Power spectrum and coherence length measurements of a compact terahertz free-electron laser

We investigated the possibility of developing advanced imaging technology, such as 3-D holographic THz (terahertz) tomography, by using a compact high-power FEL (free-electron laser) operating as user facility at the Korea Atomic Energy Research Institute. The wavelength range of the FEL output pulses is 100 ∼ 1200 μm, which corresponds to 0.3 ∼ 3 THz in frequency. In particular, we measured the dependence of the power spectrum of the compact FEL beam on the length of the FEL resonator. The coherence lengths of the FEL pulses were measured by using a Michelson interferometer, and the interference intensity was recorded as a function of the optical path difference. We also confirmed that the spectral width of the FEL had an influence on its coherence length by changing the detuning length of the resonator. The measured coherence lengths of the THz FEL are around 1 cm. We anticipate that our measured results will be useful for coherent imaging of biomedical specimens.