Research Of Terahertz Imaing Radar And Application In Coal Mine Safety

Terahertz(THz) wave is one of the most subjects in current optics and electronics research, which occupies a middle spectral region between microwaves and infrared light waves(wavelength ranges from 0.03 mm to 3mm). Compared with microwave and millimeter wave, THz has characteristics of short wavelength, non-ionizing, high penetration capability. Ultra wideband and high resolution imaging radar can be achieved by using the short wavelength and high penetration, which makes it possible to detect small objects. In recent years, terahertz has been widely used in the field of safety detection, non-destructive detection and biological medicine. As one of applications in the field of safety detection, THz wave has never been studied in the coal mine. With the increasing intensity of coal mining, fracture water caused by the frequent occurrence of coal mine drilling is direct threat to the safety of people's lives and property. Therefore, it is of great importance to find a reasonable method to detect coal fracture. Based on this viewpoint, THz imaging radar has been studied in this paper, the main contents and conclusions are as follows:1.In view of the low emission power of terahertz radar at home and abroad, a continuous wave imaging radar with working frequency of 0.11 THz is developed. The operating frequency of the system is selected in conjuction of the attenuation of terahertz wave in the atmospheric window, and the working mechanism of the system is compared. The noise factor and the receiver sensitivity of the continuous wave imaging radar system are studied by using the network of signal noise, and the evaluation method of the system stability is introduced. The transmitter and receiver structure of the system are studied. Moreover, the signal flow of the system is analyzed, and the whole structure of the radar system is established. The key parameters of the system are analyzed and tested. The results show that the radar can realize the continuous output of frequency 0.11 THz and power 200 m W.2. Based on the fundamental principle of complex permittivity measurement by using free space method, dielectric properties of Shanxi anthracite and Shandong bituminous coals are researched in the frequency region 75-500 GHz. The free space test system is built by using the vector network analyzer combined with the millimeter wave frequency multiplication module. The test results show that the dielectric constant of coal decreases with the increase of frequency in 75-110 GHz. The dependence of coal on THz frequency in 75-110 GHz is explained by the theory of dielectric polarization. During the process of measurement in frequency region 140-500 GHz, A comparative test is carried out by using terahertz time domain spectroscopy system and free space system. The results show that in the 140-500 GHz, the variation of anthracite and bituminous coals exhibits obvious difference with increasement of frequency, and the characteristics of different frequency bands are not different as well.3. By using the dielectric property measurement system, the influence of water content below 10% on the dielectric property of coals is studied. It is found that the dielectric constant of coals increases with the increasing of moisture content. The transmission coefficient presents exponential decrease with the increase of the coal moisture content, and the reflection coefficient presents the Gauss function distribution.4. The three kinds of common coal cracks(deep fracture, plane fracture and hidden fracture) were studied. The continue wave imaging method is researched and the imaging system is established. Coal sample with fractures is made in thelaboratory environment, and then the imaging is simulated by using electromagnetic wave simulation software FEKO. The simulation results show that the reflection method is effective in detecting the depth of the crack, and the three kinds of crack can all be imaged under the transmission mode. The experimental results show that the resolution of 75-110 GHz can achieve the imaging effect of 3mm coal fracture.5. Based on the continuous wave imaging radar system,waveguide probe antenna in the frequency region 75-110 GHz is designed for neare-field imaging in order to obtain the super imaging resolution. The effect of antenna material, electromagnetic wave propagation mode and antenna size on the THz wave enhancement is simulated in detail. According to the simulation results, the antenna with different sizes is produced. The experimental results show that the probe can achieve the imaging effect of 0.3mm in the working wavelength of 3mm.Based on the study of the above contents, the main innovation of this paper lies as follow:For the first time we put forward the method of coal fracture detection by using terahertz technology, and built a terahertz continuous wave radar system. Parameters of the system and function modules are discussed. The continuous wave radar can realize 200 m W power output at 0.11 THz frequency point.The dielectric properties of coal in terahertz wave band are studied. The test system is built by combining the vector network analyzer and the millimeter wave frequency extension module. The relationship between anthracite and bituminous coal with frequency variation is obtained. The results are explained by using the principle of media polarization. Moreover, the transmission coefficient and reflection coefficient of coal sample with variation of frequency were obtained. Fill the gaps in the measurement of the dielectric constant of coal in the terahertz band at home and abroad.The effect of moisture content in the coal on the dielectric constant is studied at 0.11 THz frequency point. The law of the variation of dielectric constant with water content is revealed with the explanation of propagation of terahertz wave and the state of water in coal.The imaging resolution of the three common cracks is measured and analyzed by researching the continuous wave imaging method and establishing the measurement system.The THz waveguide probe antenna is developed. The antenna is tested based on the continuous wave imaging system. The experiment results show that the imaging resolution can achieve 1/10 wavelength in the THz band, which provides the reference for the THz non optical near-field imaging.