Expressions are given for the phase modulation by the optical Kerr effect and the resulting frequency broadening of an optical pulse with elliptical polarization. The compression method is discussed for a quantitative comparison with experimental investigations. Starting with single pulses of 20 ps duration and 0.4 cm -1 frequency width, generated by a mode-locked laser and an optical switching device, the phase modulation in an external cell containing CS2 produced a frequency broadening of 5 cm-1 typically. A compression factor of 5 is achieved in a system consisting of one optical grating and a reflection prism. Satisfactory agreement between theory and experiment is obtained for the frequency modulation Δω) and the compression length l. Our data suggest that the time dependence of our input pulses is symmetric with wings of higher power than those of a Gaussian pulse.