Study On High Performance Near Infrared Fourier Transform Spectroscopy System

In the near infrared region,the absorption coefficient of the object is small,the signal amplitude is small,and the thermal body radiates infrared light.So the interference is large,and the spectrum overlap is serious in the near infrared region.The main purpose of this paper is to reduce the air background noise and improve the detection signal-to-noise ratio of NIRS by adding chopper and lock-in amplifier.A chopper is added to the optical path to modulate the light into a high frequency signal.The signal detected by the photoelectric detector enters the input signal end of the lock-in amplifier,and the sinusoidal signal with the same frequency as the chopper enters the reference signal end of the lock-in amplifier.The two signals have the same frequency.After the lock-in amplifier,the noise will be reduced,so as to improve the signal-to-noise ratio.By improving the signal-to-noise ratio,we can obtain the strong spectral information in the near-infrared region,which can be used in the weak light detection,so that it can have a better application in the detection of silica and other samples;in addition,the system can be further improved and applied to the field of polarization spectrum.In the aspect of experiment,a near-infrared Fourier transform spectrum system is built,and its main achievements are as follows:(1)Carry out simulation analysis through MATLAB.By changing the wavelength density,wavelength range and scanning step length in the program,it is found that the above three parameters affect the interferogram.The results show that a more ideal interferogram can be obtained only when the wavelength dispersion density is larger,the wavelength range is wider and the scanning step is smaller.(2)Design and select the required components of the near-infrared Fourier transform spectrum system,and build a near-infrared Fourier transform spectrometer,which improves the ability of the system to detect the near-infrared signal.(3)By changing the frequency of chopper,the moving mirror speed and the time constant of lock-in amplifier,the best parameters are obtained.When the frequency of chopper is 10 KHz,the time constant of lock-in amplifier is 5 ms and the moving mirror moving speed is 0.01 mm/s,the signal-to-noise ratio of interferogram reaches the maximum.On this basis,a polarization spectrometer structure of polarization maintaining reflection type near infrared Fourier transform is proposed and applied for patent.