Comprehensive Ranging Disambiguation for Amplitude-Modulated Continuous-Wave Laser Scanner With Focusing Optics

Amplitude-modulated continuous-wave (AMCW) laser scanner with focusing optics is expected to realize high-precision 3-D measurement, which requires accuracy of mm or less. Since such a modulation scheme employs periodical intensity modulation, the longitudinal resolution and the unambiguous range are in a tradeoff. Another problematic situation is the case that the target object is large so that the scanning range exceeds the unambiguous range. The acquired 3-D point clouds contain phase jumps at the boundary of the unambiguous range imposed by the modulation frequency. In this article, we propose multiple solutions to cope with these problems. First of all, our system utilizes dual-frequency modulation to overcome the tradeoff between the resolution and the unambiguous range. With appropriate electronics for demodulation, the hardware imposed a longitudinal resolution of 19.2 ± 38 $\mu \text{m}$ and the unambiguous range of 48 ± 614.3 $\mu \text{m}$ were realized. The unambiguous range can be shifted within 5-m range using a mechanical focusing optics. However, such an attractive laser scanner still suffers from ranging ambiguity. The noise on the laser light modulated by the lower modulation frequency results in phase deviation, which can incur ranging errors at the integer times of the half cycle of the higher modulation frequency. We have coped with such ranging ambiguity by synthesizing ranging errors with the data in the correct range. Especially, defocused 3-D point clouds contaminated by severe ranging errors were analyzed and restored. The measurement range can thus be elongated by >20 times of the depth-of-focus using such data processing without manipulating the mechanical focusing optics. Next, we developed an algorithm as a remedy to prevent phase jumps in the unambiguous range. With exploitation of the relationship between the intensity and spatial information, the phase unwrapping was performed to recover the spatial continuity. Thereafter, the unambiguous range can be elongated to be >48 cm. With all the abovementioned configuration and data processing, we have overcome the ranging ambiguity inherent in the AMCW laser scanner.