We have previously reported a new design for drift-free liquid-crystal polarization modulators (LCMs) based on liquid-crystal variable retarders (LCVRs). Here, we study their performance on Stokes and Mueller polarimeters. LCMs have polarimetric responses similar to LCVRs and can be used as temperature-stable alternatives to many LCVR-based polarimeters. We have built an LCM-based polarization state analyzer (PSA) and compared its performance to an equivalent LCVR-based PSA. Our system parameters remained stable over a wide range of temperature, precisely from 25°C to 50°C. Accurate Stokes and Mueller measurements have been conducted, paving the way to calibration-free polarimeters for demanding applications.

Polarization imaging has found many applications ranging from material sciences to biomedical applications to astronomy. A widely used class of polarimeters is based on Liquid Crystal Variable Retarders (LCVR). Indeed, LCVRs are ideal for imaging application: they are versatile polarization modulators with fast response times and a large aperture. However, the main drawback of such systems is their strong dependence on temperature. As a consequence, they require frequent and time-consuming calibration procedures. In this work, we propose a new design for a temperature-stable Variable Retarders cell compatible with LCVR-based polarimeter designs. We formalize a phenomenological model for the temperature dependency of LCVRs and derive theoretical expressions for the working points of the temperature stable cells. We used a heated enclosure to validate the proposed design experimentally. Stable operation of a single cell built from commercially available LCVRs is demonstrated on a wide range of temperatures (25-50°C). Two cells were then combined to obtain a Polarization State Analyzer (PSA), acting either as a standalone Stokes polarimeter or as part of a Muller polarimeter in combination with a Polarization State Generator (PSG). In both cases, excellent stability is demonstrated compared to similar LCVR based polarimeters.

We report a new design for temperature-stable polarization modulators. Each modulator is composed of two liquid crystal variable retarders (LCVRs) positioned in such a way that their temperature drifts mutually compensate. We propose a model for the temperature-dependent polarization response of LCVRs, which permits us to establish expressions for the operating point of the system and for its accessible retardance range. We have validated such a model experimentally by thorough analyses of LCVR temperature responses, and we have built a polarization modulator that is stable over a wide range of temperature with commercially available LCVRs.

A better understanding of tumor development is crucial for treating cancer. Polarimetric imaging is an interesting alternative for monitoring subcutaneous tumors as it is non-invasive. In this study, a Mueller spectro-polarimeter is used to monitor tumor development on mice injected with non-pigmented breast cancer cells or with pigmented murine melanoma cells. Three stages of non-pigmented tumor development are revealed with three polarimetric parameters. These stages also appear for pigmented tumors, although less clearly. A halo of high depolarization surrounding the non-pigmented tumor in the first stage allows the outlining of the tumor. Considering polarimetric parameters, a biological interpretation is proposed.

Imaging spectropolarimetry is an informative technique that can be useful as a tool to detect and analyze cancerous tissues. However, to fit clinical standards, imaging spectropolarimeters must be fast, drift-free, and without any recurrent calibration, which is not the case for most imaging spectropolarimeters based on nematic liquid crystal phase modulators. Here, we present an instrument based on a novel architecture of differential liquid crystal variable retarders cells. A complete spectropolarimeter was built using this architecture and is now part of a clinical study at the dermatology department of the Strasbourg University Hospital.

We present a new method that allows efficient spectral calibration for a polarization state analyzer. The procedure does not require any additional polarization optical element other than the polarization state analyzer itself. It uses a double-pass technique that can be achieved up to a very good precision. The method is illustrated using real measurements done at several wavelengths with a rotating wave plate polarization state analyzer. Alignment of axis as well as true retardation at a specific wavelength are easily obtained by a standard function fitting.

