Two-Photon Dual-Comb LiDAR for Multi-Target Ranging

Dual-comb ranging techniques employ a 'probe' comb of repetition frequency, f _rep to sample the distance between reference (R) and target (T) optics, and a 'local oscillator' (LO) comb with a repetition frequency, f _rep + ∆f _rep to temporally gate the returning probe pulses. When conventional detection is used, the result is a series of interferograms requiring high-bandwidth digitisation and post-processing to derive the distance values. Conversely, in two-photon dual-comb LiDAR, the probe and LO combs are combined with orthogonal polarisations and detected via two-photon absorption to create optical cross-correlations [3]. The carrier-free nature of these signals allows them to be conditioned to act as triggers for a microcontroller, enabling an ultra-precise data collection paradigm where the microcontroller acts as a stopwatch recording the time between the detection of subsequent cross-correlations, τ _i [4]. The raw data take the form of a string of integers written to the serial port, allowing real-time distance calculations. The transition from digitising the full optical signal to time-stamping pulses using a microcontroller-based stopwatch significantly reduces the data burden associated with dual-comb techniques, whilst maintaining a comparable level of measurement precision.