Béla Ráczkevi

Holographic illumination for comparative measurement

Abstract The displacement, the shape of an object, or the refractive index distribution change in the case of transparent objects can successfully be measured by either holographic or speckle techniques. However, the demand for comparative measurements of two objects, e.g. master and test by interferometry has both conceptual and practical importance. One way of doing that is using holographic illumination for the test object. Applying holographic technique in recording test interferograms provides high resolution of records. On the other hand, the holographically illuminated test object may also be inspected by electronic speckle pattern interferometry more easily but with different quality. The paper presents results of applications of hologram interferometric and electronic speckle correlation techniques in recording process of the difference patterns showing that both techniques can be effective in comparative measurements of displacements.

Pre-excitation studies for rubidium-plasma generation

The key element in the Proton-Driven-Plasma-Wake-Field-Accelerator (PWFA) project is the generation of highly uniform plasma from Rubidium vapor. A scientifically straightforward, yet highly challenging way to achieve full ionization is to use high power laser which can assure the barrier suppression ionization (BSI) along the 10 m long active region. The Wigner-team in Budapest is investigating an alternative way of uniform plasma generation. The proposed Resonance Enhanced Multi-Photon Ionization (REMPI) scheme can be probably realized by much less laser power. In the following we plan to investigate the resonant pre-excitations of the Rb atoms, both theoretically and experimentally. In the following our theoretical framework is presented together with the status report about the preparatory work of the planned experiment.

Analogue and Digital Holography: Absolute and Comparative Measurements and Measuring Range Extension (Analoge und Digitale Holografie: Absolute und vergleichende Messungen sowie Messbereichserweiterungen)

Summary In the paper a brief survey of research at the Department is given that had a twofold aim: to support the project DISCO in indirect and direct ways as well. In this respect the physical state of the art of the analogue basis of DISCO is discussed: principle of the comparative technique named difference holographic interferometry (DHI) is analysed and applications for comparative deformation and shape control in analogue holography are presented. Peculiarities of holographic illumination as well as requirements for repositioning of the master and test objects at comparative measurements are treated. Investigations of the measuring range extension in holographic interferometry are described. Detailed analysis is devoted to the puzzle-read-out extension technique. Finally, application of the principles of DHI in comparative deformation control by digital holography and the developed portable laboratory device for comparative deformation control are presented.

The accuracy of the phase difference formula in holographic interferometry

Abstract According to the wave optical description of holographic interferometry, the visibility and the phase of interference fringes are strongly influenced by the aperture of the observing optical instrument. An extra term appears in the phase difference formula compared to the usual one calculated by the method of corresponding points of the undeformed and the deformed object surfaces. Neglecting this extra term could cause (especially at large deformations) a significant error in fringe pattern analysis — even as much as half a period of fringes. In this paper the existence of the extra phase term and its dependence on the aperture of the observing system has been proven experimentally. We have demonstrated that the extra phase term is a periodic function of the ratio of the object displacement to the focal depth of the viewing system. Further on, it has been shown that the visibility is periodic too, although with decreasing maxima. It has the same period as the extra phase term.

Holographic illumination for comparison in interferometry

Conventional holographic interferometric and speckle pattern interferometric techniques are tailored basically to compare the two states of the very same object, which may have optically rough surface or optically distorting transparent walls around. The direct comparison of the behavior of two different--but macroscopically quite similar--objects does not fit naturally in the process. At present, this type of extension can go three different ways. In holographic interferometry, one can mix the interferograms of the two objects at some sublevel of interferogram making and the moire effect of the individual fringe systems can be produced. In speckle pattern interferometry, the reference surface can be replaced by the other object itself and it serves as a live reference: changing according to the test object. Finally, although really first if time of birth is regarded, holographically recorded and reconstructed images of a master object can be used for illumination of test objects--in an otherwise conventional holographic or speckle interferometric arrangement. The present paper deals with this latter technique. Some new developments are reported in difference holographic interferometry and partly as a consequence of this--a really successful realization is introduced in holographically illuminated difference ESPI.

Stabilization and time resolved measurement of the frequency evolution of a modulated diode laser for chirped pulse generation

We have developed experimental methods for the generation of chirped laser pulses of controlled frequency evolution in the nanosecond pulse length range for coherent atomic interaction studies. The pulses are sliced from the radiation of a cw external cavity diode laser while its drive current, and consequently its frequency, are sinusoidally modulated. By the proper choice of the modulation parameters, as well as of the timing of pulse slicing, we can produce a wide variety of frequency sweep ranges during the pulse. In order to obtain the required frequency chirp, we need to stabilize the center frequency of the modulated laser and to measure the resulting frequency evolution with appropriate temporal resolution. These tasks have been solved by creating a beat signal with a reference laser locked to an atomic transition frequency. The beat signal is then analyzed, as well as its spectral sideband peaks are fed back to the electronics of the frequency stabilization of the modulated laser. This method is simple and it has the possibility for high speed frequency sweep with narrow bandwidth that is appropriate, for example, for selective manipulation of atomic states in a magneto-optical trap.

Light intensity versus small aperture in two-wavelength contouring by analog difference holographic interferometry

In two-wavelength contouring by difference holographic interferometry, the test object is illuminated holographically by real images of the master object belonging to the two different wavelengths. In this process, however, the illuminating holograms have to reconstructed together. The illuminations belonging to the other wavelengths are unwanted, but fortunately they are reconstructed with some direction shift. Thus they can be filtered out by a proper aperture in the Fourier plane of a lens. Because the shift of the corresponding spot in the Fourier plane is relatively small, the similarly small filtering aperture leads to a significant reduction of the master object wave intensities in the recording steps. This in practice may result in a low-quality master hologram and consequent poor holographic illumination as well. To overcome this, the paper suggests two techniques to increase the light intensity of the filtered master object waves. First, a shape change of the aperture is proposed, and its extension to an aperture system. Second, a special optimization of the optical arrangement geometry is suggested to maximize the applicable aperture size in the Fourier plane. The two techniques provide an order of magnitude of intensity increase, even up to more than half of the nonfiltered value.

One-wavelength in-plane rotation analysis in electronic speckle pattern interferometry

The analysis of in-plane rigid-body rotations requires phase-shifting methods to determine the direction of rotation in conventional electronic speckle pattern interferometry (ESPI). The phase-shifting procedure makes the real-time measurements impossible. A quasi-real-time method is published recently, where the usual symmetric illumination is combined with a wavelength change before the second exposure. The present paper proposes a device sparing an alternative to this. The symmetric illumination is retained but the wavelength change is replaced by simple illumination direction changes.

Beyond the upper limit of holographic and speckle interferometry

Holographic interferometry has the advantage of high sensitivity but at practical loads, the fringe systems get soon too dense to be observed conveniently. One special way of overcoming this problem is provided by the comparative methods. In the present paper, comparisons of deformations up to the millimeter region will be reported in difference holographic interferometry and an extension possibility to electronic speckle pattern interferometry will be demonstrated, too. The most direct approach would be to magnify the image to the required great extent and then build up the complete fringe system from the observed tiny parts. A practical method will be suggested here which alleviates this cumbersome procedure. In addition, if the very dense fringes are already washed away by the speckles -- some integration along the fringes proves to be of real help.