Research On Optimization And Application Of THz-TDS System Based On Electro-optical Sampling

The free electron laser terahertz source(HUST FEL-THz source)of Huazhong University of Science and Technology has the advantages of high power,tunable wavelength,high resolution and high sensitivity,and is suitable for application research of terahertz time domain spectroscopy(THz-TDS).The traditional terahertz time-domain spectroscopy system is limited by the environmental noise,the phase mismatch between the femtosecond laser and the THz pulse during the electro-optical sampling process,and the crystal dispersion effect,which will cause the distortion of the output THz time domain waveform and the reduction amplitude of the electro-optical signal and other issues.In view of the above problems,this thesis optimizes the existing terahertz time-domain spectroscopy system in the laboratory from two aspects:control system and experimental system.In terms of control system,the use of LabVIEW to write wavelet filter and SQL-Server database sub VI realizes the functions of data preprocessing and system management.In terms of the experimental system,based on the electro-optical sampling theoretical model,the detection results of the electro-optical sampling are optimized by using a broad-spectrum laser and pre-chirp dispersion compensation.Finally,the optimized THz-TDS system was used to study the THz spectrum of irradiated silicone rubber materials.The main research contents are as follows:The Pockels effect and the frequency domain electro-optic response process in the crystal are elaborated.By analyzing the dispersion effect of the femtosecond laser propagating in the electro-optic crystal,the calculation model of the laser time-domain pulse width is derived,and the pulse width broadening effect of the laser after passing through the crystal is found.Combined with the existing problems and shortcomings of the current THz-TDS detected system,a wavelet filtering and database system module is designed for optimal management.Using LabVIEW to write wavelet filter sub VI for data preprocessing,which effectively improves the signal-to-noise ratio of the system output signal.At the same time,a SQL-Server database management system was established based on LabVIEW,which completed data writing,data deletion,and data query operations.According to the definition of coherence length,suitable femtosecond lasers and electro-optic crystals are selected for electro-optic detection in various frequency bands.By simulating the electro-optical response process,the optimization effect of broadband sampling laser on electro-optic response functions of Zn Te,Ga P and Ga As crystals is analyzed.In the experiment,10 fs pulse width femtosecond laser and 3 mm Zn Te crystal were used for electro-optic sampling,which effectively smoothed the amplitude and phase curve of the electro-optic response function of the crystal.By analyzing the dispersion propagation process of the femtosecond laser in the crystal,it is found that the use of prechirp to compensate the group delay dispersion can effectively eliminate the time-domain signal distortion caused by the crystal dispersion effect.In the experiment,the DCMP 175 chirped mirror was used to compensate the femtosecond laser with group delay dispersion.When the dispersion compensation amount is-2100 fs~2,the maximum amplitude electro-optical signal is obtained,which is 55.89%higher than before the dispersion compensation.The traditional method of extracting the optical parameters of the sample and the unequal optical path method are introduced,and the THz spectrum analysis of the irradiated silicone rubber is carried out using the optimized THz-TDS system.The experimental results show that the optimized THz-TDS system can distinguish the influence of the type of soaking agent and the radiation dose on the optical parameters of silicone rubber to a certain extent.