Fig. 1. Sparse representation method based on LASSO

Fig. 2. Schematic of some atoms in the dictionary. (a) Some atoms in non-dispersive dictionary A; (b) some atoms in dispersion dictionary D

Fig. 3. Terahertz wave reflection model

Fig. 4. THz reference signal generated by DGMM for 3000 data points with a sampling period of 0.0333 ps

Fig. 5. L-curve diagrams and their values for regularization problems under different noise levels. (a) LASSO; (b) BPDN

Fig. 6. Reconstruction results of pulse response function for non overlapping echo signals using three methods. (a) Absence of noise; (b) R_SN=40 dB; (c) R_SN=20 dB; (d) R_SN=10 dB

Fig. 7. Reconstruction results of pulse response function for overlapping echo signals using three methods. (a) Absence of noise; (b) R_SN=40 dB; (c) R_SN=20 dB; (d) R_SN=10 dB

Fig. 8. Variation of the minimum TOF with noise level distinguishable by different algorithms

Fig. 9. Effect of dispersion on the reflected THz echo signal

Fig. 10. Comparison between the reconstruction results of three methods and the preset value. (a) Absence of noise; (b) R_SN=40 dB; (c) R_SN=20 dB; (d) R_SN=10 dB

Fig. 11. THz-TDS system. (a) Physical image of the THz system and sample placement window; (b) switch for displacement table; (c) display interface of the software program for controlling data collection, display, and storage written through LabVIEW, the first part includes control buttons related to system initialization, start/stop operation, and data storage, the second part is about setting and displaying the spectral range, the third part is to control the displacement table for displacement or imaging scanning, the fourth part is the display of scanning results

Fig. 12. Experimental spectrograms and pulse response functions reconstructed using three algorithms. (a) THz time-domain spectra of PTFE films with different thicknesses; (b) PTFE200; (c) PTFE100; (d) PTFE50

Fig. 13. Physical diagram of GFRP model and schematic of THz detection principle. (a) Physical image; (b) schematic of detection principle

Fig. 14. Experimental spectrograms and pulse response functions reconstructed using three algorithms. (a) THz time-domain spectrograms of three sets of models; (b) GFRP1; (c) GFRP2; (d) GFRP3

Fig. 15. Physical image of composite material model, schematic of imaging area, and diagram of terahertz detection principle. (a) Physical image; (b) schematic of the imaging area, triangulation dots represent random sampling points in defect‑free areas, round dots represent random sampling points in the defect area; (c) schematic of terahertz detection

Fig. 16. Original data graph and B-scan imaging graph of the model before and after processing for two groups of sampling points. (a) Terahertz time-domain spectrogram before processing; (b) terahertz time-domain spectrogram after processing; (c) B-scan imaging image before processing; (d) B-scan imaging image after processing

Fig. 17. Imaging pseudo color images of GFRP samples with mixed defects before and after processing. (a) Original imaging result; (b) imaging result without amplitude correction; (c) terahertz time-domain spectrogram before and after correcting the amplitude of sampling points in the defect area; (d) imaging result after amplitude correction

Table 1. DGMM preset values for different echoes in THz signals

Table 2. Localization of TOF of PTFE films with different thicknesses by three algorithms

Table 3. Actual thickness of three sets of PTFE films and detected thicknesses obtained by different algorithms respectively

Table 4. Localization of TOF of PTFE thin film in detection layer by three algorithms

Table 5. Actual thickness of three sets of GFRP models and detected thicknesses obtained by different algorithms respectively

Table 6. Weber contrast of images