Showing papers by "Giovanni Pierattini published in 2004"

Controlling image size as a function of distance and wavelength in Fresnel-transform reconstruction of digital holograms

Abstract: A method for controlling the size of amplitude and phase images reconstructed from digital holograms by the Fresnel-transform method is proposed and demonstrated. The method can provide a constant reconstruction pixel width in the reconstructed image plane, independent of the recording and reconstruction distance. The proposed method makes it possible to maintain the size of an object for a sequence of digital holograms recorded at different distances and, therefore, to subtract phase maps for an object recorded at different distances. Furthermore, the method solves the problem of superimposition in multiwavelength digital holography for color display and holographic interferometry applications.

Recovering image resolution in reconstructing digital off-axis holograms by Fresnel-transform method

Abstract: The resolution of amplitude and phase reconstruction of images from digital holograms by the Fresnel transformation method is limited by the pixel width in the reconstructed image plane. The reconstruction pixel depends on distance, wavelength, and number of pixels in the hologram. The spatial frequencies, in the reconstructed image, are band limited by the size of the reconstruction pixel. Undersampling can occur, in the image plane, in the reconstructed amplitude and/or wrapped phase maps. Recovery of reconstructed undersampled spatial frequencies, is possible by the fictitious enlargement of the digital hologram. Correct profile reconstruction of a silicon microelectromechanical structure is demonstrated, applying the proposed method.

In-situ visualization, monitoring and analysis of electric field domain reversal process in ferroelectric crystals by digital holography.

Abstract: In-situ monitoring of domain reversal in congruent lithium niobate by a digital holographic technique is described. While the ferroelectric polarization is reversed by electric field poling, the two-dimensional distribution of the phase shift, due mainly to the linear electro-optic and piezoelectric effects, is measured and visualized. Digital holography is used to reconstruct both amplitude and phase of the wavefield transmitted by the sample to reveal the phase shift induced by adjacent reversed domains during the poling. The resulting movies of both amplitude and phase maps, for in-situ visualization of domain pattern formation, are shown. The possibility of using the technique as tool for monitoring in real-time the periodic poling of patterned samples is discussed.

Evaluation of the internal field in lithium niobate ferroelectric domains by an interferometric method

Abstract: We report on the evaluation of internal electric field of a ferroelectric engineered-domain in a LiNbO3 wafer crystal by detecting optical path length variation with a noninvasive interferometric ...

Talbot self-image effect in digital holography and its application to spectrometry.

Abstract: For the first time to the authors' knowledge, the Talbot effect has been observed and investigated in digital holography. By numerical reconstruction of holograms, the Talbot self-imaging phenomenon is observed by reconstruction of the amplitude of the image at different distances and (or) wavelengths. A simple spectrometer based on Talbot self-imaging in digital holography is proposed and demonstrated.

Investigation of electric internal field in congruent LiNbO3 by electro-optic effect

Abstract: We investigate the effect of the defect-induced internal field on the electro-optic behavior of z-cut congruent lithium niobate crystals. We measure, by a spatially resolved interferometric techn ...

Real-time phase-contrast analysis of domain switching in lithium niobate by digital holography

Abstract: We present a method for in-situ visualization of electric field domain reversal in congruent lithium niobate (LN) through an electro-optic interferometric technique. The crystal refractive index n changes by the linear electro-optic and piezoelectric effects along the z crystal axis, due to the external electric field. This variation depends on the domain orientation so that two adjacent antiparallel domains present a refractive index difference equal to 2Dn which is used for in-situ visualization of the reversed domain pattern during formation. A digital holographic (DH) technique is employed for a two-dimensional (2D) reconstruction of the wavefield transmitted by the sample in amplitude and phase during the process. The corresponding amplitude-map and phase-map movies are presented. The amplitude-map gives qualitative information about the spatial evolution of the domain boundaries while the phase-map provides measurement of the 2D distribution of the phase shift induced along the z axis. The phase-map movies provide unequivocal information about the spatial distribution of the reversed domain regions. This technique can be used as in-situ monitoring method alternative to the measurement of the poling current which provides information only about the amount of charge delivered to the sample, ignoring the spatial distribution of the domain boundaries.

