| Authors: |
Elena Sunchugasheva, Daniil Shipilo, Andrey Ionin, Alexey Shutov, Nikolay Panov, Leonid Seleznev, Vera Andreeva, Sergey Ryabchuk, Nikolay Ustinovskii, Vladimir Zvorykin, Daria Mokrousova, Semen Goncharov and Olga Kosareva |
| Abstract: |
A femtosecond laser pulse with peak power higher than the critical power for self-focusing propagates in air in nonlinear regime and undergoes spatio-temporal localization, known as filamentation. Laser pulse filamentation is accompanied by a plasma channel formation. Pulses with power higher than 10 times higher than the critical one propagate in a multiple filamentation regime, which is highly stochastic process depending on media turbulence and pulse intensity variations. In the recent article [Shipilo, D.E., et al, Laser Phys. Lett. 13, 116005 (2016)], we studied the fusion of several regularized coherent femtosecond filaments experimentally and numerically. The fusion of filaments was obtained by transmitting a laser beam through a mask with four circle holes in vertexes of the square followed by the focusing lens. We used IR emission of Ti:Sa laser system with central wavelength of 744 nm, pulse duration of 100 fs and pulse energy up to 3 mJ. The measured far-field image of the four-filament fusion region was dramatically different from the diffraction pattern during a linear regime of laser pulse propagation. Nonlinear case lead to a bright on-axis maximum formation. Plasma density, as well as peak intensity, in this axial filament exceeded corresponding parameters for one filament. Numerical simulation showed a good qualitative correlation with experimental results. We made the next step towards to a fusion of regularized filaments in a larger scale: we conducted experiments devoted to regularization of the filaments by similar technique – using amplitude mask of 2D array of holes. In the experiment laser pulse with central wavelength of 248 nm (third harmonic of Ti:Sa laser system emission) was amplified in wide-aperture excimer KrF amplifier Garpun-MTW up to energy of 200 mJ. Laser beam passed through one of the amplitude masks (2D array of square or circle holes) and propagated over a long distance. Power of the sub picosecond laser pulse after the mask was thousand times higher than the critical one, therefore multifilamentation occurred. We observed the formation of regularized structure of a few hundred filaments, one filament per hole for all taken masks. This array of filaments sustained over 20 m of propagation in turbulent atmosphere.
So, we studied the fusion of filaments regularized in a small scale and obtained increase of the intensity and plasma density in the fusion area. Also, we implemented an approach of multiple filamentation regularization with help of different amplitude masks to reduce media fluctuations influence. In further experiments, we are going to study the fusion of these regularized filaments.
This work is supported by RFBR grants (15-02-99630, 15-32-20966, 14-22-02021, 15-02-09410), RF President Grant NSh-9695.2016.2, Dynasty Foundation. |