Collinear, two-color optical Kerr effect shutter for ultrafast time-resolved imaging

Imaging with ultrashort exposure times is generally achieved with a crossed-beam geometry. In the usual arrangement, an off-axis gating pulse induces birefringence in a medium exhibiting a strong Kerr response (commonly carbon disulfide) which is followed by a polarizer aligned to fully attenuate the on-axis imaging beam. By properly timing the gate pulse, imaging light experiences a polarization change allowing time-dependent transmission through the polarizer to form an ultrashort image. The crossed-beam system is effective in generating short gate times, however, signal transmission through the system is complicated by the crossing angle of the gate and imaging beams. This work presents a robust ultrafast time-gated imaging scheme based on a combination of type-I frequency doubling and a collinear optical arrangement in carbon disulfide. We discuss spatial effects arising from crossed-beam Kerr gating, and examine the imaging spatial resolution and transmission timing affected by collinear activation of the Kerr medium, which eliminates crossing angle spatial effects and produces gate times on the order of 1 ps. In addition, the collinear, two-color system is applied to image structure in an optical fiber and a gasoline fuel spray, in order to demonstrate image formation utilizing ballistic or refracted light, selected on the basis of its transmission time.Comment: 13 pages, 10 figure

SURFACE TENSION MEASUREMENT AT HIGH PRESSURE

ABSTRACT A non-intrusive method, based on the properties of capillary waves, is used to measure surface tension at high pressure. Results are given for diesel fuel, gasoline and n-heptane. Large variations with pressure are showed. A model is proposed to understand the influence of pressure upon the liquid-gas interface. Results of both approaches are in good agreement

Ballistic Imaging of High-Pressure Fuel Sprays using Incoherent, Ultra- short Pulsed Illumination with an Ultrafast OKE-based Time Gating

We present an optical Kerr effect based time-gate with the collinear incidence of the pump and probe beams at the Kerr medium, liquid carbon disulfide, for ballistic imaging of the high-pressure fuel sprays. The probe pulse used to illuminate the object under study is extracted from the supercontinuum generated by tightly focusing intense femtosecond laser pulses inside water, thereby destroying their coherence. The optical imaging spatial resolution and gate timings are investigated and compared with a similar setup without supercontinuum generation, where the probe is still coherent. And finally, a few ballistic images of the fuel sprays using coherent and incoherent illumination with the proposed time-gate are presented and compared qualitatively.Comment: 7 pages, 7 figures, Presented at the 17th International Symposium on Applications of Laser Techniques to Fluid Mechanics held at Lisbon, Portugal from 7th to 10th of July, 201

Quantitative comparison of fuel spray images obtained using ultrafast coherent and incoherent double-pulsed illumination

We present a quantitative comparison between the high-pressure fuel spray images obtained experimentally using classical imaging with coherent and incoherent ultrafast illuminations recorded using a compatible CMOS camera. The ultrafast, incoherent illumination source was extracted from the supercontinuum generated by tightly focusing the femtosecond laser pulses in water. The average velocity maps computed using time-correlated image-pairs and spray edge complexity computed using the average curvature scale space maps are compared for the spray images obtained with the two illumination techniques and also for the numerically simulated spray using the coupled volume of fluid and level set method for interface tracking (direct numerical simulation or DNS). The spray images obtained with supercontinuum-derived, incoherent, ultrafast illumination are clearer, since the artifacts arising due to laser speckles and multiple diffraction effects are largely reduced and show a better correlation with the DNS results.Comment: 8 pages, 9 figures, Presented at the ILASS-Europe 2014, 26th Annual Conference on Liquid Atomization and Spray Systems held at Bremen, Germany from 8th to 10th September 201

Simultaneous high-speed internal and external flow measurements for a high-pressure diesel nozzle

We present an extensive experimental study focused on understanding the impact of cavitation in a high-pressure diesel nozzle on the macroscopic properties of fuel spray. Several high-speed videos of the liquid flow through a transparent, asymmetric cylindrical nozzle with a single orifice (phi = 0.35 mm) are recorded along with the videos of the resulting spray in the near-nozzle region, issued with an injection pressure of 300 bar at a frame-rate of 75 kHz. The high-repetition images of the internal flow are then used to estimate the onset of cavitation inside the transparent nozzle and the probability of development of cavitation in different regions of the nozzle with an average estimate of the amount of cavitation with time. On the other hand, recorded spray images are used to study spray penetration, cone-angles and velocity from the start of fuel injection. A novel approach is proposed for the measurement of perturbations that occur in form of big liquid structures along the spray boundary.Comment: in 27th European Conference on Liquid Atomization and Spray Systems, Sep 2016, Brighton, United Kingdom. 201

Polarized light scattering by inhomogeneous hexagonal monocrystals. Validation with ADEOS-POLDER measurements

Various in situ measurements of the light-scattering diagram in ice clouds were performed with a new nephelometer during several airborne campaigns. These measurements were favorably compared with a theoretical scattering model called Inhomogeneous Hexagonal Monocrystal (IHM) model. This model consists in computing the scattering of light by an ensemble of randomly oriented hexagonal ice crystals containing spherical impurities of soot and air bubbles. It is achieved by using a combination of ray tracing, Mie theory, and Monte Carlo techniques and enables to retrieve the six independent elements of the scattering matrix. This good agreement between nephelometer measurements and IHM model provides an opportunity to use this model in order to analyze ADEOS-POLDER total and polarized reflectance measurements over ice clouds. POLDER uses an original concept to observe ice cloud properties, enabling to measure reflectances and polarized reflectances, for a given scene, under several (up to 14) viewing directions. A first analysis of ice cloud spherical albedoes over the terrestrial globe for November 10, 1996, and April 23, 1997, shows a rather good agreement between measurements and modeling. Moreover, polarized reflectances are also calculated and show a satisfactory agreement with measurements

Vectorial Complex Ray Model for Light Scattering of Nonspherical Particles

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Fraunhofer diffraction of irregular apertures by Heisenberg uncertainty Monte Carlo model

WOS:000369457900017International audienceGeometrical optics and the Monte Carlo method are very flexible in dealing with the interaction of light with non-spherical particles, but usually diffraction is not considered. To cover this gap, the Heisenberg Uncertainty Monte Carlo (HUMC) model is applied to calculate separately the diffraction of a ray or a photon. In this paper, we report an improvement of the HUMC model by specifying the phase of the photon subject to the Fraunhofer diffraction condition. After validating the model by comparing its results with analytical results for apertures of simple shapes, the HUMC model is then applied in simulations of Fraunhofer diffraction by apertures of complex shapes, such as those composed of one or two elliptical openings. We have shown that the diffracted intensity distributions of simple apertures obtained by the HUMC model are in good agreement with the results calculated from analytical expressions. The simulations of diffraction by apertures composed of two square or elliptical openings prove that the HUMC model is a powerful and flexible tool for predicting the Fraunhofer diffraction by a complex optical system. (C) 2015 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved