Research On Material Variable-angle Reflection Measurement Based On Fiber-type Terahertz Spectrometer

Terahertz wave is in the transition band from electronics to photonics in spectroscopy.It is a bridge between electronics and Photonics and has unique spectral properties.Terahertz spectroscopy is an important part of the application of terahertz technology.It has important applications in the fields of material composition identification,explosive detection,biomedical diagnosis and product quality detection and control.It is very important to study the measurement principle and method of terahertz spectrometer and optimize its structure and function.Compared with the traditional free space terahertz time-domain spectrum system,the fiber-optic terahertz time-domain spectrum system has the advantages of small size,compact structure and easy to carry.After improvement,it can measure the variable angle reflection spectrum of materials.In this paper,the design structure,dispersion management and polarization control of the existing terahertz spectrometer in the laboratory are improved,and a femtosecond laser is developed based on the principle of nonlinear polarization rotation mode locking,which can be used to pump and detect terahertz pulse.At last,the variable angle reflection of different materials is measured by the improved terahertz spectrometer.The experimental results are consistent with the calculation results of Fresnel reflection law.The main research contents are as follows:(1)This paper introduces the basic principle and type of terahertz time domain spectrometer,and expounds the improvement scheme of the existing optical fiber terahertz time domain spectrometer in the laboratory in detail.Firstly,the swallow tail guide rail structure is used to replace 60 mm cage coaxial structure expands the range of angle adjustment;secondly,the negative dispersion fiber is used to compensate the dispersion and compress the broadened pulse;finally,by precisely adjusting the polarization direction of laser and the fast and slow axis direction of optical fiber transmission,the spectrum curve is optimized and the terahertz time-domain spectrum system which can realize the variable angle reflection spectrum measurement is obtained.Furthermore,the design of all optical fiber system is proposed by using polarization maintaining dispersion compensation fiber instead of common dispersion compensation fiber.(2)In this paper,the mode-locked principle of femtosecond laser is introduced.A femtosecond laser is designed and built by using the ring cavity structure of nonlinear polarization rotation mode-locked.In the process of laser development,the repetition frequency of the output pulse sequence is controlled by controlling the total length of the resonator,the pump efficiency is improved by increasing the length of the doped fiber,and the optimal mode-locked position is determined by testing the pump threshold and adjusting the polarizer angle.After comprehensive optimization and debugging,the femtosecond laser with the center wavelength of 1561 nm,pulse width of 87 fs and repetition frequency of 79.8 MHz is finally obtained.(3)On the basis of the above research,the reflection spectra of different materials are measured by using the fiber type variable angle terahertz time-domain spectral system.The complete reflectivity curve can be measured at different angles,whether it is transparent and high resistance silicon wafer or weak reflection terahertz absorbing material,and the measured value is in good agreement with the calculation result of Fresnel reflection theory.Furthermore,the scattering spectra of terahertz composite absorbers under normal incidence are measured,and the scattering patterns of different angles are obtained,which provides a reference for the uncertainty evaluation of terahertz radiation power measurement.