A fast-Fourier-transform method of topography and interferometry is proposed. By computer processing of a noncontour type of fringe pattern, automatic discrimination is achieved between elevation and depression of the object or wave-front form, which has not been possible by the fringe-contour-generation techniques. The method has advantages over moire topography and conventional fringe-contour interferometry in both accuracy and sensitivity. Unlike fringe-scanning techniques, the method is easy to apply because it uses no moving components.

Fourier-transform method of fringe-pattern analysis for computer-based topography and interferometry

Femtosecond laser micromachining can be used either to remove materials or to change a material's properties, and can be applied to both absorptive and transparent substances. Over the past decade, this technique has been used in a broad range of applications, from waveguide fabrication to cell ablation. This review describes the physical mechanisms and the main experimental parameters involved in the femtosecond laser micromachining of transparent materials, and important emerging applications of the technology. Interactions between laser and matter are fascinating and have found a wide range of applications. This article gives an overview of the fundamental physical mechanisms in the processing of transparent materials using ultrafast lasers, as well as important emerging applications of the technology.

Femtosecond laser micromachining in transparent materials

This article describes the principles and major applications of digital recording and numerical reconstruction of holograms (digital holography). Digital holography became feasible since charged coupled devices (CCDs) with suitable numbers and sizes of pixels and computers with sufficient speed became available. The Fresnel or Fourier holograms are recorded directly by the CCD and stored digitally. No film material involving wet-chemical or other processing is necessary. The reconstruction of the wavefield, which is done optically by illumination of a hologram, is performed by numerical methods. The numerical reconstruction process is based on the Fresnel–Kirchhoff integral, which describes the diffraction of the reconstructing wave at the micro-structure of the hologram. In the numerical reconstruction process not only the intensity, but also the phase distribution of the stored wavefield can be computed from the digital hologram. This offers new possibilities for a variety of applications. Digital holography is applied to measure shape and surface deformation of opaque bodies and refractive index fields within transparent media. Further applications are imaging and microscopy, where it is advantageous to refocus the area under investigation by numerical methods.

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Digital recording and numerical reconstruction of holograms

Fringe projection techniques: Whither we are?

Digital holography is an emerging field of new paradigm in general imaging applications. We present a review of a subset of the research and development activities in digital holography, with emphasis on microscopy techniques and applications. First, the basic results from the general theory of holography, based on the scalar diffraction theory, are summarized, and a general description of the digital holographic microscopy process is given, including quantitative phase microscopy. Several numerical diffraction methods are described and compared, and a number of representative configurations used in digital holography are described, including off-axis Fresnel, Fourier, image plane, in-line, Gabor, and phase-shifting digital holographies. Then we survey numerical techniques that give rise to unique capabilities of digital holography, including suppression of dc and twin image terms, pixel resolution control, optical phase unwrapping, aberration compensation, and others. A survey is also given of representative application areas, including biomedical microscopy, particle field holography, micrometrology, and holographic tomography, as well as some of the special techniques, such as holography of total internal reflection, optical scanning holography, digital interference holography, and heterodyne holography. The review is intended for students and new researchers interested in developing new techniques and exploring new applications of digital holography.

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Principles and techniques of digital holographic microscopy

We present an experiment of modulation of the light propagating in a waveguide-liquid crystal (LC) planar stack, by an external light source. The results confirm that the molecular reorientation in the LC layer, induced by an external optical field, gives rise to a change in the refractive index which in turn modifies the propagation constants of the guided modes, so that the light can be partially leaked off the guide. It is shown how the modulation depends on the physical parameters of the materials and of the light.

Modulating light with light: A slab waveguide-liquid crystal device

We propose a method based on an interferometric technique to measure the molecular distortion induced by an incident light beam on a nematic liquid crystal film. An analysis of the fringe pattern generated by means of a Fourier transform method allows comparison of the measurements with the theoretical molecular distribution. From this analysis an estimation of the elastic anisotropy of the liquid crystal is derived which is in good agreement with previous results.

A Fourier-transform-based interferometric technique for measuring the elastic anisotropy of a nematic liquid crystal

We investigate the domain structure obtained on overpoled both sides polished z-cut Lithium Niobate crystal sample by applying an electric field on periodic liquid electrodes. Lithium Niobate sample was spatially patterned with photoresist by use of interference photolithographic process. The sample was then etched in a mixture of hydrofluoric and nitric acid to reveal the reversed domains. The different etch rate of the two opposite z faces of the crystal domains allowed to measure the surface profile of the sample by using a profilometer. Overpoling the crystal caused the appearance of high density and submicrometer spacing single dot domains vertically aligned along the direction of the interference fringes in the patterned region. ( Summary only available )

Investigation on overpoled lithium niobate patterned crystal

Real-time and double-exposure microholographic interferometry for observing the dynamics of phase variations in transparent specimens

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.

Giovanni Pierattini

Papers

Modulating light with light: A slab waveguide-liquid crystal device

A Fourier-transform-based interferometric technique for measuring the elastic anisotropy of a nematic liquid crystal

Investigation on overpoled lithium niobate patterned crystal

Real-time and double-exposure microholographic interferometry for observing the dynamics of phase variations in transparent specimens

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