Random Light Fields Research Articles

Real-time tracing and location for critical parts of moving objects with single-pixel detection

A scenario based on single-pixel input of recognizing and tracing the critical parts of the moving targets has been proposed. The one-dimensional single-pixel matrix is achieved by bucket measurement by making use of pre-training illumination suitable for the target datasets. The designed recognition network can export the real-time position coordinates of the key components of the moving targets through single-pixel detection. Compared with the general random illumination light field, the results of the pre-training illumination show significant improvement in both recognition efficiency and accuracy. The recognition efficiency can reach 100Hz in the experimental detection of moving model planes. It may have important application prospects in military target tracking and hyperspectral remote sensing.

The Effect of Anisotropic Gaussian Schell-Model Sources in Generalized Phase Space Stokes Parameters

Phase-space transforms describe spatial and angular information about light sources where one example is the Wigner functions in wave optics. Stokes parameters, on the other hand, supply information about the polarization of light beams. The generalized phase space Stokes parameters of 2D stochastic electromagnetic beams are already developed. In this article, the application of anisotropic light sources in generalized phase space Stokes parameters is theoretically investigated and numerically analyzed. There are several different ways of studying electromagnetic light beams depending on the spatial domain. But, most measure of the polarization of random light fields is carried out within the Stokes parameters context. In this account we study the electromagnetism, Stokes parameters, phase space, and the anisotropy properties of random light beams at once. We find here that when an anisotropy introduced in phase space then the cross terms of the Wigner matrix depart from the diagonal terms, which is not the same in configuration space. As a result, anisotropy has a different effect in Phase space, i.e. an anisotropic source introduces a phase and a variance change only in the cross terms of Wigner matrix. This is due to the use of anisotropy in the shifted kernel of Wigner transform.

Single-shot generalized Hanbury Brown-Twiss experiments using a polarization camera for target intensity reconstruction in scattering media.

To see through a random light field in real-time, single-shot generalized Hanbury Brown-Twiss experiments using a polarization camera are proposed. The target intensity distribution is obtained from a complex coherence function which is calculated from auto-correlation and cross correlation functions of phase-shifted speckle intensity distributions. The phase-shifted speckle intensity distributions are simultaneously obtained through a strategy of parallel phase-shifting digital holography. Experimental results show that the proposed method can image a moving object in a random light field using a measured complex coherence function through the van Cittert-Zernike theorem.

Holography with higher-order Stokes correlation

We present and experimentally demonstrate a holography technique with a higher-order Stokes correlation of the random light field. The proposed technique is capable to reconstruct the complex-valued object from the random light field, and overcome twin image issues in the higher-order correlation holography without angular shift, phase shifting, and iterations. A basic principle of the proposed technique is presented and its application is demonstrated by establishing a connection between the higher-order Stokes fluctuations correlations and the far-field complex coherence function of the random field in a lensless configuration. The viability of our technique is demonstrated by considering incoherent reconstruction of the hologram and reconstructing the complex-valued information encoded into the hologram as a distribution of complex coherence function.

Experimental investigation of chirped amplitude modulation heterodyne ghost imaging.

We have constructed a chirped amplitude modulation heterodyne ghost imaging (CAM-HGI) experimental system that demonstrates a robust ability against background light in experiments. In the experiments, the background light is simulated by irradiating a spatiotemporal random modulated light field onto the target. The effects of background light, modulation depth and modulation duration of the signal light source on CAM-HGI are investigated experimentally. The results show that the quality of CAM-HGI can be improved by increasing the modulation depth and the modulation duration of the signal light source, and more importantly, an image with a good signal-to-noise ratio (SNR) can be achieved even when the irradiation SNR is lower than -30 dB. This technique of CAM-HGI has an important application prospect for laser imaging in strong background light environments.

Measuring Complex Degree of Coherence of Random Light Fields with Generalized Hanbury Brown–Twiss Experiment

We advance a protocol to measure the complex spatial degree of coherence of a partially coherent light field obeying Gaussian statistics through a generalized Hanbury Brown--Twiss (HBT) experiment. The proposed generalized HBT experiment amounts to combining a partially coherent field with a pair of coherent reference fields and measuring the intensity-intensity cross-correlation of the cumulative field. The real or imaginary part of the complex spatial degree of coherence can be extracted directly from the intensity-intensity cross-correlation by adjusting the phase delay between the two reference fields to be $0$ or $\ensuremath{\pi}/2$. We test our method by carrying out a proof-of-principle experiment to measure the complex spatial degree of coherence of inhomogeneous light fields obeying Gaussian statistics. We find excellent agreement between the experimental results and our general theory. We show that our approach is extremely robust against the environmental fluctuations (e.g., the atmospheric turbulence) during the measurement. We demonstrate experimentally that a moving target hiding behind a ground-glass disk can be tracked with the aid of our complex-spatial-coherence measurement protocol.

