# Special Issue: Novel Approaches for Nondestructive Testing and Evaluation
## 3 Fast Terahertz Coded-Aperture Imaging Based on Convolutional Neural Network by [PERSON] et al. Terahertz coded-aperture imaging (TCAI) has many advantages such as forward-looking imaging, staring imaging, low cost, etc. However, it is difficult to resolve the target under a low signal-to-noise ratio (SNR), and the imaging process is time consuming. In this study [2], the authors provided an efficient solution to tackle this problem. A convolution neural network (CNN) was leveraged to develop an off-line, end-to-end imaging network whose structure is highly parallel and free of iterations. Additionally, it can simply have a general and powerful mapping function. Once the network is well trained and adopted for TCAI signal processing, the target of interest can be recovered immediately from the echo signal. Additionally, the method to generate training data was shown, and the authors found that the imaging network trained with simulation data was of good robustness against noise and model errors. The feasibility of the proposed approach was verified by simulation experiments, and the results show that it has a competitive performance with state-of-the-art algorithms. Indirect Method for Measuring Absolute Acoustic Nonlinearity Parameter Using Surface Acoustic Waves with a Fully Non-Contact Laser-Ultrasonic Technique by [PERSON] et al. This paper [3] proposed an indirect method to measure absolute acoustic nonlinearity parameters using surface acoustic waves by employing a fully non-contact laser-ultrasonic technique. For this purpose, the relationship between the ratio of relative acoustic nonlinearity parameters measured using the proposed method in two different materials (a test material and a reference material) and the ratio of absolute acoustic nonlinearity parameters in these two materials was theoretically derived. Using this relationship, when the absolute nonlinearity parameter of the reference material was known, the absolute nonlinearity parameter of the test material could be obtained using the ratio of the measured relative parameters of the two materials. For experimental verification, aluminum and copper specimens were used as reference and test materials, respectively. The relative acoustic nonlinearity parameters of the two materials were measured from surface waves generated and received using lasers. Additionally, the absolute parameters of aluminum and copper were measured using a conventional direct measurement method, with the former being used as a reference value and the latter being used for comparison with the estimation result. The absolute parameter of copper estimated by the proposed method showed good agreement with the directly measured result. Proposal of UWB-PPM with Additional Time Shift for Positioning Technique in Nondestructive Environments by [PERSON] et al. The ultra-wideband (UWB) technology has many advantages in positioning and measuring systems; however, the powers of UWB signals rapidly reduce while traveling in propagation environments; hence, detecting UWB signals are difficult. Various modulation techniques are applied for UWB signals to increase the ability for detecting the reflected signal from transmission mediums, such as pulse amplitude modulation (PAM), pulse position modulation (PPM), etc. In this paper [4], the authors proposed an ultra-wideband pulse position modulation technique with an optimized additional time shift (UWB-PPM-ATS), to enhance the accuracy in locating buried objects in nondestructive environments. Moreover, the Levenberg-Marquardt-Fletcher algorithm (LMFA) was applied to determine the medium parameters and buried object location simultaneously. The influences of the proposed modulation technique on determining the system's parameters, such as propagation time, distance, and properties of the medium were analyzed. Calculation results indicate that the proposed UWB-PPM-ATS provided higher accuracy than conventional methods such as UWB-OOK and UWB-PPM, in both homogeneous and heterogeneous environments.