Generation of periodic filament arrays by self-focusing of highly elliptical ultrashort pulsed laser beams
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Abstract:
We have studied the spontaneous breakup of highly elliptical laser beams into one- and two-dimensional arrays of light filaments in fused silica. Although the multiple filamentation process is initiated by random intensity modulation across the beam (amplitude noise), the output patterns exhibit a certain periodicity and high shot-to-shot reproducibility. The experimentally observed periodicity of the multiple filament arrays depends on the input beam intensity and this behavior is reproduced in detail by the numerical simulations. With the help of a simple analytical model we propose that the formation of periodic multiple filament structures could be understood within a simple framework of multistep phase-matched four-wave optical parametric amplification.Keywords:
Filamentation
Self-focusing
The effect of transverse elliptical intensity distribution on multiple filament formation in a femtosecond laser pulse is numerically studied. Two types of filament control are demonstrated. First, the beginning of filamentation can be varied by changing the degree of ellipticity. Second, random multiple filaments can be arranged in a spatial order predetermined by the specific shape of the ellipse. The latter is valid for pulses with a peak power more than ten times higher than the critical power for self-focusing in the medium.
Filamentation
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A method to determine the amplitude and phase fluctuations in each point of a wavefront is presented. The basic idea of the method is the possibility of investigating the amplitude and phase at each point of a wavefront by superposing a spherical reference wavefront that has its origin in the point under investigation. The information on the fluctuations is extracted in the form of an electrical signal. Application to the study of phase fluctuations produced by atmospheric turbulence is considered.
SIGNAL (programming language)
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We describe a new procedure for retrieving both amplitude and phase of an optical beam from radial shearing measurements. Information from the sheared interferogram is used to estimate and improve the beam and wavefront shape in successive iterations. We present computer simulations and experimental results that show the performance of the method.
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Shearing (physics)
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Self-focusing and self-defocusing of 1.6-nsec CO(2) laser pulses were observed in SF(6) gas for several selected 10-microm P-branch lines. The lines P(8), P(10), P(14), and P(16) self-focused, P(28) defocused, and either effect occurred at P(20), depending on the incident energy. The self-focusing was found to depend on incident fluence rather than on intensity and weakly on He buffer. Under the appropriate conditions beam filamentation occurred.
Filamentation
Self-focusing
Self-phase modulation
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In the framework of the nonlinear Schr\"odinger equation, the radiation aspect of self-focusing in two transverse dimensions appears to be (together with self-trapped propagation) an essential part of any self-focusing process. The final beamlets created in multiple filamentation are shown to be very close to canonical solitonlike solutions. This fact is used to establish an upper bound on the amount of radiation emitted. From this a parameter is defined which proves to be useful to characterize the self-focusing behavior as a function of the initial conditions.
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Self-focusing
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Up to now the long range filaments are considered as a balance between Kerr focusing and defocusing by plasma generation in the nonlinear focus. However, the above explanation of filamentation in far-away zone finds considerable difficulties. There are basically two main characteristic which remain the same at these distances - the super broad spectrum and the width of the core, while the power in a stable filament drops to the critical value for self-focusing. With such power the plasma and higher-order Kerr terms are too small to prevent selffocusing. We suggest here a new mechanism for stable soliton pulse propagation in far-away zone, where the power of the laser pulse is a little above critical, and the pulse admits super-broad spectra.
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Self-phase modulation
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A phase-locked laser loop has been used as an amplitude and phase measuring device for coherent optical wavefronts. A relative phase resolution on the order of one degree and an amplitude resolution accurate to one dB or better were obtained. The system and measuring techniques used are described, and the results obtained are illustrated by several examples.
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Degree (music)
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We numerically demonstrated the filamentation dynamics of a 6 fs, 800 nm pulse focused in argon at atmospheric pressure by a zone plate and a concave mirror. In comparison with a concave mirror, the zone plate has a frequency-dependent focal length and can be used to control the wavefront of the laser beam in the frequency domain. A separate supercontinuum in the ultraviolet region extending from 250 to 300 nm and peaked at ~280 nm can be generated by using a proper zone plate.
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Abstract Spatially resolved wavefront measurements are presented during nonlinear self-collapse and provide the first detailed characterization of wavefront evolution during filament formation. The wavefront dynamics of key nonlinear processes including Kerr self-focusing, ionization and plasma defocusing, and dynamic spatial replenishment are identified and resolved in both the filament core and reservoir regions. These results are analyzed and interpreted with respect to numerical simulations and provide insight into fundamental aspects of filamentation. They also inform applications based on phase manipulation, such as external beam guiding, and present a new method for measuring the nonlinear index of refraction, n 2 .
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We have studied the large-scale beam breakup and filamentation of femtosecond pulses in a Kerr medium. We have experimentally monitored the formation of stable light filaments, conical emission, and interactions between filaments. Three major stages lead to the formation of stable light filaments: First the beam breaks up into a pattern of connected lines (constellation), then filaments form on the constellations, and finally the filaments release a fraction of their energy through conical emission. We observed a phase transition to a faster filamentation rate at the onset of conical emission. We attribute this to the interaction of conical emissions with the constellation which creates additional filaments. Numerical simulations show good agreement with the experimental results.
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