We propose a novel scheme to generate ultra-wideband (UWB) pulse by employing a Sagnac interferometer comprising a phase modulator. This structure performs a dual-input and dual-output intensity modulator (IM), ultimately resulting in the flexibility to select the shape and the polarity of the generated UWB pulse. The experiment results show a good agreement with the theoretical investigation in terms of both pulse profile and spectrum, which conforms to the definition of UWB signals by the U.S. Federal Communications Commission. Furthermore, the proposed scheme is independent of the voltage bias point.