Beam metrology and control for the Nulling Interferometry Cryogenic Experiment

We present our progress in stabilising the warm precursor of the Nulling Interferometry Cryogenic Experiment (nice), a laboratory testbed demonstrating key mid-infrared nulling technologies for the Large Interferometer for Exoplanets (life). To fulfil the preliminary requirements of nice, the optical path length difference (opd) between the beams has to be controlled to within 0.8 nm rms, while beam pointing and shear have to be controlled to within ≈ 1 µm and ≈ 1 µrad rms, respectively. A heterodyne laser metrology system was implemented to monitor opd and shear in nice, with pointing measurements following soon. The metrology beams follow the science beams with minimal non-common paths, and modulation techniques enable simultaneous measurements of all critical paths through the testbed at 1 kHz analog bandwidth. We use quadrant photodiodes and a small number of components in a compact layout, simplifying future cryogenic or space-based implementations. The noise of the metrology measured in a mock-up is < 0.8 nm rms in opd, and < 85 nm rms in beam position, with crosstalk between the beams lower than the sensor noise, despite performing multiple measurements on the same detector with overlapping metrology beams. In the warm precursor of nice, we achieve closed-loop opd control to within 14 nm rms, which enables an improvement in null depth and stability by a factor of ≈ 10, reaching a 2 ∙ 10⁻⁴ average null over a two-minute period. The metrology and control systems thus provide both an accurate diagnostic and characterisation tool for nice, as well as a means to achieve deeper and more stable nulls. The main limitation is a 48 Hz vibration from mechanical resonances in the setup, which we will mitigate with an improved mechanical design.