Photorefractive BaTiO3 Research Articles

Soliton-like interactions of bright, low-power, low-divergence Gaussian fields in an unbiased self-defocusing photorefractive BaTiO<sub>3</sub> crystal

The soliton-like interactions of two low divergence fields propagating in unbiased self-defocusing photorefractive media are examined. The interactions appear to be a result of factors that are not dissimilar to those between two steady state screening solitons. Analysis of the conditions for the interactions show a cutoff for the degree of beam divergence required for the soliton-like interactions to occur. The analysis also indicates that the strength of the interaction increases as the beam divergence decreases.

Optical bistability by two laser beams in photorefractive crystals

For a nonlinear medium, a given incident laser beam may produce different transmitted light waves. This phenomenon (the so-called optical bistability) corresponds to multiple solutions of the boundary value problems of the nonlinear Helmholtz equation. Optical bistability can be useful in the design of optical switches. Devices that display this behavior could potentially play a major role in the development of optical communication systems and computing. In this article we present experimental results concerning the optical bistability in photorefractive BaTiO 3:Fe crystal. Two laser beams were used to interact with the photorefractive crystal which resulted in the bistability of the intensity of transmitted wave. This was achieved without the application of any optical resonator. High contrast optical bistability is found experimentally in the pump-ratio dependence of the output intensity.

Kinetics of two wave mixing gain for moving grating technique in photorefractive BaTiO_3 crystal

We present analytic expressions for time-dependent space charge field and two wave mixing gain under the external applied field accompanying the grating translation. The variations of complex space charge field are analyzed in a complex plane, and the explicit expressions for the resonance and optimum frequencies (or moving grating velocities), which maximize the magnitude and imaginary part of space charge field, respectively, are also obtained. In addition, two wave mixing experiment was conducted with a grating translation technique without the external applied field in a BaTiO(3) crystal and the transient behaviors of measured gain look like damped harmonic oscillations, showing excellent agreement with the theory for the entire time range.

Optical encryption and decryption using a sandwich random phase diffuser in the Fourier plane

We describe the encryption and decryption of a two-dimensional (2-D) image by using a sandwich diffuser in the Fourier plane of the first lens of a 4-f imaging system. The diffuser is made with two random phase diffusers, sandwiched together. The encrypted image is holographically recorded in a photorefractive BaTiO3 crystal. Decryption is done by generating a phase conjugate image through the phase conjugation technique. During decryption, if only one of the two random phase diffusers constituting the sandwich diffuser is used, it becomes impossible to retrieve the image. Also no decryption is possible if any of the diffusers constituting the sandwich is shifted in-plane with respect to the other. It is possible to generate the image only if the diffusers are sandwiched together again after matching point to point, and then the sandwich diffuser is kept at the same position and orientation in which it was kept during the encryption. Experimental method has been described to overcome the practical difficulty for the generation of the key in the case of the sandwich diffuser. Simulation and experimental results are presented in support of the proposed idea. To evaluate reliability of the technique, the mean-square-error (MSE) between decrypted and original image has been calculated.

Hanbury-Brown and Twiss type double-slit interference with photorefractive fanning light

This paper reports that when an intense extraordinary-polarized laser beam illuminates a photorefractive BaTiO3 crystal, the dynamic beam fanning light is formed to be a thermal-like light source with a long correlation time and wide spectral bandwidth. The experimental results of the first- and second-order double-slit interference with such photorefractive fanning light source, can be understood with the theoretical simulation in terms of Hanbury-Brown and Twiss effect.

Optical detection of the Casimir force between macroscopic objects

We report the optical detection of mechanical deformation of a macroscopic object induced by the Casimir force. An adaptive holographic interferometer based on a photorefractive BaTiO3:Co crystal was used to measure periodical nonlinear deformations of a thin pellicle caused by an oscillating Casimir force. A reasonable agreement between the experimental and calculated values of the first and second harmonics of the Casimir force oscillations has been obtained.

Phase-Conjugate External-Cavity Diode Laser

An external-cavity diode laser based on self-pumped phase-conjugate optical feedback is reported. An antireflection-coated diode laser is subjected to strong phase-conjugate feedback from a photorefractive Ce-doped BaTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> crystal. This provides narrow laser linewidth and good stability. Inherent linewidth of 25 kHz and relative frequency stability of 1.5times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-8</sup> (at 100 s) are demonstrated. Due to phase-conjugate feedback, the laser is insensitive to mechanical drifts and misalignment of the feedback element

