Research On Detection Technology Based On Terahertz Time-domain Spectrum

So far,terahertz(THz)band is still the lowest human utilization of spectrum resources.Terahertz waves have some unique advantages due to its combined characteristics of both electronics and photonics.In recent decades,advances in material science and ultrafast photonics have promoted the rapid development of terahertz technology.As an emerging spectral analysis method,terahertz time-domain spectroscopy technology can easily obtain the optical parameters of materials,and has been applied to many research fields such as non-destructive testing,drug analysis,and explosives identification,etc.The current terahertz time-domain spectroscopy technology has the following challenges:firstly,the traditional Ti-sapphire solid-state laser is widely used as a femtosecond light source in the terahertz time-domain spectroscopy system,leads to hugh system volume,inconvenient in maintenance and high cost;secondly,the use of spatial-coupled structure leads to complex optical path,low stability and inconvenience in movement,which greatly limits the applications of terahertz time-domain spectroscopy;thirdly,in the terahertz time-domain spectroscopy,it is difficult to obtain good classification and recognition for some biological samples without obvious absorption peaks and complex composition.In view of the above technical challenges,three new aspects of terahertz time domain spectroscopy are studied in this paper,namely,the novel passive mode-locking femtosecond fiber laser source,the optical fiber-coupled terahertz spectroscopy system and terahertz time-domain spectroscopy analysis method based on pattern recognition.The main innovations of this paper are as follows:1.In view of the large size,complex structure and inconvenient maintenance of the traditional Ti:Sapphire laser source,the terahertz time-domain spectroscopy system is difficult to miniaturize.In response to this problem,the paper developed a small and high-performance femtosecond fiber laser for system miniaturization.Firstly,a passive mode-locked erbium-doped fiber laser based on nonlinear amplifier loop mirror(NALM)with an all polarization-maintaining dispersion management cavity is proposed and developed.The all polarization-maintaining structure ensures that the laser is not sensitive to disturbances in the external environment.The designed figure "8”cavity erbium-doped fiber laser can realize self-starting operation.After de-chirpe,the output pulse width is 500fs,and the pulse energy reaches 0.8nJ.Through the pre-chirped amplification and compression system,the pulse width of the laser can be further compressed to less than 100fs,and the average output power can be increased to 120mW.The performance of the laser can meet the general requirements of the terahertz time domain spectrum system.Secondly,based on the above-mentioned figure "8" cavity laser,by reducing the repetition rate and adjusting the dispersion inside the cavity,the laser operates in the large positive dispersion region.A high output pulse energy figure“8”fiber laser can be realized.Erbium-doped femtosecond fiber laser with output pulse energy of 4nJ,repetition rate of 1.96MHz and pulse width of 660fs is realized.In addition,the RMS error of the laser output power is 0.17%under 10 hours of continuous mode-locked operation,which indicates good output power stability.The pulse energy of the laser is much higher than that of the same type of laser reported.This kind of laser has high application potential in optical rectification terahertz time domain spectrum systems.Thirdly,in order to improve the repetition rate of the figure“8”NALM mode-locked laser,this paper studies the NALM laser with figure "9" cavity.A stable self-starting mode-locking of the figure "9" cavity is realized.The output power of the laser is 2.3mW,the repetition rate is 40MHz,and the pulse width of the output pulse after de-chirping is 120fs.2.In the aspect of miniaturization of terahertz time-domain spectroscopy system,a fiber-coupled terahertz time-domain spectroscopy measurement system using fiber femtosecond light source is developed in this paper,and the related automatic control and data acquisition and analysis program of the system are developed.After experimental testing,the system size is 48cm×45cm×12 cm,the peak dynamic range is about 60dB,the SNR is about 493 and the detection bandwidth is about 2.5THz.The system has good repeatability and provides a useful reference for the future development of a portable terahertz time-domain spectral measurement system.3.In the aspect of terahertz time-domain spectral detection,terahertz spectral information of transgenic rapeseed seeds is extracted,and terahertz absorption spectrum of two transgenic and non-transgenic rapeseed seeds are obtained.Because of the high terahertz spectral similarity of the three,this paper carried out pattern recognition method to identify and analyze the transgenic rapeseed seeds.The classification accuracy rate of Adaboost combined with Naive Bayes algorithm reaches 96.6%,which is higher than that of PCA-SVM and PCA-RF algorithm.The experimental results show that Adaboost combined with Naive Bayes algorithm is more suitable for classification and identification of transgenic rapeseed.