  VFEL lasing in system with dynamical undulator is described. In this system
radiation of long wavelength creates the undulator for lasing on shorter
wavelength. Two diffraction gratings with different spatial periods form VFEL
resonator. The grating with longer period pumps the resonator with long
wavelength radiation to provide necessary amplitude of undulator field. The
grating with shorter period makes mode selection for short wavelength
radiation. Lasing of such a system in terahertz frequency range is discussed.
