Displacement measuring technique for satellite-to-satellite laser interferometer to determine Earth's gravity field

We present a new displacement measuring technique with simplicity, robustness, high sensitivity and wide measurement range. A set of a frequency shifter and a voltage–frequency converter is used to lock a homodyne interferometer on the half-bright fringe by eliminating the Doppler fringe resulting from mirror motion. The mirror displacement is directly retrieved from the feedback signal of a fringe control loop. By developing a table-top interferometer, we successfully demonstrated signal recovery without significant degradation. The achieved displacement sensitivity and measurement range of the interferometer were 24 nm Hz^−1/2 and 1.3 mm at a Fourier frequency of 0.1 Hz, respectively. This technique was found to have a potential for application to precise displacement measurements. It is, in particular, suitable for a satellite-to-satellite laser interferometer to determine Earth's gravity field