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This paper proposes a novel simple image encryption and decryption technique based on the principle of interference and a binary-phase computer-generated hologram (BPCGH). To obtain a stable interference pattern, a technique involving a photorefractive material is also proposed. The proposed scheme is composed of digital encryption and optoelectronic decryption. For the encryption process, a BPCGH that reconstructs the original image perfectly is designed, using a simulated annealing algorithm, and the resulting hologram is regarded as the image to be encrypted digitally. In addition, a reference image with key information is created using a random two-phase generator. The hologram is then encrypted using the reference image and the interference rule. For the decryption process, the interference intensity is obtained by interfering the reference image and the encrypted image, binarized with software, and then transformed into phase information. Finally, the original image is recovered by an inverse Fourier transformation of the binary-phase information. During this process, since the intensity information generated by the interference of the two images is very sensitive to external vibrations, a stable interference pattern is achieved using a self-pumped phase-conjugate mirror made of the photorefractive material BaTiO 3 .

Characterization of a Pico-Second Phase Conjugate Nd:YVO4 Laser System

We present a full characterization of a pico-second pulse generation from a Nd:YVO4 double-pass amplifier with a phase-conjugate mirror based on a photorefractive BaTiO3 crystal with an external loop geometry. With this system, 7.4 W diffraction-limited pico-second output was generated. A corresponding optical extraction efficiency of >20% was achieved.

Hologram multiplexing with photorefractive beam-fanning speckle

We propose a hologram multiplexing method with a speckled reference beam generated by the photorefractive beam-fanning effect. In this method, a bulk photorefractive crystal takes the role of generating various speckle fields as well as storing holograms. The speckle field of a reference beam used for holographic recording is generated by the photorefractive beam-fanning effect in the storage crystal. This speckle field can be changed by varying the spatial intensity distribution, the spatial phase distribution, the incident angle or the incident position of the original reference beam at the crystal surface. We experimentally demonstrate hologram multiplexing and retrieving with the speckled reference beams generated by the beam-fanning effect in a photorefractive BaTiO 3 crystal. Here we vary the speckle field by changing the incident position of the original reference beam. We can successfully reconstruct stored images from four multiplexed holograms with the reference beams speckled by the beam-fanning effect. This method has the great possibility for all-optical control of hologram multiplexing, besides the simplification of an optical system compared with conventional speckle multiplexing methods using external diffusers such as ground glass and multimode optical fiber.

STUDY OF SELF-PUMPED PHASE CONJUGATIONS AND THEIR SUPPRESSIONS AT THE COMMON INPUT OF TWO LASERS

Two He-Ne laser beams at some distance, with their included angle only a few tenths of a degree, are intersected on the surface of a photorefractive BaTiO 3: Ce crystal at the same incidence point to simultaneously conduct their self-pumped phase conjugations (SPPCs). Our experimental results and theoretical analysis show that, when a stronger laser and a weaker laser are applied to the crystal, the mutual competition of their build-up gratings can greatly interfere the weaker laser with the formation of its SPPC. The weaker SPPC response time is of the order of minutes, quite long compared with that of its normal circumstances.

Ba0.77Ca0.23TiO3 (BCT): a new photorefractive material to replace BaTiO3 in applications

The most popular photorefractive material BaTiO 3 suffers from a crippling defect when applications are considered: its phase transition around room temperature that can destroy its photorefractive characteristics, if not the crystal itself. Some years ago, University of Osnabrück [1] , proposed and successfully grew Ba 0.77 Ca 0.23 TiO 3 an alloy derived from BaTiO 3 , in which the phase transition was pushed away towards low temperature. The remaining question was then: does this new material keep the good photorefractive properties that made the success of BaTiO 3 ? We will present here some theoretical and experimental results that allow us to answer this question and that show that despite some differences in the photorefractive properties between BaTiO 3 and BCT, the new crystal is an extremely promising materials for photorefractive applications, such as phase conjugation or dynamic holographic intracavity laser mode selection.

Light-induced absorption in photorefractive BaTiO3 crystals

We present the experimental results on the measurement of temporal and steady-state light-induced absorption change in undoped and rhodium-doped barium titanate (Rh:BaTiO3) crystals at different wavelengths and intensities. Theoretical calculations based on a two-center charge transport model agree well with the experimental results which supplement the earlier studies carried out using photorefractive BaTiO3 crystals. We also report a wavelength-specific light-induced effect that modifies the absorption dynamics in a peculiar manner. This can be attributed to light-induced thermal effects. However, the possibility of additional photorefractive centers becoming active cannot be ruled out.

Increasing the conductivity of photorefractive BaTiO 3 single crystals by doping Ru

BaTiO 3 crystals are important photorefractive materials due to their self-pumped phase-conjugation and large electro-optic coefficient. High gain, fast response time and proper beam fanning BaTiO 3 crystals are desirable. We fabricated photorefractive BaTiO 3:Ru single crystals and then investigated their optical and photorefractive properties. In as-grown Ru-doped BaTiO 3 single crystals, the absorption shoulder is around 450 nm. The dark conductivity is dominant in the crystal and much higher than the photo conductivity at a low intensity pumping beam.

