Absolute distance measurement based on asymmetric cross-correlation of femtosecond pulse

Abstract

Optical frequency comb is a kind of new pulse source, whose repetition rate and phase are locked. Optical frequency comb plays an important role in absolute distance measurement and time-frequency metrology. Lots of laser ranging methods such as time-of-flight and multi-heterodyne interferometry based on femtosecond laser pulse have been used in distance measurement. In this paper, a high-precision distance measurement system based on optical sampling by cavity tuning is set up to realize a long absolute distance measurement. And a kind of error compensation method is proposed based on the asymmetric cross-correlation patterns. In traditional optical sampling by cavity tuning measurement system, the fiber link is inserted into the reference path to extend the non-ambiguity distance, which does not have a good performance in arbitrary distance measurement. In our system, we use a 116-meter-long fiber which is inserted into the measuring path to extend the non-ambiguity distance. Besides, dispersion compensation technique is used to control the shape of the laser pulse. An asymmetric optical pulse is used as the light source, so that we can obtain extremely asymmetric cross-correlation patterns. The cross-correlation patterns can be acquired by sweeping the repetition frequency. We use an arbitrary waveform generator to provide the scanning voltage, and the scanning voltage can adjust the repetition rate of the pulse and has a frequency of 1 Hz. There will be two peaks on the envelope of cross-correlation pattern, and both peaks can be used to obtain the distance information. When the laser propagates in vacuum and the system is stabilized, the distance between these two peaks is constant, and we can use this distance to obtain the important factor N, which is used to describe the number of the pulse. As a result, we can realize absolute distance measurement without the help of other measurement systems. However, due to the dispersion of the medium, the distance between these two peaks is not constant, which means that the asymmetry of the cross-correlation patterns in dispersion medium will influence the measurement results. And the deviation is relevant to the peak-to-peak distance. We use the difference among the peak-to-peak distances at different positions to correct the measurement results. A comparison of our results with those from a commercial He-Ne laser interferometer shows that they are in agreement within 2 μm over 50 m distance, corresponding to a relative precision of 1.9×10-7.