Correlator-aided alignment of phase key in encrypted holographic storage systems

An encrypted holographic scheme with a compact alignment system for a phase key is proposed. Alignment for a phase key is performed by a holographic correlator. A filter in the correlator system contains the phase information used for encryption. Thus, it can be used to check the authorization of the phase key. Owing to the shift-invariant property of the holographic correlator, the location of the correlation peak is highly related to the alignment of the phase key. Therefore, the phase key can be repositioned on the desired location by searching for the specific position of the correlation peak. With the help of the correlator, alignment of the phase key can be done in 1 min. Experimental results using a correlator system in support of our proposed idea are demonstrated as well.     

Exploring the limits of optical storage using persistent spectral hole-burning: holographic recording of 12000 images

Received: 18 February 1997/Revised version: 23 April 1997     

The role of carrier mobility in holographic recording in LiNbO3

We investigate the role of carrier mobility in holographic recording in LiNbO₃ crystals. Both normal holographic recording (single wavelength, single trap) and two-center recording are considered, and the differences between the performances of the two methods are explained. We show that increasing mobility by using stoichiometric crystals or by doping with Mg does not improve sensitivity considerably, but does reduce M/# by at least one order of magnitude. Received: 22 February 2001 / Revised version: 5 March 2001 / Published online: 27 April 2001     

Compact and portable holographic camera using photorefractive crystals. Application in various metrological problems

We present a compact holographic interferometer that uses a photorefractive crystal of the sillenite family as a holographic recording medium. Its development is based on a previous prototype that showed lack of flexibility and portability. We briefly discuss the main improvements leading to a compact device. Applications of this instrument in various metrological problems are shown, among which are two that were not already considered using holography, namely measurement of a thermal expansion coefficient and detection of fingerprints. Received: 20 December 2000 / Revised version: 8 February 2001 / Published online: 20 April 2001     

Real-time holographic interferometry using sillenite photorefractive crystals. Study and optimization of a transportable set-up for quantified phase measurements on large objects

We present the development of a holographic interferometer that uses a photorefractive crystal of the sillenite family as holographic recording medium. The aim of this work is to achieve a transportable and breadboard instrument, with a flexible design and which is user friendly for the quantified measurements of displacements on large scattering objects. The state of the art of the use of photorefractive crystals in holographic interferometry is presented. Based on the latter, a method that is the best suited to our mentioned objectives is chosen: the real-time interferometric technique associated with the crystal configuration exhibiting diffraction anisotropy. On this basis, we have studied and compared two imaging systems, we have experimentally determined the holographic figures of merit of three sillenite crystals samples, and, finally, we have shown the use of two measurement quantification methods. The result of these investigations is a certified and transportable holographic camera prototype containing all the necessary equipment for its operation, which can be easily adapted to various applications. Among these, we show its use in the detection of defects in composite structures, in the measurement of time-evolving deformations, and in the measurement of vibration modes. Received: 4 December 1998 / Revised version: 6 March 1999 / Published online: 12 April 1999     

High-sensitivity read-write volume holographic storage in reduced KNbO3 crystals

Reduced KNbO₃ is a photoconductive ferroelectric in which holograms can be recorded by the photorefractive effect. Read-write volume hologram storage and erase sensitivities ofS ⁻¹=100 J/cm² andS ⁻¹=84 J/cm² (S=d(Δn)/d(I₀t)‖_t=0) have been measured at zero applied electric field, where the charge transport is shown to be due to diffusion of photoexcited electrons. By applying an electric field along thec-axis, the migration length of the photoexcited electrons becomes comparable to the holographic grating spacing. This leads to storage sensitivities comparable to high-resolution photographic plates. Experimental data on storage and erase sensitivity as a function of the grating spacing, applied electric field, writing light intensity and temperature are reported and interpreted on the basis of the theoretical results of Young et al. and Amodei. Changes of the intensity ratio of the writing beams by self diffraction (beam coupling), reflections from surfaces and the residual dark conductivity are assumed to cause experimental results which deviate from the theoretical models. It is shown, that in reduced KNbO₃ and other ferroelectric photoconductors having photocarrier transport lengths much larger than the unit cell dimension, photovoltaic currents do not contribute significantly to the build-up of space-charges leading to the photorefractive effect.     

