Holographic RAM for optical fiber communications

Transcription

1 Header for SPIE use Holographic RAM for optical fiber communications Pierpaolo Boffi, Maria Chiara Ubaldi, Davide Piccinin, Claudio Frascolla and Mario Martinelli * CoreCom, Via Amp re 3, 2131-Milano, Italy * also with Dept. of Electronics and Information, Politecnico di Milano, P.za Leonardo Da Vinci 32, Milano ABSTRACT Preliminary experimentation of angle-multiplexed high-efficiency volume holograms at 155 nm is presented. It exploits two-color technique by recording digital bytes at 488 nm in iron-doped lithium niobate and retrieving them in near infrared. The stored base constitutes a first holographic memory for optical fiber communication systems. Keywords: Volume holographic memory, two-color technique, optical communication devices, infrared base 1. INTRODUCTION Volume holography stands out as having many potentialities in order to implement optical memories providing very large storage density and high speed access 1.. A 2-D page is recorded in a photorefractive medium (fig. 1a-b) through the interference of two coherent optical beams (the beam carrying out the information to be stored and the reference one). When the hologram is recorded inside a thick medium, Bragg diffraction causes the reconstruction strength to be extremely sensitive to the incidence angle of the readout beam. Many holograms, each of them constituted by a different page, can be multiplexed within the same material volume by changing the angle of the beams during recording (angle multiplexing) 2.. The superimposition of 1, holograms in a 1cm 3 volume has been demonstrated 3.. In addition to high storage density, holography exploits one of the most powerful advantages of free-space optics: the massive parallelism in signal processing operations. Conventional storage media such as magnetic hard disks and CD ROMÕs operate in a serial way, only 1 bit at a time. Instead parallel readout, inherent in holographic memories, allows reading a whole page up to 1 million bits of information 4. at a time. Some authors 5. call compact and inexpensive modules executing wide write-read functions Holographic Random Access Memory (HRAM). Furthermore, the multiplexed volume holograms constitute a base comparable in real time with an incoming page under test (fig. 1c). Optical peaks of correlation between the input and stored pages are produced in parallel providing recognition operation necessary in any hypothesis of optical signal processing 6.. On the other hand, in the last years the tremendous deployment of optics in communications has taken place. Optical fiber represents the best transmission support able to transport very high bit-rate flows. Optical communication systems operate at the wavelengths where fiber shows minimum attenuation, in particular 155nm is nowadays a standard transmission wavelength. In order to develop network nodes allowing fast processing of optical communication signals, optical devices such as addressable memories, look-up tables and digital word recognizers are needed. Volume holography could play an important role in the implementation of such devices. Unfortunately the substantial lack of photorefractive materials sensitive in near infrared region 7. has restrained the development of holography in communication devices. Correspondence: Pierpaolo Boffi boffi@corecom.it Tel Fax

2 In this paper we present preliminary experimental results of a holographic memory operating at 155nm based on two-color technique 8.. We exploit the opportunity of recording volume holograms in classical photorefractive materials by means of light at maximum sensitivity wavelengths (blue-green spectral range) and reading such holograms at different wavelengths. Stored holograms contain digital 8-bit words: each digital word recorded at 488 nm in a LiNbO 3 :Fe crystal is reconstructed by a 155nm optical beam at just one suitable readout direction. a) b) reference beam readout beam retrieved input thick medium c) correlation peaks under test Fig.1a) Recording of 2-D digital information page into a thick photorefractive material, b) retrieval of the stored digital information page and c) implementation of correlation operation between the stored base and a test page. 2. THEORETICAL PRINCIPLES As already pointed out before, volume holography offers the chance of recording multiple holograms inside the same volume of photorefractive material: in fact the additional thickness size of the storage medium is responsible for enhanced angular selectivity. Therefore only a small incidence angular range is possible for the read-out beam direction, in order to originate a diffracted signal. The well-known square-sinc law of diffraction efficiency 9. defines a relationship between η and the angular mismatch θ = θ-θ B (where θ B is the incidence angle which satisfies Bragg law) A meaningful parameter related to angular selectivity can be derived η sinc 2 ( f ( ϑ)) (1) ϑ B λ = (2) L sin( ϑ ) B

