A terahertz (THz) radiation source plays an important role in many applications, such as in biophysics, detecting and imaging. However, an effective method to generate coherent THz radiation is still under development. Based on nonlinear optical techniques, a beat-wave laser with THz spectral modulation can be a useful tool to generate a coherent THz wave. We have previously generated a sub-ns coherent THz wave from a difference frequency generator pumped by a THz beat-wave laser near 1.55-