Dual-comb time-of-flight measurement via attosecond-resolved active pulse tracking

Dual-comb ranging (DCR) enables rapid and precise time-of-flight (ToF) measurements, with interferometric phase providing additional precision refinement. Here, we demonstrate a ToF DCR method with active pulse tracking (APT) based on balanced optical cross-correlation (BOC). The BOC provides attosecond-resolved timing discrimination and generates a bipolar error signal, which is used by a field-programmable gate array (FPGA)-based servo to scan, acquire, and track the returning pulse train. After acquisition, the local comb remains synchronized with the signal comb, enabling displacement tracking over the tested range. Distance changes are reconstructed from the accumulated change in phase difference between the signal comb and local comb repetition-rate signals recorded by the FPGA. In the locked state, the BOC error signal gives an in-loop timing jitter of 4.16 fs, and the BOC-equivalent length Allan deviation reaches 15 nm at an averaging time of 0.1 s. The capability for tracking moving targets is also tested. A sinusoidal displacement with an independently calibrated reference amplitude of 166 nm was measured with a fitted APT amplitude of 169 nm. The system can still track the target when the sinusoidal oscillation frequency is changed. This work may have potential applications in various nanometer precision dynamics measurement scenarios.