• Chinese Journal of Lasers
  • Vol. 48, Issue 11, 1104001 (2021)
Zhenkun Liang1、2、3, Xiao Li1, Zhibin Wang1、2、3、*, Kewu Li1、2、3, and Xiaoyang Zang1、2、3
Author Affiliations
  • 1School of Electrical and Control Engineering, North University of China, Taiyuan, Shanxi 0 30051, China
  • 2Frontier Interdisciplinary Research Institute, North University of China, Taiyuan, Shanxi 0 30051, China
  • 3Nantong Intelligent Optical Electromechanical Research Institute, North University of China, Nantong, Jiangsu 226000, China
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    DOI: 10.3788/CJL202148.1104001 Cite this Article Set citation alerts
    Zhenkun Liang, Xiao Li, Zhibin Wang, Kewu Li, Xiaoyang Zang. Photo-Elastic Modulation Based on Adaptive Regulation of Driving Voltage[J]. Chinese Journal of Lasers, 2021, 48(11): 1104001 Copy Citation Text show less

    Abstract

    Objective With the advantages of high modulation precision, high efficiency, and wide spectral range, photo-elastic modulation technology has attracted considerable attention in many fields, such as optical communication, polarization analysis, and spectral analysis. A series of devices with a photo-elastic modulator as the core are widely used in material detection, scientific research, aerospace, and national defense construction. The basic principle of the photo-elastic modulator is to use the anisotropic crystal of a piezoelectric crystal to provide the external mechanical force, so as to make the photo-elastic crystal produce birefringence. It is a thermal-mechanical-electrical coupling resonant device consisting of photo-elastic and electro-optic crystals. In the working process, the driving circuit provides a high-voltage sine wave signal for the piezoelectric crystal in the vertical direction, making the piezoelectric crystal vibrate horizontally and causing the elastic crystal to deform accordingly, therefore producing periodic birefringence to achieve phase modulation of incident light. The phase modulation amplitude is positively correlated with the voltage amplitude output by the drive circuit. The driving voltage amplitude is positively correlated. When the amplitudes of the driving voltage and phase modulation increase, the piezoelectric quartz and photo-elastic crystal consisting of the photo-elastic modulator will produce thermal loss during mechanical vibration. This will result in the shift of the resonance frequency, making it become a challenge to keep the optical path difference consistent for a long time, which reduces the photo-elastic modulator's stability and modulation efficiency. Besides, the change in the external environment will reduce the modulator's stability. Thus, the stability technology of the photo-elastic modulator plays a significant role.

    Methods In this study, starting from the structure of the photo-elastic modulator, according to the vibration model of the photo-elastic modulator, the influencing factors of the temperature drift of the photo-elastic modulator are analyzed. The conventional optical system with a photo-elastic modulator as the core is developed. According to the Stokes parameter, Muller matrix, and digital phase-locking technology, the relationship between the driving signal and the phase modulation amplitude is finally obtained, and the correlation function diagram is drawn. Thus, a drive voltage adaptive adjustment method based on a field-programmable gate array (FPGA) is proposed. Direct digital synthesis (DDS) technology is used to control the square wave signal of a photo-elastic modulator. After being amplified by the LC resonant circuit, a high-voltage sine wave is generated to drive the photo-elastic modulator. The incident light is modulated by the photo-elastic modulator and received by the detector. The optical signal is converted into an electrical signal, which is transmitted to FPGA through an analog-to-digital sampling module. By using digital phase-locked technology, we multiply and accumulate the acquisition signal with the quadruple frequency reference signal in the memory to obtain the quadruple frequency correlation component in the acquisition signal, and similarly multiply and accumulate the acquisition signal and the double frequency reference signal to obtain the double frequency correlation component. The relationship between the ratio of the quadruple frequency component to the double frequency component and the phase modulation amplitude is analyzed. The duty cycle is adjusted in real-time according to the ratio of the quadruple and second harmonic frequencies in each cycle. If the ratio obtained in the next cycle is smaller than that in the previous cycle, the duty cycle will increase; otherwise, it will decrease. Finally, the phase modulation amplitude will be stabilized at a fixed range. In the system, two key points need to be determined in advance. One is the resonant frequency of the photo-elastic modulator, and the other is the unit adjustment of the driving square wave duty cycle. Since the external temperature and thermal effect of the photo-elastic modulator affect the change of its resonant frequency, it is essential to determine the resonant frequency of the photo-elastic modulator in the test environment first. The key to the single adjustment of the duty cycle in the feedback control is that, to prevent excessive feedback regulation, and it is essential to ensure that the duty cycle adjustment amplitude is less than the setting value of the phase modulation amplitude. However, the duty cycle adjustment amplitude should be larger than the fluctuation of the phase amplitude after temperature drift, therefore preventing the lack of control due to the lack of amplitude modulation. At room temperature of 25 ℃, the photo-elastic modulator was powered for 15 min. According to the sweep frequency test, the resonant frequency under current temperature is determined. The two tests are conducted to determine the conversion relationship among the duty cycle, driving voltage, and phase modulation amplitude, so as to determine the regulation value of the duty cycle unit.

    Results and Discussions The stability of the system is tested by experiments. Compared with the previous studies (Table 1), the accuracy has been significantly improved. When the incident light wavelength is 632.8 nm, the phase modulation amplitude accuracy is 0.82% at the half-wave state and 0.44% at the quarter-wave state [Fig. 11 (a) and Fig. 12]. The constant temperature control method is more stringent for the device and environment. The frequency control method is not conducive for data processing after application. The voltage regulation has none of the above shortcomings, and the accuracy is higher than that of the above two methods.

    Conclusions Based on the temperature drift model and digital phase-locked technology, the phase modulation amplitude stability control of the photo-elastic modulator is realized. The experimental results show that the voltage self-regulation method has higher accuracy, a wide application range, and more convenient for subsequent data processing than the existing temperature control and frequency regulation methods. It has important theoretical significance for improving the accuracy and reliability evaluation of phase modulation amplitude control system.

    Zhenkun Liang, Xiao Li, Zhibin Wang, Kewu Li, Xiaoyang Zang. Photo-Elastic Modulation Based on Adaptive Regulation of Driving Voltage[J]. Chinese Journal of Lasers, 2021, 48(11): 1104001
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