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

Effect of cavitation on velocity in the near-field of a diesel nozzle

The entire process of atomization of the fuel in an internal combustion engine plays a very important role in determining the overall efficiency of these engines. A good atomization process could help the fuel to mix with the air properly leading to its efficient combustion, thereby reducing the emitted pollutants as well. The recent trend followed by the engineers focused on designing fuel injectors for more efficient atomization is to increase the atomization pressure while decreasing the nozzle orifice diameter. A consequence of this is the development of cavitation (formation of vapor cavities or bubbles in the liquid) inside the injector close to the nozzle. The main reason behind this is the sudden changes in the pressure inside the injector and these cavities or bubbles are usually formed where the pressure is relatively low.This work mainly focuses on studying the formation of cavitation and its effect on the velocity of the spray in the near nozzle region using asymmetrical transparent nozzle equipped with a needle lift sensor with nozzle diameter of 0.35 mm at 300 bar of injection pressure. The experiment consists in recording of several image-pairs, which are separated by about 300 ns, capturing the dynamics of the spray, a few millimeters from the nozzle in the direction of the flow. These image-pairs are then used to compute the velocity from the displacement of the liquid structures and ligaments by correlating the first image with the second. About 200 of such velocity graphs are then averaged to obtain a velocity map and is compared with the similar average velocity maps obtained at different times from the start of the injection. The angular spread of the spray from each of these images is calculated as well. The images showing cavitation inside the injector are also recorded at these same instants of time so as to understand the effects of cavitation on the velocity and angular spread of the spray close to the nozzle.Comment: 13th International Conference on Liquid Atomization and Spray Systems, Aug 2015, Tainan, Taiwan. 2015, https://iclass2015.tw

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

Differing self-similarity in light scattering spectra: A potential tool for pre-cancer detection

The fluctuations in the elastic light scattering spectra of normal and dysplastic human cervical tissues analyzed through wavelet transform based techniques reveal clear signatures of self-similar behavior in the spectral fluctuations. Significant differences in the power law behavior ascertained through the scaling exponent was observed in these tissues. The strong dependence of the elastic light scattering on the size distribution of the scatterers manifests in the angular variation of the scaling exponent. Interestingly, the spectral fluctuations in both these tissues showed multi-fractality (non-stationarity in fluctuations), the degree of multi-fractality being marginally higher in the case of dysplastic tissues. These findings using the multi-resolution analysis capability of the discrete wavelet transform can contribute to the recent surge in the exploration for non-invasive optical tools for pre-cancer detection.Comment: 13 pages, 14 figure

Imagerie ultra-rapide des sprays de carburant : développement de diagnostics optiques, traitement d'images