Acceptable signal recovery of the band-pass signals typically used in the off-axis digital holography systems is possible in the undersampling conditions. A typical system is considered in which the angle between two beams represents a variable parameter. For the given signal bandwidth and experimental conditions the hologram reconstruction is constrained by the sampling frequency of the array photo-detector. Reconstructions from the undersampled digital holograms are analyzed both theoretically and experimentally. It is shown how increasing the angle values beyond the Nyquist limits leads to repeatedly folding and inverting the reconstructed object image until the fading of the image. The phase point at the image fading and the non-overlapping intervals for correctly preserving the useful information are defined and evaluated. Amplitude distributions are analyzed on the example of the time-averaged holograms acquired for an oscillating membrane. Based on removing the zeroth-order reconstruction term, significant extensions of these intervals are also demonstrated.

When using the interferometric techniques, the optical path changes induce the wavefront deformations that in turn cause the appearance of fringes. One general characteristic of such an approach is the measurement sensitivity. The actual sensitivity of a holographic interferometer is a function of, on one hand, the parameters of the measuring system (such as the wavelength of the light used) and, on the other hand, the environmental conditions in which the measurements are being made. The later depend predominately on statistical fluctuations inherent to the particular laboratory conditions. In many applications the sensitivity is near the limits of the deformation detectability. In such cases, it is of vital interest to increase the number of interferometric fringes thus improving the quality of the acquired data. In this paper, we give an overview of the sensitivity increase through various experimental and numerical approaches. We also present a new numerical iterative method in which every cycle doubles the number of interferometric fringes. The method has shown to be especially useful in applications with sub-wavelength wavefront deformations.

In digital holography, primary holographic fringes are recorded using a matricial CCD sensor. Because of the low spatial resolution of currently available CCD arrays, the angle between the reference and object beams must be limited to a few degrees. Namely, due to the digitization involved, the Shannon's criterion imposes that the Nyquist sampling frequency be at least twice the highest signal frequency. This means that, in the case of the recording of an interference fringe pattern by a CCD sensor, the inter-fringe distance must be larger than twice the pixel period. This in turn limits the angle between the object and the reference beams. If this angle, in a practical holographic interferometry measuring setup, cannot be limited to the required value, aliasing will occur in the reconstructed image. In this work, we demonstrate that the low spatial frequency metrology data could nevertheless be efficiently extracted by careful choice of twofold, and even threefold, undersampling of the object field. By combining the time-averaged recording with subtraction digital holography method, we present results for a loudspeaker membrane interferometric study obtained under strong aliasing conditions. High-contrast fringes, as a consequence of the vibration modes of the membrane, are obtained.

Corona discharges are generally characterized by a low optical density whose detection is often near or under the limits of interferometric techniques. In this paper, we propose a method of digital holographic interferometry that enables detection with enhanced sensitivity. This sensitivity increase is obtained by post-processing the digital holographic recordings with a set of point-wise image operations. The procedure is described mathematically and illustrated experimentally. Examples are given for an opaque object and for DC corona discharges generated in the symmetrical point-plane geometry.

This work reports on recent results in the area of the time-averaged digital holographic interferometry. Two techniques are described, one that removes the zero-order reconstruction term thus allowing recording of large objects, and the other by which the hidden stationary deformations can be detected. Quantifying hidden deformations is important because its evaluation allows direct measurement of a stationary bias strain in the dynamic analysis of the vibration object. These two techniques are explained mathematically and illustrated by the experimental results.

Time-averaged holographic interferometry is a known technique frequently used for analyzing vibration properties of objects. The development of array photo-detectors allowing long integration times enabled the capture of time-averaged holograms. A new technique called 'subtraction digital holography' has been recently developed for suppressing the zero-order disturbance in off-axis digital holography. In this work, we combine the time-averaged principle with subtraction digital holography technique. Results for a torsional micro-electro-mechanical system (MEMS) and an oscillating membrane demonstrate clear hologram reconstructions covered with high-contrast fringes that describe the vibration modes.

To record high-resolution holograms successfully, it is necessary to minimize scattering in the recording material. This scattering, in silver halide materials, is caused by initial and final (i.e. after processing) grain-size distributions and by the thickness as well as by the index of refraction variations of the recording medium. Using atomic force microscopy (AFM) we investigated surface topography of representative silver halide holographic recording material. We have applied the trypsin treatment, which was usually used to enhance a surface relief holographic recording, to eliminate the superficial cover layer of gelatine and to render underneath buried grain-size investigation possible. The average grain-size was measured before and after trypsin treatment and the results were compared with Electron-Microscopy measurements. Local hardness of samples under the influence of standard as well as of the surface relief sensitive processing is measured as well.

