Using anomalous large diffusion we report a recording of reflection type gratings in a PVK-based photorefractive polymer without any external electric field. Diffraction efficiency of the gratings was measured to be 7%. An efficient modulation of beams during two-beam coupling up to 12% was demonstrated by using phase mutilation technique. This practical scheme can be used in dynamic vibration measurements.

We provide a direct experimental observation of space charge waves in photorefractive crystals with point group 23 based on their penetration into an area with uniform light illumination. Those are the results of excitation of eigen modes of space charge filed excited by moving light fringes. We have found experimental conditions when the quality factor of the space charge waves attempts dramatical increase with respect to what current theory predicts. This leads to self- excitation of the waves and results in self-oscillation of light. The competition between space charge waves and fundamental grating results in appearance of K/2, K/4 components in the fundamental grating and to spontaneously occurring beams called subharmonics. The subharmonic beams are generated in a pattern of domains that evolve dramatically as they move through the crystal. The domains are result of non- homogeneity of the space charge waves and separated by narrow boundary regions, where the phase of subharmonics changes by pi. We experimentally prove the domains move with the group velocity of running space charge waves.

We investigate role of temperature-dependent ionic and re- created photocarrier (hole) gratings in self-enhanced readout from thermally fixed holographic gratings in undoped barium titanate. It is shown that the temperature dependence of the diffraction efficiency below approximately 80 degrees Celsius is mainly determined by temperature-dependent behavior of the dielectric and electro-optic constants. On the other hand, the trend of the ionic grating predominantly determines the temperature dependence of the diffraction efficiency above approximately 80 degrees Celsius even though a magnitude of the hole grating monotonically increases and is more than ten times larger than that of the ionic grating. This finding explains origin of the previously reported behavior of the temperature-dependent diffraction efficiency in barium titanate [D. Zhang et al., Appl. Opt. 34, 5241(1995)]. An activation energy of the mobile ions is also determined to be 1.05 eV from which shelf life is estimated to be over 150 days at 25 degrees Celsius.

The temperature dependence of the self-pumped phase conjugation phenomenon in the no-doping photorefractive barium titanate crystals is investigated. We found the no-signal time of a few seconds before observing the self-pumped phase conjugation. The no-signal time decreases as the temperature goes up until 110 degrees Celsius. Before 80 degrees Celsius, the rise time of self-pumped phase conjugation goes down exponentially. Higher than 80 degrees Celsius, we could not observe the steady signal of self-pumped phase conjugation, and it fluctuates between zero and saturate intensity randomly. The duration of zero intensity is longer and more when the temperature goes up from 80 degrees Celsius to 110 degrees Celsius. We use the mobile-ion model to explain the phenomenon. The increase of the mobility of charge carriers and the decrease of the relative dielectric constant with temperature can be used to explain the faster response time of self-pumped phase conjugation. That the absorption constant does not change and electro-optic coefficient decrease with temperature is used to explain the decrease of the diffracted intensity of self-pumped phase conjugation with temperature. The mobility of electrons is much larger than that of holes in the barium titanate crystals at elevated temperatures.

Twelve lithium niobate crystals doped with Ce, Co, Cr, Cu, Fe, Mn, Ni, Rh, Tb, Fe:Ce, Fe:Cr, and Fe:Mn have been grown and tested. The transmission spectra and dark conductivity of the crystals are presented. Holograms have been written in each of the crystals with wavelengths from 457 nm to 671 nm. The quality and stability of the holograms vary dramatically among the crystals. The properties of the holograms and a comparison of sensitivities between the crystals are discussed.

A model for beam propagation through a nonlinear material that takes into account inhomogeneous induced refractive index changes due to the nonlinearity is first developed. The theory based on this model can be used to analyze the propagation of Gaussian beams through photorefractive lithium niobate. A focused Gaussian beam of circular cross-section incident on the sample emerges as an elliptic Gaussian after interaction in this material. We observe that the value of the nonlinearity coefficient derived from a z-scan of photorefractive lithium niobate compares favorably with that found by varying the power P of a Gaussian beam focused at a fixed longitudinal position within the sample and monitoring the far-field beam ellipticity. We use the value of the nonlinearity coefficient to determine the donor to acceptor ratio of dopants in photorefractive lithium niobate samples. We show how our theory can be extended to include two species of impurities in the photorefractive material.

