Instantaneous Speckle Research Articles

Real time characterization of atmospheric turbulence using speckle texture

A new method is proposed for the characterization of the atmospheric turbulence strength parameter () based on the principal component analysis (PCA) of the texture of the speckle intensity pattern obtained on long-range propagation of a focused charge +1 vortex beam. Employing the split-step propagation method, datasets containing instantaneous intensity images of the focused vortex beam are generated for three distinct values, specifically , , and , representing medium to high turbulence levels for a 2 km propagation distance. The gray level co-occurrence matrix (GLCM) methodology is employed to extract key texture attributes, like contrast, correlation, homogeneity, and energy from the intensity images. These extracted texture parameters serve as inputs for training a PCA model, enabling the identification of associated values. The PCA analysis exhibits distinct clustering of the first three principal components for each of the three values, forming individual clusters on the PCA plot. Standard deviational ellipses are drawn to clearly demarcate these clusters on the PCA plot. The texture-based PCA classification of atmospheric turbulence was also performed for a focused Gaussian beam. The comparison of PCA plots between vortex and Gaussian beams showed that a pronounced clear separation of values is obtained for the vortex beam. This indicates that the non-zero orbital angular momentum of the vortex beam also plays an important role in achieving the distinct separation of values on the PCA plot. The proposed method can provide efficient real-time turbulence estimation solely on the basis of texture of the instantaneous intensity speckle with prior training and therefore may simplify the estimation of turbulence strength parameter.

Vector speckle grid: instantaneous incoherent speckle grid for high-precision astrometry and photometry in high-contrast imaging

Context. Photometric and astrometric monitoring of directly imaged exoplanets will deliver unique insights into their rotational periods, the distribution of cloud structures, weather, and orbital parameters. As the host star is occulted by the coronagraph, a speckle grid (SG) is introduced to serve as astrometric and photometric reference. Speckle grids are implemented as diffractive pupil-plane optics that generate artificial speckles at known location and brightness. Their performance is limited by the underlying speckle halo caused by evolving uncorrected wavefront errors. The speckle halo will interfere with the coherent SGs, affecting their photometric and astrometric precision. Aims. Our aim is to show that by imposing opposite amplitude or phase modulation on the opposite polarization states, a SG can be instantaneously incoherent with the underlying halo, greatly increasing the precision. We refer to these as vector speckle grids (VSGs). Methods. We derive analytically the mechanism by which the incoherency arises and explore the performance gain in idealised simulations under various atmospheric conditions. Results. We show that the VSG is completely incoherent for unpolarized light and that the fundamental limiting factor is the cross-talk between the speckles in the grid. In simulation, we find that for short-exposure images the VSG reaches a ∼0.3–0.8% photometric error and ∼3−10 × 10−3λ/D astrometric error, which is a performance increase of a factor ∼20 and ∼5, respectively. Furthermore, we outline how VSGs could be implemented using liquid-crystal technology to impose the geometric phase on the circular polarization states. Conclusions. The VSG is a promising new method for generating a photometric and astrometric reference SG that has a greatly increased astrometric and photometric precision.

Мгновенные спекл-модулированные интерференционные изображения и когерентные эффекты в оптической микроскопии тонких слоев

The formation of instantaneous speckle-modulated interference images of thin transparent layers in an optical microscope under an illuminating quasi-monochromatic or frequency broadband wave field with wide angular spectra and coherent effects are considered. Instantaneous speckle-modulated interference images of the layer in the transverse and longitudinal sections of the total wave field reflected by the layer are obtained by numerical simulation of the spatial distributions of the complex amplitude of the wave fields reflected by the layer faces and their superposition. The results of computer simulation of instantaneous speckle-modulated and experimentally observed averaged interference images of the layer under quasi-monochromatic and frequency broadband illuminating layer field with different width of the angular spectrum are presented. The mutual correlation of instantaneous speckle fields reflected by the layer faces and the effects of the influence of the width of the angular and frequency spectra of the illuminating field on the contrast of experimentally observed interference images of the layer are studied numerically. On the basis of the instantaneous speckle-modulation of the wave field the physical mechanism of the influence of angular width and the width of the frequency spectra of the illuminating field on the mutual coherence of the interfering wave fields is investigated.

Instantaneous Speckle Structures in a Partially Coherent Optical Wave Field with Broad Frequency and Angular Spectra

We consider the hypothesis of instantaneous volume speckle structures in a partially coherent optical wave field of frequency-broadband radiation from an extended light source. We discuss the spatiotemporal correlation properties of instantaneous speckle structures and the relation of these properties with the spatiotemporal coherent properties of the wave field. We show that the instantaneous speckle structures determine spatial fluctuations of the wave field and its spatial coherent properties. Using numerical calculations of the field of wave perturbations in the near-field diffraction region of the extended light source, we obtain images of instantaneous speckle structures in the longitudinal cross section of the wave field and investigate the correlation properties of the spatial distribution of the instantaneous intensity of the wave field in the direction of its propagation as functions of the width of the frequency spectrum and the width of the angular spectrum of the field. We show that the longitudinal length of instantaneous speckles can be determined either by the width of the frequency spectrum of the field, or the width of its angular spectrum, or, jointly, by the width of the frequency spectrum and the width of the angular spectrum of the field. We find conditions under which instantaneous speckle structures propagate, experiencing decorrelation changes during their propagation, to a distance that is determined by the width of the angular spectrum of the field.

