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An accurate metrology system is required to stabilize the differential path lengths in the Nulling Interferometry Cryogenic Experiment (nice) to within 0.45nm peak-to-peak to achieve broadband mid-infrared nulls with long exposure times, which are required for potential future space missions, such as the Large Interferometer for Exoplanets (life) mission, that aim to directly image and characterize temperate terrestrial exoplanets. For this purpose, a differential heterodyne laser distance metrology is developed to enable differential path length measurements that are stable over multiple days with sub-nanometer accuracy at a bandwidth of 1 kHz. The system aims to solve several challenges that arise in the context of NICE, such as the need for long-term stability, the high intensity attenuation through the NICE beam path, and the requirement that the metrology be able to deliver low-latency feedback for closed-loop operation to compensate vibrations and drifts of the nulling testbed. The metrology uses a 633nm HeNe laser and operates at ambient temperature and pressure with a beat frequency of 10 kHz, which is generated by acousto-optic modulators. To improve long-term stability, the compact optical layout is optimized for low susceptibility to temperature variations. Over periods of 2 s, the intrinsic instability of the metrology is ≈ 80pm RMS when sampling at 10 kHz, and it is stable to within ≈ 0.5nm peak-to-peak for 2 hours. When correcting the distance measurements for the temperature of the metrology board, it is stable to within ≈ 1nm peak-to-peak for 14 hours. The metrology fulfils the stability and bandwidth requirements for nice for a duration of at least 2 hours. To achieve stability over even longer time periods, the metrology will later be placed in a temperature-controlled vacuum environment.
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