Far-field Coherence Research Articles

Effect of Polarization on Cross-Spectral Density Matrix

Coherence-polarization properties of different beams are experimentally measured in the far-field from the source and results are presented for incoherent sources with three different polarization features, such as unpolarized, diagonally polarized, and spatially depolarized. These results highlight the role of polarization tailoring on far-field coherence-polarization properties of the incoherent vector source. The effect of polarization on far-field coherence is analyzed using a beam cross-spectral density (CSD) matrix, and the role of polarization tailoring on the CSD matrix is demonstrated. Two-dimensional spatial distributions of all four elements of the CSD matrix are experimentally realized using a field-based interferometer with Sagnac geometry in combination with a four-step phase-shifting technique.

Propagation of Gaussian Schell-model Array beams in free space and atmospheric turbulence

Based on the extended Huygens-Fresnel principle, the evolution behavior of the spectral density and the spectral degree of coherence of the beam produced by a recently introduced novel class of Gaussian Schell-model Arrays (GSMA) source in free space and turbulence atmospheric are explored and comparatively analyzed. And the influence of the fractal constant of the atmospheric power spectrum and refractive-index structure constant on the spectral density and the spectral degree of coherence of beams are analyzed. It is shown that the optical lattice profile is stable when beams propagate in free space, but the spectral density eventually is suppressed and transformed into a Gaussian profiles when it passes at sufficiently large distances through the turbulent atmosphere. The distributions of the spectral degree of coherence in far field eventually transformed into a shrink Gaussian profile relative to free space which means that the degree of spatial coherence turns worse.

Generation of Spatially Broadband Twin Beams for Quantum Imaging

We generate spatially multimode twin beams using 4-wave mixing in a hot atomic vapor in a phase-insensitive traveling-wave amplifier configuration. The far-field coherence area measured at 3.5 MHz is shown to be much smaller than the angular bandwidth of the process and bright twin images with independently quantum-correlated subareas can be generated with little distortion. The available transverse degrees of freedom form a high-dimensional Hilbert space that we use to produce quantum-correlated twin beams with finite orbital angular momentum.

Coherence vortices of partially coherent beams in the far field

Based on the propagation law of cross-spectral density function, studied in this paper are the coherence vortices of partially coherent, quasi-monochromatic singular beams with Gaussian envelope and Schell-model correlator in the far field, where our main attention is paid to the evolution of far-field coherence vortices into intensity vortices of fully coherent beams. The results show that, although there are usually no zeros of intensity in partially coherent beams with Gaussian envelope and Schell-model correlator, zeros of spectral degree of coherence exist. The coherence vortices of spectral degree of coherence depend on the relative coherence length, mode index and positions of pairs of points. If a point and mode index are kept fixed, the position of coherence vortices changes with the increase of the relative coherence length. For the low coherent case there is a circular phase dislocation. In the coherent limit coherence vortices become intensity vortices of fully coherent Laguerre–Gaussian beams.

Numerical simulation of laser beam propagation in three-dimensional random media: beam splitting and patch formation

Abstract The results of numerical simulations of far-field, focused beam propagation in a homogeneous, Kolmogorov field are compared with theoretical predictions. It is shown that the spot structure at the focal plane depends on two parameters: the diffraction parameter ηD, which is defined as the ratio of the Fresnel length 2Z/kD 0 to the initial beam diameter D 0, and an analogous turbulence parameter ηtu=2Z/k(ρ0 LT)2, where Z is the focal distance, k is the wavenumber in vacuum, and ρ0 LTis the long-term far-field coherence length. For ηtu≫1 the beam is broken into multiple patches of typical size that is smaller than the transmitter diffraction scale. It is found that patches are also formed when ηtu≤1 provided the initial diameter D 0 is much bigger than ρ0 LT. The structure of the spot when ηtu≪1 depends on the value of ηD. For ηD≫1, the beam intensity pattern can be very distorted, however the spot is not dispersed.

Jet noise: An overview of the self and shear noise theory

Lighthill's theory of aerodynamic sound furnishes an exact set of differential equations. In order to obtain reasonable estimates of sound generated by aerodynamic sources certain simplifications are introduced. One model that has been quite successful is Ribner's self and shear noise formalism. The equivalent source terms therein are proportional to the unsteady momentum flux directed at an observer. The theory has been extended to deal with forward flight affects, heated turbulent flows. In addition there are results for nearfield pressures as well as the farfield coherence. All this can be obtained without any alteration of the underlying formalism. The turbulent flow is described by a “locally homogeneous isotropic” correlation function with specified length and time scales. Despite its simplicity the model is capable of predicting many features of the sound field generated by subsonic jets. [Work supported by NSERC, Canada.]

An analysis of the far-field coherence and radiant intensity of light scattered from liquid crystals

The dependence of the far-field coherence and radiant intensity of light scattered from a liquid crystal on the correlation length for the fluctuations of molecules in the crystal are analyzed theoretically. The results are useful for understanding data obtained in scattering experiments that study the hydrodynamic properties of liquid crystals excited into internal flow by a dc electric field or by application of sound vibrations. It is found that if the molecular correlation length is small relative to the dimensions of the illuminated volume, the scattered field behaves according to the van Cittert–Zernike theorem, and the motion of the molecules is better studied by measurement of the radiant intensity. If instead the molecular correlation length is much longer than the dimensions of the illuminated volume, the van Cittert–Zernike theorem does not apply, and the spatial correlation properties of the motion of the molecules can be studied by measurement of the far-field coherence.