Research Of Optical Fiber Type Terahertz Time-domain Spectroscopy System

Terahertz spectroscopy is an extremely important part of terahertz applications and is widely used in many areas such as explosives detection,material composition identification,medical diagnostics,and product quality control.To study the miniaturization of the terahertz spectrometer,the rapid and accurate measurement is of great significance for the practical use of terahertz.The optical fiber type terahertz time-domain spectroscopy system is small in size,practical and portable,and has good prospects as a detection instrument for industrial and on-site large-scale applications.In order to obtain a high-performance optical fiber type terahertz time-domain spectroscopy system,this paper carried out systematic research on the structural design,dispersion control,polarization control and control program of the spectrometer,the specific contents are as follows:In this paper,the terahertz characteristics,basic applications and basic principles of free space terahertz time-domain spectrometer are briefly introduced.The basic principle and structure design of optical fiber type terahertz time-domain spectrometer are described in detail.The spectrometer is constructed by combining an optical fiber femtosecond laser with an optical fiber coupled terahertz photoconductive antenna.In the system structure design,the principle of using negative dispersion fiber to compensate the pulse width is introduced,and the principle of using device for signal amplification and data acquisition is introduced.Two rotating structures,dovetail guide rail structure and 60 mm cage coaxial system,which can realize the conversion between transmission and reflection,are explained.The influence of laser polarization state on terahertz time-domain waveform,intensity and spectral characteristics was studied experimentally.By controlling and optimizing the polarization state of femtosecond laser,the phenomenon of pulse splitting in time-domain is eliminated.A single pulse terahertz time-domain waveform with a signal-to-noise ratio of over 12000 is obtained.In this study,the control program of slow scan and fast scan for optical fiber type terahertz time-domain spectrometer was also written based on Lab VIEW software.The influence of integrating time on the waveform shape and the noise in the time-domain of phase-locked amplifier are studied with two kinds of control programs.The results show that different scanning speeds require different optimal integration time to ensure that the waveform is undistorted,with a high signal-to-noise ratio and a typical bandwidth of 4 THz.Based on the above research,the reflectance of the terahertz composite absorber is measured by the optical fiber type terahertz time-domain spectrometer.The measured results show that the relationship between the reflectivity at different angles conforms to Fresnel's law of reflection.