Diffusion of Anisotropic Particles in Random Energy Landscapes—An Experimental Study

If a colloidal particle is exposed to an external field, its Brownian motion is modified. In the case of an anisotropic particle, the external potential might not only affect its translation but also its rotation. We experimentally investigate the dynamics of a trimer, which consists of three spherical particles, within a random potential energy landscape. This energy landscape has energy values drawn from a Gamma distribution, a spatial correlation length similar to the particle size and is realised by a random light field, that is a laser speckle pattern. The particle translation and rotation are quantified by the mean squared (angular) displacement, the van Hove function and other observable quantities. The translation shows an intermediate subdiffusive regime and a long-time diffusion that slows down upon increasing the modulation of the potential. In contrast, the mean squared angular displacement exhibits only small deviations from a linear time dependence but a more detailed analysis reveals discrete angular jumps reflecting the symmetry of the trimer. A coupling between the translation and rotation is observed and found to depend on the length scale.

Creating and controlling non-Rayleigh speckles

A method is developed to control and create spatially random light fields, a type of complex light which occurs in all varieties of waves.

Creating and controlling complex light

Random light fields—commonly known as speckles—demonstrate Rayleigh intensity statistics and only possess local correlations which occur within the individual speckle grains. In this work, we develop an experimental method for customizing the intensity probability density function (PDF) of speckle patterns while simultaneously introducing nonlocal spatial correlations among the speckle grains. The various families of tailored speckle patterns—created by our method—can exhibit radically different topologies, statistics, and variable degrees of spatial order. Irrespective of their distinct statistical properties, however, all of these speckles are created by appropriately encoding high-order correlations into the phase front of a monochromatic laser beam with a spatial light modulator. In addition to our experimental demonstration, we explore both the theoretical and practical limitations on the extent to which the intensity PDF and the spatial intensity correlations can be manipulated concurrently in a speckle pattern. This work provides a versatile methodology for creating complex light fields and controlling their statistical properties with varied applications in microscopy, imaging, and optical manipulation.

Light Induced Inverse-Square Law Interactions between Nanoparticles: "Mock Gravity" at the Nanoscale.

The interaction forces between identical resonant molecules or nanoparticles, optically induced by a quasimonochromatic isotropic random light field, are theoretically analyzed. In general, the interaction force exhibits a far-field oscillatory behavior at separation distances larger than the light wavelength. However, we show that the oscillations disappear when the frequency of the random field is tuned to an absorption Fröhlich resonance, at which the real part of the particle's electric polarizability is zero. At the resonant condition, the interaction forces follow a long-range gravitylike inverse square distance law which holds for both near- and far-field separation distances.

Non-hogel-based computer generated hologram from light field using complex field recovery technique from Wigner distribution function.

We propose a novel method that synthesizes computer-generated holograms from light field data. Light field, or ray space, is the spatio-angular distribution of light rays coming from three-dimensional scene, and it can also be represented using a large number of views from different observation directions. The proposed method synthesizes a hologram by applying the complex field recovery technique from its Wigner distribution function to the light field data. Unlike conventional approaches, the proposed method synthesizes holograms without hogel configuration, generating a converging parabolic wave for each object point with continuous wavefront. The proposed method does not trade the spatial resolution with angular resolution like conventional hogel-based approaches. Moreover, the proposed method works not only for random phase light field like conventional approaches, but also for arbitrary phase distribution with corresponding carrier waves. Therefore, the proposed method is useful in synthesizing holographic contents for a wide range of applications. The proposed method is verified by simulations and optical experiments, showing successful reconstruction of three-dimensional objects.

Dimensionality of random light fields

BackgroundThe spectral polarization state and dimensionality of random light are important concepts in modern optical physics and photonics.MethodsBy use of space-frequency domain coherence theory, we establish a rigorous classification for the electricfield vector to oscillate in one, two, or three spatial dimensions.ResultsWe also introduce a new measure, the polarimetric dimension, to quantify the dimensional character of light. The formalism is utilized to show that polarized three-dimensional light does not exist, while an evanescent wave generated in total internal reflection generally is a genuine three-dimensional light field.ConclusionsThe framework we construct advances the polarization theory of random light and it could be beneficial for near-field optics and polarization-sensitive applications involving complex-structured light fields.

Ghost Imaging for Online Angiography

Ghost imaging (GI) has characteristics that make it promising for applications in life sciences and other fields, such as its high sensitivity and strong anti-interference compared with traditional imaging. This paper presents a new approach for online angiography using GI. Two signals are correlation-calculated to detect the object image: one of them is the random light field generated by a computer, and the other enters the optical fiber path after being transmitted to the detected object via a modulator. A new approach for the real-time imaging of intravascular flow, vascular wall structures, and components of atherosclerotic plaque is proposed, which has the advantages of a high sensitivity and anti-interference.

