Distance measurement in air with a femtosecond frequency comb

A femtosecond frequency comb is a powerful tool in a wide range of metrological applications, one of them being in the field of distance measurement. Due to the locking of the repetition rate of the laser to a time standard, the distance between successive pulses is accurately known, providing direct traceability to the SI definition of the meter. Since the interpulse distance is typically of the order of 1 m, the range of non-ambiguity is large. Such accuracy is easily obtained with other methods like fime-of-flight measurements. Several schemes for distance measurement have been proposed and demonstrated. This paper reports on measurement of distances up to 20 m in air, that have been acquired using a scheme based on a Michelson interferometer. The interferometer consists of a long measurement arm and a short reference arm that can be adjusted such that the path length difference between both arms is a multiple of the interpulse distance. Once this is the case, a cross-correlation function can be measured by scanning the reflector of the reference arm with a piezo element. The path-length difference between both arms is then determined from the the center of the cross-correlation function and the interpulse distance, taking into account the refractive index of air.