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.
We demonstrate a high-average-power, single-longitudinal-mode, and tunable terahertz-wave (THz-wave) source based on difference frequency generation (DFG) in a MgO:LiNbO<sub>3</sub> (MgO:LN) crystal. The DFG waves are generated using a pair of Yb-doped, pulsed fiber lasers with a master oscillator power fiber amplifier configuration. The average power of the THz-wave output reaches 1.35 mW at 1.0 THz (300 μm) at a linewidth of 7.2 GHz, and the tunability ranges from 0.34 to 1.25 THz under a pulse repetition frequency of 500 kHz. With this scheme, we constructed a compact THz-wave generation head for imaging applications. The combination of MgO:LN-DFG and the stable and robust fiber laser sources is highly promising for developing high-average-power THz-wave sources, particularly in the high-transmission sub-THz region. This approach may enable new applications of THz-wave spectroscopy in imaging and remote sensing.