Wavefront aberration analysis in two-beam reversal shearing interferometry by elliptical Zernike polynomials

Abstract: We present a modal phase-reconstruction method for analyzing wave front aberrations of rotationally symmetric optical components in two beam shearing interferometry. The optical configuration requires only two mutually coherent off-axis plane wave fronts transmitted through or reflected by the optical component under test. Finite moire beating between the interferograms and the CCD array is used to subtract the linear carrier introduced by defocus and the tilt making the presense of high order aberrations more evident. The difference wave front is described by elliptical Zernike polynomials as a function of the amount of lateral displacement between the two aberrated wave fronts. The method allows for accurate wave front reconstruction inside the quasi-elliptical overlap area between the laterally sheared interfering beams. Results of numerical experiments and applications of the technique for measuring aberrations of simple biconvex spherical lenses are presented.

Investigation of internal electric field in LiNbO 3 crystal with two anti-parallel ferroelectric domains by interferometric technique

Abstract: Large built-in internal field is present in congruent LiNbO 3 and is due to bulk dipolar defect complexes of nonstoichiometric crystals. This field influence electric field switching of domains as well as other optical and electrical properties of crystals. Moreover it's time-temperature dependent and this feature can bias the stability of LiNbO 3 based devices. We investigate how the internal field could affect the electro-optic effect. To this aim an improved interferometric technique is used to obtain a spatially resolved measurement of the electro-optic coefficient of z-cut LiNbO 3 crystals. The samples are mounted in one arm of a reflective grating interferometer and resulting fringes patterns are visualized and stored by a CCD camera. Then this as-recorded data are processed by digital holographic technique in order to obtain 2D phase maps as function of the applied voltage across the crystal thickness. Hence spatial distribution of r 13 is achieved for crystal either in virgin state or in domain reversed one. Moreover, samples just after poling with two anti-parallel ferroelectric domains structure have been studied. Results show that the virgin area and the reversed one have quite different r 13 coefficient values. We suppose that this difference is due to the internal field, however further measurements are currently under investigation to confirm this hypothesis.

Testing silicon MEMS structures subjected to thermal loading by digital holography

Abstract: We have applied digital holography (DH) as interferometric tool for measuring the out of plane deformation of Micro-Electro-Mechanical structures. DH has been adopted as method for determining with high accuracy deformations due to the residual stress introduced by fabrication process evaluating MEMS behavior subjected to thermal load. A thermal characterization of these structures requires to cope two fundamental problems. The first one regards the loss of the focus due to thermal expansion of the MEMS sample support. With an out-of-focus image, a correct reconstruction of the sample image can not be obtained. To overcome the problem an auto-tracking procedure has been adopted. The other problem regards the direct comparison of images reconstructed at two different distances. In fact, in DH the numerical reconstruction image is enlarged or contracted according to the reconstruction distance. To avoid this problem, we have adopted a novel but very simple method for keeping constant the image size by imposing the reconstruction pixel constant through the fictitious enlargement of the number of the pixel of the recorded digital holograms. These procedures have been employed in order to characterize MEMS with different shapes and dimensions. The measured profiles obtained by DH can be employed to evaluate both the residual stress induced during the fabrication processes and its dependence on the temperature.

Recent advancements in digital holographic microscopy and its applications

Abstract: Digital holograms recorded with a charge coupled device camera array are numerically reconstructed in amplitude and phase through calculation of the Fresnel-Kirchhoff integral. The flexibility offered by the reconstruction process in digital holography allows exploitation of new possibilities of application in microscopy. Through the reconstruction process we will show that it is possible to control image parameters as focus distance, image size and image resolution. We report on recent developments obtained in the numerical reconstruction process of holograms. Novel prospective of application of digital holography in single and multi-wavelengths operation either for display and metrological applications can be explored by those recent achievements. Examples of applications of digital holographic microscopy for characterizing silicon MEMS structures by adopting new procedures are illustrated and discussed.

