Long-range frequency-modulated continuous-wave LiDAR employing wavelength-swept optical frequency comb with ranging disambiguation

Rapid-aging civil infrastructures have become social issues worldwide. Developed countries have faced difficulties e.g. cost increase in infrastructure maintenance, shortage of highly-skilled human resources. Thereafter, infrastructures have been inspected infrequently e.g. every 1~10 year. 3D mapping powered by light detection and ranging (LiDAR) platforms has recently been brought to public attention. In infrastructure maintenance, LiDAR having high-precision and long-measurement range is expected to enhance inspection frequencies and lower the maintenance cost. Time-of-flight (ToF) approach is generally utilized for long-range measurement, however it suffers from low depth resolution. Although development of dual-comb interferometry enabled high-precision ToF rangefinders, sophisticated stabilization is required for two lasers. On the other hand, frequency-modulated continuous-wave (FMCW) LiDAR can perform ranging with a single laser. However, its measurement range is limited by its receiver bandwidth. Although FMCW LiDAR with low-speed analog-to-digital converters can conduct long-distance ranging by reducing its wavelength-sweeping rate, the measurement refresh rate is sacrificed. In this article, we propose a long-range FMCW LiDAR employing wavelength-swept optical frequency comb which overcomes ranging limitation caused by its receiver bandwidth. In addition, the spectrum of such optical frequency comb is apodised to circumvent its inherent ranging ambiguity. We have performed proof-of-concept experiments using the proposed LiDAR. With optical fibers for emulating beam propagation, we have succeeded in ranging corresponding to 1,605-m free-space propagation with a 10-MHz FMCW receiver. Our proposal will make an important contribution to infrastructure maintenance.