Advances in healthcare imaging products and multimedia entertainment systems are demanding higher media capacity (TB/disk) and faster data transfer speeds (lGb/s) from future removable disk based data storage systems. One promising direction to satisfy these requirements with low cost systems is the use of optical volumetric multi-layer disk media. At Call/Recall we have demonstrated the scalability of two-photon recordable photochromic doped polymeric Write Once Read Many (WORM) disk media to more than 1 00 layers with negligible interlayer crosstalk and excellent stability of the written bits"2'3'4'5'6. The media employed in our WORM system, consist ofphotochromic organic molecules designed and synthesized so that they change their structure, upon excitation at the absorption band of the molecule5'7'8'9'. A spot is written in the volume of the medium only at points of temporal and spatial intersection of two photons with sufficient energy to record by altering the structure of the photochromic molecule. A high power short pulse laser beam is tightly focused for recording. Recording occurs only within a small volume around the focus of the laser beam due to twophoton absorption. The recording response ofthe material follows the square ofthe optical system point spread ftinction (PSF) resulting in a recorded bit size that is 30% less than the Rayleigh criterion PSF. The recorded bits are read by fluorescence when excited by suitable optical radiation absorbed within the written spot volume. The doped polymer media is low cost, flexibly shaped and molded, and its properties may be customized (by changing the dopant molecules) to match evolving application and technology requirements. We had previously reported bit dimensions of 0.5 x0.5x4.5jim 2 withNA 0.75 and 532nm wavelength exhibiting raw bit-error rates (BER) of iO'. Here we show a ftjrther decrease in the bit size to 0.4 x 0.4 x 2jtm with a higher NA1 .4 objective lens at 532nm wavelength, resulting in ultra high density volumetric disk recording using up to 100 layers and providing a potential for 400bits4im2 "effective area! density". This paper discusses the theory and approach as well as experimental results we used and obtained to demonstrate the feasibility of high NA recording of volumetric multilayer disks by two photon absorption.

Two-photon absorption (TPA) is a promising technique for high density 3D-addressing for writing and read-out of data, provided that suitable two-photon sensitive materials facilitating fast recording and read-out will be developed. Free-base porphyrins and other metal-free tetrapyrroles, such as phthalocyanies and naphthalocyanines possess a unique fast intrinsic photo-tautomerization mechanism, which consists in switching the position of a pair of protons in the core of the molecule. In the past photo-tautomerization was used for holographic storage, but can be also applied for bit-oriented volumetric information storage using laser-excited fluorescence for readout. However, the utility of the photo-tautomerization for two-photon storage was severely restricted so far by the fact that all known tetrapyrroles have rather low TPA cross section, with values not exceeding 1 - 10 GM (1GM = 10-50 cm4 s photon-1). Recently we have discovered a new class of porphyrins, where TPA cross section is dramatically amplified by certain chemical modification of the chemical structure, and that some of the new porphyrins have the ability of photo-tautomerization by simultaneous absorption of two photons. In this paper we discuss the photophysics and nonlinear optics of the new porphyrins that can lead to fast volumetric re-writable optical storage. We present a quantitative comparison of the new compounds with previously known TPA chromophores and introduce a merit figure, which takes into account both TPA cross-section as well as the efficiency of light-induced changes. We show the combination of high cross sections of two-photon absorption, up to 1000 GM in near-IR range of wavelength, with the fast photo-tautomerization, offers, for the first time, a sufficiently high merit figure needed for implementation of high-density, high speed volumetric two-photon re-writable optical storage.

