Surface Scanning of Structured Metal Artefacts With Two-Photon Dual-Comb LiDAR

Dual-comb ranging (DCR) employs one frequency comb to ‘probe’ the distance under test, and a second ‘local oscillator’ (LO) comb to time-gate the returning probe reflections. In the conventional implementation, this time-gating creates interferograms, which are digitised at > 100MSa/s to determine the distance under test [1]. Conversely, our non-interferometric implementation of DCR detects intensity cross-correlations which, after conditioning, can be detected as rising edges on a microcontroller, thereby allowing the time-of-flight information to be extracted in real-time with minimal data burden [2], [3]. Recently, we showed that the weak optical returns from metal surfaces can be amplified in an Erbium-doped fibre amplifier (EDFA) to obtain ranging precisions comparable to those previously recorded with optical targets (i.e. mirrors and retroreflectors) [4]. In this development, we repeatedly probe the distance to the target, while translating the target surface, to create a surface-scanned image.