Effect of Chirp and External Focusing on the Spectrally Isolated Raman Component During Filamentation in Air

Spectrally isolated Raman component produced in filamentation of a near-infrared femtosecond laser pulse in air has been studied experimentally. The effect of laser pulse chirp and geometrical focusing on the Raman component formation are detailed in our study. Qualitative transformation of the frequency-angular spectra with the chirped laser pulse duration and the chirp sign has been found. The optimum conversion of the laser pulse into the Raman component reaches 25% for the loosely focused (numerical aperture NA = 0.002) and slightly negatively chirped laser pulse.