Photorefractive materials are of considerable interest for the development of optical devices. Recently, attention has been drawn to the photorefractive polymer owing to its excellent properties. In this paper, photorefractive polymer composite based on photoconductor poly(N-vinylcarbazole) (PVK) is described. As an electro-optic chromophore (S)-(-)-N-(5-nitro- 2-pyridyl)prolinol(PNP) is used in the composite. In order to decrease the grass transition temperature N-ethylcarbazole (ECZ) is added. A small amount of 2-4-7 trinitro-9 fluorenon (TNF) is also added to increase the photosensitivity in visible region. The composition is PNP:PVK:ECZ:TNF 50:33:16:1 wt%. The mixture of these materials is dissolved in THF and is stirred sufficiently. The PR samples are prepared by casting the mixture onto the glass plate and pressed another glass at about 140 C. The glass plates are coated with transparent electrode for application of the external electric field. The film thus obtained is light yellow and the optical absorption coefficient of this film is 25 cm^-1 at 633 nm. Two coherent writing beams from He-Ne laser are overlapped in the sample to create a fringe pattern. From the measured increase and decrease in the writing beam powers, the two beam coupling gain coefficient is calculated. In our experiment gain coefficient is 101 cm^-1 and 68 cm^-1 for p- polarized and s-polarized light respectively at 100 V/10^-6 m.

Electron paramagnetic resonance (EPR) for specially doped LiNbO₃ crystals was determined. Study of the charge transfer and the photosensitivity in these crystals determined the ions, which participate on the charge transfer processes. In doubly doped crystals with Fe:Cr and Fe:Ce the decay of the EPR signal was less than the decay time in LiNbO₃:Fe:Mn. some inherited impurities in these crystals like Mn and Fe were detected. The effect of the inherited impurities on the decay and growth of the EPR signal was estimated by comparing the results with homegrown LiNbO₃ crystals.

In this paper we derive the eigenmodes for the nonlinear self- organization of light in photorefractive potassium niobate. This is the first step of the two-step process involved in the self-organization of light into a hexagonal pattern observed in this and other materials. Complete steady state solutions are derived and amplitude and phase variations of the counterpropagating main beam and its spatial sidebands (responsible for ring formation) are shown. Our computations show the bifurcation diagrams of the amplitudes and phases as a function of the photorefractive gain parameter. Dependence of the results on the amount of linear scattering is also demonstrated.

In almost all cases, vibration will nullify photorefractive induced effects and thus provide no benefit for us. This paper shows that we can make use of vibration to eliminate the effect of one grating and retain the effect of the remaining gratings in a multiple grating environment, such as those created via multiple two-wave mixing. If the two grating periods are different, this type of elimination is equivalent to spatial frequency filtering. We report here the first results on real-time spatial frequency filtering for the gratings formed via two-wave mixing within photorefractive crystals.

Holographic memories can be read-out either with the reference or the signal beam. Reference beam read-out reconstructs the stored data whereas signal beam read-out performs a search of the stored data base. This dual mode of holographic memories is explored for the various methods that have been developed for multiplexing holograms.

Bragg selectivity in volume holography can be exploited to store many holographic pages within the same physical volume. The detailed character of this Bragg selectivity is also responsible for the crosstalk among stored pages and is governed by the 3D envelope function of the volume hologram. Other workers have discussed these crosstalk effects in detail. In this paper we discuss the effect of reference beam apodization on the angular selectivity of photorefractive volume holograms. Apodization using phase-only beam forming is studied and we focus on the use of apodization during hologram retrieval. The trade-offs among storage capacity, information density, and noise are discussed for both the apodized and unapodized configurations. The presence of conventional hologram crosstalk as well as effects due to material absorption and recording angle jitter are included.

We have designed and implemented a compact holographic memory module that uses a dual spatial-light modulator/detector optoelectronic integrated circuit in the lens-less phase- conjugate geometry. In this paper we discuss optical design and systems issues and present recent experimental results.

The crosstalk noise between volume holograms multiplexed with various orthogonal phase codes is analyzed. The methods for the generation of Hadamard codes of different orders are also briefly described. Compared with a common Hadamard matrix of order of a power of 2, a Hadamard matrix whose order is not a power of 2 generally does not have a single bad code. To achieve the maximum possible storage capacity of a phase-code multiplexed holographic memory, the orthogonal phase codes need to be carefully selected by considering both the available pixel numbers of spatial light modulators and the crosstalk effect between stored holograms.

We report our observation of the generation of a conical ring when two extraordinarily polarized beams are incident upon a heavy-doped LiNbO3. We demonstrate that large refractive index change, multiple scatterings and diffractions through Bragg- mismatching condition exist in the crystal. It leads to the generation of the conical ring.

The mathematics relations of the system characteristics versus the material properties and read/write conditions are obtained by analysis the frequency-domain optical storage system. Through computer simulation analyses, the optimal parameter space is obtained. In addition, absorption area of material, quantum efficiency and recording light intensity are analyzed quantitatively.

