An unpolarized light beam phase modulator emitting in a given wavelength range comprises at least one cell containing a liquid crystal having a helical structure and means for applying a voltage to said cell, said liquid crystal having a torsion elastic constant greater than its twisting elastic constant so as to ensure continuous stable behavior voltage-wise, said liquid crystal having a sufficient number of turns and in which the axis of the turns is in the light-propagation direction, making it possible to obtain an optical effect generated by the liquid crystal on the electrical field that is identical regardless of the direction of this electrical field, said cell being transparent in said wavelength range.