Time-of-flight measurement with femtosecond light pulses

The time-of-flight of light pulses has long been used as a direct measure of distance^1,2, but state-of-the-art measurement precision using conventional light pulses or microwaves peaks at only several hundreds of micrometres^3,4. Here, we improve the time-of-flight precision to the nanometre regime by timing femtosecond pulses through phase-locking control of the pulse repetition rate using the optical cross-correlation technique ^5,6. Our experiment shows an Allan deviation of 117 nm in measuring a 0.7-km distance in air at a sampling rate of 5 ms once the pulse repetition is phase-locked, which reduces to 7 nm as the averaging time increases to 1 s. This enhanced capability is maintained at long range without periodic ambiguity, and is well suited to lidar applications such as geodetic surveying⁷, range finders⁸ and absolute altimeters⁹. This method could also be applied to future space missions involving formation-flying satellites for synthetic aperture imaging^10,11 and remote experiments related to general relativity theory¹².