Study On Stability Of Photo-Elastic Modulator Based On Optimal Hill Climbing Search Algorithms

Photo-elastic modulation technology is optical polarization modulation technology based on the birefringence.When Photo-Elastic Modulator(PEM)works in resonance,the piezoelectric crystal imposes periodic mechanical stress on the elastic crystal.Then the elastic crystal begins to vibrate and produce periodically varying birefringence,which can realize to adjust the phase of the incident light.At present,the commonly used PEM has some features,such as a broader spectral window(from deep ultraviolet to terahertz band),a higher operating frequency(generally 40 to 80 kHz),a larger pass-through area and aperture,a higher modulation purity and modulation efficiency.The PEM has been widely used in polarization related fields,such as spectral polarization imaging,transient spectral measurement,biochemical sensing and so on.However,the PEM will be affected by its own thermal dissipation in the the process of the working,resulting in the temperature change of the elastic crystal itself.Changing with the temperature,the inherent frequency drifts.It leads to the instability of the PEM and the fluctuation of phase delay.Firstly,this paper studies the working principle of the PEM,the dynamic model of the PEM and the principle which the temperature causes the resonant frequency drift of the PEM.Secondly,the factors affecting the stability of the PEM are analyzed.Then,the high-voltage driving circuit of the PEM,the DDS square-wave clock signal generator based on the FPGA and the peripheral circuit of the FPGA are designed.In this paper,digital phase-locked technology is used to collect the frequency doubling data of the signal exported by the detector.In terms of data analysis and processing,the data transmitted to the upper computer through the serial port of FPGA is analyzed and calculated by VC++ program.In the process of data processing,an optimized hill-climbing search algorithm is used to calculate the ratio of the double frequency to the quadruple frequency.And the functional relationship between the double frequency ratio and is obtained by linear fitting with thephase delay.Thus,the resonant frequency of the whole photo-elastic modulation system is adjusted in real time.So the phase delay of PEM is kept within the allowable error,and the PEM is in stable working state.Finally,the function test of each module of the improved PEM stability control system is carried out,and an experimental platform is built to test the stability of the improved PEM.The phase delay error can be reduced to 0.01% by the test in the control system.The phase delay error can make the improved PEM work stably.That proves the system can stabilize the improved PEM and meet the actual experimental requirements.