Research On The Key Technologies Of Dispersion Compensation For Femtosecond Laser In Fiber THz-TDS

Terahertz time-domain spectroscopy(THz-TDS)technology has excellent technical characteristics and application prospects in the field of non-destructive testing.How to improve the radiation efficiency of terahertz waves is a core issue in the development of terahertz time domain spectroscopy.Fiber optic THz-TDS systems can reduce the use of spatial optical components,thereby improving the stability and flexibility of the system,making it easier to integrate.However,femtosecond laser transmission in optical fibers will be affected by factors such as dispersion,which affects the terahertz radiation characteristics of the system.In this paper,the dispersion compensation technology of femtosecond laser is studied,and two dispersion compensation systems,grating pair and grism pair,are designed.The internal optical path and dispersion characteristics of the system are analyzed from both theoretical simulation and experimental research.Firstly,in this paper,the transmission characteristics of femtosecond laser in optical fibers were studied,the factors that affect the pulse width of femtosecond laser and the impact of pulse width on terahertz radiation efficiency were analyzed.Secondly,the diffractive optical path inside the grating pair and grism pair system was analyzed.Based on the ray tracing method,the relationship between the structural parameters of the grating pair and grism pair and the dispersion characteristics was numerically simulated.Then,the autocorrelator based on autocorrelation method was studied,the internal optical path composition and the basic principle of measuring pulse width were analyzed,and a zoom optical fiber coupling system was designed to realize the transformation of pulse transmission mode.Finally,an integrated transceiver dispersion compensation experiment was set up to test the optical path;and the dispersion compensation effects of grating pairs and grism pairs were compared.The simulation results show that with the larger the grating constant,the larger the total dispersion of the system is gained.No matter what is the grating constant,the dispersion of the grating pair and grism pair is proportional to the component spacing.When the grating constant is 300lp/mm,the dispersion of the grating is proportional to the incident angle,and the dispersion of the grism is inversely proportional to the incident angle.When the grating constant is 1200lp/mm,the dispersion of grating and grism is inversely proportional to the incident angle.When the grating constant is 600lp/mm,the dispersion of the grating decreases first and then increases with the increase of the incident angle.The experimental results show that both grating and grism can effectively compensate the dispersion in the fiber after reasonable selection.Compared with the pure grating,the dispersion compensation ability of the grism is stronger,however,its optical path tuning is more complicated,and it is not conducive to integration.Compared with traditional dispersion compensation fibers,dispersion compensation elements such as grating pair and grism pair can freely tune the amount of dispersion to achieve precise control of system dispersion.It has important application value for overcoming the technical problem of low terahertz radiation efficiency caused by the transmission of pulses in optical fibers in THz-TDS systems,and achieving highly stable transmission of femtosecond laser pulses in optical fibers without broadening and distortion.