Speckle Grain Research Articles

Dynamic ultrasound modulated optical tomography by self-referenced photorefractive holography

Photorefractive Bi(12)SiO(20) single crystal is used for acousto-optic imaging in thick scattering media in the green part of the spectrum, in an adaptive speckle correlation configuration. Light fields at the output of the scattering sample exhibit typical speckle grains of 1 μm size within the volume of the nonlinear crystal. This heterogeneous illumination induces a complex refractive index structure without applying a reference beam on the crystal, leading to a self-referenced diffraction correlation scheme. We demonstrate that this simple and robust configuration is able to perform axially resolved ultrasound modulated optical tomography of thick scattering media with an improved optical etendue.

Polarimetric imaging beyond the speckle grain scale

We address an experimental Stokes imaging setup allowing one to explore the polarimetric properties of a speckle light field with spatial resolution well beyond the speckle grain scale. We detail how the various experimental difficulties inherent to such measurements can be overcome with a dedicated measurement protocol involving a careful speckle registration step. The setup and protocol are then validated on a metallic reference sample, and used to measure the state of polarization (SOP) of light in each pixel of highly resolved speckle patterns (>2000 pixels per speckle grain) resulting from the scattering of an incident coherent beam on samples exhibiting different polarimetric properties. Evolution of the SOP with spatial averaging and across adjacent speckle grains is eventually addressed.

Scattering through fruits during ripening: laser speckle technique correlated to biochemical and fluorescence measurements

This paper reports monitoring fruits maturation using speckle technique. Performed measurements aim the assessing of biological inner fruit variation effect on the speckle image. We show that the speckle grain size is both affected by the glucose level inside the fruits and by the chlorophyll content. Moreover, the determination of circular polarization degree and circular grain size indicate that a Rayleigh diffusion regime gradually becomes predominant in fruits. Principal component analysis is used to highlight high correlation between results and strengthen the establishment of speckle as a novel non invasive method to monitor fruits ripening.

Anderson localization in optical lattices with speckle disorder

We study the localization properties of non-interacting waves propagating in a speckle-like potential superposed on a one-dimensional lattice. Using a decimation/renormalization procedure, we estimate the localization length for a tight-binding Hamiltonian where site-energies are square-sinc-correlated random variables. By decreasing the width of the correlation function, the disorder patterns approaches a $\delta$-correlated disorder, and the localization length becomes almost energy-independent in the strong disorder limit. We show that this regime can be reached for a size of the speckle grains of the order of (lower than) four lattice steps.

Far-field angular distribution of the mean scattered intensity from strong diffusers by means of a near-field intensity pattern

The speckle grain close to a diffuser medium carries information about the angles at which light is scattered. We propose the extension of a technique that allows obtaining the angular distribution of the scattered intensity in the far-field speckle pattern from the near-field speckle pattern produced by strong diffusers (whose roughness is greater than the illuminating wavelength). To illustrate this technique we applied it to the light scattered from ground glass diffusers. The experimental results are compared with those obtained by direct measurement of the angular distribution of the mean scattered intensity in the far field showing good agreement with each other. The advantages over the classical far-field methods are outlined.

Intensity Distribution and Phase Vortices of Speckle Fields Generated by Multi-Aperture Random Scattering Screens

The intensity distribution and phase vortices of the speckle fields generated by multi-aperture random scattering screens are simulated, and it is found that the vortices exhibit layer-like structures and the dislocation phenomena occur in the local phase patterns produced by the two-pinhole aperture, whose phase distributions appear as striped structures. For three- or four-pinhole aperture, there are many circular bright spots appearing in the speckle grains, and there is one vortex between the neighboring circular bright spots. The positive and negative phase vortex lattices appear in the phase distributions, and the regions circled by the isothetic phase lines form irregular quadrilaterals or hexagons. Moreover, the relative positions of the vortices or bright spots can be adjusted by changing those of the pinhole apertures.

