Terahertz Nondestructive Testing Imaging Technology Based on Linear Frequency Modulation Mechanism

A terahertz nondestructive testing imaging system with frequency modulated continuous wave is developed based on all-solid-state electronic devices and the zero intermediate frequency mechanism. The effective frequency range of the proposed system is 9.375-13.75 GHz, and the frequency ranges from 0.225 to 0.330 THz after frequency multiplication by a factor of 21 in the free space. A single horn antenna is combined with a directional coupler to achieve transceiver integration, and the terahertz beam is collimated and focused using a pair of lenses. The beam quality of the quasi-optical system is optimized and the signal-to-noise ratio of the system is improved, by which the best spatial resolution is ensured under current parameters. At the same time, nonlinear correction of the system is carried out by the phase focusing method so that the depth resolution is close to the theoretical resolution. Combining rectilinear scanning and rotating scanning, the system is capable of acquiring three-dimensional data in the cylindrical coordinate system. The high voltage insulation terminal is tested and evaluated by the proposed system, and the full-view perspective image is reconstructed, which realizes the digital twin of the three-dimensional structure of the detected target in real space and image space. In addition, the proposed system can accurately display abnormal areas inside the insulation terminal, which is conducive to the intuitive evaluation of the internal state of a detected target, and thus can further meet the requirements of nondestructive testing in the industrial field.