Photorefractive BaTiO3 Research Articles

Two examples of mirrorless parametric oscillation in photorefractive barium-titanate

Abstract In this contribution we report on two examples of mirrorless oscillation in photorefractive BaTiO3 pumped by two pairs of two counterpropagating light waves, each of them containing an ordinary and an extraordinary component of polarization. The oscillation waves are located on a cone with an apex angle defined by the phase matching conditions for this wave mixing process. The threshold behaviour is investigated by measuring the dependence of oscillation efficiency and starting time on the intensity ratio of the orthogonal polarized components of the pump beams.

In-plane displacement measurement using objective speckles in photorefractive two-beam coupling: effect of multiple exposures

A technique is proposed for the real-time measurement of in-plane displacement utilizing objective speckles and two-beam coupling in a photorefractive BaTiO 3 crystal. Speckles generated by a diffuse object in free-space geometry are used as the signal in two-beam coupling. The set-up is such that the phase shift between the refractive index grating and the interference grating is + π 2 (amplification mode). Due to the in-plane displacement, Young's fringes are obtained with a bright speckle background as a result of the signal amplification and beam fanning. The slow process of two-beam coupling amplification in a BaTiO 3 crystal is exploited for recording multiple exposures. This method of recording multiple exposures provides a simple and convenient way of analyzing the rigid body motions with enhanced accuracy by sharpening the fringes and with acceptable contrast. Theory has been developed to obtain the intensity distribution in the fringes formed in real time at the observation plane. Experiments have been conducted to show the validity of the theoretical results. Good agreement is established between the theory and the experiments.

Parallel optical image addition and subtraction in a dynamic photorefractive memory by phase-code multiplexing

We propose and demonstrate experimentally a method for utilizing a dynamic phase-encoded photorefractive memory to realize parallel optical addition, subtraction, and inversion operations of stored images. The phase-encoded holographic memory is realized in photorefractive BaTiO(3), storing eight images using Walsh-Hadamard binary phase codes and an incremental recording procedure. By subsampling the set of reference beams during the recall operation, the selectivity of the phase address is decreased, allowing one to combine images in such a way that different linear combination of the images can be realized at the output of the memory.

Recent development of optical novelty filter and dynamics of optical novelty filter with use of The CAT-type self-pumped phase conjugate mirror

The recent development of the optical novelty filter is briefly surveyed. Principles, methods and photorefractive crystals used in these studies are classified and tabulated. The novelty filter with the CAT-type self-pumped phase conjugator was made with photorefractive BaTiO3 crystal. Dynamical properties of this novelty filter were examined by using a periodically modulated phase hologram.

Multiwave-pumped mirrorless parametric oscillation in BaTiO3

We report on the observation of mirrorless oscillation in photorefractive BaTiO3 pumped by two pairs of two nearly counterpropagating light waves, each of them containing an ordinary and an extraordinary component of polarization. The oscillation waves are located on a cone with an apex angle defined by the phase-matching conditions of this process.

Coherent refreshment and updating for dynamic photorefractive optical memories using phase conjugation

We propose and demonstrate experimentally a new, phase-locked refreshment method for reading out dynamic holographic photorefractive memories without destruction of the stored information. This refreshment method can be used for various multiplexing techniques as angular-, phase encoded- or wavelength multiplexing and for different recording schedules as sequential or incremental recording. In a first experimental realization, an image stored in a photorefractive BaTiO 3-memory can be sustained during readout when it is fed back into the crystal using a double phase conjugation oscillator, realized with photorefractive BaTiO 3 as well. Experimental results on the diffraction efficiency of the refreshed images, signal-to-noise ratio and optimal refreshing time are given as well as a demonstration of multiple refreshment implementing a series of refreshment cycles. Moreover, our method is experimentally compared to phase-locked hologram sustainment by redirecting both the image and the reference beam back into the memory via phase conjugation in photorefractive BaTiO 3.

Buildup of spontaneous hexagonal patterns in photorefractive BaTiO_3 with a feedback mirror

Spontaneous formation of optical patterns is demonstrated in a system consisting of a BaTiO3 crystal and a feedback mirror. The buildup and the dependence on the feedback path length of the patterns are described. A possible mechanism of the pattern formation is discussed.

Nondestructive waveguide loss-measurement method using self-pumped phase conjugation for optimum end-fire coupling

We present a new nondestructive method for measuring the propagation loss coefficient in waveguides based on optimum end-fire coupling by self-pumped phase conjugation in a photorefractive BaTiO3 crystal. In a first pass through the waveguide, the laser beam is adapted to the waveguide mode profile, and hence the phase-conjugated beam is automatically coupled back into the guide with the optimum coupling efficiency. The propagation loss coefficient is determined by measurement of the transmittance of the phase-conjugated beam in the second pass through the wave-guide. We apply this method to measure the propagation loss coefficient of ion-implanted KNbO3 channel waveguides with an accuracy of better than 5%.

