The time-of-flight of light pulses has long been used as a direct measure of distance, but the state-of-the-art measurement
precision using conventional light pulses or microwaves reaches only several hundreds of micromeres. This is due to the
bandwidth limit of the photodetectors available today, which is in the picosecond range at best. Here, we improve the
time-of-flight precision to the nanometer regime by timing femtosecond pulses through phase-locking control of the
pulse repetition rate using the optical cross-correlation technique that exploits a second-harmonic birefringence crystal
and a balance photodetector. The enhanced capability is maintained at long range without periodic ambiguity, being well
suited to terrestrial lidar applications such as geodetic surveying, range finders and absolute altimeters. This method
could also be applied to future space missions of formation-flying satellites for synthetic aperture imaging and remote
experiments related to the general relativity theory.
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