J. Goltz

Enhanced four-wave mixing in photorefractive BaTiO3 by use of tilted pump waves

Abstract We present an experimental investigation of the phase conjugate reflectivity as a function of angular deviation from collinearity of the pump waves in BaTiO 3 . In that case of four-wave mixing the coupling process between the interacting waves is not precisely phase matched with the consequence that the conjugate beam intensity increases compared to the collinear case. We observed an enhanced phase conjugate reflectivity up to 36% relative to exact collinear alignment for deviations about 0.4 mrad from ideal pump-beam collinearity. This result is compared with our theoretical predictions.

Nearly degenerate four-wave mixing in photorefractive crystals, an analytical treatment

Abstract We extend the standard model of four-wave mixing in photorefractive crystals to the case where the Bragg-condition is broken by a slight departure from degeneracy. Our analytical treatment shows that for this conditions an increased phase conjugate reflectivity can be obtained.

Four-wave mixing in photorefractive crystals with depleted pumps

We present an analytical solution and experimental results for four-wave mixing in photorefractive crystals with depleted pumps. The approximation that we introduce permits a theoretical treatment without numerical computation. The measurements, which were performed using a BaTiO3 crystal, are compared with this model.

Dynamics of two-wave mixing with running gratings in photorefractive crystals

Abstract In this study we extend previous works of the dynamics of two-wave mixing in photorefractive crystals in respect of two points: (i) We allow different frequencies of the two beams. (ii) We calculate both hologram build up and erasure, where the writing time may be arbitrary.

Angular selectivity of volume holograms recorded in photorefractive crystals; An analytical treatment

Previous examinations of the angular selectivity of volume holograms recorded in photorefractive crystals are not fully analytical. These examinations have shown that the minima of the diffraction efficiency versus angular mismatch curve are nonzero for volume holograms attenuated along the axis perpendicular to the grating vector. The periodic variation of this curve becomes obscure as the coupling strength increases and for strong coupling the curve becomes monotonic. We can describe this behaviour with an analytical solution which is based on the undepleted pumps approximation. Furthermore, our solution shows that all minima of the order n

Equivalence of the method of thin slice decomposition and calculation of Maxwell's equations for screw-like orientated liquid crystals

Abstract For the description of wave propagation in thick optically anisotropic media, in general the solution of Maxwells equations is necessary. By decomposition of the medium into thins slices and subsequent analytic treatment with the amplitude transmittance theory, the mathematical effort can be reduced considerably. We demonstrate the applicability of this method by the calculation of wave interactions in screw-like oriented liquid crystals.

Dynamics of hologram readout in photorefractive crystals for broken Bragg-condition

Abstract We calculate erasure during hologram readout in photorefractive crystals for the case that the Bragg-condition is broken. Our analytical solution is compared with experimental investigations in BaTiO 3 with an angular tilt between writing and readout pump beam.

Critical coupling strength for enhanced four-wave mixing by use of moving interference gratings in photorefractive crystals

Abstract Increased four-wave mixing reflectivity in BaTiO 3 has been observed in the case of moving interference gratings earlier [Phys. Rev. Lett. 55 (1985) 821]. In diffusion dominated photo-refractive crystals this increased reflectivity is obtained only for a coupling strength above a critical value. In our analytical study we calculate this coupling strength for week input signals as a function of the pump beam intensity ratio. Then we discuss the reflectivity versus grating (temporal-) frequency dependence.

Beam coupling in volume holograms recorded with N+1 plane waves

Abstract We calculate beam coupling in volume holograms recorded with N object waves and one reference wave, all assumed to be plane. We consider the case where the coupling among the object beams is described by the same coupling coefficient for all object beams. Arbitrary signal to reference beam intensity ratio is allowed. We find that for readout with the recording reference wave, the total diffracted energy decreases with the number of signal waves until the limit value is reached. However we find a special case where 100% diffraction efficiency is obtained by deviation from the Bragg-condition.

Coherent optical correlator for diffuse scattering objects using a spatial light modulator

Abstract When the principle of matched filtering is applied to correlate in reflection scattering objects, usually unsurmountable problems arise due to unwanted signal parts generated by the rough surface of the objects. This kind of noise will be eliminated by incoherent illumination of the object. To still have the convenience of complex filtering, a spatial light modulator with a deformable oil layer is used which modulates the phase of a He-Ne-laser according to the intensity of the incoherent object image. It will be shown that this spatial light modulator is suitable for application in principle. To get reproducible results the environment of the oil layer must be well controlled.