Interference Effects in the Conical Emission of Femtosecond Filament in Fused Silica

It is shown both experimentally and theoretically that interference effects play the key role in the formation of frequency-angular spectrum of the filament conical emission. For the first time we investigated experimentally the transformation of the conical emission frequency-angular spectrum with increasing of the filament length inside fused silica. We discovered the appearance of fine structure of the conical emission rings produced by lengthy filament. It is shown that the conical emission frequency-angular spectrum is produced by interference of coherent radiation from one or several moving point sources in the filament. The shape of the conical emission spectrum depends on the medium material dispersion, the spectrum structure is determined by length and relative location of filament emitting regions.