Polarized-light-controlled holographic recording in an azobenzene-doped polymer film

A new polarized-light-controlled holographic recording based on the optical enhancement/restraint of self-diffraction has been demonstrated in an azobenzene-doped polymer film. It is found that a continuous variation of the polarization status as well as of the intensity of the pumping light results in a continuous variation of self-diffraction efficiency. The mechanism originates from the photo-induced anisotropy and polarization-dependent absorption. Both positive and negative replicas of an incident image were presented in real time by means of this incoherent–coherent optical conversion technique. Received: 30 March 2000 / Revised version: 28 September 2000 / Published online: 27 April 2001     

Diffraction from angular multiplexing slanted volume hologram gratings

The diffraction characteristics of the volume holographic gratings made by multiexposures with angular multiplexing during its construction is investigated. Because of the reflection by the interface between the emulsion and the substrate or the air there is an extra unslanted periodic structure inside a slanted grating, it will affect properties of the slanted volume holographic gratings. When the angle between the surface of the substrate and the grating plane of the slanted grating is less than a certain value, an extra peak accompanying the main peak will appear in the diffraction spectrum. But, when the angle is larger than the certain value, one designed and expected main peak will disappeared while the extra peak is kept and observed. This phenomenon limits the capacity of the volume hologram for the application in wavelength-division multiplexing (WDM).     

Enhanced angular selectivity in volume holograms with speckle reference waves

A speckle-multiplexing scheme with enhanced angular selectivity for holographic storage is proposed. Angular selectivity in the proposed holographic storage system is theoretically and experimentally investigated. We find the effect of speckle reference wave on angular selectivity strongly depends on the techniques to perform multiplexing for holographic storage. Angular selectivity of a holographic storage system can be effectively enhanced as long as angular deviation of reading wave induces a lateral displacement of the speckle pattern on the hologram plane. When angular deviation of reading wave only induces a speckle wavefront tilt on the hologram plane, the speckle wave is not helpful to enhance the angular selectivity and the angular selectivity becomes to depend on material thickness (Bragg condition) only.     

Optical holographic memory using angular-rotationally phase-coded multiplexing in a LiNbO3:Fe crystal

We propose a new holographic memory scheme based on random-phase-coded multiplexing in a photorefractive LiNbO₃:Fe crystal. Experimental results show that rotating a diffuser placed as a random-phase modulator in the path of the reference beam provides a simple yet effective method of increasing the holographic storage capabilities of the crystal. Combining this rotational multiplexing with angular multiplexing offers further data-storage possibilities. The advantage of using post-image versus pre-image phase modulation of the object beam is demonstrated. Received: 15 August 2000 / Published online: 10 January 2001     

Self-adaptive Nd:YLF holographic laser with selectable wavelength operation

We demonstrate, for the first time to our knowledge, operation of a self-adaptive holographic laser with wavelength selection, which uses Nd:YLF as the gain medium. The holographic laser was self-starting and, by virtue of the temporal dynamics in writing the gain grating, passively Q-switched the cavity to produce pulses with 20–50 ns duration. A pulse energy of 184 mJ was obtained from the system with beams having a TEM₀₀ spatial profile. Single-longitudinal-mode operation was observed with near-transform-limited bandwidth. Received: 30 July 2002 / Revised version: 1 October 2002 / Published online: 26 February 2003 RID="*" ID="*"Corresponding author. Fax: +44-20/7594-7744, E-mail: s.y.lam@ic.ac.uk     

Determination of displacement derivative in digital holographic interferometry

Digital holography (DH) and digital shearography (DS) both play an important role in non-destructive evaluation. In this paper, a novel method based on digital holographic interferometry (DHI) and complex phasor (CP) is proposed to determine displacement derivative. An algorithm is employed to filter the imaginary and real parts of complex values without the need of direct phase manipulation. Two-dimensional short time Fourier transform (STFT) is employed subsequently to process wrapped phase maps. An experiment is conducted to demonstrate the validity of the proposed method.     

Orthogonal geometry of wave interaction in a photorefractive crystal for linear phase demodulation

We present a novel configuration of an interferometer which is implemented by using vectorial wave mixing in the orthogonal geometry of wave interaction in a cubic photorefractive crystal, for the detection of small phase transients. The orthogonal geometry of wave interaction allows the linear regime of the phase-to-intensity transformation even when phase transients are encrypted in a depolarized wave without using any polarizing element. The use of a depolarized beam without a polarizing element strongly diminishes the noise level, thus increasing the sensitivity to phase transients.     

