A novel photonic generation technique for frequency-tunable microwave and millimeter-wave sources is presented.Making use of the polarization hole burning effect and the contrary propagations of two light beams in the Er-doped fiber ring cavity,a dual-wavelength fiber ring laser with stable output and tunable wavelength spacing is obtained at room temperature,outputting a single longitudinal mode with a single frequency along each direction.A millimeter-wave signal ranging from 472.5 MHz to 26.5 GHz is gene...