Laser ranging system based on double intensity modulation

Abstract

Long-range, high-precision, and high-refresh rate absolute distance measurement based on double intensity modulation is proposed and experimentally demonstrated. In this scheme, a Mach-Zehnder modulator is utilized to perform bidirectional modulation by a reversible optical path. In the Mach-Zehnder modulator, interference demodulation is completed by bidirectional modulated light with time difference. By adjusting the driving frequency of the modulator, the curve of light intensity versus driving frequency is achieved. Consequently, the distance to be measured can be obtained by extracted the frequency interval between two adjacent light intensity minimum points. In the traditional double polarization modulation ranging, the optical carrier is polarized by a polarizing beam splitter (PBS) before phase modulator. Moreover, a quarter wave or Faraday rotating mirror need to be placed to adjust the polarization in front of the target object. Therefore, the polarization state is an indispensable factor in the traditional double polarization modulation ranging. Due to the advantage of the intensity modulation, absolute distance measurement is achieved without additional polarization control, greatly simplifying the system. Theoretical analysis of the system is developed, which is then demonstrated by experiments. In the experiments, we achieved the following results. Firstly, the relationship between the intensity of the output light of the system and the modulation frequency is theoretically analyzed, which proved to be a cosine form. Secondly, swing method is introduced to realize high-speed absolute distance measurement during the analytical distance algorithm, and we achieved a refresh rate of 2 kHz in the experiments. Thirdly, the relationship between measurement ranging precision and frequency stability is analyzed. When the modulation frequency is 8.9 GHz, the experimentally measured frequency stability is on the order of 10^–7. And when the distance to be measured is 2.73 m, the standard deviation of ranging reaches 1 μm, which represents the precision of the system. That is, the relative measurement precision is also on the order of 10^–7, which is consistent with theoretical analysis. Finally, when the distance to be measured increases from 1.57 m to 100.83 m, the measurement precision increases from 1 μm to 30 μm. It is worth mentioning that the relative measurement precision of the system is always stable in the order of 10^–7. Our scheme has some significant advantages, such as long-range, high-precision, high-refresh rate, and a simple and compact configuration. Moreover, our method can be used in important applications such as precision instruments, metrology, and aerospace.