The pattern matching for fingerprints requires a large amount of data and computation time. Practical fingerprint identification systems require minimal errors and ultrafast processing time to perform real time verification and identification. By utilizing the two-dimensional processing capability, ultrafast processing speed and noninterfering communication of optical processing techniques, fingerprint identification systems can be implemented in real time. Among the various pattern matching systems, the joint transform correlator (JTC) has been found to be inherently suitable for real time matching applications. Among the various JTCs, the fringeadjusted JTC has been found to yield significantly better correlation output compared to alternate JTCs. In this paper, we review the latest trends and advancements in fingerprint identification system based on the fringeadjusted JTC. Since all pattern matching systems suffer from high sensitivity to distortions, the synthetic discriminant function concept has been incorporated in fringe-adjusted JTC to ensure distortion-invariant fingerprint identification. On the other hand a novel polarization-enhanced fingerprint verification system is described where a polarized coherent light beam is used to record spatially dependent response of the scattering medium of the fingerprint to provide detailed surface information, which is not accessible to mere intensity measurement. It is shown that polarization-enhanced database improves the accuracy of the fingerprint identification or verification system significantly. Keywords: Fringe-adjust joint transform correlation, finger print identification, polarization, synthetic discriminant function

Spectral-spatial holographic crystals have the unique ability to resolve fine spectral features (down to kilohertz) in an optical waveform over a broad bandwidth (over 10 gigahertz). This ability allows these crystals to record the spectral interference between spread spectrum waveforms that are temporally separated by up to several microseconds. Such crystals can be used for performing radar range-Doppler processing with fine temporal resolution. An added feature of these crystals is the long upper state lifetime of the absorbing rare earth ions, which allows the coherent integration of multiple recorded spectra, yielding integration gain and significant processing gain enhancement for selected code sets, as well as high resolution Doppler processing. Parallel processing of over 10,000 beams could be achieved with a crystal the size of a sugar cube. Spectral-spatial holographic processing and coherent integration of up to 2.5 Gigabit per second coded waveforms and of lengths up to 2047 bits has previously been reported. In this paper, we present the first demonstration of Doppler processing with these crystals. Doppler resolution down to a few hundred Hz for broadband radar signals can be achieved. The processing can be performed directly on signals modulated onto IF carriers (up to several gigahertz) without having to mix the signals down to baseband and without having to employ broadband analog to digital conversion.

Results have been presented on isolating rare earth atoms in small numbers in semiconductor nanoparticles so as to use their organized arrays as hardware for quantum computing. We have tailored atomic states of rare earths, fabricated nanoparticles where these atomic systems are incorporated in small numbers and have patterned arrays of nano-holes on semi-conducting and polymer surfaces to encapsulate these rare earth doped nanoparticles. Results are presented on fabrication, microscopy and spectroscopy of these structures.

For practical pattern recognition and tracking systems, it is often useful to have a high-speed random access memory (RAM), which complements a holographic correlator. Recently, we have demonstrated a super-parallel holographic optical correlator, which uniquely identifies N images from a database using only 2 number of detector elements. In this paper, we show how this correlator architecture, operated in reverse, may be used to realize a super-parallel holographic random access memory. We present preliminary results establishing the feasibility of the super-parallel holographic random access memory, and show that essentially the same set of hardware can be operated either as the super-parallel holographic optical correlator or as a super-parallel holographic random access memory, with a minor reorientation of some of the elements in real time. This hybrid device thus eliminates the need for a separate random access memory for a holographic correlator based target recognition and tracking system.

We have proposed an all-optical authentic memory with the two-wave encryption method. In the recording process, the image data are encrypted to a white noise by the random phase masks added on the input beam with the image data and the reference beam. Only reading beam with the phase-conjugated distribution of the reference beam can decrypt the encrypted data. If the encrypted data are read out with an incorrect phase distribution, the output data are transformed into a white noise. Moreover, during read out, reconstructions of the encrypted data interfere destructively resulting in zero intensity. Therefore our memory has a merit that we can detect unlawful accesses easily by measuring the output beam intensity. In our encryption method, the random phase mask on the input plane plays important roles in transforming the input image into a white noise and prohibiting to decrypt a white noise to the input image by the blind deconvolution method. Without this mask, when unauthorized users observe the output beam by using CCD in the readout with the plane wave, the completely same intensity distribution as that of Fourier transform of the input image is obtained. Therefore the encrypted image will be decrypted easily by using the blind deconvolution method. However in using this mask, even if unauthorized users observe the output beam using the same method, the encrypted image cannot be decrypted because the observed intensity distribution is dispersed at random by this mask. Thus it can be said the robustness is increased by this mask. In this report, we compare two correlation coefficients, which represents the degree of a white noise of the output image, between the output image and the input image in using this mask or not. We show that the robustness of this encryption method is increased as the correlation coefficient is improved from 0.3 to 0.1 by using this mask.

