Finite Spatial Coherence Research Articles

Partial-field decomposition analysis of full-scale supersonic jet noise using optimized-location virtual references.

Supersonic jet noise reduction efforts benefit from targeted source feature extraction and high-resolution acoustic imaging. Another useful tool for feature extraction is partial field decomposition of sources into independent contributors. Since such decomposition processes are nonunique, care must be taken in the physical interpretation of decomposed partially coherent aeroacoustic fields. The optimized-location virtual reference method (OLVR) is a partial field decomposition designed to extract physically meaningful source and field information through the strategic placement of virtual references within a reconstructed field. The OLVR method is applied here to obtain spatially distinct and ordered partial sources at multiple frequencies of a full-scale, high-performance supersonic jet engine operating at 100% engine power. Partial sources are shown to mimic behaviors of the total source distributions including monotonic growth and decay. Because of finite spatial coherence, multiple partial sources are used to reproduce far-field radiation away from the main lobe, and the number of required sources increases with increasing frequency. An analytical multiwavepacket model is fitted to the partial sources to demonstrate how OLVR partial fields can be leveraged to produce reduced-order models.

Gaussian-Schell analysis of the transverse spatial properties of high-harmonic beams.

High harmonic generation (HHG) is an established means of producing coherent, short wavelength, ultrafast pulses from a compact set-up. Table-top high-harmonic sources are increasingly being used to image physical and biological systems using emerging techniques such as coherent diffraction imaging and ptychography. These novel imaging methods require coherent illumination, and it is therefore important to both characterize the spatial coherence of high-harmonic beams and understand the processes which limit this property. Here we investigate the near- and far-field spatial properties of high-harmonic radiation generated in a gas cell. The variation with harmonic order of the intensity profile, wavefront curvature, and complex coherence factor is measured in the far-field by the SCIMITAR technique. Using the Gaussian-Schell model, the properties of the harmonic beam in the plane of generation are deduced. Our results show that the order-dependence of the harmonic spatial coherence is consistent with partial coherence induced by both variation of the intensity-dependent dipole phase as well as finite spatial coherence of the driving radiation. These findings are used to suggest ways in which the coherence of harmonic beams could be increased further, which would have direct benefits to imaging with high-harmonic radiation.

Effect of finite spatial coherence length on small-angle scattering

This study shows that forward scattering at the origin of reciprocal space contributes to the scattering intensity profiles of ultra-small-angle scattering. The forward scattering corresponds to a Fourier transform of the X-ray coherent volume on a sample. This contribution is usually ignored in the study of small-angle scattering, while it is fully considered in the fields of X-ray imaging, such as coherent X-ray diffraction imaging and X-ray ptychography. This effect is explicitly illustrated in the context of small-angle scattering, and the effect of a finite spatial coherence length on small-angle scattering is discussed.

Influence of finite spatial coherence on ptychographic reconstruction

X-ray ptychography is an ultrahigh-resolution scanning coherent diffractive imaging technique, allowing quantitative measurements of extended samples and a simultaneous reconstruction of the illuminating wavefront. Recent development of the mixed-state reconstruction algorithm has triggered a certain interest in utilizing partially coherent X-ray sources for ptychography. Here, we study how finite spatial coherence influences the reconstructed image of a test structure. Our work shows that use of a highly coherent illumination provides images with better spatial resolution and fewer artefacts than the approach with partial coherence.

Numerical study of acoustic radiation due to a supersonic turbulent boundary layer

AbstractDirect numerical simulations are used to examine the pressure fluctuations generated by fully developed turbulence in a Mach 2.5 turbulent boundary layer, with an emphasis on the acoustic fluctuations radiated into the free stream. Single- and multi-point statistics of computed surface pressure fluctuations show good agreement with measurements and numerical simulations at similar flow conditions. Consistent with spark shadowgraphs obtained in free flight, the quasi-homogeneous acoustic near field in the free-stream region consists of randomly spaced wavepackets with a finite spatial coherence. The free-stream pressure fluctuations exhibit important differences from the surface pressure fluctuations in amplitude, frequency content and convection speeds. Such information can be applied towards improved modelling of boundary layer receptivity in conventional supersonic facilities and, hence, enable a better utilization of transition data acquired in such wind tunnels. The predicted acoustic characteristics are compared with the limited available measurements. Finally, the numerical database is used to understand the acoustic source mechanisms, with the finding that the supersonically convecting eddies that can directly radiate to the free stream are confined to the buffer zone within the boundary layer.