A color digital holographic interferometry movie was produced by applying the subtraction digital holography method in a quasi-Fourier off-axis experimental setup. The movie was numerically recorded and replayed from three sets of digital holograms obtained with three different laser lines (476 nm, 532 nm, and 647 nm). The movie shows convective flows induced by thermal dissipation in a tank filled with oil.

Quantitative measurements of shapes, displacements and deformations of opaque objects, as well as of refractive properties of transparent media, through spatial and temporal fringe patterns analysis is done by applying two basic techniques. The first one is the phase modification technique (shifting or modulating). The second one is the Fast Fourier Transform technique (FFT), assisted by some sort of heterodyning. FFT is also the choice far a single frame, not-modified (not-heterodyned), interferogram analysis. In this case however, because the lack of a spatial carrier, the sign of the phase cannot be determined. A solution to this problem is a technique that requires only two (but nevertheless two) phase-shifted interferograms. In this paper we propose a holographic interferometry method based on wavelength multiplexing in which no spatial carrier neither second, phase shifted interferogram is required. Using the Lippmann-Denisyuk single beam holographic setup, an interferogram from a deformable object is recorded with multiple laser wavelengths in a panchromatic emulsion. A reconstruction of the 3D abject in (natural if RGB laser lines are used) colors is observed, superimposed with a multicolor fringe pattern. The object phase is then computed from the multiple single-wavelength fringe patterns, allowing the measurement of the object deformation. This work is aiming at quantitative analysis of highly dynamic objects.

In interferometry, spatial heterodyning is performed by multiplying the optical phase to be determined by a reference phase. Thus the spatial frequency spectrum corresponding to the signal investigated is better identified and manipulated. We propose a holographic interferometry method that is spatially heterodyned by deliberate rigid-body movement of the object investigated. The object deformation is calculated from the fringe pattern corresponding to two different positions of the object. This work is aimed at a quantitative deformation analysis of highly dynamic objects through pulsed holographic interferometry.

Interest in wavelength multiplexing in holography derives naturally from the need for realistic color rendition as well as from resolution requirements. Imaging in coherent illumination is compromised by speckle. Speckle is prejudicial to quality, sharpness, contrast—in a word, to the fidelity of reproduction or holographic reconstruction. In incoherent light these patterns are canceled by spatial phase averaging. Incoherent light can indeed be regarded as a superposition of a very large number of coherent components whose phase factors are distributed at random. It is demonstrated that the averaging effect, ultimately caused by the law of large numbers, is achieved by the superposition of only three components, thus allowing simultaneously a true color rendition and an improvement in spatial resolution. The spatial statistical behavior of the amplitude of the sum of three intrinsically coherent waves, when they are incoherently superposed in an imaging system, is investigated. A random variable representing the amplitude of this sum is introduced. Then the cumulative probability function and the probability density function of the resulting amplitude are calculated. The white-light (infinite-wave illumination) case and the purely coherent (one-wave) case are analyzed. The results are interpreted with a heuristic vector model.

We describe a method to transform a laser intensity gaussian distribution into a uniform distribution, using computer generated holograms; we explain the modelisation, the production and the realisation on DCG plates.

In this paper we propose an optical design for implementation of neuronic Hopfield network. We describe the algorithm and its potential possibilities as associative (or content addressable) memory. We then describe the optical set (using a magneto-optic spatial light modulator) and explaning its operating mode: the binary transparency of the SLM does not allow a direct and accurate experimental realisation of the theoretical algorithm. However, there is a particular setup that can implemented it powerfully but with a reduction of the effective number of neurons. The operating speed is then evaluated from the characteristics of the SLM "Sight-Mod" manufactured by SEMETEX corp.: the maximun operating frequency seems limited by the speed of the optical valve.