Experimental demonstration of an on-axis laser ranging interferometer for future gravity missions

We experimentally demonstrate an interferometric architecture for next-generation gravity missions, featuring a laser ranging interferometer (LRI) that enables monoaxial transmission and reception of laser beams between two optical benches with a heterodyne frequency of 7.3 MHz. Active beam-steering loops, utilizing differential wavefront sensing signals, ensure coalignment between the receiving beam and the transmitting beam. With spacecraft attitude jitter simulated by hexapod-driven rotations, the interferometric link achieves a pointing stability below 10 μ rad / Hz in the frequency range between 0.2 mHz and 0.5 Hz, and the fluctuation of the transmitting beam’s polarization state results in a reduction of 0.14% in the carrier-to-noise-density ratio over a 15 h continuous measurement. Additionally, tilt-to-length coupling is experimentally investigated using the periodic scanning of the hexapod. Experimental results show that the on-axis LRI enables the interspacecraft ranging measurements with nanometer accuracy, making it a potential candidate for future GRACE-like missions.