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1

Two methods for determination of the effective wavenumber of Gaussian beams in absolute gravimeters

Křen P., Pálinkáš V.

Metrology and Measurement Systems

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2018

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Vol. 25, nr 4

701--713

EN

This paper presents two methods for evaluation of the effective wavenumber of nearly-Gaussian beams in laser interferometers that can be used for determination of a so called diffraction correction in absolute gravimeters. The first method, that can be simply used in situ, is an empirical procedure based on the evaluation of the variability of g measurements against the amount of light limited by an iris diaphragm and transmitted to a photodetector. However, precision of this method depends on the beam quality similarly as in the case of the conventional method based on measurement of a beam width. The second method, that is more complex, is based on beam profiling in various distances and on calculation of the effective wavenumber using the second spatial derivative of a non-ideal beam field envelope. The measurement results achieved by both methods are presented on an example of two absolute gravimeters and the determined diffraction corrections are compared with the results obtained by measurements of beam width. Agreement of methods within about 1 μGal have been obtained with average diffraction corrections slightly exceeding +2 μGal for three FG5(X) gravimeter configurations.

2

A method of directly extracting multiwave angle-domain common-image gathers

Han J., Wang Y.

Acta Geophysica

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2017

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Vol. 65, no. 5

1009--1017

EN

Angle-domain common-image gathers (ADCIGs) can provide an effective way for migration velocity analysis and amplitude versus angle analysis in oil-gas seismic exploration. On the basis of multi-component Gaussian beam prestack depth migration (GB-PSDM), an alternative method of directly extracting multiwave ADCIGs is presented in this pa-per. We first introduce multi-component GB-PSDM, where a wavefield separation is proceeded to obtain the separated PP- and PS-wave seismic records before migration imaging for multiwave seismic data. Then, the principle of extracting PP- and PS-ADCIGs using GB-PSDM is pre-sented. The propagation angle can be obtained using the real-value travel time of Gaussian beam in the course of GB-PSDM, which can be used to calculate the incidence and reflection angles. Two kinds of ADCIGs can be extracted for the PS-wave, one of which is P-wave incidence ADCIGs and the other one is S-wave reflection ADCIGs. In this paper, we use the incident angle to plot the ADCIGs for both PP- and PS-waves. Finally, tests of synthetic examples show that the method introduced here is accu-rate and effective.

3

Pomiary charakterystyk wiązek laserowych stosowanych do obróbki materiałów

Sarzyński A.

Prace Instytutu Elektrotechniki

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2012

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Z. 256

251-267

PL

W pracy zamieszczono wyniki pomiarów charakterystyk laserów stosowanych do obróbki materiałów. Zamieszczono schemat układu pomiarowego. Opisano charakterystyczne cechy wiązek Gaussowskich i sposób obliczania parametru M2, charakteryzującego jakość wiązki laserowej. Podano prawa podobieństwa dla różnych układów ogniskujących. Mierzono rozkłady gęstości energii w przekroju poprzecznym wiązki, czasowy kształt impulsu, stabilność mocy wiązki w funkcji czasu oraz wyznaczano parametr M2. Pomiary przeprowadzono dla lasera He-Ne, lasera włóknowego ciągłego działania SPI-100C o mocy 100 W, włóknowego lasera impulsowego Ylia M20EG oraz pikosekundowego lasera Nd:YAG. Podano przykłady czynników zwiększających błędy pomiarowe. Zamieszczono przykłady zastosowania wyników pomiarów do procesu znakowania laserowego.

EN

The results of measurements of characteristics of laser beams applied in processing of materials are described in the paper. The scheme of experimental setup is described. The specific properties of Gaussian laser beams are posted. The procedure for determination of laser beam quality parameter, the so called M2 parameter is discussed. The laws of similarity of beams focused by different lenses are included. Distributions of energy density in the beam cross-section, temporal pulse shape, stability of laser Power versus time were measured. The M2 parameter was calculated. The measurements were performed for He-Ne, fiber CW SPI-100C, pulsed Ylia M20EG and picosecond lasers. Some examples of factors influencing measurements errors are listed. Some examples of laser marking process are provided.

4

A new spherical aberration coefficient C4 for the Gaussian laser beam

Bencheikh A., Bouafia M., Ferria K.

Optica Applicata

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2011

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Vol. 41, nr 4

855--861

EN

Laser beam quality is related to the aberration effect. Quartic phase aberration, more commonly known as spherical aberration, can result from aberrated optical components such as beam expanding telescopes, focusing or collimating lenses, or other conventional optical elements. In general, any kind of quartic aberration will lead to increased far field beam spread, degraded laser beam focusability and increased values of the beam quality. Currently, a well established quality parameter for laser beams is the M2 factor which is proportional to the coefficient of quartic phase aberration denoted C4. In many recent papers, authors used C4 given in geometrical optics approach to evaluate the laser beam quality M2 which belongs to the Gaussian beam optics and the two disciplines are not to be confused. In this paper, we present a new mathematical set for the spherical aberration coefficient C4, especially for Gaussian beams in the context of Gaussian beam optics. A numerical analysis of a set of lenses is done to show the importance of the new C4.

5

Ponderomotive self-focusing of a short laser pulse under a plasma density ramp

Kant N., Saralch S., Singh H.

Nukleonika

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2011

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Vol. 56, No. 2

149-153

EN

The ponderomotive self-focusing of a short laser pulse in an underdense plasma under a plasma density ramp is analyzed. The pulse may acquire a minimum spot size due to the ponderomotive self-focusing. Beyond the focus, the nonlinear refraction starts weakening, and the spot size of the laser pulse increases, resulting in an oscillatory self-focusing and defocusing behavior of the beam with the propagation distance. In order to minimize the defocusing, we introduce a localized upward plasma density ramp. Due to the upward plasma density ramp, the laser beam retains a minimum spot size. Self-focusing becomes stronger with a mild ripple as the propagation distance increases. The conditions for the ponderomotive self-focusing for suitable parameters of the laser beam and the plasma are determined. The plasma density ramp of the considered type may be observed in gas jet plasma experiments.

6

Digital holography of Gaussian beams

Jóźwicki R., Prytulak M.

Elektronika : konstrukcje, technologie, zastosowania

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2004

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Vol. 45, nr 8-9

287-290

EN

There are two methods to reconstruct images in digital holography, they are based on the Fresnel approximation and the convolution approach. Both of them require however digital reconstruction of image field distributions at a chosen image plane. Such methods do not allow the formulation of general conclusions concerning the image quality. In this paper full registration and reconstruction processes of gaussian beams in analytical forms are described. The formulae received allow the determination of fields in the whole paraxial space of the holographic system. Additionally, it can be accomplished for four components of the diffraction field independently.

PL

W cyfrowej holografii istnieją dwie metody rekonstrukcji obrazów, a mianowicie przybliżenie Fresnela i metoda splotowa. Obie te metody wymagają jednak cyfrowego wyznaczania zespolonych pól w wybranej płaszczyźnie. Takie podejście nie pozwala na formułowanie ogólnych wniosków dotyczących jakości odwzorowania holograficznego. W artykule opisano analitycznie cały proces odwzorowania holograficznego dla wiązek gaussowskich. Otrzymane zależności umożliwiają wyznaczanie pól rejestrowanych i rekonstruowanych w całej przestrzeni przyosiowej. Ponadto analiza odwzorowania może być dokonana niezależnie dla każdego z czterech składowych pola rekonstruowanego.