Vector sensors and vector sensor line arrays: Comments on optimal array gain and detection

This paper examines array gain and detection performance of single vector sensors and vector sensor line arrays, with focus on the impact of nonacoustic self noise and finite spatial coherence of the noise between the vector sensor components. Analytical results based on maximizing the directivity index show that the particle motion channels should always be included in the processing for optimal detection, regardless of self noise level, as long as the self noise levels are taken into account. The vector properties of acoustic intensity can be used to estimate the levels of nonacoustic noise in ocean measurements. Application of conventional, minimum variance distortionless response, and white-noise-constrained adaptive beamforming methods with ocean acoustic data collected by a single vector sensor illustrate an increase in spatial resolution but a corresponding decrease in beamformer output with increasing beamformer adaptivity. Expressions for the spatial coherence of all pairs of vector sensor components in homogeneous, isotropic noise show that significant coherence exists at half-wavelength spacing between particle motion components. For angular intervals about broadside, an equal spacing of about one wavelength for all components provides maximum directivity index, whereas each of the component spacings should be different to optimize the directivity index for angular intervals about endfire.

Derivation of a Monte Carlo method for modeling heterodyne detection in optical coherence tomography systems.

A Monte Carlo (MC) method for modeling optical coherence tomography (OCT) measurements of a diffusely reflecting discontinuity embedded in a scattering medium is presented. For the first time to the authors' knowledge it is shown analytically that the applicability of an MC approach to this optical geometry is firmly justified, because, as we show, in the conjugate image plane the field reflected from the sample is delta-correlated from which it follows that the heterodyne signal is calculated from the intensity distribution only. This is not a trivial result because, in general, the light from the sample will have a finite spatial coherence that cannot be accounted for by MC simulation. To estimate this intensity distribution adequately we have developed a novel method for modeling a focused Gaussian beam in MC simulation. This approach is valid for a softly as well as for a strongly focused beam, and it is shown that in free space the full three-dimensional intensity distribution of a Gaussian beam is obtained. The OCT signal and the intensity distribution in a scattering medium have been obtained for several geometries with the suggested MC method; when this model and a recently published analytical model based on the extended Huygens-Fresnel principle are compared, excellent agreement is found.

Long intrinsic radiative lifetimes of excitons in quantum wires

Calculations of exciton radiative lifetimes in quantum wires free of defects are presented. Only excitons near zero wave number can spontaneously undergo radiative decay while conserving energy and momentum. The lifetime of the lowest-energy exciton in 100-\AA{}-diam GaAs quantum wires, ${\mathrm{\ensuremath{\tau}}}_{\mathrm{spon}}$=150 ps, is an order of magnitude longer than in quantum wells. This is due to the finite spatial coherence in the lateral direction. Thermalization effects lead to an effective lifetime of \ensuremath{\tau}(T)=140 \ensuremath{\surd}T ps ${\mathrm{K}}^{\mathrm{\ensuremath{-}}1/2}$.

Observability of atmospheric glories and supernumerary rainbows

The finite spatial coherence width of sunlight at the Earth imposes restrictions on the production of scattering phenomena based on the interference of light waves. With the spatial coherence properties of sunlight taken into account, the visibility of the supernumerary rainbow sequence adjacent to the primary rainbow and the radii of the water droplets that produce the optimum glory intensity were calculated. A substantial reduction was found in the contrast of all the supernumeraries beyond the first few, and the peak observability of the glory occurred for water droplets with radii between 10 and 20 μm.