S. Riehemann

Phase shifting holographic double exposure interferometry with fast photorefractive crystals

Abstract A technique for quasi real-time holographic interferometry with reflection type objects is presented. It is based on fast sequences of holographic double exposure interferograms recorded in sillenite-type photorefractive BTO crystals. The application of the phase shifting method for automatic quantitative evaluation of the obtained interferograms under the special constraint of short storage times in fast photorefractive crystals is investigated. Approaches to overcome this difficulty by applying a time dependent external electric field are discussed. Experimental results for dynamic analysis of reflecting type objects are presented.

Optical and Photorefractive Characterization of BTO Crystals Doped with Cd, Ca, Ga, and V

Single crystals of Bi12TiO20 (BTO) are grown by a modified Czochralski technique. Doping with cadmium, calcium, gallium, and vanadium is performed by adding corresponding oxides to the melt. Optical parameters like refractive index, optical absorption, optical activity, light induced absorption, electrooptic coefficient, and photoconductivity are determined. Photorefractive properties of the samples are characterized with the help of a conventional two-beam interference setup. Diffraction efficiency and decay time constant of elementary holograms are determined. The influence of the utilized dopands is discussed.

Utilization of photorefractive crystals for holographic double-exposure interferometry with nanosecond laser pulses

A holographic double-exposure interferometer for deformation and vibration analysis of diffusely reflecting objects in quasi real-time is presented. Writing of holographic gratings with 3 ns laser pulses in photorefractive sillenite-type crystals enables recording of sequences of double-exposure interferograms. A repetition rate of 12.5 Hz is achieved with the set-up used. Automatic phase evaluation of the interferograms is performed with a spatial-heterodyne technique. Photorefractive recording in a BTO:Cd crystal with pulsed illumination is investigated and discussed in view of the application in double-exposure interferometry. The performance of the interferometer is investigated by observing deformations of a vibrating test object.

Diffraction efficiency enhancement of holographic gratings in Bi 12 Ti 0.76 V 0.24 O 20 crystals after recording

An enhancement of the diffraction efficiency of holographic gratings in Bi12Ti0.76V0.24O20 crystals by a factor of almost 40 is obtained after recording by application of an external dc electric field and illumination with a spatially homogeneous light beam. The enhancement increases with increasing applied field, larger spatial frequency, and longer writing time of the holographic grating, and with intensity of the homogeneous illumination. A theoretical description of the enhancement effect is proposed, assuming a modulated photoconductivity, which is generated by a strongly modulated concentration of photorefractive traps or donors.

Non-exponential build-up and decay of holographic gratings in Bi12Ti0.76V0.24O20 crystals

Abstract In investigations of build-up and decay of elementary holograms in Bi 12 Ti 0.76 V 0.24 O 20 (BTVO) crystals, non-exponential dynamics is observed. In combination with measurements of time constants, saturation/maximum values of diffraction efficiency and dark-decay of the holograms, we conclude that two phase-shifted photorefractive gratings are responsible for the detected time-dependence.

A high resolution real-time temporal heterodyne interferometer for refractive index topography

Abstract A Mach-Zehnder interferometer for two-dimensional refractive index topography is presented which utilizes the temporal heterodyne technique. High scanning rates and excellent phase resolutions are obtained using an image dissector camera and a lock-in amplifier. The setup is applied for investigation of the photorefractive effect. A macroscopic light pattern is recorded and thermally fixed in iron-doped LiNbO3. The evolution of the refractive index pattern during development by homogeneous illumination is observed with the interferometer. The measured refractive index profiles are in good agreement with the results of computer simulations.

Holographic double-exposure interferometry with tetragonal KTa1−xNbxO3: Fe crystals

Abstract Iron-doped tetragonal potassium tantalate-niobate (KTN:Fe) crystals are used as photorefractive storage media for holographic double-exposure interferometry. Holographic recording in the investigated crystals is influenced by thermal treatments. Deformations of a diffusely reflecting test target are observed with the help of holographic interferograms utilizing a phase-stepping technique.

High-resolution imaging with reflection-type holograms in SBN:Ce

Abstract Reflection-type holograms are recorded in SBN:Ce single crystals. The angular dependence of their diffraction efficiency is investigated experimentally and discussed theoretically. Holograms of a test chart show a spatial resolution of 4.4 μm in the reconstructed image.

Optimization of Tetragonal KTa1−xNbxO3: Fe crystals as Storage Materials for Double-Exposure Holographic Interferometry

Double-exposure holographic interferometry with as-grown and oxidized iron-doped tetragonal potassium tantalate-niobate (KTN: Fe) crystals as a photorefractive storage material is presented. The influence of oxidation treatments on the light-induced refractive index change and decay time constant is investigated for different external electric fields and spatial frequencies. The experimental findings can be explained in terms of a charge transport model which assumes one kind of photorefractive centres with hole conductivity only. The fringe contrast and exposure time of double-exposure holographic interferograms are determined. The displacement of a test target is evaluated from interferograms utilizing a quasi-heterodyne technique. For that purpose a fully automatically working computer program is developed.