## 1 Introduction
In addition, THz waves are harmless to the human body below a certain power, unlike radiographic methods which can affect the human body despite providing high-resolution images of voids. There are two kinds of THz A-scan methods: time-domain and frequency-domain methods. In time-domain THz A-scan methods, THz pulses generated by a femtosecond laser are used to directly acquire time-domain A-scan data while scanning the time delay between the THz pulse and the femtosecond optical sampling pulse [12]. In addition to the conventional mechanical delay line combined with lock-in detection, fast delay scanning methods using signal averaging have been demonstrated, such as asynchronous optical sampling, electronically controlled optical sampling, optical sampling by laser cavity tuning, and specifically designed mechanical delay tools [13; 14; 15; 16; 17]. Fast delay scanning methods have been used for high-speed THz spectroscopy or tomography with applications in real-time detection of explosives, high-speed nondestructive testing (NDT) of composites, etc [17; 18]. The cost and size of a time-domain THz A-scanner can be reduced using a femtosecond fiber laser. The short duration or broad spectrum of THz pulses can provide high A-scan resolution. In general, there is a trade-off between A-scan rate and range, depending on time delay scanning methods. In frequency-domain THz A-scan methods, A-scan data are obtained from the fast Fourier transform (FFT) of frequency-domain THz data acquired while the frequency of continuous-wave (CW) THz radiation is scanned [19; 20; 21; 22; 23; 24]. The frequency-modulated CW (FMCW) THz method using electronic devices has the advantages of fast acquisition, small size, and long A-scan range, where the frequency modulation rate can be more than kHz and electronic devices are employed, including an oscillator with a narrow linewidth [21; 23; 24]. However, the FMCW THz method using electronic devices has the disadvantages of high cost and low A-scan resolution, due to the use of high-frequency electronic devices and a frequency modulation range of less than 100 GHz. The FMCW THz method using electronic devices was exploited for THz radar imaging applied to stand-off detection of concealed objects [23] and THz tomography applied to NDT of composites [24]. The advantages of the THz photomixing method are its low cost and compact size [25; 26]. Also, it can have high A-scan resolution and long A-scan range due to its wide frequency sweep range of more than 1 THz and a narrow linewidth. However, it usually requires a relatively long frequency sweep time due to the slow tuning of the laser frequency. The THz photomixing method was used in the coherent homodyne detection scheme for THz spectroscopy and thickness measurement [20, 22, 27, 28, 29, 30, 31]. We previously developed a method for high-speed broadband frequency sweep of CW THz radiation [32]. Particularly, the use of a wavelength-swept (WS) laser and a distributed feedback laser diode (DFB-LD) in THz photomixing enabled THz frequency sweep with a THz sweep range and a kHz sweep repetition rate. However, instead of the WS fiber laser, a WS semiconductor laser is used in this study. Compared to the WS fiber laser with a coherence time of several tens of picoseconds, the WS semiconductor laser has the advantages of a long coherence time of about 700 ps and high frequency-sweep linearity [32, 33]. In this study, we demonstrate that our mobile frequency-domain THz A-scanner, based on the high-speed broadband THz frequency sweep, is useful for nondestructive inspection of external tendons for voids below the duct.