We proposed a compact variable all-optical buffer using slow-light in semiconductor nanostructures. We discuss the general design principle via dispersion engineering. The buffering effect is achieved by slowing down the optical signal using an external control light source to vary the dispersion characteristic of the medium via electromagnetically induced transparency effect. We demonstrate that the semiconductor quantum dot structures can be used as a slow-light medium. In such structure, the total buffering time is variable and controlled by an external pump laser. We present a theoretical investigation of the criteria for achieving slow light in semiconductor quantum dots. New pump scheme is proposed to overcome the sample nonuniformity. Finally, optical signal propagation through the semiconductor optical buffer is presented to demonstrate the feasibility for practical applications.

Lately, active research has been conducted in slowing down light pulses to the order of tens of meters per second or below. An interesting application would be an optical buffer for all optical routers, where solid state devices operating under room temperature are desired. In this paper, we investigate the possibility of such a device with a Ce:BaTiO3 crystal. Group velocity as slow as 2 mm/s is obtained in the two-wave coupling experiment. By modulating the angular-multiplexed pump beams in the experiment, we also demonstrate the modulation of the output waveform, which could be used to address the problem of pulse broadening that so far limits the application of room temperature slow light.

We proposed the integrated optical pick-up with a catadioptric system which has a super resolution effect and with ferrofluid cooling structure. All of optical elements such as an objective lens, a laser diode and a photodetector are integrated into the moving part of the actuator to miniaturize the pick-up. Laser beam is double reflected between first reflecting region on top of the objective lens and second reflecting region on a reflecting mirror placed under the objective lens to miniaturize the optical system. The moving part having the laser diode and the photodetector needs high cooling performance to realize the optical system. We developed the cooling structure with ferrofluid held between a magnet and a coil of the actuator. Ferrofluid works as a cooling path to conduct the heat in the moving part towards external parts. We achieved the results as described below. Temperature of the laser diode is approximately equivalent to conventional pick-up against the heat of about 200mW generated in the moving part. Thermal resistance of 120 degree/Watt is available for practical use. The cooling structure leads the results of optical characteristics. As a super resolution effect, spot size of the integrated optical pick-up with wavelength of 660nm and a numerical aperture (NA) of 0.55 is equivalent to spot size of conventional pick-ups with wavelength of 660nm and a NA of 0.65. Focal and tracking error signals for servo control are available for practical use. The cooling performance is enough for realizing the integrated optical pick-up.

In this report, we demonstrate a noise reduction recording in a photorefractive memory with a dynamic refreshing technique by mutually pumped phase conjugate mirrors (MPPCMs). The dynamic refreshing, that means the continuous rewriting of an original hologram with the optical feedback in synchronism with the readout, largely contributes to the maintenance of the recorded hologram by all-optical process. Two MPPCMs are used in the dynamic refreshing technique; one is built for the optical feedback, the other is used as the storage. The nondestructive readout without any fixing technique can be achieved by the continuous rewriting with the beam resonance between the phase conjugate mirrors. Our technique additionally offers the noise reduction of an input image in the recording process without any external image processing systems. The noise reduction is caused by the characteristics of the MPPCM that the incident intensity ratio required for the MPPCM generation is determined by the coupling strength of the crystal. Moreover, the application of the MPPCM to the photorefractive memory brings great advantages of the high quality image retrieving and the simple optical configuration because the illumination of two incoherent beams into a PR crystal can generate the mutual phase conjugate beams in the MPPCM. We show the noise reduction effect is controllable by the appropriate adjustment of the incident beam intensities. We also experiment on the photorefractive memory with this technique using barium titanate crystals and show the high-quality noise-reduced image can be read out over 10 times longer than the conventional readout technique.

