1,720,978 research outputs found
25 ns software correlator for photon correlation spectroscopy
No full textA 10 ns time resolution, multi-tau software correlator, capable of computing simultaneous autocorrelation (A-A, B-B) and cross (A-B) correlation functions at count rates up to ∼10 MHz, with no data loss, has been developed in LabVIEW and C++ by using the National Instrument timer/counterboard (NI PCIe-6612) and a fast Personal Computer (PC) (Intel Core i7-4790 Processor 3.60 GHz). The correlator works by using two algorithms: for large lag times (τ ≥ 1 μs), a classical time-mode scheme, based on the measure of the number of pulses per time interval, is used; differently, for τ ≥ 1 μs a photon-mode (PM) scheme is adopted and the correlation function is retrieved from the sequence of the photon arrival times. Single auto- and cross-correlation functions can be processed online in full real time up to count rates of ∼1.8 MHz and ∼1.2 MHz, respectively. Two autocorrelation (A-A, B-B) and a cross correlation (A-B) functions can be simultaneously processed in full real time only up to count rates of ∼750 kHz. At higher count rates, the online processing takes place in a delayed modality, but with no data loss. When tested with simulated correlation data and latex spheres solutions, the overall performances of the correlator appear to be comparable with those of commercial hardware correlators, but with several nontrivial advantages related to its flexibility, low cost, and easy adaptability to future developments of PC and data acquisition technology
Fast multi-tau real-time software correlator for dynamic light scattering
No full textWe present a PC-based multi-tau software correlator suitable for processing dynamic light-scattering data. The correlator is based on a simple algorithm that was developed with the graphical programming language LabVIEW, according to which the incoming data are processed on line without any storage on the hard disk. By use of a standard photon-counting unit, a National Instruments Model 6602–PCI timer–counter, and a 550-MHz Pentium III personal computer, correlation functions can be worked out in full real-time over time scales of ∼5 μs and in batch processing down to time scales of ∼300 ns. The latter limit is imposed by the speed of data transfer between the counter and the PC’s memory and thus is prone to be progressively reduced with future technological development. Testing of the correlator and evaluation of its performances were carried out by use of dilute solutions of calibrated polystyrene spheres. Our results indicate that the correlation functions are determined with such precision that the corresponding particle diameters can be recovered to within an accuracy of a few percent rms. © 2001 Optical Society of America
Three-dimensional coherence of light speckles: Theory
No full textWe provide a detailed analysis of the three-dimensional spatial coherence properties of light speckles, based on very general assumptions. We show that, while in the deep Fresnel region close to the source the longitudinal coherence of speckles is ruled by the laws of ordinary diffraction, on approach to the Fraunhofer zone the longitudinal coherence length tends to become infinite. We offer both a quantitative and a qualitative description of the emergence of these different behaviors.We provide a detailed analysis of the three-dimensional spatial coherence properties of light speckles, based on very general assumptions. We show that, while in the deep Fresnel region close to the source the longitudinal coherence of speckles is ruled by the laws of ordinary diffraction, on approach to the Fraunhofer zone the longitudinal coherence length tends to become infinite. We offer both a quantitative and a qualitative description of the emergence of these different behaviors. © 2008 The American Physical Society
Three-dimensional coherence of light speckles: Experiment
No full textWe provide an experimental detailed study of the three-dimensional coherence properties of light speckles produced by different tunable pseudothermal sources. Our findings confirm the theoretical prediction of the companion article [A. Gatti et al., Phys. Rev. A 78, 063806 (2008)], according to which the longitudinal coherence of the speckles is ruled by ordinary diffraction laws only in the deep-Fresnel zone close to the source, deviates from this behavior in the Fresnel zone, and tends to become infinite when approaching the Fraunhofer zone. A quantitative comparison with theory is presented for Gaussian speckles in all the three regimes and for Airy speckles in the deep-Fresnel zone. Potential applications to three-dimensional imaging techniques are briefly discussed.We provide an experimental detailed study of the three-dimensional coherence properties of light speckles produced by different tunable pseudothermal sources. Our findings confirm the theoretical prediction of the companion article [A. Gatti , Phys. Rev. A 78, 063806 (2008)], according to which the longitudinal coherence of the speckles is ruled by ordinary diffraction laws only in the deep-Fresnel zone close to the source, deviates from this behavior in the Fresnel zone, and tends to become infinite when approaching the Fraunhofer zone. A quantitative comparison with theory is presented for Gaussian speckles in all the three regimes and for Airy speckles in the deep-Fresnel zone. Potential applications to three-dimensional imaging techniques are briefly discussed
Hardware simulator for photon correlation spectroscopy”, Rev. Sci. Instrum., 74, (2003) 4273-4279
No full textA hardware simulator, used for testing digital correlators in photon correlation spectroscopy, was described. An instrument capable of delivering a continuous stream of transistor-transistor logic pulses over one or more channels was also realized. The photodetector defects were simulated and their elimination by cross correlation techniques was checked
Differential Ghost Imaging
No full textWe present a new technique, differential ghost imaging (DGI), which dramatically enhances the signal-to-noise ratio (SNR) of imaging methods based on spatially correlated beams. DGI can measure the transmission function of an object in absolute units, with a SNR that can be orders of magnitude higher than the one achievable with the conventional ghost imaging (GI) analysis. This feature allows for the first time, to our knowledge, the imaging of weakly absorbing objects, which represents a breakthrough for GI applications. Theoretical analysis and experimental and numerical data assessing the performances of the technique are presented
Heterodyne speckle velocimetryof Poiseuille flow
No full textWe review the technique named heterodyne speckle velocimetry and present two applications for
testing the method with a fluid moving under the conditions of Poiseuille flow. The fluid was seeded
with small tracking particles diameter 300 nm and fluxed between the two parallel planes of a cell
with constant or variable cross section. In the first case the velocity distribution was constant along
the direction parallel to the planes and was in excellent agreement with the expected Poiseuille
profile along the orthogonal direction. In the second case, where velocity gradients were present also
along the planes, the technique was able to reconstruct both the orthogonal Poiseuille profile and the
in-plane two dimensional mapping of the velocity vectors, with the possibility of measuring the fluid
flux within an accuracy of 1%
Dynamic heterodyne near field scattering
No full textThe technique heterodyne near field scattering (HNFS), originally developed for low-angle static light scattering, has also been implemented for carrying out dynamic light scattering. While the classical dynamic light scattering method measures the intensity-intensity correlation function, dynamic HNFS gives directly the field-field correlation function, without any assumption on the statistical properties of the sample, as the ones required by the Siegert relation. The technique has been tested with calibrated Brownian particles and its performances compared to those of the classical dynamic light scattering method.The technique heterodyne near field scattering (HNFS), originally developed for low-angle static light scattering, has also been implemented for carrying out dynamic light scattering. While the classical dynamic light scattering method measures the intensity-intensity correlation function, dynamic HNFS gives directly the field-field correlation function, without any assumption on the statistical properties of the sample, as the ones required by the Siegert relation. The technique has been tested with calibrated Brownian particles and its performances compared to those of the classical dynamic light scattering method. © 2008 American Institute of Physics
Heterodyne speckle velocimetry
No full textWe present a simple method for fluid velocimetry based on the velocity of the heterodyne speckles
generated by tracking particles illuminated with coherent light. It works in real time and provides
instantaneous two-dimensional velocity mappings in the direction orthogonal to the optical axis,
independently of the particle concentration and size, also for subwavelength particles. It also
provides the velocity distribution of the fluid over the entire sample thickness. The method has been
quantitatively tested by using the motions of rigid diffusers and applied for mapping the flow of a
confined fluid
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