Abstract

Using a lens of focal distance 10 cm, we sent plane-polarized optical pulses of wavelength 532 nm and duration 30 ps into a transparent cell of length 1 cm, filled with carbon disulfide at standard pressure and temperature. If a pulse generates at the focus of the lens an input intensity of at least I0=0.3 GW/cm2, then stimulated light scattering takes place, and we observe a strong backward-propagating signal. By monitoring its spectrum and transverse spatial profile as a function of input intensity, we found quantitative information on the optical Kerr effect. At input intensities of I0, 1.2I0, and 1.8I0, self-focusing leads to the formation of one, two, and four filaments, respectively. Each of these is subject to self-phase modulation and thus generates in the backward spectrum a frequency band of a granular structure. The latter can be perfectly reproduced by evaluating the Fourier transform of a phase-modulated electric field on the basis of the method of stationary phase. This allows us to calculate intensity and lifetime of a filament. If the input intensity exceeds the value of 1.8I0, fluctuations in refractive index destabilize the filamentation process. Backward spectra no longer consist of separate bands, and their shape varies at random during each series of laser shots. For input intensities higher than 3I0 the combined action of stimulated scattering and self-phase modulation causes the structure of spectra to become smooth. This explains why at an input intensity of 30I0 one observes for each laser shot a continuous backward spectrum, which possesses a large band that extends to relative wave numbers of approximately -200 cm-1.

© 1998 Optical Society of America

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