Phase extraction in dynamic speckle interferometry: proposal of a road map

Of all the two-beam interference patterns, the ones obtained in speckle interferometry (SI) are the most difficult to be phase-demodulated. Many solutions exist in classical smooth-wave interferometry and alike techniques, both in static and dynamic regimes. In SI, the three constituents of the signals - the background, the modulation and the phase - are all basically random variables. There is no way to make a prediction of the evolution of these variables outside the small size of the correlation volumes - the volumes defined by the average speckle grain. To some extent, the classical methods can be adapted to SI. Here, we prefer to develop a series of new processing tools tailored to the specificities of the dynamic SI signals: the cooperative use of the empirical mode decomposition (EMD), the Hilbert transform (HT), and the three dimensional piecewise processing (3DPP) for recovering efficiently the phase of these signals.

The effect of optically-induced random anisotropic disorder on a two-dimensional electron system

We have studied the effect of optically-induced random, anisotropic disorder on the magnetoresistance of a Al 0.3Ga 0.7As/ GaAs two-dimensional electron system by exposing the heterojunction to an asymmetric laser speckle pattern. Changes in the amplitude of the Shubnikov–de Haas oscillations can be explained in terms of easy and hard conductivity paths parallel and perpendicular to the long axis of the oval speckle grains. We also observe corresponding changes in the electron scattering rates.

Surface characterization from doubly scattered light.

When a translucent diffuser is illuminated by a speckle pattern, a new speckle pattern is produced. We show that the decorrelation of this intensity pattern by displacement of the diffusing surface is related to the standard deviation of the slope's distribution when the illuminating speckle grain is chosen appropriately small. The experimental results are compared with those obtained by measuring the angular distribution of the mean scattered intensity, and they show good agreement with each other.

Photorefractive detection of tagged photons in ultrasound modulated optical tomography of thick biological tissues

We present a new and simple method to obtain ultrasound modulated optical tomography images in thick biological tissues with the use of a photorefractive crystal. The technique offers the advantage of spatially adapting the output speckle wavefront by analysing the signal diffracted by the interference pattern between this output field and a reference beam, recorded inside the photorefractive crystal. Averaging out due to random phases of the speckle grains vanishes, and we can use a fast single photodetector to measure the ultrasound modulated optical contrast. This technique offers a promising way to make direct measurements within the decorrelation time scale of living tissues.

Analysis and applications of the speckle patterns registered in a photorefractive BTO crystal

The features of the volume speckle pattern recording in a sillenite BTO crystal are investigated. The modulation of the speckle grains appears when the image of a random diffuser is formed onto the crystal by an optical system whose pupil consists of two identical apertures. In fact, the volume nature of the grains generates speckle overlapping regions where the modulation takes place. The 3-D modulated speckle pattern stored in the crystal leads to a space-charge field by drift of photocarriers, resulting in a refractive index modulation. By observing the diffracted read-out light is established that the index modulation in the speckle overlapping regions lead to a remarkable angular selectivity. The statistical nature behavior of the speckle distributions is considered in the theoretical analysis. A comparison between the theoretical predictions and the experimental results is done. The analysis leads to the adequate selection of the write-in parameters as it is illustrated in the optical processing applications presented.

High resolution digital holography

In digital Fresnel holography, present specifications of charge-coupled device cameras require the incident beams to be quasi-parallel. That implies large speckle grain size and low lateral resolution in reconstructed images. Better lateral resolutions are demonstrated in our work, down to 8 μm, allowing the observation of sub-millimetre objects by digital holography. The experimental set-up built samples the incident light distribution with a definition of 500 pixels mm −1. The maximum acceptable angle is then widened and the hologram recording and reconstruction distances are drastically reduced, as well as the speckle size. Interferometric holography was implemented for the measurement of the deformation of a sub-millimetre silicon cantilever and results in phase-contrasts imaging are also reported. The design of dedicated complementary metal-oxide-semiconductor active pixel sensor cameras is also discussed.

Optical operations based on speckle modulation by using a photorefractive crystal

An image multiplexing technique, based on the internal modulation of speckle grains by using different multiple aperture pupils during exposure, is proposed. A BSO crystal is employed as the recording medium. A theoretical analysis of the multiplexing procedures is outlined. Selective retrieving of each encoded image and the superposition of several images is discussed. Experimental results in connection with image binary operations are presented.

