We study theoretically a new possibility of unipolar pulse generation in a 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 a circle or helix, and demonstrate the possibility of obtaining rectangular unipolar pulses with an arbitrarily long duration above the minimum value equal to the natural period of the low-frequency vibrations in the Raman-active medium.