Two-color holography in reduced near-stoichiometric lithium niobate

We explored a number of factors affecting the properties relevant to holographic optical data storage by using a two-color recording scheme in reduced, near-stoichiometric lithium niobate. Two-color, or photon-gated, recording is achieved by use of 852-nm information-carrying beams and 488-nm gating light. Readout at 852 nm is nondestructive, with a gating ratio of ~10(4). Recording sensitivity, gating ratio, dynamic range, and dark decay were measured for crystals of differing stoichiometry, degree of reduction, wavelength of the gating light, temperature, and optical power density. The two-color sensitivity per incident photon is still somewhat less than that of the one-color process at 488 nm for ~1 W/cm(2) of gating light but is essentially the same in terms of absorbed photons. Two-color recording is an attractive way of achieving nondestructive readout in a read-write material, and it allows selective optical erasure.     

Digital holographic data storage in a high-performance photorefractive polymer composite

Binary information in the form of a 256 x 256 pixel array has been stored holographically within a polymeric photorefractive composite of 130-mum thickness. Devices used consisted of 55-wt. % organic chromophore 1-(2?-ethylhexyloxy)-2, 5-dimethyl-4-(4?nitrophenylazo) benzene and up to 11.2-wt. % 2, 4, 7-trinitro-9-fluorenone (TNF) dispersed within a poly(N-vinyl carbazole) matrix. In a degenerate four-wave mixing arrangement, the refractive-index modulation and speed were 10(-3) and 200 ms, respectively. The high TNF concentration leads to short digital hologram recording times of 2 s without detriment in optical quality. Although not yet optimized, a figure of merit, M#, for holographic storage in this composite has been measured to be 0.017, and this is estimated to reach 0.18 in a revised experimental geometry.     

Phase-conjugate resonant holographic interferometry applied to NH concentration measurements in a two-dimensional diffusion flame

Resonant holographic interferometry is a diagnostic technique based on the dispersion of light having a frequency close to that of an electronic transition of a molecule. We propose a novel single-laser, two-color setup for the recording of resonant holograms and apply it to two-dimensional (2D) species concentration measurements in a combustion environment. The generation of the second color is achieved by optical phase conjugation from stimulated Brillouin scattering in a cell. The frequency shift of ~8.5 GHz introduced by the phase conjugation matches approximately the linewidth of many molecular transitions at typical flame temperatures and can be implemented to produce holograms of good contrast and diffraction efficiency. Phase-conjugate resonant holographic interferometry is demonstrated in a 2D NH(3) -O(2) flame, yielding interferograms containing information on the NH radical concentration distribution in the flame. Experimental results are quantified by application of a numerical computation of the complex refractive index.     

Holographic zone plates for f x theta and collimating lenses

A holographic zone plate generated by subtracting the spherical wave phase is proposed, that is, a hologram generated by using two divergent or convergent spherical waves. This holographic zone plate can correct the increase in focal power in a radial direction in contrast with the interferometric zone plate. Use of this holographic zone plate for f x O and collimating lenses is analyzed. For an f x O lens, it is shown that an optimum combination of recording spherical waves exists that satisfies both scan linearity and image field flatness. For a collimating lens for a laser diode, it is shown that aberration can be corrected for different playback wavelengths from that used during recording.     

Polarization holographic high-density optical data storage in bacteriorhodopsin film

Optical films containing the genetic variant bacteriorhodopsin BR-D96N were experimentally studied in view of their properties as media for holographic storage. Different polarization recording schemes were tested and compared. The influence of the polarization states of the recording and readout waves on the retrieved diffractive image's intensity and its signal-to-noise ratio were analyzed. The experimental results showed that, compared with the other tested polarization relations during holographic recording, the discrimination between the polarization states of diffracted and scattered light is optimized with orthogonal circular polarization of the recording beams, and thus a high signal-to-noise ratio and a high diffraction efficiency are obtained. Using a He-Ne laser (633 nm, 3 mW) for recording and readout, a spatial light modulator as a data input element, and a 2D-CCD sensor for data capture in a Fourier transform holographic setup, a storage density of 2 x 10(8) bits/cm2 was obtained on a 60 x 42 microm2 area in the BR-D96N film. The readout of encoded binary data was possible with a zero-error rate at the tested storage density.     

