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

Abstract

Spectral and spatial properties of infrared femtosecond laser pulses with a wavelength between 1.2 and 1.6 μm 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.