Research On Dual-photoelastic-modulator Based Beat Frequency Modulation And Fourier-bessel Transform Spectrometer

Spectral measurement is widely used in environmental monitoring, food monitoring,aerospace, biomedical engineeing, and military field. Fourier transform spectroscopy basedphotoelastic-modulator (PEM), featuring of high spectral modulation frequency, widespectrum, high sensitivity, and seismic.So it has good prospects in the spectral measurements.This dissertation analyzes the statusof PEM spectroscopy technique. Existing PEMmodulation frequency is high (tens to hundreds of kHz). When large optical path differenceand short wavelength, the interference signal frequency will be higher. Signal frequency is sohigh that they can not effectively be detected and collected. Combined with the requirementsof the project, a new beat frequency modulation spectroscopy method based on dualphotoelastic-modulator and Fourier-Bessel transform is proposed. The dual-PEM areoperated as an electro-optic circular retardance modulator, operating the PEMs at slightlydifferent resonant frequencies, generates a differential signal at a much lower heterodynefrequency that modulates the incident light. This method not only retains the advantages ofthe existing PEM, but also the frequency of modulated photocurrent decreased by2~3ordersof magnitude (10Hz~500Hz) and can be detected by common array detector, and the incidentlight spectra can be obtained by Fourier-Bessel transform of low frequency component in themodulated signal. For one thing, study on the inversion algorithm of beat frequencymodulation method based on dual photoelastic-modulator. According to the characteristics ofthe interference signal which modulated by dual-photoelastic-modulator. Fourier-Besselspectral inversion algorithm deduced. For another, using the method of simulation andexperiment, large optical path difference PEM was studied. This paper analyzes the materialand structure of the PEM photoelastic crystal and piezoelectric crystal elastic material and itsstructure. Finally, in view of the proposed approach has carried on the simulation andexperimental validation. The complex ray theory simulation verification, by632.8nm laserhas carried on the comparison of simulation and experiment. Through the simulationexperiment of the method arguments, the double play light tone difference frequencymodulated Fourier-Bessel transform spectrum measurement method is feasible in theory andpractice. Large optical path difference PEM experimental results show that: in the same sizeand drive voltages, A maximum optical path difference of this PEM is19.25times that ofordinary PEM’s. In short, the feasibility of the method is verified by the simulation andexperimental.