Absorption characteristic and nonvolatile holographic recording in LiNbO3:Cr:Cu crystals

The absorption characteristic of lithium niobate crystals doped with chromium and copper (Cr and Cu) is investigated. We find that there are two apparent absorption bands for LiNbO3:Cr:Cu crystal doped with 0.14 wt.% Cr2O3 and 0.011 wt.% CuO; one is around 480 nm, and the other is around 660 nm. With a decrease in the doping composition of Cr and an increase in the doping composition of Cu, no apparent absorption band in the shorter wavelength range exists. The higher the doping level of Cr, the larger the absorbance around 660 nm. Although a 633 nm red light is located in the absorption band around 660 nm, the absorption at 633 nm does not help the photorefractive process; i.e., unlike other doubly doped crystals, for example, LiNbO3:Fe:Mn crystal, a nonvolatile holographic recording can be realized by a 633 nm red light as the recording light and a 390 nm UV light as the sensitizing light. For LiNbO3:Cr:Cu crystals, by changing the recording light from a 633 nm red light to a 514 nm green light, sensitizing with a 390 nm UV light and a 488 nm blue light, respectively, a nonvolatile holographic recording can be realized. Doping the appropriate Cr (for example, NCr = 2.795 x 10(25) m(-3) and NCr/NCu = 1) benefits the improvement of holographic recording properties.     

Write-once recording for multilayered optical waveguide-type holographic cards

We propose a write-once recording technique for multilayered optical waveguide-type holographic cards. The card medium has a construction created by adding a recording layer and a holographic grating layer to the multilayered optical waveguide composed of core and cladding layers. Individual data for each medium were recorded as an arrangement of optically transparent holes formed in the recording layer. Holograms common to all media were designed in the holographic grating layer so that diffracted lights from the holograms could pass through the holes and focus on an image sensor. We succeeded in write-once recording with a memory capacity potential of more than 128 bits.     

Storage density of shift-multiplexed holographic memory

The storage density of shift-multiplexed holographic memory is calculated and compared with experimentally achieved densities by use of photorefractive and write-once materials. We consider holographic selectivity as well as the recording material's dynamic range (M/#) and required diffraction efficiencies in formulating the calculations of storage densities, thereby taking into account all major factors limiting the raw storage density achievable with shift-multiplexed holographic storage systems. We show that the M/# is the key factor in limiting storage densities rather than the recording material's thickness for organic materials in which the scatter is relatively high. A storage density of 100 bits/mum(2) is experimentally demonstrated by use of a 1-mm-thick LiNbO(3) crystal as the recording medium.     

Three-dimensional shift selectivity in reflection-type holographic disk memory with speckle shift recording

Three-dimensional shift selectivity of a reflection-type hologram with speckle shift recording is investigated experimentally and numerically. We build an experimental setup consisting of lenses with numerical apertures of 0.28 and an iron-doped LiNbO(3) with a thickness of 0.5 mm. The experimental results show that three-dimensional selectivity has a size of 0.97 microm x 0.97 microm x 8.8 microm in diffraction efficiency. We also develop a volume holographic memory simulator to evaluate the experimental results. The simulator can quantitatively evaluate bit error rate, signal-to-noise ratio, and diffraction efficiency. Numerical results are in good agreement with the experimental results. The experimental and numerical results indicate that three-dimensional shift multiplexing can increase the storage capacity.     

Application of phase-conjugate mirrors during recording of volume holographic memories

We propose, demonstrate, and analyze the application of phase-conjugate mirrors during the recording of volume holographic memories. Our results show that using phase-conjugate mirrors during recording improves the uniformity of the holographic gratings and increases the recording speed. Theoretical simulations agree with experimental results.     

Distortion compensation and elimination in holographic reconstruction

We discuss the quantitative location of objects from holographic images when the reconstruction wavelength differs from the recording wavelength. The holographic image equations are interpreted in a way that clarifies the meaning of stereo pairs of holographic images and indicates how backprojection methods can be used in holography to locate objects. Alternative methods involving the production of distortion-free regions in the holographic image field during reconstruction, the use of self-calibrating objects in the object field during recording, and triangulation can be used to locate objects.     

Line-focusing holographic mirrors for solar ultraviolet energy concentration

We report on the design and the construction of a 1.3-m(2) UV selective solar concentrator intended for photocatalytic water detoxification processes. This concentrator was made by assembling 30 cm x 30 cm line-focusing holographic mirrors. Holograms with an 80% diffraction efficiency concentrate ~40% of the incident solar energy, which is effective for exciting an aqueous suspension of the TiO(2) photocatalyst, and transmit unnecessary longwave radiation. Major problems, such as the low sensitivity of the dichromated gelatin used for recording the holograms and wavelength dispersion, are discussed.     

Hybrid holographic microscopy free of conjugate and zero-order images

The true image area that can be used for recording microscopic objects in hybrid holographic microscopy can be increased by elimination of the conjugate image and the zero-order image. We therefore added two shutters and one phase modulator to the electro-optical holographic recording system so that we could change the recording parameters and evaluate four methods of eliminating the conjugate image and the zero-order image. We found that the methods that use only the phase modulator require the recording of fewer holograms than do the methods that use the shutters and also provide reconstructed images that are less noisy.     

