Development of fiber-coupled compact terahertz time-domain spectroscopy imaging head

The development of a fiber-coupled compact terahertz time-domain spectroscopy (THz-TDS) imaging system is reported. We focus the THz waves by means of two silicon hyper-hemispherical lenses with no parabolic reflectors. The THz transmission image of an IC card clearly shows that the spatial resolution of the system is less than 0.75 mm, which is quantitatively consistent with the THz beam diameter at the focal point estimated by knife-edge measurement. The discrepancy between the effective frequency of THz broadband pulses and the frequency which gives the maximum dynamic range in the THz spectrum is considered to be due to the coherent high-frequency components of THz pulses.