Intensity-dependent beam-fanning in high-gain photorefractive barium titanate crystal

A phenomenological multi-wave mixing model has been used along with a more general solution of Kukhtarev's model to explain intensity-dependent beam-fanning in photorefractive (PR) BaTiO3 crystal. The variation of trap density owing to the photoexcitation of charge carriers has been considered to explain the intensity-dependent effect. Theoretical results predicted by the model show a confirmation with experimental results obtained from PR crystal, and also show some of the controlling mechanisms that enable use of beam-fanning for some potential applications.

Two-way two-dimensional pattern transferring upon requesting with BaTiO3 crystal

We propose a setup which is capable of transferring two-dimensional (2D) pictures from one plane to another in either way upon the requesting of the plane which wishes to obtain information from the other one. Since this picture transferring is purely optical, it is free of electromagnetic interference. The setup consists of a high-gain photorefractive BaTiO 3 crystal, and the crystal is set in a configuration of a Cat conjugator. The working principle of the setup relies on the mechanism of photorefractive self-pump phase conjugation and four-wave mixing.

Measurement of tilt of a diffuse object by double-exposure speckle photography using speckle fanning in a photorefractive BaTiO 3 crystal

A technique has been proposed for the measurement of tilt of a diffuse object using speckle fanning in a photorefractive BaTiO 3 crystal. Free space geometry has been used for recording in which the axis of rotation of the object lies away in three-dimensional space. The method is based on recording of two exposures, one before- and another after the tilt, thus producing speckle correlation fringes due to overlapping of two speckle fans. We consider a generalized case of tilt, generating curved fringes with center shifted in the transverse direction. The axis of rotation lies perpendicular to the optical axis in three-dimensional space having the transverse and axial components of the shift in the center of rotation with respect to the diffuser plane. Fourier transform of the curved fringes produces correlation output. Measure of separation between the correlation peaks in transverse and axial directions provides information about the tilt angle, the direction of tilt, and the distance of rotation axis from the optical axis. Experimental results have been presented to validate the theoretical predictions.

Generation of highly efficient self-pumped phase conjugation femtosecond pulse using photorefractive BaTiO 3:Co crystal

The generation of highly efficient self-pumped phase conjugate (SPPC) pulse of 180 fs, 810 nm Ti:sapphire laser at 76 MHz in a photorefractive BaTiO 3:Co crystal is demonstrated. Brewster prisms are inserted in front of the photorefractive crystal to make wide bandwidth of the femtosecond pulse spatially separated in order to remove an influence of erasure caused by beam fanning and to avoid spectrum narrowing. A maximum reflectivity is improved from 1.5% without prisms to 6.6% with prisms. The spectral bandwidth of the phase conjugate pulse is also improved from 2.7 to 3.8 nm. The spectral bandwidth of the phase conjugate pulse with prisms measured is approaching 5.1 nm of the incident laser pulse, whereas the phase conjugate pulse without prisms is narrower. Since the refractive index grating that contributes to the generation of the SPPC pulse is only the transmission grating, therefore, the rise-time behavior of the SPPC pulse is smoother than that of continuous wave.

Non-reciprocal transmission via phase conjugation in multimode optical fibres

We demonstrate phase conjugate correction of modal distortion of a single mode beam on double-passing a passive highly multimode fibre. Phase conjugation is achieved in a self-pumped photorefractive BaTiO 3 crystal which conjugates only the e-polarised input component. We show that only a small fraction of the multimode fibre output is required to faithfully conjugate the single mode input beam. This technique enables the demonstration of a novel non-reciprocal transmission device, which for example has immediate application in gain-grating holographic resonator designs with multimode fibres as the active gain medium.

Three-dimensional rigid body motion measurement using speckle fanning in a photorefractive BaTiO 3 crystal

Summary A simple speckle technique, utilizing speckle fanning in a BaTiO 3 crystal, is demonstrated for the measurement of rigid body displacement in an arbitrary direction. Collimated laser beam illuminates a diffuser to generate speckles, which are then recorded inside a photorefractive barium titanate crystal thus producing speckle fan. Speckle correlation fringes are formed due to interference between the reconstructed speckle fans corresponding respectively to the initial- and final positions of the diffuser. Displacements occurring in a direction making small angles with the optical axis have been measured by profiling the fringes. The measurement of displacement in a direction making large angle with the optical axis is possible only by taking the Fourier transform of the fringes. This facilitates the separation of transverse and axial correlation peaks giving the transverse and axial components of the motion.