We developed a narrow line width terahertz and mid-infrared source based on difference frequency generation(DFG) in
GaSe crystal. The source could be easily continuous-tuned in a terahertz region of 68.1-1386 μm (0.22-4.4 THz) and a
mid-infrared region of 16.1-26.5 m without altering any component. The maximal peak power of terahertz waves is 9.1
W at 143 μm (2.1 THz), which corresponds to a power conversion efficiency of 2.8~10<sup>-5</sup>. Using a FTIR spectrometer, we
verified the wavelength and the line width of the source.