Difference frequency generation (DFG) is one of the most important methods for obtaining monochromatic THz-wave radiation, with the advantages of simplicity, lack of a threshold, room-temperature operation, and wide-range tunability. We previously demonstrated a milliwatt single-longitudinal-mode and tunable THz-wave source based on DFG in a MgO:LiNbO3 (MgO:LN) crystal using a pair of Yb-doped, pulsed fiber lasers [Y. Wada et al., Proc. SPIE 10531, 1053107- 8 (2018)]. In this study, we report the improvement of the THz-wave source by optimizing the collimation optics for THz output and enhancing the pumping fiber laser sources. The modified source produces an average power of 3.6 mW and a peak power greater than 7 W with nanosecond pulses at a pulse repetition frequency of 500 kHz and a tunability range from 0.34 to 1.25 THz. This improved source enables nondestructive 2-D transmission imaging of objects behind materials as thick as 5 mm using a pyroelectric detector operated at room temperature. As a demonstration of our powerful THz source, we present some results of transmission imaging of a train pass and thin objects such as an optical fiber and a human hair. We also demonstrate the direct spectroscopic imaging of medicine tablets.
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