Examples of optical path-length laws for two batches of Fibered Differential Delay Lines (FDDL). FDDL E, F, G, H were built with the first generation of GRAVITY fibers while I, J, K, L were built with the second generation of GRAVITY fibers. An optical delay of up to 6 mm is built in both cases. The command is the piezo extension for E, F, G, H while it is a voltage for I, J, K, L to show the amount of hysteresis corrected by the Physik Instrument E-621 servo controller.