Method for modifying spatial resolution in the resolution in the reconstruction of images in digital holography

Abstract: The invention concerns a method Method for the reconstruction of holographic images, the holographic image being detected by an image detection device ( 9 ), the holographic image being transformed in a digitized hologram ( 10 ), the digitized hologram ( 10 ) being comprised of a number V r of elementary pixels, the size of which being equal to the holographic image sampling intervals, and of the V r values ( 51 ) respectively associated to the elementary pixels, the method comprising a first step ( 11,12 ) of processing the digitized hologram array, and a second step ( 13,15,16,17,18 ) of hologram reconstruction in the observation plane starting from the digitized hologram processed in the first step, the method being characterised in that the second step is carried out through discrete Fresnel Transform applied on an array of V e values corresponding to pixels having size equal to that of said elementary pixels, wherein said array of V e values ( 50, 51 ) includes said array of V r values and an integer number p=V e −V r >0 of constant values ( 50 ) equal to OS, said number V e of values being inversely proportional to the desired pixel size to be obtained for the reconstructed image ( 14 ). The invention further concerns the instruments necessary to the execution of the method and the apparatus executing it.

Controlling several image parameters in the digital holographic reconstruction process

Abstract: Recent developments in solid-state image sensors and digital computers have made it possible to directly record holograms by Charge Coupled Device (CCD) camera and numerical reconstruction of the object wave front by computer. Digital holograms recorded with a CCD array are numerically reconstructed in amplitude and phase through calculation of the Fresnel-Kirchhoff integral. Two methods are usually adopted to reconstruct digital holograms called Fresnel Transformation Method (FTM) and the Convolution Method (CM). In FTM, the reconstruction pixel increases with the reconstruction distance so that the size of image, in terms of number of pixels, is reduced for longer distances, limiting the resolution of amplitude and phase reconstruction. In CM, by contrast, the reconstruction pixel does not change, but remains equal to the pixel size of recording array. The CM is more appropriate for reconstruction at small distances whereas the FTM is useful for longer distances according to the paraxial approximation necessary to apply it. The flexibility offered by the reconstruction process in Digital Holography allows exploitation of new possibilities of application in different fields. Through the reconstruction process we will show that it is possible to control image parameters as focus distance, image size and image resolution. Those newly explored potentialities open further novel prospective of application of Digital Holography in single and multi-wavelengths operation either for display and metrological applications. We demonstrate the concept of controlling parameters in image reconstruction of digital holograms in some real situations for inspecting silicon MEMS structures.

A novel interferometric spectrometer obtained by imaging Talbot effect in digital holography

Abstract: We present here the observation of the Talbot effect in digital holography (DH). A self-imaging phenomena is observed by reconstructing the amplitude of the object wavefield by using different distances and different illumination wavelengths. The numerical reconstruction allows to determine the complex field amplitude at different wavelengths while maintaining constant the reconstruction distance. We investigate on the possibility to build a spectrometer based on the Talbot effect. In particular, the spectrometer proposed in this work can cover the whole visible spectrum ranging from 350 nm to 750 nm and, from the FWHM of the spectrograms with a resolution of about 20 nm

Method for modifying spatial resolution in the reconstruction of images in digital holography

Abstract: The invention concerns a method for the reconstruction of holographic images in digital holography, the holographic images being detected and transformed in a digitized hologram by an image detection device, the digitized hologram being made of an array having a number Vr of elementary pixels, the sizes of which being equal to the sampling intervals of the image detection device, the method comprising a first step of processing the digitized hologram array (e.g. zero-order elimination, aberration correction), and a second step of digitally reconstructing the complex object in an observation plane starting from the digitized hologram processed in the first step, the method being characterised in that the second step is carried out through discrete Fresnel Transform applied on an array of Ve pixels having sizes equal to that of said elementary pixels, wherein said array of Ve pixels (50, 51) includes said Vr pixels (51) and an interger number p=Ve-Vr>0 constant values (50). The invention further concerns the instruments necessary to the execution of the method and the apparatus executing it.

Digital holographic microscope for thermal characterization of silicon microhotplates for gas sensor

Abstract: Digital Holographic Microscope has been employed to obtain an accurate characterization of a micro-hotplate for gas sensing applications. The fabrication of these sensors needs different materials, with different properties and different technological processes, which involve high temperature treatments. Consequently, the structure is affected by the presence of residual stresses, appearing in form of undesired bowing of the membrane. Moreover, when the temperature of the sensor increases, a further warpage of the structure is observed. DHM allows to evaluate, with high accuracy, deformations due to the residual stress and how these deformations are affected by thermal loads. In particular, profiles of the structure have been evaluated both in quasi-static condition and the profile variation due to the biasing of the heater resistor has been measured.