Formation of an O-shaped structure in the red wing of the frequency–angular spectrum during filamentation on a long atmospheric path

An O-shaped structure at wavelengths of 930–960 nm in the frequency–angular spectrum of the supercontinuum generated during the filamentation of a femtosecond laser pulse with a central wavelength of 740 nm on a 75-m path in air has been observed experimentally. This feature of the frequency–angular spectrum is due to the presence of the absorption band of water vapor in the range of 930–960 nm and the anomalous dispersion region associated with this absorption. This result opens prospects for the remote single-pulse detection of impurities in air.