###### Abstract  
Composite materials are increasingly being utilized in many products, such as aircrafts, wind blades, etc. Accordingly, the need for nondestructive inspection of composite materials is increasing and technologies that allow nondestructive inspection are being studied. Existing ultrasound methods are limited in their ability to detect defects due to high attenuation in composite materials, and radiographic examination methods could pose a danger to human health. Terahertz (THz) wave technology is an emerging approach that is useful for imaging of concealed objects or internal structures due to high transmittance in non-conductive materials, straightness, and safety to human health. Using high-speed THz tomography systems that we developed, we have obtained THz tomographic images of glass-fiber-reinforced polymer (GFRP) laminates with artificial internal defects such as delamination and inclusion. The defects have various thicknesses and sizes, and lie at different depths. We present THz tomographic images of GFRP samples to demonstrate the extent to which the defects can be detected with the THz tomography systems.  
terahertz tomography; glass-fiber-reinforced polymer; high-speed; nondestructive inspection Article  
High-Speed Time- and Frequency-Domain Terahertz Tomography of Glass-Fiber-Reinforced Polymer Laminates with Internal Defects. _Applid_  
3 May 2021  
26 May 2021  
Publisher's Note  
in an imaging target is obtained by detecting reflected THz waves from the interfaces and calculating the position of the interfaces from the round-trip times. By acquiring the depth-direction information at multiple points on the plane perpendicular to the depth direction, it is possible to obtain a 3D image of the imaging target [10; 11; 12].  
Reflection-type THz tomography can be divided into time- and frequency-domain THz tomography, depending on the A-scan method. Time-domain THz tomography uses pulsed THz waves to measure time-domain data, which is used as A-scan data [13]. Time-domain data are acquired through optical sampling with varied time delays between the pump and probe pulses. Here, the A-scan rate depends on the scan rate of the time delay. Continuous THz waves are employed in frequency-domain THz tomography, where frequency-domain data are measured as the frequency of the continuous THz wave is varied. A-scan data are obtained through fast Fourier transform (FFT) of frequency-domain data [14]. Here, the A-scan rate depends on the frequency scan rate of the continuous THz wave. In addition, C-scan methods include moving the imaging target using translation stages [15] and steering a beam without moving the imaging target [16].  
We previously developed a high-speed time-domain THz tomography system using electronically controlled optical sampling (ECOPS) combined with beam steering [16; 17]. ECOPS is used for fast A scan, and beam steering can speed up C scan more than moving an imaging target. A telecentric f-\(\theta\) lens and a galvanometer scanner are used for beam steering. In addition, we developed a high-speed frequency-domain THz tomography system that relies on fast frequency sweep of continuous-wave (CW) THz radiation [18; 19]. High-speed broadband THz frequency sweep is performed for fast A scan by using the THz photomixing technique with a wavelength-swept laser (WSL) and a distributed-feedback laser diode (DFB-LD) [19].  
In this paper, we test and compare the performance of the time- and frequency-domain THz tomography systems for nondestructive inspection of GFRP laminate samples with various artificial internal defects.