Мгновенные спекл-структуры в частично когерентном оптическом волновом поле с широкими частотным и угловым спектрами

AbstractWe consider the hypothesis of instantaneous volume speckle structures in a partially coherent optical wave field of frequency-broadband radiation from an extended light source. We discuss the spatiotemporal correlation properties of instantaneous speckle structures and the relation of these properties with the spatiotemporal coherent properties of the wave field. We show that the instantaneous speckle structures determine spatial fluctuations of the wave field and its spatial coherent properties. Using numerical calculations of the field of wave perturbations in the near-field diffraction region of the extended light source, we obtain images of instantaneous speckle structures in the longitudinal cross section of the wave field and investigate the correlation properties of the spatial distribution of the instantaneous intensity of the wave field in the direction of its propagation as functions of the width of the frequency spectrum and the width of the angular spectrum of the field. We show that the longitudinal length of instantaneous speckles can be determined either by the width of the frequency spectrum of the field, or the width of its angular spectrum, or, jointly, by the width of the frequency spectrum and the width of the angular spectrum of the field. We find conditions under which instantaneous speckle structures propagate, experiencing decorrelation changes during their propagation, to a distance that is determined by the width of the angular spectrum of the field.

Velocity measurements of heterogeneous RBC flow in capillary vessels using dynamic laser speckle signal.

We propose an approach to measure heterogeneous velocities of red blood cells (RBCs) in capillary vessels using full-field time-varying dynamic speckle signals. The approach utilizes a low coherent laser speckle imaging system to record the instantaneous speckle pattern, followed by an eigen-decomposition-based filtering algorithm to extract dynamic speckle signal due to the moving RBCs. The velocity of heterogeneous RBC flows is determined by cross-correlating the temporal dynamic speckle signals obtained at adjacent locations. We verify the approach by imaging mouse pinna in vivo, demonstrating its capability for full-field RBC flow mapping and quantifying flow pattern with high resolution. It is expected to investigate the dynamic action of RBCs flow in capillaries under physiological changes.

Speckle Decorrelation and Dynamic Range in Speckle Noise–limited Imaging

The useful dynamic range of an image in the diffraction-limited regime is usually limited by speckles caused by residual phase errors in the optical system forming the image. The technique of speckle decorrelation involves introducing many independent realizations of additional phase error into a wave front during one speckle lifetime, changing the instantaneous speckle pattern. A commonly held assumption is that this results in the speckles being moved at the rate at which the additional phase screens are applied. The intention of this exercise is to smooth the speckles out into a more uniform background distribution during their persistence time, thereby enabling companion detection around bright stars to be photon noise limited rather than speckle limited. We demonstrate analytically why this does not occur and confirm this result with numerical simulations. We show that the original speckles must persist and that the technique of speckle decorrelation merely adds more noise to the original speckle noise, thereby degrading the dynamic range of the image.

Self- and cross-phase modulation of high-intensity laser beams emerging from a diamond-turned KDP wedge

The Laboratory for Laser Energetics plans to install KDP wedges on each beam line of the OMEGA laser system in order to improve the on-target laser uniformity by decreasing the instantaneous speckle through spatial averaging of the two resultant orthogonally polarized beams. The proposed wedge-production method, diamond turning, produces small residual scratch marks, causing each beam to acquire a pseudorandom phase perturbation. In addition, the orthogonally polarized beams interfere such that their combined polarization state continuously cycles through all elliptical states along any transverse plane. Since the nonlinear refractive index depends on the polarization state, intense beams accumulate a periodic phase perturbation that is greater for linear polarization. Propagation of both types of phase perturbation yields an intensity modulation that tends to be larger in the neighborhood of linear polarization, through a combination of diffraction and self- and cross-phase modulation. However, one- and two-dimensional calculations using a symmetrized split-step Fourier method demonstrate that diamond-turned KDP wedges are not a significant source of intensity modulation under OMEGA laser conditions. Installation of diamond-turned rather than polished wedges will reduce costs without adversely affecting the system performance.

Optical smoothing of the high power PHEBUS Nd-glass laser using the multimode optical fiber technique

The use of an optical fiber for the coherence reduction at the laser front-end changes the common point laser source into an extended source. The required optical arrangement taking into account the divergence of the extended source beam is described. The smoothing efficiency is measured by the reduction of one order of magnitude of the contrast of the energy modulations in the focal spot. The output power is limited to 4 TW, which is about half the laser capability, by a saturation-like effect in the large amplifier section. It is believed that the hot spots of the instantaneous speckle patterns in the glass lead to nonlinear losses, responsible for the observed power limitation. This hypothesis is supported by the measurement of the transmission of laser glass samples on a separate experiment with high intensity picosecond pulses.

A method for processing stellar speckle interferometry data

It is shown that the difference between the average autocorrelation of instantaneous speckle patterns and the average cross correlation of such patterns is proportional to the autocorrelation of the image effectively uninfluenced by a turbulent atmosphere. This result leads to a useful method for the reduction of astronomical speckle data.