Long-range high-precision laser ranging using a joint measurement method

A joint measurement method for long-range and high-precision laser ranging is proposed, in which the frequencymodulated- continuous-wave (FMCW) laser ranging method is used for long-range coarse ranging, while the phase-shift laser ranging method is used for high-precision ranging. The key of the joint measurement method is to ensure that the resolution of the FMCW method is smaller than the unambiguous range of the phase-shift method. In this paper, a super resolved FMCW laser ranging method based on a low-duty-cycle linear-frequency-modulated (LFM) signal is used to achieve phase unwrapping with small bandwidth. A demonstrated experiment is carried out, in which a 2-4 GHz LFM signal with the duration of 1 μs and duty cycle of 10%, and a 5 GHz single-frequency signal are used to simultaneously modulate the lightwave in a dual-parallel Mach-Zehnder modulator (DPMZM). The theoretical measurement range and resolution are 300 m and 0.16 mm, respectively. The absolute length of a 140-m fiber spool and a variable optical delay line (VODL) varied with a step of 0.2 cm in a range from 7 to 20 cm is measured. The standard deviation of the displacements between the measured distances and the exact values is 0.377 mm. The proposed method promises a long range laser ranging with high resolution and measurement rate.