3 where θ B is the angular deviation of first zeros in diffraction efficiency curve with respect to the Bragg angle θ B, λ w the operating wavelength, L the grating effective length. Thus a rotation step during recording of at least θ B is needed to superimpose multiple holograms with negligible cross-talk (fig.2): in practice an angular separation of 4-5 times the angle to the first null of the selectivity curve is chosen normalized diffraction efficiency [%] angular mismatch [ ] Fig.2 Angular selectivity curves of two angle-multiplexed holograms separated by θ B. Besides, two-color technique allows to retrieve angle-multiplexed holograms at a wavelength different from the writing one by simply varying the incidence beam read-out angle, according to the relation λw λr = sin( ϑ + ϑ ) sin( φ + φ ) (3) where θ 1,θ 2 are respectively the incidence angles for the object and reference beam during recording at λ w (with respect to the normal to the crystal face), while φ 1, φ 2 define the directions of the read-out and diffracted beams at the new read-out wavelength λ r. In order to read out angle-multiplexed holograms at λ r, the read-out angle φ 1 must vary always satisfying Bragg condition. 3. EXPERIMENTATION The memory is stored in a -cut LiNbO 3 :Fe crystal (.15% mol doped) by means of a standard set-up for transmission recording/retrieval. An Argon laser is used to generate the coherent writing optical beams at 488 nm. The angle in air between writing beams, θ 1 +θ 2, is about 3 so that good angle selectivity in holograms retrieving at 155 nm is achieved. In order to obtain good diffraction efficiency in the near infrared, high refractive index variation has to be induced during photorefractive recording, very long recording time being consequently necessary. Figure 3b shows 7% diffraction efficiency experimented in readout at 155 nm after a 3 min recording time at 488 nm: the grating dynamic curve is reported in fig. 3a.

4 Diffraction efficiency [%] Exposure time [min] Diffraction efficiency [%] Angular mismatch [ ] a) b) Fig.3 Diffraction efficiency vs. a) exposure time during recording at 488 nm and b)angular mismatch at 155 nm. In our first experimentation four intensity-modulated (IM) digital bytes have been recorded in the holographic crystal (fig. 4). The crystal thickness (1 cm) allows to superimpose the four different holograms into the same volume, with negligible mutual interference. In order to obtain angle-multiplexing, during recording the 488 nm reference beam is rotated horizontally with respect to the crystal (.6 in rotation step). A readout beam at 155 nm interrogates the recorded base according to two-color technique (the new incident angle φ 1 is about 55 ). The stored byte corresponding to the direction which satisfies Bragg law at readout wavelength is retrieved. The stored digital bytes are space-encoded along one direction only, perpendicular to writing beams incidence plane, granting complete reconstruction of the information 8. Fig.4 Left: recording at 488 nm of angle-multiplexed bytes. Right: readout at 155 nm. The experimental results related to the implementation of such a holographic memory readable at optical fiber communication wavelengths are presented in figure 5. The picture reports the retrieved digital bytes, acquired by means of an infrared camera and reconstructed each one by the related readout direction (angular separation of the readout 155 nm beams is about.12 ).

5 Fig.5 Infrared camera acquisition of the digital bytes diffracted by.12 angle-spaced 155 nm beams. 4. CONCLUSIONS In conclusion, a highly efficient holographic memory operating at optical communication wavelengths has been preliminarily experimented. It exploits two-color technique by recording digital bytes at 488 nm and retrieving the same information at 155 nm. A compact architecture incorporating a linear array of laser diodes, liquid crystal matrix and a reduced dimension crystal (with a very large number of holograms stored inside) can implement read-write functions in real time in order to develop a realistic and low cost HRAM. Furthermore the stored volume holograms can be employed reversibly as a base for optical byte recognition, giving rise to output parallel correlation peaks corresponding to the incoming information to be checked. REFERENCES 1. D. Psaltis and F. Mok, ÒHolographic memoriesó, Sci. Am. 273, pp.7-76, X. An and D. Psaltis, ÒExperimental characterization of an angle-multiplexed holographic memoryó, Opt. Lett., 2, pp , G.W. Burr, F.H. Mok and D. Psaltis, ÒStorage of 1, holograms in LiNbO 3 :FeÓ, in Digest on Conference on Lasers and Electro-Optics 1994, Optical Society of America, Washington, D.C., paper CMB7, R.M. Shelby, J.A. Hoffnagle, G.W. Burr, C.M. Jefferson, M.P. Bernal et al., ÒPixel-matched holographic storage with megabit pagesó, Opt. Lett. 22, pp , E. Chuang, W. Liu, J. Drolet and D. Psaltis, ÒHolographic random access memory (HRAM), Proc. of the IEEE, 87, pp , G.W. Burr, S. Kobras, H. Hanssen and H. Coufal, ÒContent-addressable storage by use of volume hologramsó, Appl. Optics, 38, pp , A. Partovi, J. Millerd, E.M. Garmire, M. Ziari, W.H. Steier, S.B. Trivedi, M.B. Klein, ÒPhotorefractivity at 1.5 µm in CdTe:VÓ, Appl. Phys. Lett. 57, pp , E. Chuang, D. Psaltis, ÒStorage of 1 holograms with use of a dual-wavelength methodó, Appl. Opt. 36, pp , P. Yeh, Introduction to photorefractive nonlinear optics, John Wiley & Sons, 1993.