Figure 2

The figure shows a measurement of the length fluctuations of an optical resonator used to test the suspension point stabilization. The purple curve is taken from Fig. 1d and shows the expected length change of the optical resonator at its suspension point with a SPI system and matches the measured length change of the resonator in blue at frequencies below its suspension resonance. The green, red, and yellow curves show the length of the optical resonator without length, pitch, and yaw controls at the seismic platform. It is clear that pitch and yaw control by the SPI is essential for good performance. A simplified schematic shows the setup of the experiment, and the photo shows two of the three suspended mirrors of the optical resonator. The third mirror is located on an AEI-SAS about 10 m away.