Research Of Low-losses Optical Resonator And Transport System In FEL-THz Source

Free Electron Laser Terahertz(FEL-THz) source has many advantages, such as high output power, continuously tunable laser wavelength over a wide range, good beam quality, etc. It is one of the best ways to obtain tunable and high-power terahertz radiation. The optical resonator is an important component of the free electron laser device. In order to make the terahertz radiation amplified and saturated, the gain must be greater than the losses in optical resonator. So minimization of the optical resonator losses is essential to get high power terahertz radiation.In order to increase the gain and reduce the losses of optical resonator in terahertz band, waveguide structures are considered to be used in the traditional open optical resonator. This thesis focuses on the propagation modes in the waveguide optical resonator, the transmission losses of the waveguide and the coupling losses between waveguide and free space. The transmission losses of terahertz wave in the atmospheric environment are very large, so a low-losses terahertz beam transmission line systems are designed.First of all, the ohmic losses and reflection losses of the optical resonator with waveguide structure in terahertz band are analyzed and calculated respectively. Validated with the results of the HFSS software, it shows the transmission losses of waveguide are very low.Then the mode expansion method is used to analyze and calculate the coupling losses in the waveguide. The relationship between the minimum coupling losses and the most suitable radius of mirror curvature are studied. The mirror of the optical resonator is optimal to minimize coupling losses at the wavelength ranged from 50 ?m to 100 ?m. The results of analysis and calculation above show that the losses of the optical resonator meet the low-losses requirements and the FEL-THz source can work well.Finally,since the generated terahertz beam should be transmitted to the diagnosis or application devices with long-distance and low-loss beam line. The thesis designs a terahertz beam line transport system based on the properties of Gaussian beam. It can retain good beam quality when the terahertz beam goes through this system with a wavelength from 50 ?m to 100 ?m.