Characterization of Two-Color Holography Performance in Reduced LiNbO(3):In

The two-color recording sensitivity S and the figure of merit M/# are measured in a reduced and congruent LiNbO(3):In crystal of 2.2-mm thickness. The results are compared with those before reduction treatment. It is found that S increases by an order of magnitude through reduction treatment but the M/# is unchanged. Measured values for S and M/# in the reduced crystal are found to be 3 x 10(-3) cm/J and 0.04, respectively, with a total writing intensity of 4.0 W/cm(2) at 780 nm and a gating intensity of 1.15 W/cm(2) at 488 nm. These values are close to those of near-stoichiometric LiNbO(3) crystals under similar recording conditions. We also show that photoconductivity plays an important role in the improvement of S in LiNbO(3):In.     

Two-photon recording of microholograms in undoped lithium tantalate

Two-photon recording of microholograms was demonstrated in undoped congruent lithium tantalate crystal with high holographic sensitivity, large refractive index change, and fast hologram writing time. Nonvolatile heterodyne readout was carried out at a wavelength of 660 nm. Recording of a 3 x 3 x 3 matrix of microholograms was realized and it showed the cross-talk effect was negligibly small.     

一种基于聚乙烯醇/丙烯酸-2-羟基乙酯的新型全息存储材料

本文研究了一种基于聚乙烯醇/丙烯酸-2-羟基乙酯的新型全息存储材料;研究了不同浓度的丙烯酸-2-羟基乙酯对光聚物材料衍射效率的影响;在厚度为95μm的光聚物薄膜中当曝光能量为231mJ/cm2时获得了近75%的高衍射效率。应用Matlab软件以光化学漂白理论、角度响应曲线理论为拟合模型,进行了基于Levenberg-Marquardt算法的非线性曲线拟合;通过非线性曲线拟合研究了不同浓度的丙烯酸-2-羟基乙酯对摩尔吸收系数、量子产率、折射率调制度、光聚物薄膜厚度等多个全息参数的影响;实验表明,丙烯酸-2-羟基乙酯的加入可有效增加光聚物薄膜的厚度并且改善光聚物薄膜的结构和性能,达到提升聚乙烯醇/丙烯酰胺光聚物初始材料的全息特性,改进光聚物材料全息光学数据存储容量的目地。     

Single-exposure, photorefractive holographic surface contouring with multiwavelength diode lasers

We studied the use of multiwavelength diode lasers for surface profilometry through holographic recording in sillenite Bi12TiO20 crystals. When such lasers are used, the holographic image from single-exposure recordings appears covered with interference fringes providing information on the surface relief of the object. By taking advantage of the narrow interference fringes due to the multiwavelength emission of the laser, we obtained interferograms by holographic recording with two reference beams, which improves the surface analysis by visual inspection and enhances the profilometry sensitivity.     

Computer-originated planar holographic optical elements

We present novel, to our knowledge, methods for the analytical design and recording of planar holographic optical elements in thick materials. The recording of each planar holographic element is done by interference of two aspherical waves that are derived from appropriately designed computer-generated holograms such that the element has the desired grating function for minimizing aberrations and closely fulfills the Bragg condition over its entire area. The design and recording methods are described, along with calculated results of representative elements.     

Achromatized transmission-type holographic screen for a multiview stereoscopic image system

The main drawback of the use of transmission-type holographic screens is poor color reproduction caused by their high spectral dispersion. For overcoming this drawback, a long, narrow diffusing slit is used as an object when recording the screen. The necessary size and position of the slit relative to the photoplate and to the recording and reconstruction beams are determined by the phase relations of the beams. By use of the slit, holographic screens of 30 cm x 40 cm are recorded with a diverging reference beam and are used to display a multiview full-color stereoscopic image. The images displayed on the screen show no sign of color separation except near the edges of the screen. The image brightness on the screen is high enough that it can be watched in a normally illuminated room.     

Optical compensation of hologram distortion avoiding interpage crosstalk on reconstructed image in angle-multiplexed holograms

We are studying a form of holographic data storage with phase conjugation, and we compensated for hologram distortion due to shrinkage of photopolymer materials in the holographic medium by controlling the wavefront of the reference beam. When a high NA lens and narrow angle interval of angle multiplexing are employed to obtain a high data recording density, some wavefronts cause interpage crosstalk on the reconstructed image. We tried to determine the moving range of actuators in a deformable mirror for controlling the wavefront. As a result, we found that the distortion in the hologram could be compensated while avoiding interpage crosstalk and that the bit error rates of the reproduced data could be decreased. We also found that the optimized wavefront could compensate for distortions in several neighboring data pages. This method can ensure a high data recording density in holographic data storage.