Study On Time Domain Diagnosis Technology Of FEL-THz Source Based On Electro-Optical Sampling

Due to the outstanding advantages such as high output average power,continuously adjustable wavelength,good beam quality and stable time-domain structure,the Free Electron Laser Terahertz Source(FEL-THz source)has achieved rapid development and widespread application in the field of terahertz science and technology.Accurate measurement of time-domain pulse waveforms of picosecond-order electron bunches and terahertz pulses in FEL-THz sources is one of the key technologies for achieving stable operation and saturated output.Moreover,it is necessary for the THz application experiments utilizing the FEL as a radiation source.This thesis conducts an in-depth study on the time-domain electro-optical diagnostic systems based on the free electron laser terahertz source facility built by Huazhong University of Science and Technology.The key issues involved in the development of the diagnostic system are: the complex electro-optical mixing process of sampling laser pulse and THz pulse in electro-optic crystal;the optimization design of the diagnostic system which needs to consider the special parameters of optical pulse and electron bunch and many other factors such as the configuration of optical components and long-wave diffraction effects in optical beamlines;modeling of complex system response functions and reconstruction of the distortion measurement signal by appropriate algorithm;fs-level synchronization between femtosecond lasers and the accelerator RF source;optical calibration of measurement system.This thesis has carried out relevant theoretical analysis,optimized design and experimental research on the above key issues.The main research contents including:(1)The theory of electro-optic process in THz electro-optical sampling and the propagation theory of femtosecond laser and THz pulse are introduced.This thesis gives a detailed theoretical derivation of the electro-optic mixing process in crystal.Also,the dispersion propagation of ultrashort femtosecond laser is described.Besides,both of the gaussian beam transformation method and numerical calculation method based on fast Fourier transform are presented to decrible the THz propagation.(2)In the theoretical analysis of the electro-optical process,the broadband spectrum effect of the sampling laser was found in electro-optical process.The broad spectrum of the sampling laser can effectively eliminate the distortion of the electro-optical signal due to dispersion propagation and phase mismatch in the electro-optic crystal.It is theoretically interpreted as the smooth effect of the broad spectrum of the sampling laser on the electro-optic response function of the crystal.A THz electro-optical sampling experiment using a 100 nm bandwidth laser in thick ZnTe crystals is performed to confirm the above broadband spectrum effect.(3)According to the requirements for time domain diagnostic of the compact FEL-THz sources,optimal design of the electro-optical diagnosis platform is performed.The key parameters such as the polarization direction of the sampled laser and the THz laser,the phase mismatch in the crystal,and the type and thickness of the crystal are optimized.Two beamline for FEL-THz radiation and transition radiation are designed separately.For the THz beamline,the simulation results using the Gaussian beam model show that the power transmission efficiency is over 72% in 1.5~6 THz range.For the Transition radiation beamline,the simulation results using the virtual photon diffraction model show that the power transmission is greater than 40% in 0.1~1 THz range.(4)A one-dimensional analysis model of THz electro-optic sampling detection was systematically established.Using this model,an efficient deconvolution algorithm is proposed.The algorithm can effectively reconstruct the THz electric field even under low signal-to-noise ratio(20 dB)and large system response function error(5%).Besides,the algorithm also successfully reconstruct the longitudinal distribution of non-Gaussian ultrashort femtosecond electron bunches(FWHM = 100 fs)in X-FELs.(5)In the development and testing of the electro-optical sampling platform,the high-precision synchronization between the sampling femtosecond laser and the accelerator signal source(2856 MHz)was achieved,and the jitter was tested to be less than 30 fs.Based on this electro-optical sampling platform,an electro-optical sampling experiment of the ps-level THz pulse generated by the photoconductive antenna was successfully performed,and the optical parameters of the common THz vacuum window material in the frequency range of 0.1 to 2.5 THz were measured.