Difference of cross-spectral densities

Atmospheric turbulence effects on hollow Gaussian Schell-model array beams

Two types of hollow array beams with circular and rectangular distributions on propagating in atmospheric turbulence are investigated and analyzed comparatively with that in free space. Analytical formulas for the cross-spectral density function of two kinds of hollow array beam propagation in linear isotropic random media are derived and used to examine the behavior of the spectral densities. It is found that such beams possess stable hollow arrays with any dimension and lobes in free space, while such distributions only maintain small distances in atmospheric turbulence and ultimately tend to a Gaussian shape due to the turbulence destroying the hollow array profiles. The effects of the turbulence parameters on the behavior of the spectral density are analyzed in depth.

Reproducing Kernel Hilbert spaces for wave optics: tutorial

An introduction to the Hilbert spaces that are endowed with a reproducing kernel is presented on using the mathematical tools of Fourier optics and coherence theory. After giving the basic definition of such spaces, some examples are worked out to show how the inner product can take different forms depending on the particular function space one works with. The basic rule to build a reproducing kernel Hilbert space (RKHS) is then presented together with the basic properties of those spaces. Eigenfunctions and eigenvalues of the reproducing kernel are then illustrated and lead to the important integral representation of the reproducing kernel. The latter is used to present pseudomodal expansions and generalized forms of sampling. The concluding section offers some thoughts on the applications of RKHSs in wave optics. An appendix presents an introduction to treatments using more advanced concepts of functional analysis.

Linear Combinations of the Complex Degrees of Coherence

We propose a method for structuring the spatial coherence state of light via mixed linear combinations of N complex degrees of coherence (CDC) and specify the conditions under which such combinations represent a valid CDC. Several examples demonstrate that this method opens previously unknown avenues for modeling random sources, radiating to light fields with unique features.

Christoffel-Darboux sources

A new, to the best of our knowledge, class of partially coherent sources is introduced on the basis of the Christoffel-Darboux (CD) formula read as the expression of a possible cross-spectral density. It will be seen that such an interpretation is possible because the CD formula gives the reproducing kernel of a suitable Hilbert space. After discussing general properties of CD kernels, a specific example is worked out using Hermite polynomials. A connection with the density matrix will be highlighted.

Shaping the focal intensity distribution using a partially coherent radially polarized beam with multiple off-axis vortices

A new kind of partially coherent vector vortex beam, namely, the partially coherent radially polarized (PCRP) beam with multiple off-axis vortices, is introduced, and the average intensity distributions of such vortex beam focused by a thin lens are investigated theoretically. It is novelty that the off-axis vortices will induce the focal intensity redistribution and reconstruction, while this remarkable characteristic will be vanished in the case of a very low coherence. In view of this distinctive feature, a new method has been put forward to shape or modulate the focal intensity distribution by elaborately tailoring the multiple off-axis vortices as well as the coherence length. More importantly, some peculiar focal fields with novel structures, such as bar-shaped, triangle-shaped, square-shaped, and pentagon-shaped hollow profiles or flat-top foci, are obtained. Our results indicate that modulating the multiple off-axis vortices provides an additional degree of freedom for focus shaping.

Besinc Pseudo-Schell Model Sources with Circular Coherence

Partially coherent sources with non-conventional coherence properties present unusual behaviors during propagation, which have potential application in fields like optical trapping and microscopy. Recently, partially coherent sources exhibiting circular coherence have been introduced and experimentally realized. Among them, the so-called pseudo Schell-model sources present coherence properties that depend only on the difference between the radial coordinates of two points. Here, the intensity and coherence properties of the fields radiated from pseudo Schell-model sources with a degree of coherence of the besinc type are analyzed in detail. A sharpening of the intensity profile is found for the propagated beam by appropriately selecting the coherence parameters. As a possible application, the trapping of different types of dielectric nanoparticles with this kind of beam is described.

Hyperbolic sine-correlated beams

An explicit expression is given for the cross-spectral density that characterizes a new family of partially coherence sources with hyperbolic sine correlated function. Beam conditions for such sources are established. The propagation properties of such partially coherent beams are studied by numerical simulations. It is demonstrated that, unlike the reciprocity theorems relating to radiation from classical Schell-model sources, such beams possess both the invariance of coherent distribution and of hollow intensity shape.

Pseudo-Schell model sources

Partially coherent pseudo-Schell model sources are introduced and analyzed. They present radial symmetry and coherence characteristics depending on the difference between the radial distances of two points from the source center. As a consequence, all points belonging to circles centered on the symmetry center of the source are perfectly correlated. We show that such sources radiate fields with peculiar behaviors in paraxial propagation. In particular, when compared to beams produced by Gaussian Schell-model sources with comparable coherence parameters, their irradiance can present sharper profiles and higher peak valuesmono and a better beam quality parameter. Furthermore, when a pseudo-Schell model source presents a vortex, the propagated beam preserves a null of the intensity along its axis.

