KEYWORDS: Terahertz spectroscopy, Interference (communication), Denoising, Tunable filters, Quartz, Signal processing, Signal detection, Smoothing, Reflectance spectroscopy, Signal to noise ratio
When using the terahertz pulse signal for thickness detection, the system echo response directly affects the extraction of effective information in the signal. The double Gaussian inverse filtering (DGIF) method can effectively separate the sample reflection signal and the system echos. However, DGIF method will introduce high-frequency noise, seriously degrade the signal quality, and affect the detection result. In this paper, variational mode decomposition (VMD) combining DGIF algorithm and sparrow search algorithm (SSA) is used to denoise the signals of terahertz time-domain spectroscopy system. Firstly, the DGIF method is used to reconstruct the sample signal. Secondly, using the minimum envelope entropy as fitness value, the SSA was used to optimize the key parameters of VMD. Thirdly, the reconstructed terahertz time-domain spectral signal is decomposed by the optimization parameters to obtain intrinsic mode function. The kurtosis value and energy entropy of intrinsic mode function (IMF) is further extracted as the comprehensive evaluation index. The signal synthesized by the main mode functions is the reconstructed time domain signal after noise reduction. A quartz standard specimen was measured and compared with the original signal, DGIF signal, VMD signal and DG-SSA-VMD signal. Combined with the SNR and RMSE evaluation indexes, it is verified that the DG-SSA-VMD method can avoid the inconvenience of manually selecting key parameters, reduce the thickness extraction error, and has the best comprehensive denoising effect, which is conducive to further detection and imaging applications.
In high-voltage substations, flashover will occur along the surface of wet insulators under bad weather conditions, resulting in pollution flashover accidents of power system, which will seriously threaten the safe and stable operation of power system. Room Temperature Silicone Rubber (RTV), as a new technology and material, has been rapidly developed and widely applied due to its outstanding characteristics such as long lasting and maintenance-free. However, its surface uniformity and defects in the curing process will affect the service life of the coating. In this paper, a terahertz time-domain spectroscopy system was used to analyze the defects, material parameters, thickness, and curing process of RTV coating. Firstly, RTV coating samples with different absences and different thicknesses were produced, and the imaging results of RTV coating prefabrication defects, the optical parameters of cured RTV in the terahertz band and the thickness of the samples were obtained by the terahertz spectroscopy system. In addition, the results of the optical parameters of the RTV material over time (experimental conditions: 20°C, 15%RH) were verified by the experiment over a 3 h period. Different methods were used to extract the optical parameters of the materials for comparison. The results obtained in this paper provide a theoretical model for the application of terahertz time-domain spectroscopy in the nondestructive testing of RTV coating in power systems, and can realize the lifetime analysis of RTV coating.
Accurate measurement of the permittivity of materials is important in the imaging, communication, radar, material analysis and other fields. The method of extracting material parameters under different model conditions of frequency domain continuous wave signal based on quasi-optics vector network analyzer system is realized. It provides reference conditions and practical basis for evaluating the reliability of dielectric parameter measurement and defect detection of terahertz band materials. The process of terahertz wave interaction with the tested sample produces intractable spike noise due to interfacial echo resonance. In the signal transmission, the singularity is due to the wavelength and the sample optical path parameters forming the resonant mode, which is different from other noise signals.We combine wavelet denoising and variational mode decomposition, and introduce frequency expansion, which not only removes frequency-dependent noise, but also avoids frequency curling, which improve the accuracy of the measurement results.
In this paper, the S-parameters of 2mm silicon wafer, 14.345mm wood I, 14.215mm wood II and 13.80mm wood III were measured based on 220-325 GHz quasi optical Vector Network Analyzer measurement system. Basic theory based on free space electromagnetic component equations, boundary conditions and S parameters. The electromagnetic field of "air-MUT-air" three-layer structure between calibrated reference planes is analyzed in detail, and the formula of complex permittivity is derived. The results of complex permittivity of silicon wafer extracted by this method are basically consistent with those in the literature. The measurement of woods can accurately distinguish heartwood from sapwood. This study is helpful for us to better understand the interaction mechanism between terahertz waves and wood media, and can lay a foundation for the identification of woods.
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