Absolute distance measurement system using a femtosecond laser as a modulator

The generation of broadband microwave frequency comb from a femtosecond pulse train by direct photodetection opens the possibility for high-accuracy length measurements of long distances. We demonstrate a relatively simple realization of this measurement principle: an electronic distance measurement system based on a time-of-flight approach, driven by a femtosecond fibre laser source as a modulator. By the evaluation of the phase shifts of two distinct comb frequencies, a coarse and a fine measurement of the absolute distance can be performed. The range of the measurement system is demonstrated up to a length of 100 m. The experimental comparison of the femtosecond laser system with a conventional reference counting interferometer shows a precision better than +/-10 mu m at 100 m, corresponding to a relative measurement uncertainty of 1 X 10-7 L. The limiting factors for the measurement uncertainty of the system are theoretically investigated and shown to be of the same order of magnitude.