Characterization of the KNSBN: Cu crystal with two-wave coupling at visible and infrared wavelengths

We have optically characterized the newly-developed photorefractive crystal KNSBN: Cu by two-wave coupling at visible and near-infrared wavelengths. The effective electrooptic coefficients(k _g)r ₁₃ and(k _g)r ₃₃, the effective charge carrier densityN _eff, the dark conductivity _d, the product of mobility and electron-trap recombination time _R, the formation and decay rate of index grating and the absorption coefficient of the crystal were determined from visible to near-infrared wavelengths. Their wavelength dependences were also studied. It is interesting to notice that the KNSBN: Cu crystal has a very largeN _eff and large gain coefficient for ordinary ray (o-ray) in the visible region, and the photorefractive properties of the crystal are very different in the visible and near-infrared spectral regions.     

Approach to wavelet multiresolution analysis using Coiflets and a two-wave mixing arrangement

An approach to the wavelet multiresolution analysis is realized using Coiflet wavelets and a two-wave mixing arrangement via transmission gratings in a bismuth silicon oxide (BSO) crystal. A Coiflet filter acts not only as a band-pass filter but as a high-pass or low-pass filter. 2-D wavelet bases are generated by a tensorial product of the 1-D Coiflet bases and are separable in x and y. Therefore, the 1-D band-pass property produces direction ability in the 2-D tensorial products. In this way, unlike the Mexican hat-based filters, the Coiflet-based filters allow selecting horizontal, vertical or diagonals details of an original image. The decomposition of a signal in two levels of multiresolution is experimentally demonstrated in this scheme. The original image is discomposed in an average image and several detail images depending on the level of multiresolution. Coif12 wavelets are used to achieve the multiresolution analysis. The photorefractive implementation of the wavelet analysis is numerically simulated and experimentally performed. An agreement between numerical and experimental results is obtained.     

Multichanneled puzzle-like encryption

In order to increase data security transmission we propose a multichanneled puzzle-like encryption method. The basic principle relies on the input information decomposition, in the same way as the pieces of a puzzle. Each decomposed part of the input object is encrypted separately in a 4f double random phase mask architecture, by setting the optical parameters in a determined status. Each parameter set defines a channel. In order to retrieve the whole information it is necessary to properly decrypt and compose all channels. Computer simulations that confirm our proposal are presented.     

A method for single-pass distortion correction using holography in rhodium-doped barium titanate

We present a holographic method of one-way image transmission with ‘real-time’ correction of distortion through an optical system. The technique is demonstrated using a crystal of photorefractive rhodium-doped barium titanate as the holographic medium and corrects for phase distortions imposed upon a US Air Force test chart. This technique is suitable for applications in both cw and pulsed regimes, and for either fixed or changing distortions. Received: 26 October 1998 / Revised version: 19 December 1998 / Published online: 12 April 1999     

Optical color image encryption with redefined fractional Hartley transform

We propose a new method for color image encryption by wavelength multiplexing on the basis of two-dimensional (2-D) generalization of 1-D fractional Hartley transform that has been redefined recently in search of its inverse transform. A color image can be considered as three monochromatic images and then divided into three components and each component is encrypted independently with different wavelength corresponding to red, green or blue light. The system parameters of fractional Hartley transform and random phase masks are keys in the color image encryption and decryption. Only when all of these keys are correct, can the image be well decrypted. The optical realization is then proposed and computer simulations are also performed to confirm the possibility of the proposed method.     

All-optical continuous-tunable image switch based on nonlinear photoinduced anisotropy in bacteriorhodopsin film

Using nonlinear photoinduced anisotropy in bacteriorhodopsin (bR) film, we presented and demonstrated an image switch in which the output can be tuned continuously by the intensity of a pumping beam. A laser with wavelength 532 nm was used as the pumping beam, and a He-Ne laser at wavelength 632.8 nm was used as the probe beam. Without pumping light, a little of polarized probe beam can transmit the crossed polarizers and the output is very low. With the presence of pumping light, owing to photoinduced anisotropy in the bR film, a portion of the probe beam transmits the crossed polarizers, depending on the intensity of the pumping beam. For the low-intensity probe beam (0.44 mW/cm²), the output is dependent on a wide range of pumping beam (2-30 mW/cm²). On the contrary, for the high-intensity probe beam (0.80 mW/cm²), the output is dependent on a narrow range of pumping beam (2-7 mW/cm²).     

Finite element modelling of dielectric elastomer minimum energy structures

This paper presents an experimentally validated finite element model suitable for simulating the quasi-static behaviour of Dielectric Elastomer Minimum Energy Structure(s) (DEMES). A DEMES consists of a pre-stretched Dielectric Elastomer Actuator (DEA) adhered to a thin, flexible frame. The tension in the stretched membrane causes the frame to curl up, and when a voltage is applied, the frame returns to its initial planar state thus forming a useful bending actuator. The simulation method presented here incorporates a novel strain energy function suitable for simulating general DEA actuator elements. When compared against blocked force data from our previous work, the new model provides a good fit with an order of magnitude reduction in computational time. Furthermore, the model accurately matched experimental data on the free displacement of DEMES formed with non-equibiaxially pre-stretched VHB4905 membranes driven by 2500 V. Non-equibiaxially pre-stretching the membranes allowed control of effective frame stiffness and bending moment, this was exploited by using the model to optimise stroke at 2500 V in a hypothetical case study. Dielectric constant measurements for non-equibiaxially stretched VHB4905 are also presented.     

Variable resolution with pupil masks

There are many applications in which pupil-plane masks are useful for point-spread-function (PSF) apodization or superresolution. A limitation of this technique is that once a mask is fabricated, the corresponding PSF characteristics are fixed. To overcome this drawback we introduce a technique for easily varying the performance of pupil-plane masks. This technique is based on the modification of the transmittance of each of the mask zones and, thus, can be implemented using a spatial light modulator or linear polarizers, e.g., we apply the technique to binary phase-only masks and we check that the figures of merit that characterize the PSF can be easily controlled. We study different configurations that allow us to modify resolution or peak intensity in a continuous way and we derive analytical expressions for these figures of merit.     

Multiplexing encrypted data by using polarized light

We investigate the feasibility of multiplexing, employing polarized light, a set of security encrypted data. The encryption approach is based on the double random pure-phase enciphering method. Phase conjugation operation is conducted in the reconstruction stage with the aid of a photorefractive crystal which stores the encrypted information. When storing each encrypted image, a polarization change is introduced in the system. This induces decorrelation on the speckle patterns inside the storing medium. We apply this approach for multiple image encryption. We show experimental results that confirm our approach.     

Combined analyser-based and propagation-based phase-contrast imaging of weak objects

A new theoretical method combining analyser-based and propagation-based hard X-ray phase-contrast imaging is investigated. Unlike the previous theoretical model of the combined imaging method constructed under the assumption of slow variation of the individual transfer functions (large Fresnel numbers), a new model proposed in this paper uses the assumption of a weak scatterer (analogous to the first Born approximation). Consequently, the results are not limited to the case of short propagation distances or low-resolution imaging. An explicit expression for the combined transfer function is derived and analytical and numerical examples solving related inverse imaging problems are presented.