An acousto-optic method for spectral processing of rf signals is proposed. This method is based on a multichannel cell with frequency separated channels within a given band. The optimum structure of such a system is a multichannel cell with the slow shear mode in the (110) direction in TeO₂ and far- axis anisotropic diffraction. A system with 12 channels covering the frequency band of 84 - 96 MHz with the bandwidth of each channel of approximately 0.5 MHz and frequency separation of approximately 1 MHz is experimentally studied. An optical beam which spreads in the plane orthogonal to that of the acousto-optic interaction must be used in this system. The influence of the transducer electrode shape on the acoustic crosstalk in the adjacent channels is studied. The experimental results are in good agreement with the calculated data. The expansion of acousto-optic processing requires that multichannel acousto-cells be used. Narrow-band acousto-optic interaction regimes can be used for frequency-domain filtering of rf signals in multichannel cells. This scheme can be used for the parallel analysis of an rf signal spectrum. This paper describes the process of the design and manufacturing of a multichannel acousto-optic filter for an rf signal with a narrow bandwidth of each channel and estimates its possible parameters. Each channel of the filter is tuned to its own frequency different from those of the adjacent channels within a given overall bandwidth of the whole device.

Optical 3-D waveguides can be fabricated by moving a focus of a strong laser beam in photorefractive materials. If Bragg reflectors as small as the waveguides can be fabricated, they can be used as spectral filters, beam splitters, or beam combiners along with the waveguides or network of waveguides. Small complex optical systems such as a very small interferometer can be made in the photorefractive media. We first demonstrate our fabrication experiments of a straight, curved and array of photorefractive waveguides. Then we present the concept of small Bragg reflector in a lithium niobate crystal and show the experimental results. We confirmed the spectral selectivity of this micro-Bragg reflector by using light from a tunable dye laser. We also confirmed that we can fabricate waveguide structures simultaneously with the micro-Bragg reflector, by using a crystal whose c-axis is directed 45 degrees off the optical axis of the microscope objectives. Unfortunately, however, the reflectivity of the micro-Bragg reflector is not large enough for the immediate use.

We present and describe different techniques based on the photorefractive effect that are used for the optical detection of ultrasonic signals. These techniques use the wavefront adaptation properties of the photorefractive effect. They are: the photorefractive beam combiner, the double phase conjugate heterodyne detection and the adaptive photodetector based on non steady state photoelectromotive force. Their respective advantages and drawbacks, are overviewed. We insist on the latest development and performances obtained with the photorefractive beam combiner that seems to use the most promising technique for the detection of ultrasonic signals on rough surfaces. We show that a sensor with near optimum sensitivity can be developed with the same photorefractive crystal at different wavelength in the range of 1 micrometer to 1.55 micrometer.

Wideband optical signal processing for adaptive antennas calls for a high-performance spatial light amplifier (SLA). The latter can be implemented as an upgrade of micro-channel SLA by replacing bulk crystal with a single-crystal fiber array (SCFA). This paper is focused on SCFA fabrication. The design process from specifying SCFA parameters to selecting fabrication technique (dice-and-fill or laser-heated pedestal growth) is reviewed. The results achieved with the dice-and- fill method are presented.

Two-wave mixing in a Bi₁₂TiO₂₀(BTO) fiberlike crystal at a laser-diode (LD) wavelength of 685 nm is presented. The energy-exchange effect between two coherent beams is observed in BTO single-crystal optical fiber under external alternating voltage. A laser-diode optical-heterodyne interferometry with a photorefractive two-wave mixing in BTO crystal is performed by using a frequency-ramped LD. The photorefractive grating is averaged with the time constant of a crystal. The experimental result is shown to measure a motion of a minor attached to a piezoelectric transducer.

Optical fiber and fiber taper are used for reference waves in writing and reading volume holograms. The object wave interferes with the speckle pattern reference wave coming from fiber tip in the holographic medium to make a volume hologram. The spatial selectivity of the volume hologram is improved by the proposed scheme. Both experimental results and theoretical analysis are discussed.

In this paper, the application of fiber and waveguide crystal gratings to narrow band, fast speed, tunable filter is presented. First, we present a brief introduction and motivation of this work. After that a brief theory is reviewed. Furthermore, several approaches of fabricating this type of filter are discussed. Finally, some preliminary results are provided.

A novel method for optical reconfigurable interconnections by moving holograms with ring form of interferometric fringes on photorefractive disk in coherent optical processor architecture is proposed. Reconfigurable interconnections are realized by deflection of light on holograms with ring form of fringes and different space frequency which insert to Fourier plane under the disk rotation. Multiplex holograms recorded on photorefractive disk could provide shift operations between two images for optical computing algorithms.

The results on experiments on 3-D speckle-shift holograms multiplexing in thick-layered gelatin are presented. The details of thick-layered dichromated gelatin synthesis are described. The axial scheme of a speckle-shift hologram is suggested and considered. The results of the experiments on holograms multiplexing are discussed. The experiments have confirmed the possibility of multiplexing the holograms by shifting the light-sensitive material by the distance equal to the size of a speckle of the reference wave. The experiment has also shown that in the case when holograms are multiplexed in a thick-layered gelatin their lifetime decreases as the number of holograms increases.

