  We study theoretically a new possibility of unipolar pulses generation in
Raman-active medium excited by a series of few-cycle optical pulses. We
consider the case when the Raman-active particles are uniformly distributed
along the circle, and demonstrate a possibility to obtain a unipolar
rectangular video pulses with an arbitrarily long duration, ranging from a
minimum value equal to the natural period of the low frequency vibrations in
the Raman-active medium.
