ИСТИНА

Our work discusses spatial interaction effects between several filaments in stochastic and regularized multifilamentation regimes in air (so called superfilamentation) as well as a single filament spectral fea- tures. The analysis is based on the measurements of radiation frequency-angular spectrum (FAS) and spatial mode of a filament. A filament was formed by radiation of a Ti:Sa laser system delivering pulses with 55 fs duration and 2–10 mJ energy at 10 Hz repetition rate with 7 mm FWHM beam diameter. The filament was generated in air under additional focusing by 3 m lens. Frequency-angular spectra were measured using an imaging spectrometer accepted radiation at the end of a filament path reflected from silica wedge. We used a home-made imaging spectrometer based on Czerny–Turner scheme and an ACTON SP-500i imaging spectrometer. Regular filament structure was produced by amplitude and phase masks [1] placed into the beam before the lens. We have studied angular structure of a radiation spectrum in a single filament. In particular, we discuss features of conical emission, separated IR Stokes spectral component, four-wave mixing, etc. Based on anti-Stokes component angle the electron density of plasma is estimated. The result was 3.2 · 10 17 cm −3 . For both stochastic and regularized multiple filaments the existence of single on-axis component was firstly revealed. Its central wavelength varies from 840 to 900 nm. In the case of filament with regular structure on-axis radiation is observed in whole spectral range. All these components could be interpreted in terms of fusion of several filaments and superfilament formation. This work is supported by Russian Fund for Basic Research (grants ## 15-32-20966 and 15-02-99630). References: [1] Generation of hermite–gaussian modes of high-power femtosecond laser radiation using binary-phase diffractive optical elements / A. S. Larkin, D. V. Pushkarev, S. A. Degtyarev et al. // Quantum Electronics. — 2016. — Vol. 46, no. 8. — P. 733–737.