Fuel atomization plays a very crucial role in determining the overall efficiency of diesel internal combustion engines. This work focuses on developing fast optical diagnostic tools for the study of the liquid fuel atomization with an aim to characterize the fuel sprays in the near-field of the injector nozzle. At first, classical imaging techniques using continuous illumination with a high-speed camera and an ultra-short pulsed illumination with a high-resolution CCD camera are reviewed. Next, the possibility of supercontinuum (SC) derived illumination is investigated. It was observed that the spray images obtained with such an illumination were almost free from laser speckle, which tremendously improved the clarity of these images. An application of the classical imaging technique to a preliminary study of cavitation inside a transparent injector is presented.In the next part of the thesis optical techniques to reduce the noise originating due to the multiple scattering of light from its interaction with the fuel spray are studied. Optical Kerr effect based time-gate in its primitive crossed-beam configuration is reviewed and a novel approach with collinear overlap of the pump and probe beams for time-resolved imaging of fuel spray with time resolution ~1 ps is proposed. Ballistic images of fuel spray in the near-nozzle region with high spatial resolution are obtained. The possibility of using SC-derived illumination with the optical time-gate configuration is also discussed. Preliminary time-gated spray images obtained by using SC-derived probe beam for spray illumination in the optical-gate setup shows that laser speckle is substantially reduced maintaining a similar time resolution.The change in the optical polarization properties of the Kerr medium on introduction of the pump pulse are completely characterized by measuring its Mueller matrix (MM). The polarization parameters - depolarization, diattenuation, and retardance are then quantified by decomposing the measured MM using polar decomposition formalism.L'atomisation du carburant joue un rôle très important dans l'efficacité globale du moteur à combustion interne utilisant le Diesel. Ce travail se concentre sur le développement d'outils de diagnostics optiques rapides pour l'étude de l'atomisation du combustible liquide, avec pour but de caractériser les jets de carburant en proche sortie de l'injecteur. Dans un premier temps, les techniques d'imagerie classiques utilisant (i) un éclairage continu avec une caméra à haute vitesse et (ii) un éclairage pulsé ultra-court avec une caméra CCD haute résolution sont examinées. L'utilisation d'un supercontinuum (SC) pour éclairer le jet est testée. On observe alors que les images de spray obtenues avec ce type d'éclairage sont presque exemptes de speckle, ce qui en améliore considérablement la netteté. Une technique d'imagerie classique est ensuite appliquée à l'étude de la cavitation à l'intérieur d'un injecteur transparent et une première approche de ce problème est présentée.Dans la partie suivante de la thèse, la problématique de la réduction du bruit dû à la diffusion multiple de la lumière sur le jet de carburant est posée. Les avantages et les inconvénients d'un montage utilisant une porte optique à effet Kerr avec des faisceaux pompe et sonde non colinéaires sont présentés. Une nouvelle approche avec des faisceaux pompe et sonde colinéaires aboutissant à une résolution temporelle de ~1 ps est proposée. Des images balistiques de sprays de carburant en proche sortie d'injecteur ayant une excellente résolution spatiale sont ainsi obtenues. La possibilité d'utiliser un éclairage de type SC avec une porte optique est également discutée. Les images de sprays réalisées par ce montage montrent que l'on réduit le specke tout en gardant une bonne résolution spatiale.Enfin, les propriétés polarimétriques du milieu Kerr utilisé lorsqu'il est soumis au faisceau pompe sont caractérisées par la mesure de sa matrice de Mueller (MM). Les paramètres de polarisation dépolarisation, diatténuation et retardance sont alors quantifiés par décomposition de la MM mesurée, en utilisant le formalisme de décomposition polaire

Elementary and collective excitations as probes for order parameter symmetry in Fe-based superconductors

We show that the superconducting quasiparticle energy of Fe-based superconductors would be different for a sign-changing order parameter than that for a non–sign-changing order parameter. This particularly happens in the low-doping region when the electron and hole Fermi surfaces nest and superconductivity coexists with either magnetic or orbital or both density waves. This difference in the nature of superconducting quasiparticles is also detectable from collective mode excitations. We further show that while the magnetic spin density wave order competes with superconductivity, the orbital density wave grows when the superconductivity order parameter changes sign accross the electron-hole Fermi surface. However, when the superconducting order parameter does not change sign both the density waves compete with superconductivity. Irrespective of the nature of the superconducting order parameter the spin and orbital density waves are concurrent indicating simultaneous magnetic and structural transition. The significance of the various results is discussed comparing the various experimental observations

Estimation of Longitudinal Dimensions of Sub-Picosecond Electron Bunches with the 3-Phase Method

International audienceAn estimation of the longitudinal dimensions for short electron bunches in an accelerating field is an important diagnostic and can be extremely helpful in evaluating the performance of an accelerator. We investigate a method for close estimation of bunch length for sub-picosecond electron bunches from the measurement of their energy spreads. Three or more measurements for the bunch energy spread are made by varying the phase of the accelerating structure and later a reconstruction of the bunch longitudinal dimensions, namely bunch length, initial energy spread and chirp at the entrance of the accelerating structure are obtained using the least square method. A comparison of the obtained results with ASTRA simulations is also included to validate the 3-phase method for sub-ps electron bunches. It is a simple method from both understanding (easy reconstruction using transport matrices) and experimental point of views (multiple measurements of energy spread with varying phase of the accelerating structure)