Ghost imaging for online angiography

Ghost imaging (GI) has characteristics that make it promising for applications in life sciences and other fields, such as its high sensitivity and strong anti-interference compared with traditional imaging.This paper presents a new approach for online angiography using GI.Two signals are correlation-calculated to detect the object image:one of them is the random light field generated by a computer, and the other enters the optical fiber path after being transmitted to the detected object via a modulator.A new approach for the real-time imaging of intravascular flow, vascular wall structures, and components of atherosclerotic plaque is proposed, which has the advantages of a high sensitivity and anti-interference.

Spatial Distribution of Phase Singularities in Optical Random Vector Waves.

Phase singularities are dislocations widely studied in optical fields as well as in other areas of physics. With experiment and theory we show that the vectorial nature of light affects the spatial distribution of phase singularities in random light fields. While in scalar random waves phase singularities exhibit spatial distributions reminiscent of particles in isotropic liquids, in vector fields their distribution for the different vector components becomes anisotropic due to the direct relation between propagation and field direction. By incorporating this relation in the theory for scalar fields by Berry and Dennis [Proc. R. Soc. A 456, 2059 (2000)], we quantitatively describe our experiments.

Dynamics of a small particle in a fluctuating random light field.

The dynamics of an electric dipole in a light field consisting of electromagnetic plane waves with polarizations randomly distributed and fluctuating phases is theoretically analyzed. The expression for the optical random-force fluctuations is derived and found to be proportional to the scattering cross section and to the square of the intensity divided by the frequency of the electromagnetic field. Under these fluctuations, and in the absence of damping, the dipole behaves like a super-diffusive particle with a kinetic energy growing linearly with time. The analytic predictions are tested against numerical simulations for the particular case of a resonant dipole.

Edge detection based on gradient ghost imaging.

We present an experimental demonstration of edge detection based on ghost imaging (GI) in the gradient domain. Through modification of a random light field, gradient GI (GGI) can directly give the edge of an object without needing the original image. As edges of real objects are usually sparser than the original objects, the signal-to-noise ratio (SNR) of the edge detection result will be dramatically enhanced, especially for large-area, high-transmittance objects. In this study, we experimentally perform one- and two-dimensional edge detection with a double-slit based on GI and GGI. The use of GGI improves the SNR significantly in both cases. Gray-scale objects are also studied by the use of simulation. The special advantages of GI will make the edge detection based on GGI be valuable in real applications.

Generation and electromagnetic coherence of unpolarized three-component light fields

Conditions for controlled generation of completely unpolarized, genuine three-component random light fields, both radiating and evanescent, in multi-beam illumination at a planar dielectric interface are explored. The associated electromagnetic degrees of coherence are also analyzed. Our results reveal the possibility to tailor fields with polarization properties identical to those of universal blackbody radiation, yet with tunable spatial coherence characteristics. Such unconventional, fully unpolarized three-component electromagnetic fields, not addressable by the traditional beam-field formalism, could be exploited in surface-photonic light-matter interactions.

Controlling dispersion forces between small particles with artificially created random light fields.

Appropriate combinations of laser beams can be used to trap and manipulate small particles with optical tweezers as well as to induce significant optical binding forces between particles. These interaction forces are usually strongly anisotropic depending on the interference landscape of the external fields. This is in contrast with the familiar isotropic, translationally invariant, van der Waals and, in general, Casimir–Lifshitz interactions between neutral bodies arising from random electromagnetic waves generated by equilibrium quantum and thermal fluctuations. Here we show, both theoretically and experimentally, that dispersion forces between small colloidal particles can also be induced and controlled using artificially created fluctuating light fields. Using optical tweezers as a gauge, we present experimental evidence for the predicted isotropic attractive interactions between dielectric microspheres induced by laser-generated, random light fields. These light-induced interactions open a path towards the control of translationally invariant interactions with tuneable strength and range in colloidal systems.

Step-by-step guide to the realization of advanced optical tweezers

Since the pioneering work of Arthur Ashkin, optical tweezers (OT) have become an indispensable tool for contactless manipulation of micro- and nanoparticles. Nowadays OT are employed in a myriad of applications demonstrating their importance. While the basic principle of OT is the use of a strongly focused laser beam to trap and manipulate particles, more complex experimental setups are required to perform novel and challenging experiments. With this article, we provide a detailed step-by-step guide for the construction of advanced optical manipulation systems. First, we explain how to build a single-beam OT on a homemade microscope and how to calibrate it. Improving on this design, we realize a holographic OT, which can manipulate independently multiple particles and generate more sophisticated wavefronts such as Laguerre–Gaussian beams. Finally, we explain how to implement a speckle OT, which permits one to employ random speckle light fields for deterministic optical manipulation.