Objective   The emergence and development of laser ranging technology has solved the problem that traditional measurement methods cannot take into account large-scale and high-precision measurement. It has the advantages of high resolution, large measurement range, and easy integration, which promotes the development of remote sensing, radar, equipment manufacturing and other related fields. Although the existing laser ranging technology has made great progress in terms of ranging range and resolution, it is easily affected by interfering signals in space and equipment time base frequency errors during the ranging process, resulting in laser signal. The echo is easily disturbed by noise during the propagation process, which affects the measurement accuracy. Therefore, a large-scale, high-precision, and traceable ranging method is an urgent requirement in current practical engineering applications. Aiming at the problems of being susceptible to environmental noise interference and frequency traceability in laser ranging applications, this paper studies the method of dual-frequency optical scanning ranging without optical interference that traces the Beidou time base. This method has strong anti-interference ability against noise. The Beidou time base improves the accuracy and stability of laser modulation, and provides a new research idea for absolute distance measurement.

  Methods   In this paper, a theoretical model of dual-frequency optical scanning ranging without optical interference is established, and the relationship among distance, signal phase and scanning frequency is obtained. A Beidou/GPS dual-mode clock is designed as the frequency reference of the ranging experiment device to achieve the effect of remote source tracing (Fig.6). An experimental device of distance measurement was built, and a dual-frequency laser was prepared based on a fiber electro-optic modulator, which has the advantages of large frequency tuning range and fast scanning speed (Fig.10). A demodulation scheme combining electronic heterodyne detection and self-mixing is designed, and the polarization orthogonal dual-frequency laser is used as the carrier of the Michelson interferometer to achieve the effect of non-optical interference, and the high-frequency photocurrent signal is demodulated to the low-frequency ranging signal, reducing environmental noise and electronic noise (Fig.11).

  Results and Discussions  The Beidou/GPS clock uses the second pulse signal output by the dual satellite navigation system receiver as a reference, and uses the PID algorithm with dead zone to correct and compensate the frequency drift of the constant temperature crystal oscillator. Long-term frequency stability can be obtained without destroying the stability of the crystal oscillator. The frequency accuracy reaches 0.03 ppm (Fig.7). In the experiment, a device measuring dual-frequency optical sweeping distance was built that traces back to the Beidou time base. Using the signal analysis method of curve fitting, a large number of experimental data were calculated, and the measured distance in the ranging experimental device was 9.843 6 m. The measurement uncertainty is 1.25 mm, better than the untraceable 1.72 mm (Tab.3).

  Conclusions   In this paper, the research on the absolute distance measurement of dual-frequency light is carried out, and the theoretical model of frequency-sweeping distance measurement by dual-frequency light is established. A method of using EOM to generate dual-frequency synthetic laser is proposed, and the time base calibration of the modulation signal source is performed through remote traceability, so that the time-frequency accuracy of dual-frequency light can meet the experimental requirements. A dual-frequency optical scanning ranging device was built, and a Beidou/GPS clock and electronic signal demodulation scheme was designed. Through Beidou/GPS clock timing calibration, the signal source time base frequency accuracy reaches 0.03 ppm (1 ppm=1×10^-6). In the process of data processing, the measured distance is obtained by curve fitting the demodulated signal, and the Gaussian distribution of a large number of experimental data is counted to obtain an absolute distance measurement result of 9.843 6 m, with a measurement uncertainty of 1.25 mm (Fig.14). This method not only avoids the frequency error caused by the internal time base of the signal source due to the factors such as crystal oscillator aging and temperature which affect the measurement results, but also has a good ability to suppress environmental noise and electronic noise, and can achieve large-scale absolute distance measurement. The measurement accuracy is improved by an order of magnitude due to the use of AOM frequency sweep ranging, so it has a wide application prospect.