Research On Double Closed Loop Control Technology For Large Optical Path Difference PEM-FTS

The photo-elastic modulated Fourier transform spectrometer has the advantages of high test rate,flexible sampling method,and fast analysis speed.It is widely used in semiconductor industry,spectrum measurement,biomedical applications,etc.Its core device is a photo-elastic Modulator Driven by a high-tension sine shaped signal,and the resonant frequency tends to drift with its own temperature changes.The higher the driving voltage,the more serious the drift is,which makes the photo-elastic modulator unstable and affects the efficiency of modulation.In order that we can improve the stability and the ability of modulation in photo-elastic modulator,this thesis puts forward a double closed-loop command system to track frequency and compensate voltage according to FPGA,which is mainly made up of the following aspects.First,the article analyzes the constitution and operating principle of the photo-elastic modulator,founds a heat consumption dynamic model and a equal circuit model,According to the characteristics of PEM frequency drift and unstable operation,the design scheme of the double closed-loop control system of photo-elastic modulation is proposed.Additionally,We can use FPGA as the control core to devise the system hardwar-e,which includes self-trace regulation circuit according to circuitry tickling;the driven voltage regulation circuit on account of OPD;the devise of high-pressure driving circ-uit of the LC resonance as well as the ambient circuit of FPGA,and completed the te-st of the hardware circuit.Moreover,the paper has finished the high-accuracy DDS generating signal program design according to FPGA,the design of phase difference detection algorithm on digital phase distinguish,the design of maximal optical path difference（OPD）monitor program depending on high-speed comparator,and the voltage automatic adjustment system based on digital potentiometer,etc.So controlling the stability of the photo-elastic modulator is achieved.Finally,an tested platform for the photo-elastic modulation double closed-loop control system was built,and the stability test of the elastic-optical modulation interference device was measured.The results of the experiment indicate that the maximal optical path difference of the photo-elastic modulated interference device is just up to 2.21%when the spectral resolution is set to 70cm^-1.It is proved that this method can effectively stabilize the elastic-optical modulation interference device,so that the maximum optical path difference is maintained under the system set index with a small error.