| Terahertz electromagnetic waves,whose waveband ranges between microwave and infrared waves,are of high academic research value due to its various unique advantages and properties.Specifically,with their non-ionizing property and ability to penetrate dielectrics,terahertz waves are of significant advantages in imaging and can therefore be applied in nondestructive testing and biomedical imaging.The gyrotron based on the theory of electron cyclotron resonance maser is known to be the terahertz radiation source with the highest output power,which is capable of generating high-power continuouswaves and is applied in nuclear magnetic resonance,etc.In this thesis,gyrotron is used as the radiation source for terahertz imaging.The main contents of this thesis are as follows:1.A brief overview of the existing imaging systems and the terahertz sources/detectors applied in these systems is given as an introduction.The properties of the devices according to different mechanisms are also analyzed.On this basis,a terahertz imaging system with a cold cathode gyrotron as the radiation source is designed.In the system,the object to be scanned is placed on a motorized translation stage,and a terahertz waveguide detector and an oscilloscope are used to receive and monitor terahertz signals respectively.2.The design of the terahertz imaging system and the introduction of its components are given in Chapter 2 and Chapter 3.The imaging system is controlled by Lab View control programs,which are optimized repeatedly according to the actual experiments.The scanning mode of the imaging system is designed according to the 10-watt pulsed terahertz wave of 0.22THz generated by the gyrotron.The object to be measured is moved and temporarily halted at each scanning grid point sequentially under the control of the program,and terahertz pulses are emitted by the gyrotron while the object is stationary.The pulse is received by the geophone after penetrating the mentioned object,and the signal is further passed to the oscilloscope that is connected to the computer.This allows the geophone signals to be viewed in real time and enables the waveform to be read and saved by the computer throughout the duration of the scan.3.The analysis of the influence of scanning step size and diffraction on the imaging performance is given in Chapter 4.In general,reducing the step-width improves the resolution of the image,but the corresponding scanning time also increases significantly.Various objects that include both dielectric and non-dielectric objects were scanned in the experiments,in which suitable step lengths were programmed individually.The scanned data were further imported to MATLAB for post-processing.The interpolation algorithm is used to further improve the resolution,and the Sigmod function is applied to sharpen the image and eliminate the diffraction fringes. |