Optical triple-in digital logic using nonlinear optical four-wave mixing

A new programmable optical processor is proposed for implementing triple-in combinatorial digital logic that uses four-wave mixing. Binary-coded decimal-to-octal decoding is experimentally demonstrated by use of a photorefractive BaTiO(3) crystal. The result confirms the feasibility of the proposed system.

Electro-optic and dielectric properties of photorefractive BaTiO_3 and KNbO_3

Recently determined complete sets of materials parameters describing the dielectric, elastic, piezoelectric, elasto-optic, and electro-optic properties of BaTiO3 and KNbO3 crystals at room temperature are used to calculate the effective electro-optic coefficients and dielectric constants required for describing photorefractive phenomena. We show a substantial deviation of the new values from the electro-optic coefficients for homogeneously applied electric fields that were used previously in describing the photorefractive effects. We derive angular dependences of the effective electro-optic coefficients and the effective dielectric constants relevant for grating recording in both crystals and verify them experimentally.

Low-noise preamplifier for multistage photorefractive image amplification

We present a two-beam coupling configuration in photorefractive BaTiO(3) that provides a low-noise amplification of the signal to be detected. A two-wave mixing gain of 100 is reached, in conjunction with very low beam fanning background in the signal direction. The extensions of this configuration to photorefractive heterodyne detection and to multistage image amplif ication are theoretically and experimentally studied.

Angular distribution of the forward light scattering from a quartz fiber

Using a new technique based on the fanning of a coherent light beam in a photorefractive BaTiO(3) crystal, we have measured the angular distribution of forward light scattering by quartz fibers of radii from 15 to 30 µm. Data have been obtained over the angular range 0° to 0.3° and are in good agreement with theory.

Dynamics of light-induced absorption in BaTiO_3 and application for intensity stabilization

Illumination of a photorefractive BaTiO3 crystal provides an induced absorption increase, which is explained by the two-center charge transport model. Detailed investigations of buildup and decay of light-induced absorption are performed. Intensity changes of the incoming wave are attenuated by these absorption changes. The requirements for distinct intensity stabilization are theoretically estimated. Intensity stabilization is experimentally demonstrated, and a decrease of the intensity fluctuations of as much as 30% has been achieved.

Intensity dependent two-wave mixing at large modulation depth in photorefractive BaTiO 3 crystal

The intensity-dependent two-wave mixing gain at large modulation depth is measured in a BaTiO 3 crystal. We derive an expression of nonlinear space-charge field from Kukhtarev's materials equations for single charge carrier without assuming the linear recombination of free charge carriers. The experimental results are interpreted by taking into account both an empirical correction function at large modulation depth [Refregier et al., J. Appl. Phys. 58 (1985) 45] and intensity-dependent factor of the space-charge field, which is caused by nonlinear recombinations of free charge carriers.

Real-time image processing with a cat conjugator

We demonstrate the real-time image-processing operations of edge enhancement, theta filtering, and pattern recognition with a self-pumped phase conjugator with an external erase beam to modify the index gratings formed inside the photorefractive BaTiO(3) crystal.

Demonstrator concepts and performance of a photorefractive optical novelty filter

We present the design, implementation and detailed investigations on the characteristics of an optical novelty filter based on two-wave mixing in photorefractive BaTiO 3. The system is cabable on novelty filtering two-dimensional video images using a commercially available liquid crystal display. The setup was realized as a compact demonstrator device. A contrast of up to 40:1 and a limiting frequency of 1.6 Hz were obtained.

Photorefractive BaTiO 3: an efficient material for laser wavefront correction

We present two different setups, a beam clean-up arrangement and a self-pumped phase-conjugating mirror, for correcting the phase aberrations introduced on laser pulses. Experimental demonstrations conducted with photorefractive BaTiO 3 crystals are presented.

ＢａＴｉＯ３結晶を用いた近赤外光による２光波混合

ＢａＴｉＯ３結晶を用いた近赤外光による２光波混合

Anomalous photorefractive response in two-beam coupling with focussed beams in photorefractive crystals and waveguides

We report on anomalous two-beam coupling results observed in photorefractive BaTiO3 crystals and ion-implanted waveguides when focussed beams have been used. The two-beam coupling gain, investigated as a function of the input beam ratio, showed an enhanced magnitude, when compared with the standard theory, and exhibited a strong beam ratio dependence. In order to explain the significant difference we observed between theory and experiment, we discuss several models, based on intensity-dependent processes, and show that only a modification with the enhanced photoinduced space-charge field being a nonlinear function of modulation, has given an agreement with the experimentally determined results.

High-quality self-pumped phase conjugation of nanosecond pulses at 532 nm using photorefractive BaTiO_3

We implement a feedback loop oscillator, using a BaTiO(3) crystal that provides stable high-quality phase conjugation for nanosecond pulses, with an efficiency close to the theoretical maximum.