Two-color holography is an effective solution to the volatile readout problem in volume holographic data storage based on photorefractive materials. Popular materials for two-color holography are reduced doped and nondoped near-stoichiometric lithium niobate crystals. However, the lifetime at room temperature is from several weeks to several months depending on the reduction state of the material. Moreover, reductive treatment will degrade the nonvolatility of two-color holograms. The important issue for two-color holography is how to increase the lifetime. In this contribution, lifetimes of two-color nonvolatile holograms recorded in as-grown near-stoichiometric lithium niobate and tantalate crystals were compared by extrapolating the high-temperature data. The dark-decay time constants obey an Arrhenius dependence on absolute temperature and yield activity energy of 1.06 eV around in all measured crystals. Lifetimes of holograms in nondoped and slightly doped crystals depend on the proton concentration. Lifetimes of hologram in lithium tantalate are one order of magnitude longer than those in lithium niobate at the same proton concentration. The lifetime of two-color holograms in lithium tantalite is longer than 20 years.

We propose a fault-tolerant holographic memory (FTHM) composed of a pair of photorefractive crystals. This memory offers not only non-destructive readout but also data restoring function by only pure optical operations without any electrical controls. In writing process, the same holographic data are simultaneously recorded as index gratings to the crystals laid out in series. In reading process, a reading beam is diffracted by the index gratings in each crystal. Here, some of the diffraction beams are detected as an output beam, and the others are used as a feedback beam. The hologram in each crystal is continuously refreshed by the feedback beam from the other crystal since the feedback beam has the same information as the original holographic data. When the data refreshing effect by the feedback beams sufficiently exceeds the erasure effect by the exposure of the reading beam, the stored data are always maintained. Furthermore, even if a certain fault such as vibration and stray beam incidence happens, the lost data in one crystal are all-optically restored as long as the corresponding holographic data remain in the other crystal. The experiment with a two-dimensional image is carried out for the purpose of checking the data restoring function in FTHM. The two-dimensional image divided in quarters is recorded as into a pair of 45°-cut BaTiO3 crystals, and the original holographic data is successfully restored by the refreshing effect in the case that a quarter of the image in the one crystal is partially lost.

Newer silicon foundry processes make possible high-resolution backplanes (i.e. larger arrays with more line pairs per millimeter). Higher resolution is a benefit of the small geometry processes being developed for the electronics industry. Unfortunately, the trend is to shrink the circuits and decrease the operating voltage of the chip. For liquid crystal on silicon (LCoS) devices, the loss in voltage has a negative impact on performance. Higher voltage provides the excitation to achieve good response time with sufficient modulation depth from liquid crystal electro-optic modulators. This paper discusses the development of large array devices using the smaller geometry processes and some of the techniques used to retain good performance from the liquid crystal modulators.

The volume holographic data storage has been attractive because of its large capacity due to the three-dimensional recording and the fast data transfer rate due to the two-dimensional retrieving. Among various holographic storage media, photorefractive crystals are expected because of rewritability. To enhance the storage density, several multiplexing techniques have already been suggested. Especially, the speckle multiplexing technique allows the easy implementation of the hologram multiplexing by the simple optical setup, in which only a diffuser such as the ground glass or the multimode optical fiber is embedded to generate the deterministic speckle-encoded reference beam. In this paper, we propose a hologram multiplexing method with the reference beam speckled by the photorefractive beam-fanning effect. Compared with the conventional speckle multiplexing method, this method realizes the facile implementation of the speckle multiplexing with the simple and compact optical setup because the external apparatus for generating the speckled reference beam is not required. A bulk photorefractive crystal takes the role of generating various speckle fields as well as storing holograms. All-optically controlled hologram multiplexing and retrieving would also be realized because the beam-fanning effect can be controlled only by light. We demonstrate the hologram multiplexing and the selective retrieving with the speckled reference beam generated by the beam-fanning effect in a photorefractive BaTiO3 crystal. The speckle field of the reference beam to record multiple holograms is varied by changing the boundary condition of the incident beam such as the incident position and the spatial phase distribution. We can successfully reconstruct the desired image from four multiplexed holograms with the reference beam speckled by the photorefractive beam-fanning effect in both procedures.