Anisotropic self-diffraction under Bragg mismatching

Anisotropic self-diffraction (ASD) under Bragg mismatch has been studied. We derive a solution that can describe well the diffraction characteristics of the anisotropic self-diffraction under Bragg mismatch. The solution is useful for estimating the Bragg constraint when the ASD is applied to optical information processing. Both the theory and the experiment are presented. Received: 11 July 2000 / Revised version: 2 January 2001 / Published online: 30 March 2001     

Hologram multiplexing in a highly photosensitive photopolymerizable material in a sol-gel matrix

In this paper, we present a photopolymerizable material in a sol-gel matrix suitable for recording a large number of multiplexed diffraction gratings with low total exposure energy. The matrix of this material is an inorganic porous silica glass, a material that does not shrink when radical photopolymerization is initiated. The photosensitive component is based on acrylamide, N,N′-methylenebisacrylamide, triethanolamine and yellow eosine as a photoinitiator. In the reported study, 50 holograms were angularly multiplexed with diffraction efficiencies between 0.1 and 1.9% resulting in a dynamic range of M/# = 3.9. The total exposure was 4.5 mJ/cm² and the energy used in recording each hologram was 95 μJ/cm². This indicates a very high sensitivity for this material in the range of 3.3 to 15.5 cm/mJ. Due to this good holographic performance, the material is suitable for holographic data storage applications.     

Phase measurement improvement in temporal speckle pattern interferometry using empirical mode decomposition

When the Hilbert transform method is used to recover the phase distribution in temporal speckle pattern interferometry, the influence of the fluctuations of the bias and the modulation intensities on the calculated phase must be driven heuristically. In this paper we show that the Empirical Mode Decomposition method can overcome these drawbacks and consequently introduces an improvement in the evaluation of the phase distribution. An example is used to illustrate the phase measurement improvement that can be obtained by the application of the proposed approach.     

Hologram multiplexing in acrylamide hydrophilic photopolymers

Unslanted diffraction gratings are recorded in a 900 μm thick acrylamide photopolymer by means of peristrophic multiplexing. A solid state Nd:YAG (λ = 532 nm) laser is used as the recording beam, with a total incident intensity of 5 mW/cm², and a He-Ne laser as the reconstruction beam. The dye concentration in the photopolymer is optimized so that it does not limit the dynamic range. Nine holograms are recorded using constant exposure time scheduling and variable exposure time scheduling. From the results obtained it may be deduced that optimization of the dye allows us to work in the linear response region of the photopolymer and at the same time obtain high values of diffraction efficiency for each hologram. An exponential increase in exposure time as the number of holograms increases enables the values of diffraction efficiency to be homogenized with regard to the case of constant exposure scheduling. In this way, better use is made of the dynamic range of acrylamide hydrophilic photopolymer.     

Estimation of trap concentration in photorefractive crystals by a technique of adaptive holographic interferometry

12 GeO₂₀, Bi₁₂SiO₂₀, Bi₁₂TiO₂₀). Received: 17 June 1998/Revised version: 3 August 1998     

Effect of the incorporation of N,N′-methylene-bis-acrylamide on the multiplexing of holograms in a hydrophilic acrylamide photopolymer

The behaviour of a hydrophilic acrylamide photopolymer, with and without crosslinker (N,N′-methylene-bis-acrylamide), in solid layers 700 μm thick is analyzed. Slanted diffraction gratings are multiplexed at constant and variable exposure. Variable exposure time scheduling, with increased exposure time by steps is used in order to determine the effect on the dynamic range and the homogenization of diffraction efficiency values. This scheduling results in a greater dynamic range compared with constant time multiplexing. For crosslinked photopolymer, those exposure times are excessive (higher energetic sensitivity) and there is overlapping of the individual angular response curves for each stored diffraction grating.     

Multiple-scattering speckle in holographic optical coherence imaging

We investigate the effect of multiple scattering on the image quality of holographic optical coherence imaging, which is a full-field coherence-domain imaging form of optical coherence tomography. The speckle holograms from turbid media and from multicellular tumor spheroids are characterized by high-contrast speckle on a multiply-scattered background caused by channel cross-talk. We quantify the multiple-scattered light that is accepted by the holographic coherence gate, and identify a cross-over from single-scattered to multiple-scattered light beyond 15 to 20 optical thicknesses. Speckle reduction relies on vibrating diffusers and on fast adaptive holograms in photorefractive quantum well devices. The high anisotropy factor for tumor tissue reduces multiply-scattered light contributions for biomedical tumor imaging.