B. Fleck

Photoinduced deformation of azobenzene polyester films

Abstract.We investigate two types of azobenzene side-chain polyesters which have shown opposite behaviour in light-induced surface grating formation experiments. Thin films of these polymers prepared on a water surface undergo opposite changes of shape under the influence of polarized light. We propose this deformation to be responsible for the surface relief gratings.

An improved method for separating the kinetics of anisotropic and topographic gratings in side-chain azobenzene polyesters

Abstract.The induction of anisotropy gratings in side-chain azobenzene polyesters is accompanied by the formation of surface relief. We introduce an improved holographic method to separate the contributions of the anisotropic and the topographic part to the diffraction efficiency by analyzing the polarization of the first-order diffracted beam. The main advantage of this method is that both parts can be determined simultaneously by only one measurement. Furthermore the displacement between both gratings can be determined in a similar manner. Experimental results obtained with two different polyesters are presented.

Incoherent modification of polarization gratings in photoanisotropic media

Abstract The influence of an incoherent wave on polarization gratings written in a photoanisotropic material is investigated. For the example of a polarization grating written with two orthogonal circular polarized waves the diffraction efficiency was found to change by adding a third incoherent wave. A theoretical description of the modified grating is given. Using a simple model the alteration of the diffraction efficiency is estimated and found to be in good agreement with the experimental results.

Characterization of photoanisotropic gratings

We present a theoretical method which allows one to predict qualitatively the diffraction efficiency of holographic gratings in photoanisotropic materials in dependence on different parameters of the grating generating waves. Such a model is necessary for the optimization of experimental parameters. The description was developed under the assumption of thin gratings. Theoretical results are verified experimentally.

A Simple Interferometric Method for Measuring the Fidelity of Phase Conjugated Beams

Abstract One of the most interesting properties of the process of nonlinear optical phase conjugation is the capability of compensating for aberrations. The quality of compensation can be determined by investigation of the wavefront. A simple interferometric method for measuring fidelity of phase conjugated beams which allows a quantitative evaluation with respect to the phase distribution is described. This method was used to investigate the quality of phase conjugation by four-wave mixing in saturable absorbers. Illustrative examples are presented for the case of a phase conjugating and an ordinary mirror.

Determination of the response time of photoanisotropy in azobenzene side-chain polyesters

Abstract We present a method which allows the determination of the response time to polarized light of an azobenzene side-chain polyester. This method is based on the measurement of intensities in dependence on the delay time between a pump and a probe pulse. The described method does not need a very sophisticated time resolving measuring equipment. The time resolution is in the order 10 ps and is comparable with the laser pulse duration. The response time of the azobenzene side-chain polyester E1aP has determined to be about 40 ps.

Transient phase measurement using a self-pumped phase-conjugate mirror as an optical-novelty filter

A transient phase-measuring technique that uses a self-pumped phase-conjugate mirror as an optical novelty filter is presented. A change in the reflectivity of the mirror as a function of a change in the incident wave front permits the transient measurement of a two-dimensional phase distribution. This method allows us to investigate fast processes using media with slow response. A simple theoretical model explains the experimental results with sufficient accuracy. The results can be used for the calibration of the measuring system. The described method is used for the measurement of a temporally varying wave front.

An application of azo-dye-doped polymer films in optical signal processing using Fourier holograms

Investigations have been carried out on azo-dye-doped polymer films to determine their suitability for use in information processing by holographic methods. The holographic build-up and erasure times of the azo dye Disperse Red 13 in polymethyl methacrylate, have been measured. These measurements have been taken at different reference-to-signal-beam-intensity ratios to determine the optimum ratio for maximum diffraction of the readout beam by the photoinduced grating. The edge enhancement of various objects, both simple and complex, was demonstrated with this azo-dye-doped polymer film, illustrating the possibility of nonlinear optical filtering using such photoanisotropic materials. The special feature of the described method is the use of the two interacting beams without any additional devices. Furthermore this method allows the edge enhancement of moving objects.

Azo-dye-doped polymers as novel optical storage media in holographic interferometry

We describe a real-time holographic interferometric set-up for the determination of dynamic deformations of reflecting surfaces. The optical storage medium used is based on the novel polymer poly(vinyl alcohol) with alkoxyazobenzencarbonyl substituent. This material does not need any development and is characterized by low costs.

Holographic interferometry in high-temperature flows using the high entalphy shock tunnel in Gottingen (HEG)

For the optical flow visualization in the new high enthalpy shock tunnel of the German Aerospace Research Establishment (DLR) in Gottingen (HEG) a fully automated holographic interferometer has been installed and improved. The interferometer applies the phase shift technique, which enables computerized evaluation of the holograms, to create high quality interferograms. Because of the high resolution of the images, optical enlargement and hologram-schlieren is made possible. With the use of the simple Gladstone-Dale equation it is possible to calculate complete flow field gradients.