The pattern matching for fingerprints requires a large amount of data and computation time. Practical fingerprint identification systems require minimal errors and ultrafast processing time to perform real time verification and identification. By utilizing the two-dimensional processing capability, ultrafast processing speed and noninterfering communication of optical processing techniques, fingerprint identification systems can be implemented in real time. Among the various pattern matching systems, the joint transform correlator (JTC) has been found to be inherently suitable for real time matching applications. Among the various JTCs, the fringeadjusted JTC has been found to yield significantly better correlation output compared to alternate JTCs. In this paper, we review the latest trends and advancements in fingerprint identification system based on the fringeadjusted JTC. Since all pattern matching systems suffer from high sensitivity to distortions, the synthetic discriminant function concept has been incorporated in fringe-adjusted JTC to ensure distortion-invariant fingerprint identification. On the other hand a novel polarization-enhanced fingerprint verification system is described where a polarized coherent light beam is used to record spatially dependent response of the scattering medium of the fingerprint to provide detailed surface information, which is not accessible to mere intensity measurement. It is shown that polarization-enhanced database improves the accuracy of the fingerprint identification or verification system significantly. Keywords: Fringe-adjust joint transform correlation, finger print identification, polarization, synthetic discriminant function

Phase masks are needed in Fourier-transform holographic data storage systems (HDS) to reduce the range of light intensities found in the Fourier plane. The range of light intensities must match the dynamic range of the holographic storage medium and of the full HDS system. Descriptions, mathematical models, and tests of a variety of phase mask types have been reported in the literature: pixelated phase masks, non-pixelated phase masks, and axicons. Lacking, however, has been a systematic way of comparing the relative merits of phase mask types in order to make sound choices. To address this problem, performance criteria are proposed for both the Fourier plane and for the output image plane (e.g. the margin by which 1’s can be distinguished from 0’s). The criteria are useful both for comparisons and for design optimization. A new numerical model has been developed enabling quantitative comparisons to be made between the predicted performance of the various phase mask types. The model reported here enables more extensive investigations than could be carried out with previously reported models, including investigation of systems in which multiple bits of data are encoded by each pixel using light intensity modulation. The viability of using non-pixelated phase masks integrated with spatial light modulators is also examined. The use of non-pixelated (continuous random) phase masks instead of the more common pixelated phase masks would eliminate the need for costly precision lateral alignment, and integration eliminates the need for precise positioning in an image plane. These advantages would enable smaller, cheaper, high performance HDS optical systems.

In this paper we summarize a new category of all optical companding nonlinear correlators developed by our group in the last decade. All optical companding nonlinear correlators consist oftwo families: The first is based on energy transfer between the joint spectra of reference and signal images. The second family is based on incoherent erasure of a grating formed by coupled beams . Allof these correlators have similar features. Therefore, we take one representative case of study, namely, the photorefractive two-beam coupling correlator. We perform theoretical analysis, computer simulations and experimental demonstrations to predict the location of the best operating point of the two-beam coupling joint transform correlator. From this study we determine the best operational condition for high speed and resolution, as well as for optimal trade-off between correlation peak intensity, efficiency and noise performance. We also study the performance of compansive correlators in analogy with the limiting square law receiver. It was found that the optimal performing point corresponds to noise variance that is proportional to the transition from compression to expansion.