Ultrasonic tagging of photon paths in scattering media:?parallel speckle modulation processing

Tagging of photon trajectories in scattering media is possible by application of a localized ultrasonic field to the sample and by measurement of the induced speckle modulation. Instead of using a single optical detector, which integrates the signal of many speckle grains, we propose a more efficient detection scheme that uses a CCD camera and parallel lock-in detection to record the full modulation of the speckle. The advantage of this multiplex detection is demonstrated, as well as the imaging capabilities of the process for biological tissues.

Trapping and cooling cesium atoms in a speckle field

We present the results of two experiments where cold cesium atoms are trapped in a speckle field. In the first experiment, a YAG laser creates the speckle pattern and induces a far-detuned dipole potential which is a nearly-conservative potential. Localization of atoms near the intensity maxima of the speckle field is observed. In a second experiment we use two counterpropagating laser beams tuned close to a resonance line of cesium and in the lin lin configuration, one of them being modulated by a holographic diffuser that creates the speckle field. Three-dimensional cooling is observed. Variations of the temperature and of the spatial diffusion coefficient with the size of a speckle grain are presented.

Image multiplexing by speckle in a BSO crystal

A technique of multiple image storage in a BSO crystal by using speckle patterns is proposed. It is based on the modulation of 3-D speckle grains which appears when the image of a coherently illuminated input is formed onto the crystal by an optical system whose pupil consists of two identical holes. Each image is stored in a sequential way by changing the pupil between images. The intensity distribution of the speckle pattern imaged onto the BSO crystal leads to a space-charge field by drift of photocarriers, resulting in a refractive index modulation. The volume nature of the registered speckles is analyzed. The encoded images are retrieved by illuminating the crystal with a collimated He-Ne laser beam and by properly filtering the Fourier plane. Experimental results are presented.

Analysis of the dispersion of speckle polarization on the Poincaré sphere

A polarimeter using the Stokes - Mueller formalism has been adapted to analyse speckle polarization. The noise effects on the Mueller matrix are minimized to separate the experimental noises from the depolarization phenomenon. The polarization of speckle grains changes with the angle of incidence whereas the global depolarization of the target is increased. As a part of the depolarization results from a mixing of polarization, the polarization dispersion of the speckle grains on the Poincaré sphere is analysed by using the polarization barycentre as a parameter. It appears that the polarization dispersion increases with the angle of incidence. In order to simplify this depolarization phenomenon, we have studied a polished iron sample.

Experimental investigation of the speckle polarization for a polished aluminium sample

As speckles carry information about polarization, we have perfected a polarimeter using the Stokes - Mueller formalism from which we exploit the matrix to analyse speckle polarization. The noise effects on the Mueller matrix are minimized to separate experimental noise from the depolarization phenomenon. In order to simplify this depolarization phenomenon, we used a polished aluminium sample. We describe a statistical polarization analysis of the speckle grains which show the depolarization evolution of the speckle field. A global polarization analysis of the clipped speckle is then performed. The experimental results show that it remains polarized, independently of the observation direction, and only shows a slight polarization dispersion.

Speckle noise reduction in TV holography fringes using wavelet thresholding

Recently, a novel wavelet method to reduce speckle noise in synthetic aperture radar images was presented. The method, based on the thresholding of the wavelet coefficients of the transformed image, is computationally efficient and maintains sharp image features. The application of a similar method is explored to reduce speckle noise in TV holography fringes, which is a very important problem in the quantitative analysis of transient phenomena. Several thresholding approaches are used to test the noise reduction algorithm on computer-simulated fringes and results are assessed through the evaluation of two comparative parameters: the image fidelity and the speckle index. It is shown that the method achieves the highest speckle reduction when speckle grains have the same average size of the image pixels.

Heterodyne readout system for dual plate speckle photography: analysis of error sources and performance evaluation

We describe the design, construction, and testing of a dual plate specklegram heterodyne readout system. The choice of the optical layout and mode of operation is based on the results of a detailed discussion on the sources of error associated both with optical noise and misalignments. Noise analysis includes the conventional scintillation noise and a recently described additional source, the speckle grain noise. Stability and reproducibility tests have been conducted and the accuracy of displacement measurement is 10(-6) of the recorded specklegram size, approximately five times worse than the ultimate noise limited value.