Electro-optic sampling of optical pulses and electron bunches for a compact THz-FEL source

A systematic one dimensional analytical model describing the full electromagnetic propagation in electro-optic (EO) diagnostic is proposed and designed for a terahertz FEL source at HUST. Our model contains two main aspects: the propagation of the THz pulse (1.5-6 THz) along the transport line and the THz EO process in the detection crystal. The diffraction, Fabry-Perot and focusing effects are included in the THz propagation process. The phase mismatch, the frequency-dependent EO coupling coefficient and geometrical overlap between probe and THz pulses as well as the group velocity dispersion and finite duration of the probe pulse are considered in the THz EO process. The calculation shows that the diffraction in the TPX window and the Fabry-Perot effect in the GaP crystal are crucial for retrieving the details of the THz optical pulse. While for electron bunches, the detected Coulomb electric field will be significantly broadened due to the relatively low relativistic Lorenz factor (γ=10-20). Therefore, a convenient deconvolution algorithm is utilized to reconstruct THz electric field from the distorted EO signals, where the truncated singular value decomposition method is employed. Our algorithm is verified to work well even with a low signal-to-noise ratio (20 dB) in the future measurement.