Amplified phase-conjugate beam reflection by four-wave mixing with photorefractive Bi(12)SiO(20) crystals

Edge-enhanced correlation with four-wave mixing in bismuth silicon oxide crystal by use of moving gratings

The nonlinearity of phase-conjugate beam reflection with four-wave mixing in bismuth silicon oxide crystal by use of moving gratings at large fringe modulation formed by the incident-beam ratio is investigated. On the basis of this investigation, the edge enhancement of an object and the edge-enhanced optical correlation with improved discrimination capability are achieved by use of moving gratings at an appropriate fringe velocity. Experimental results are presented.

Edge enhancement by use of moving gratings in a bismuth silicon oxide crystal and its application to optical correlation

The technique of moving gratings in a photorefractive crystal is applied to the edge enhancement of objects and edge-enhanced optical correlation. The nonlinear dependence of the optimum fringe velocity on the fringe modulation and the variation of the enhancement of the diffraction efficiency with fringe modulation at a fixed fringe velocity appropriate to high fringe modulations are experimentally investigated. It is shown that the diffraction at high fringe modulations, which corresponds to the high-spatial-frequency components of the Fourier spectrum, is enhanced by a factor of approximately 3.7, whereas the diffraction at low fringe modulations is suppressed by a factor of 0.6. The proposed technique has the advantages of real-time enhancement, arbitrary selection of the spatial frequency to be enhanced, and improved stability of the output. Experimental results of the edge enhancement of objects and edge-enhanced correlation are presented.

Effects of optical bias on moving gratings in bismuth silicon oxide at large fringe modulation

Optical bias has been applied in the formation of moving gratings in bismuth silicon oxide at large fringe modulations. It is shown that optical bias is an effective method of overcoming the problems associated with the sudden drop in the optimum fringe velocity when the fringe modulation is close to unity. It is experimentally found that within a certain range of optical bias the absolute diffraction efficiency can be higher than that without optical bias, which is not the case when a stationary grating is used.

Holographic-recording improvement in a bismuth silicon oxide crystal by the moving-grating technique

We investigate the enhancement of the diffraction efficiency of dynamic gratings recorded in a bismuth silicon oxide crystal at large modulation by the moving-grating technique. The optimum fringe velocity for maximum diffraction efficiency and the degree of enhancement of the diffraction efficiency at optimum fringe velocity are experimentally found to be dependent on the fringe modulation. We apply this technique to real-time incoherent correlation using bismuth silicon oxide. There are two main advantages in using moving gratings: First, the signal-to-noise ratio can be improved considerably because of the improved diffraction efficiency. Second, the resonant effect reduces the effect of environmental disturbances on the peak-correlation intensity, which is significant when the threshold detection level of the correlator is set.

Photorefractive adaptive transmission system

A photorefractive system based on the combined operation of a strontium barium niobate input amplifier and a self-pumped barium titanate phase-conjugator crystal is presented and analyzed. Amplified real-time phase-conjugate wave-front generation in this system is demonstrated. The system's ability to all-optically, temporally modulate a weak probe signal and to transmit it back to its source by means of amplified dynamic phase conjugation demonstrates its potential in adaptive optical transmission schemes.

Gain enhancement by signal beam chopping for two-wave coupling with a BSO crystal

Gain enhancement via signal beam chopping for two-wave coupling with a Bi(12)SiO(20) crystal is demonstrated. In this method the gain of a two-wave coupling scheme is increased using a transient effect. This effect is created during the grating formation due to the phase difference between the interference fringe of the light intensity incident on the crystal and the photoinduced refractive index fringe in the crystal. The maximum gain of 11.7 was achieved with an applied electric field of 6.0 kV/cm, a fringe spacing of 29.5 microm, a beam intensity ratio of 1240, and a chopping frequency of 6.0 Hz. This gain is nearly as high as that obtained with the moving interference-fringe method proposed by Huignard and Marrakchi. The gain enhancement using the transient effect caused by the polarization rotation of the signal beam (using a rotating halfwave plate) is also described. Preliminary experimental results are shown.

High-fidelity image amplification and phase conjugation in photorefractive Bi(12)SiO(20) crystals

Wide-angle, large space-bandwidth-product coherent image amplification with a flat spatial frequency response is demonstrated in photorefractive Bi(12)SiO(20) crystals under nonstationary grating recording conditions. By utilizing large-divergence, mutually phase-conjugate pump beams provided by a self-pumped BaTiO(3) crystal, high-fidelity phase conjugation with high reflectivity in Bi(12)SiO(20) is demonstrated.

Near-infrared four-wave mixing with gain and self-starting oscillators with photorefractive GaAs

We report high-reflectivity four-wave mixing in photorefractive GaAs:Cr crystals at 1.06 microm using a nearly degenerate interaction with an externally applied dc electric field (moving-grating recording mode). The highest reflectivities (R approximately 5) are observed for the positive-readout configuration with cross-polarized pump beams. The variation in reflectivity with the pump-beam ratio is presented for two pump-beam polarization states and compared with theory. Self-starting optical resonators are also described.

Two-dimensional, optical temporal bandpass filter using four-wave mixing in a BSO crystal: dynamic schlieren imaging system

In four-wave mixing in BSO crystal, the frequency of the reference wave is changed to construct a 2-D optical temporal bandpass filter with arbitrary center frequency. This filter achieves real-time precise observation of the spectral components of an oscillating object field, or it converts the frequency of the time-varying phase object field into intensity. In comparison with the traditional schlieren method imaging static states of the object, this novel method may be called the dynamic schlieren method. The principle and experimental results are given, showing the basic filtering characteristics and demonstrating two typical uses of the filter.

Nondegenerate optical oscillation in a resonator formed by two phase-conjugate mirrors

We find experimentally and explain theoretically that in a resonator bounded by two phase-conjugate mirrors (PCM's), unlike in resonators with one PCM and one conventional mirror, the oscillation frequency differs from that of the pumping beams. Using BaTiO(3) for the PCM, we find extremely small deltaomega/omega approximately 10(-15).

Self-induced coherent oscillations with photorefractive Bi(12)SiO(20) amplifier

We report coherent oscillations due to self-induced moving gratings in photorefractive BSO crystals when recording with an external applied field. Different types of ring and phase-conjugate resonators pumped with low-incident- intensity beams and having short response times are demonstrated. The conditions permitting oscillation and the respective frequencies of the pump and the signal beams in the cavity are analyzed.