Partially coherent sources with radial coherence

Partially coherent sources with radial coherence are proposed. They present a circularly symmetric intensity profile and a degree of coherence whose absolute value only depends on the angular difference between the two considered points. In particular, the source is completely coherent at pairs of points belonging to the same radius. The modal structure of such sources is determined in the general case, and conditions are derived under which the field propagated in paraxial approximation remains radially coherent at any transverse plane. In such cases, the angular dependence of the correlation function is preserved upon propagation, although the intensity profile generally changes. An example of this kind of source has been experimentally synthesized by means of a simple setup, and its coherence characteristics have been tested by means of a Young interferometer.

Partially coherent sources with circular coherence

A new class of partially coherent light sources is introduced. At the source plane, they exhibit perfect coherence along any annulus that is concentric to the source center. Between two points at different distances from the center, coherence can be partial or even vanishing. Such sources can be synthesized by using a generalized form of van Cittert-Zernike theorem where axial sources are used. Beams radiated by this type of source are analyzed at the source plane and upon free propagation for some simple cases.

Random sources for rotating spectral densities

The propagating modes of a wide-sense stationary Schell-like source with arbitrary coherence state and a twist factor are determined. This suggests a convenient practical method for modeling novel classes of twisted partially coherent beam-like fields. The first example discusses the previously introduced twisted anisotropic Gaussian Schell-model source and verifies the feasibility of this method. As a second example, we introduce a new type of twisted partially coherent beam in which a radiated flat-top average intensity pattern remains invariant in shape (but not size) while it twists around the axis upon propagation.

Random sources for cusped beams

We introduce two novel classes of partially coherent sources whose degrees of coherence are described by the rectangular Lorentz-correlated Schell-model (LSM) and rectangular fractional multi-Gaussian-correlated Schell-model (FMGSM) functions. Based on the generalized Collins formula, analytical expressions are derived for the spectral density distributions of these beams propagating through a stigmatic ABCD optical system. It is shown that beams belonging to both classes form the spectral density apex that is much higher and sharper than that generated by the Gaussian Schell-model (GSM) beam with a comparable coherence state. We experimentally generate these beams by using a nematic, transmissive spatial light modulator (SLM) that serves as a random phase screen controlled by a computer. The experimental data is consistent with theoretical predictions. Moreover, it is illustrated that the FMGSM beam generated in our experiments has a better focusing capacity than the GSM beam with the same coherence state. The applications that can potentially benefit from the use of novel beams range from material surface processing, to communications and sensing through random media.

Electromagnetic Schell-model sources generating far fields with stable and flexible concentric rings profiles

A class of electromagnetic sources with multi-cosine Gaussian Schell-model correlation function is introduced. The realizability conditions for such sources and the beam conditions for the beams generated by them are established. Analytical formulas for the cross-spectral density matrix of such beams propagating in free space are derived and used to examine their statistical properties by numerical simulations. The results demonstrate that such beams possess invariant far-field concentric rings-like intensity patterns, uniform polarization state on all rings, and generally different degree of polarization values on different rings. By changing the summation index and other source parameters, the number of rings along with the behavior of various beam characteristics can be tuned at will.

Correlation-induced changes of the degree of paraxiality of a partially coherent beam

The degree of paraxiality (DOP) is a quantification of how paraxial (or nonparaxial) a light field is. In this article, we study the DOP of a partially coherent beam with a nonconventional spatial correlation function and explore the correlation-induced changes of the DOP. Analytical expressions for the DOP and the far-field divergence angle of a generalized multi-Gaussian correlated Schell-model (MGCSM) beam are derived. The properties of the DOP and the far-field divergence angle for the first type and the second type of generalized MGCSM beams without and with truncation in free space are illustrated numerically, and we find that the DOP and far-field divergence angle are closely related with the distribution of the correlation function. Modulation of the correlation function provides a novel way for modulating the nonparaxial properties of a partially coherent beam.

Gaussian Schell-model arrays

We introduce a novel class of planar, quasi-homogeneous Schell-model source for producing far fields with optical lattice average intensity patterns and derive the corresponding beam conditions. The array dimension, lobes intensity profile, and periodicity of the optical lattice can be flexibly tuned by changing the correlation parameters of the source field. It is also found that, with an appropriate choice of the source parameters, the radiant intensity may possess flat-topped intensity patterns.

Source coherence-based far-field intensity filtering

An inherent relationship between an invariant far-field beam intensity pattern and the convolution of any two legitimate degrees of coherence in the source plane is established. Two classes of random sources are introduced by modeling the source degree of coherence with the help of the convolution operation of the Gaussian Schell-model correlation function and the multi-sinc Schell-model correlation function in the polar and Cartesian symmetries. The established relationships are used to explore the far-field intensity features produced by the new sources. It is shown that the far-field intensity patterns of the novel sources have multi-sinc Schell-model transverse distributions with a Gaussian envelope, looking like the multi-sinc Schell-model beams filtered by a soft-edge Gaussian aperture. The results demonstrate the potential of coherence modulation of the source fields for far-field beam shaping applications.

Alternating series of cross-spectral densities

We establish a sufficient condition under which the alternation series of cross-spectral densities (CSD) constitutes a valid CSD. Using such a condition, we introduce a novel class of multi-sinc Schell-model sources with circular and Cartesian symmetries. It is demonstrated that far fields produced by the new sources carry interesting characteristics, being adjustable multi-rings and lattice patterns.