Increasing demands for multi-media information in a networked society requires an optical fiber communication system with ultra-high transmission capacity at all times. A lot of researchers make strong effort to increase transmission capacity in an optical fiber communication system. The main target in multi-media is certainly ultra-high speed information processing including high-resolutional image signals transmission. Especially, to realize ultra-high speed image processing with a current communication infrastructure, it is necessary to develop a new technique for ultra-fast conversion between image signals and time sequential ones. We propose the novel optical space-to-time-to-space conversion technique for the ultra-fast image transmission by using an ultra-fast pulse laser.

We demonstrate a real-time optical image recognition system using photorefractive thin plates. The effects of donor/acceptor concentration ratio in thin lithium niobate crystal plates on the photorefractive response time and the grating diffraction efficiency are presented.

A basic conception of an adaptive, remote optical sensor for small angle tilt displacement in 3D space is presented. Sensor is based on high space resolution 11,000 lines/mm adaptive photorefractive gratings has no moving parts is portable, robust and can provide measurement in plane and out-of-plane displacements simultaneously. Resolution of small angle tilt measurement of order of 10^-5 Rad has been achieved. Advantages and disadvantages of this sensor are discussed.

We present experimental results of holographic grating recording in the near ultra-violet (UV) in photorefractive LiNbO₃ crystals. The UV wavelength limits of Fe-doped LiNbO₃ crystals for UV grating filters have been established by characterizing the grating diffraction efficiency versus recording wavelength ranging from 300 to 400 nm. Methods for improving the UV performance of LiNbO₃ have also been investigated. It has been shown that the material absorption loss of Fe:LiNbO₃ crystals can be reduced and the grating efficiency can be a significantly improved by using proper post-grown oxidization treatment. Using the improved UV LiNbO₃ crystal, a sub-Angstrom bandwidth holographic grating imaging filter for solar observation at the Ca K-line (393.3 nm) has also been fabricated. Narrow bandwidth (22 pm, FWHM), large numerical aperture (f/15), large field-of-view (35 mrad), large aperture (15 mm in diameter) and high in-band diffraction efficiency (25%) have been successfully demonstrated. These results indicate an improved performance at a reduced cost as compared with currently available Lyot filters for solar magnetic field sensing at Ca K-line. Applications of the UV photorefractive holographic grating devices include solar and planetary observing, lidar receiving sub-systems for Earth remote sensing and atmospheric monitoring, and high density UV optical data storage.

Here we propose a new kind of phase-shifting interferometer by using an SPPCM with BaTiO₃. The phase shifting is derived from the inherent frequency shift of the SPPCM. The interferometer has two characteristics: self-generating phase conjugate wave and phase shifting. The interferometer is demonstrated to have good performance on the resolution of measurement.

A scheme of using carbon-dioxide laser beams to thermally fix layers of photorefractive holograms to implement a solid-state optical system in a single block of LiNbO₃ crystal is suggested. In this paper, a modified gamma network is adapted to demonstrate, and a stage of such a network is constructed in a block of crystal. To limit the heating region with the shape similar to the heating beam, two carbon-dioxide laser beams of narrow width are used to laterally illuminate a layer of crystal, and additional absorbers are attached on the crystal surfaces outside the heating region to guide the heating flows. Analytical discussion based on the heat- transfer equations and the proof-of-principle experimental result are given. It can be thus seen that it is possible to package an optical system into a miniaturized single-block of photorefractive crystal.

In this paper, an in real time and in situ fixing method by heating with a carbon-dioxide laser beam is suggested to thermally fix a small local hologram in a bulk of Fe:LiNbO₃ photorefractive crystal. To form a heating-up to 100 - 200 degree Celsius volume with the shape similar to the laser beam, heat-guiding technique is developed. Based on the heat transfer equations, different heating modes with or without metal absorbers for heat-guiding by using a continuous or pulsed laser beam are analyzed. The optimal mode may be the case of pulsed heating with absorbers. On this basis, experiments are designed and demonstrated. It is seen that the fixing process by carbon-dioxide laser beam is very short compared with the process by oven and the fixing efficiency is quite high.

Both the photorefractive (PR) and holographic characteristics of 3-amino-9-ethyl carbazole (AEC)/dispersed orange 3 (DO3)/diglycidyl 1,2 cyclohexanedicarboxylate (DCD) main chain copolymer with the carbazole and the azo chromophore incorporated in a transverse way were investigated without external applied electric field. The recorded pattern exhibited good fringe contrast, the resolution of the recorded hologram was about 20 micrometers. Problems such as dopant aggregation, fanning, and image distortion was not found in the system.