Study On Integrated Terahertz Spectrum Measurement System

Terahertz(THz)range refers to electromagnetic waves with frequencies between microwave and infrared.This range is the only spectrum in electromagnetic spectrum,which is not fully developed.The development of THz spectrum is one of the hot spot subject in current scientific and technological research.THz technology,known as one of the 10 emerging technologies that will change the world,which has found applications with very important academic value in many different fields,such as communication,radar,security monitoring,imaging,bio-medical,and spectral analysis.The traditional THz time-domain spectroscopy system(THz-TDS)uses a mechanical delay line to obtain a THz time domain waveform,and this process usually takes several seconds to several minutes.But some typical ultrafast events are irreversible,which is very difficult to be reproduced,such as material irreversible damage,protein denaturation and some life process.In order to overcome the defects of traditional THz time-domain spectroscopy technology,the single-shot measurement technology of THz pulse arises at the historic moment.In this thesis,we developed a new integrated THz spectrum measurement system.The main functions of the system include self check:THz specturm measurement module based on photoconductive antenna,THz specturm measurement module based on optical rectification,the optical-pump terahertz-probe module and THz transient measurement module.The functions of each module are complementary and practical.It can be outlined as followings:(1)Based on the photoconductive antenna as the source,the terahertz time-domain spectral system is built.Compiling a set of program system is applied to the THz-TDS,which complete system control and data acquisition of the entire platform.(2)High power broadband THz sources based on optical rectification.First of all,the principles and the phase matching conditions of broadband THz wave generation via optical rectification have been studied.Since the collinear phase matching condition in LiNbO3 during optical rectification process is not fulfilled,a tilted optical pulse front technique is used to achieve noncollinear phase matching.The broadband THz wave generation setup based on continuous tilted pulse front technique via grating is built.With 2 mJ pump pulse,the generated THz pulse energy is as high as 0.5 μJ and the energy conversion efficiency is up to 2.5×10-4,the focal spot diameter is 2.5 mm at the focus point of off-axis parabolic mirror pair.We analyzed the lens parameters,the incident light spot,the grating’ incidence angle and the chrip pulse exerts import effect on the THz conversion efficiency.In this a LiNbO3 is selected as an optical rectification crystal,a high signal-to-noise ratio(SNR)THz time-domain spectroscopy system is established based on electro-optic sampling.We analyzed the electro-optic crystal thickness,the time constant and the sampling rate exerts import effect on the THz time domain waveform.On that basis,the optical-pump terahertz-probe module has been added and verified.(3)We developed the single-shot measurement system of THz pulses based on the method of titlted pulse front.Compiling a set of program system is applied to the system,which complete system control and data acquisition of the entire platform.We theoretically analyzed the working principle of the system,the acquisition method of signal and completed the calibration of time.Analytical model for the single shot measurement of THz pulse using tilted pulse front technique is established,and some conclusions concerning the parameters preference of grating and optical system components,as well as optical design,are obtained.(4)By the test,the signal to noise ratio of the THz time domain spectrum system of photoconductive antenna is about 1500:1,and the spectrum range is 0.1-3 THz.In the lithium niobate strong source module,the highest SNR is 10000:1,the spectrum covers from 0.1 THz to about 2.0 THz under static mode.The time window is up to 90 ps,the signal to noise-ratio is 2000:1,the spectrum covers from 0.1 THz to about 2.0 THz and the spectral resolution is 0.011 THz under transient model.The single-shot measurement result agrees well with that of a traditional electrical-optic sampling method.We analyzed the electro-optic crystal thickness,the polarizer angle,the background noise and the signal strength exerts import effect on the final result.