Spectral broadening and conical emission of near-infrared femtosecond laser pulses in air

Spectral and spatial properties of infrared femtosecond laser pulses with a wavelength between 1.2 and 1.6 m propagating in air are investigated both, experimentally and theoretically. It is shown that under tight focusing conditions laser pulses experience asymmetric spectral broadening (SB) and a strong blue-shift with respect to the incident laser wavelength. The observed blue-shifted radiation is emitted conically, in contrast to the weaker red-shifted on-axis emission. The SB and frequency shifts were interpreted in terms of ionization-induced effects.