Splitting and combining properties of an elegant Hermite-Gaussian correlated Schell-model beam in Kolmogorov and non-Kolmogorov turbulence

Elegant Hermite-Gaussian correlated Schell-model (EHGCSM) beam was introduced in theory and generated in experiment just recently [Phys. Rev. A 91, 013823 (2015)]. In this paper, we study the propagation properties of an EHGCSM beam in turbulent atmosphere with the help of the extended Huygens-Fresnel integral. Analytical expressions for the cross-spectral density and the propagation factors of an EHGCSM beam propagating in turbulent atmosphere are derived. The statistical properties, such as the spectral intensity, the spectral degree of coherence and the propagation factors, of an EHGCSM beam in Kolmogorov and non-Kolmogorov turbulence are illustrated numerically. It is found that an EHGCSM beam exhibits splitting and combing properties in turbulent atmosphere, and an EHGCSM beam with large mode orders is less affected by turbulence than an EHGCSM beam with small mode orders or a Gaussian Schell-model beam or a Gaussian beam, which will be useful in free-space optical communications.

Shape-invariant difference between two Gaussian Schell-model beams

The difference between two Gaussian Schell-model cross-spectral densities can give a new genuine correlation function if suitable conditions are met. Generally speaking, the structure of such cross-spectral density changes in a complicated way upon propagation. We consider here the notable exception of shape-invariant beams, and we investigate their intensity and coherence properties. The modal analysis of this class of cross-spectral densities is exploited to devise a synthesis scheme for this type of beam.

Powers of the degree of coherence

We establish conditions under which a legitimate degree of coherence of a statistically stationary beam-like field raised to a power results in a novel legitimate degree of coherence. The general results and examples relate to scalar beams having uniform and non-uniform correlations.

Design of weak scattering media for controllable light scattering

We explore the possibility of designing correlation functions of a particle's scattering potential for producing desired scattered intensity distributions, within the validity of the first Born and far-field approximations. It is shown that for scatterers with the same average distributions of the scattering potential (refractive index) but different degrees of potential's correlation, the scattered fields with prescribed average intensity distributions can be produced. Two examples of such potentials are included: those for flat circular and ring-like scattered intensities, if considered to be a function of the scattering (azimuthal) angle. In both cases the height, width and edge sharpness can be adjusted at will. Production of the novel media is envisioned via the 3D printing or sequences of liquid crystal light modulators.

Products of Schell-model cross-spectral densities

The condition under which a product of two cross-spectral densities (CSD) constitutes a valid correlation function is established. The results are obtained for the CSDs of two one-dimensional, scalar Schell-model sources, but can be readily generalized to other situations. It is shown via a number of numerical examples how new source classes of Schell-model type and the beam-like fields they radiate can be designed.

Optical coherence gratings and lattices

We introduce a class of partially coherent temporal/spatial sources, optical coherence gratings/lattices that have a Gaussian intensity profile and statistically stationary/homogeneous, periodic temporal/spatial coherence properties. We show that temporal coherence gratings generate partially coherent pulses with periodic spectra, whereas spatial coherence lattices yield far-zone output in the form of periodic lattices of highly directional beams.

Two types of sinc Schell-model beams and their propagation characteristics

Two types of partially coherent beams with sinc Schell-model-correlated function are introduced. The evolution behaviors of scalar beams generated by these two types of sources in free space are investigated with the help of the weighted superposition method and numerical examples. It is illustrated that the far field produced by these two novel sources carry tunable flat and dark hollow profiles, respectively. Our results suggest two new kinds of modal structure of the correlation source and of the optical field that such sources produce.

Position modulation with random pulses

A new class of sources generating ensemble of random pulses is introduced based on superposition of the mutual coherence functions of several Multi-Gaussian Schell-model sources that separately are capable of shaping the propagating pulse's average intensity into flat profiles with adjustable duration and edge sharpness. Under certain conditions that we discuss in detail such superposition allows for production of a pulse ensemble that after a sufficiently long propagation distance in a dispersive medium reshapes its average intensity from an arbitrary initial profile to a train whose parts have flat intensities of different levels and durations and can be either temporarily separated or adjacent.

Random sources for optical frames

Analytical models for random sources producing far fields with frame-like intensity profiles are introduced. The frames can have polar and Cartesian symmetry and adjustable sharpness of the inner and outer edges. The frames are shape invariant throughout the far zone but expand due to diffraction with growing distance from the source. The generalization to multiple nested frames is also discussed. The applications of the frames are envisioned in material surface processing and particle trapping.

Difference of two Gaussian Schell-model cross-spectral densities

We present a number of results relating to the difference of two Gaussian Schell-model cross-spectral densities (CSDs). They allow us to specify conditions under which such a difference represents itself in a valid CSD. In particular, a sufficient condition is derived for the non-negative definiteness of the resulting CSD, for any admissible choice of the involved parameters, while a necessary and sufficient condition is obtained for the case of CSDs endowed with the property of being shape-invariant upon propagation.