Objective The phase-shift laser ranging technique, which can achieve millimeter accuracy over medium and long distances, is widely used in the aerospace field, such as docking of spacecraft and landing on extraterrestrial planets, as well as civil fields such as three-dimensional mapping and vehicle-mounted lidar. In a previous study, we proposed a phase-shift laser range finder based on optical carrier phase modulation. Comparing with the traditional intensity-modulated phase-shift laser range finder, the technique shows excellent advantages such as simplification of modulation and demodulation, ability to measure range and velocity precisely at the same time. The technique combined the advantage of traditional intensity-modulated phase-type laser ranging technology and coherent laser ranging technology. This paper aims to conduct additional research on the technique to make it usable in the field. The system has been improved in several ways, including using a separate transmitting-receiving (T-R) optical system, using a heterodyne detection method, and the proposal of a feasible algorithm to improve system sensitivity.

Methods First, the experimental system of phase-shift laser range finder based on carrier phase modulation using heterodyne detection is descripted (Fig. 1). The balanced detector’s output signal equation is deduced. Second, the algorithm’s thoughts are presented, and the algorithm’s steps are demonstrated. Finally, the effect of background light on the system signal-to-noise ratio is discussed. The condition of ignoring background light is discussed. An experimental setup using heterodyne detection is built. The parameter of the experiment equipment are as follows, the linewidth of the laser is less than 100 kHz, the frequency shift of the acoustic-optical modulator is 1 MHz, the half-wave voltage of the phase modulator is 4 V and its bandwidth is 300 MHz, the bandwidth of balanced detector is 26 MHz, receiving aperture of an optical system is 8 mm, the center wavelength and full width of half maximum are 1550 nm and 10 nm respectively. In the experiment, the light power of the erbium-doped optical fiber amplifier (EDFA) is 500 mW, the frequency and amplitude of the input signal of the phase modulator is 200 kHz and 2 V. The oscilloscope’s sample rate is 10 MHz, and every 10000 sample point is cut to process and calculate, which makes the measurement period is 1 ms. A series of the experiment is conducted. A different distance from the target is measured, and the linearity of the distance measurement is discussed. The precision of range and velocity measurements is estimated after multiple measurements. Furthermore, the range and velocity of the moving target (a pedestrian) are measured to test the system’s ability to move targets. Finally, the ability of the proposed system to resist background light interference was tested by measuring the noise’s root mean square value (RMS) under various background light conditions.

Results and Discussions A linear model can fit the relationship between measured and actual distance (Table 1), and its intercede is 20.478 m. The intercede shows the transmission length of the RF signal inside the system unit. A target at a distance of 75.7 m is measured (Fig. 7), the average (AVG) of the measured distance is 96.18 m, the root mean square error (RMSE) is 0.93 m. The AVG and RMSE of measured velocity, respectively, are 1.18×10 ^-4m/s and 5.48×10 ^-4m/s. A moving target (a pedestrian) at a distance of 71.5 m is measured (Fig. 8). AVG of the measured distance is 71.83 m, and the RMSE of the distance measurement is 1.39 m. The measured speed is approximately 1 m/s, which corresponds to the speed of pedestrian movement. When the system is looking directly at the sun, the noise of the balanced detector is acquired, with the target surface illuminated by the sun and blocking the receiving aperture. The RMS of noise hardly changed in different conditions, indicating that the proposed system can withstand interference from sunlight (Table 1).

Conclusions This paper improves the phase-shift laser range finder based on carrier phase modulation. The basic principle of the system is deduced, an algorithm can calculate range and velocity by analyzing frequency spectrum is proposed, experiment setup uses transmit-receive separate optical system and heterodyne detection methods. The range and velocity measurement in the field was carried out when the modulation frequency of phase modulator is 200 kHz, power of the emitted signal is 500 mW. The distance of the target is 75.5 m. The precision of range and velocity is 0.93 m and 5.48×10 ^-4 m/s. Notably, the measurement range of the system is 750 m, and the accuracy of the phase measurement of the system is 0.22°. The measuring frequency is 1 kHz. A moving target is measured in the same conditions. The precision of the moving target is also high. Besides, the system’s noise in different background light conditions hardly changed, which shows that the proposed system has a strong ability to resist background light interference. The anti-interference ability and sensitivity of the system have been greatly improved as a result of the field experiment results, and it can adapt to the working conditions of the field. As a result, it will have a wide range of applications in the future.