Optical frequency comb technology opened up new fields not only in frequency metrology but in length metrology. Frequency-traceable length measurement using comb provides direct realization of the definition of meter, remote calibration using a GPS technology, and precise measurements of wide range of lengths by taking advantage of high dynamic range in frequency measurements. However, the potential in length metrology has not been fully explored yet. Here we report new techniques using optical combs for small displacement and long absolute-distance measurements. The displacement measurement uses Fabry-Perot cavities and wide-range frequency measurements. A reference cavity compensates the effects of the environmental fluctuation and achieves 10-pm stability for 30-minutes measurement even in air. In the long distance measurement, phase measurements of the intermode beats of a comb realize high stability and small cyclic errors of better than 2 micrometers for 240-m distance. A developed compact setup shows extremely small slope error of less than 1×10-7 over 200-m distance.
