Study On Optical System Of HUST-FEL THz Source

This dissertation focuses on the studies of optical system for the FEL type Terahertz source constructed at Huazhong University of Science and Technology(HUST),including the design and construction of the optical cavity,the installation and alignment of free electron oscillator,the THz transmission from the downstream mirror output to THz measurement system,and the power and spectrum measurement of THz radiation.Theoretical analysis and experimental studies on the entire optical system of the FEL type THz source were carried out.Firstly,a compact low-loss partial-waveguided optical cavity structure was proposed and realized.According to optical diffraction theory and waveguide mode analysis,an accurate method for the calculation of the cross-section distribution of electromagnetic(EM)field modes in partial-waveguided optical resonator was proposed.The properties of freespace wave propagation in the optical cavity were analyzed and calculated through the scalar diffraction method.For the waveguide part in the optical cavity,the mode evolution process was analyzed with the combination of Fresnel diffraction equations and waveguide mode transmission theroy.The multiple round trip of the cavity field mode in the HUST-FEL optical resonator was simulated,and the structural parameters of partial waveguide optical cavity were optimized to reduce the cavity loss.Secondly,the alignment of the optical cavity and electron beam orbit was studied.Mechanical errors in the optical cavity installation process will increase the loss in the cavity,and the deviation of electron beam orbit will reduce the optical gain.These two factors affect the power accumulation process in the FEL and limit the output radiation power.To achieve the high-power radiation output in the FEL type THz source,the problem of three-axis collinearity(the optical cavity,the electron beam path and the magnetic field in the undulator center)was solved successfully.Theoretical analysis and three-dimensional(3D)numerical simulation were conducted to study the alignment tolerance of the three axes,and the tolerance requirements for optical alignment system were determined quantitatively.A precise three-axis alignment calibration system based on the He-Ne laser was designed and implemented.The transverse position error of the cavity mirror was controlled under 100 ?m,the central axis angle error of the cavity mirror was limited below 100 ?rad,the transverse position error of the beam orbit was realized within 120 ?m,and the angular error of the beam was restricted within 200 ?rad.The calibration system provided optical path reference in the THz wave transport line as well as the in the THz radiation measurement platform for the FEL type THz source.Finally,the studies of the low-loss THz transport in the vacuum tube and THz power and spectrum measurement method were presented.A multi-port,low diffraction loss transport line and a power spectrum measurement system with wide spectrum and large dynamic range have been designed and developed.The measurement system comprises THz spectrum analysis based on the standard Fourier transform infrared spectrometer(FTIR)with various detectors(Golay cell,pyroelectric detectors,and liquid helium bolometer).A calibrated blackbody source was utilized to achieve absolute power spectrum measurement over a wide frequency range with high signal-to-noise ratio.The platform design and construction,the optical path alignment technique and the experimental study of the THz radiation were described in detail.The system capabilities for spectrum measurement and absolute power calibration were verified by experimental results.Preliminary THz spectrum measurements were conducted in the commissioning process of the HUST-FEL THz source.