ead out with a vaiable wavelength beam in the nea infaed egion. In ou expeiment the eadout souce is a tunable lase (Photonetics model 3642 CR ) with a tunability ange fom 49 to 56 nm and mw nominal peak powe. A sketch of the witing-eading pocess is pesented in Figs.4(a)- 4(b): RECORDING AT 488 nm READ-OUT WITH 55 nm WDM SIGNALS LiNbO3 cystal λ 2 = 55.2 nm λ 3 = nm λ = nm efeence beam signal beams angle multiplexing ecoding technique WDM signal λ + λ 2 + λ 3 a) b) Fig.4 a) Recoding by means of angula multiplexing technique; b) ead-out with 55 nm WDM signals. The hologams ae witten onto the cystal by means of angle multiplexing stoage, with incidence angle vaiations of.2 between the thee witing signal beams (4.8, 5 and 5.2 witing angles as shown in Fig. 4a). The ead-out geomety is clealy illustated in Fig. 4b: fom the Bagg elation () and equation (2), a WDM infaed signal incident at a fixed angle of 55.3 is diffacted into thee spatially sepaated components of wavelengths λ =538.2 nm, λ 2 =55.2 nm and λ 3 =562.9 nm. The angula sepaation between the diffacted beams is about.3. The infomation has been stoed into the cystal in fom of digital space-encoded wods (uppe Fig.5).

5 λ =538.2 nm λ 2 =55.2 nm λ 3 =562.9 nm Fig.5 Uppe: spatial epesentation of the thee ecoded digital wods. Lowe: infaed camea acquisition of the thee diffacted wods The digital wods have been ecoded othogonally to the signal-efeence beam plane, in ode to avoid losses in the econstucted images, the ead-out wavelength being diffeent fom the witing one [9]. Lowe Fig.5 shows the infaed camea acquisitions of the thee digital wods, each one econstucted by the optical beam at suitable wavelength. These fist esults may be suely impoved in two diections, by eithe inceasing the diffaction efficiency of the ecoded hologams o naowing down the wavelength selectivity between diffacted signals. The fome aim is achievable by optimizing the ecoding conditions: diffaction efficiencies in the infaed nea to 25% can be ealistically obtained. The latte is deeply dependent fom the ecoding geomety: by choosing the appopiate witing angle it s possible to ead out supeimposed hologams in eflection, eaching wavelength selectivity of factions of nanomete []. In conclusion we have shown the feasibility of an optical digital memoy in LiNbO 3 :Fe cystal, eadable by WDM communication signals : thee fou-bits digital wods have been ecoded by means of angula multiplexing technique and they have been econstucted at diffeent infaed wavelengths, the spatial sepaation between the diffacted beams being suitable fo subsequent tanspot of the digital data in optical communication channels. Refeences. D. Psaltis, D. Bady, K. Wagne, Adaptive optical netwoks using photoefactive cystals, Appl. Opt. 27 p. 752 (988) 2. X. An, D. Psaltis, Expeimental chaacteization of an angle-multiplexed hologaphic memoy, Opt.Lett. 2, p. 93 (995) 3. Rakuljic, V. Leyva, A. Yaiv, Optical data stoage by using othogonal wavelength-multiplexed volume hologams", Opt. Lett. 7, p. 47 (992) 4. Patovi, J. Milled, E.M. Gamie, M. Ziai, W.H. Steie, S.B. Tivedi, M.B. Klein, Photoefactivity at.5 µm in CdTe:V, Appl. Phys. Lett. 57, p. 846 (99) 5. G. A. Rakuljic, V. Leyva, Volume hologaphic naow-band optical filte, Opt. Lett. 8, p. 459 (993)

6 6. R. R. Shah, D. M. Kim, T. A. Rabson and F. K. Kittel, Chaacteization of ion-doped lithium niobate fo hologaphic stoage applications, J. Appl. Phys. 47, p. 542 (976) 7. P.Yeh, Intoduction to photoefactive nonlinea optics, (John Wiley & Sons, Inc. 993 ) 8. D. Psaltis, F. Mok and H. Li, Nonvolatile stoage in photoefac. 2 (994) 9. E. Chuang, D. Psaltis, Stoage of hologams with use of a dual-wavelength method, Appl. Optics vol 36, p (997). S. Bee, K. Buse, Wavelength demultiplexing with volume phase hologams in photoefactive lithium niobate, Appl. Phys. B 66, p. 339 (998)

School of Electrical and Computer Engineering, Cornell University. ECE 303: Electromagnetic Fields and Waves. Fall 2007

School of Electrical and Computer Engineering, Cornell University. ECE 303: Electromagnetic Fields and Waves. Fall 2007 School of Electical and Compute Engineeing, Conell Univesity ECE 303: Electomagnetic Fields and Waves Fall 007 Homewok 1 Due on Nov. 8, 007 by 5:00 PM Reading Assignments: i) Review the lectue notes. ii)