Carrier-Free Dual-Comb Distance Metrology Using Two-Photon Detection

Cross-correlation time-of-flight metrology employs two frequency combs identical except for a small repetition-frequency difference, Δ f rep. One comb probes the distance to be measured using target and reference pulse sequences, while the other serves as a local oscillator (LO) that temporally gates the arrival of these pulses on a detector. The technique is a simplification of dual-comb metrology [1], and has been demonstrated with free-running combs [2], [3]. The detected interferograms are Hilbert transformed to obtain envelopes whose positions are localized by a Gaussian-fitting procedure. While the use of free-running combs substantially simplifies the technique, the unavoidable drift in their carrier-envelope offset frequencies ( f CEO ) makes the carrier frequency of the interferograms unstable, causing Hilbert-transform processing to fail when the frequency is too low. Here, we demonstrate cross-correlation time-of-flight metrology with a non-interferometric signal which is insensitive to f CEO drift. By combining the probe and LO pulses with orthogonal polarizations and detecting their cross-correlation on a two-photon detector, we obtain a carrier-free envelope signal which is insensitive to f CEO fluctuations and - because it is non-interferometric - is unrestricted by the conventional dual-comb aliasing limit.