The Nulling Interferometer Cryogenic Experiment - The Warm Phase

Context - The Large Interferometer for Exoplanets (LIFE) is a proposed space mission to characterise the atmosphere of terrestrial exoplanets, which is planned to operate in the mid-infrared wavelength region from 6 µm to 16 µm. A key requirement needed to study the feasibility of this mission is to demonstrate broadband nulling at cryogenic temperatures (15 K), at flux levels comparable to the astronomical sources that LIFE will detect. The Nulling Interferometer Cryogenic Experiment (NICE) is a technology demonstrator built to fulfil this purpose. Aim - The objective of NICE is to demonstrate a broadband null with a null depth of 10⁻⁵ and stability of 10⁻⁸ while mantaining a high system throughput, and consequently a high level of sensitivity, sufficient to detect an Earth twin at 10 pc. We describe the optical requirements, the current progress of NICE in the warm phase, and future plans. Methods - NICE is a Single-Bracewell nuller with closed loop optical path-length control, currently operating at ambient conditions. We use a 3.85 µm laser with 150 nm bandwidth to demonstrate achromatic nulling capability, and a narrowband (< 0.5 nm bandwidth) 4.5 µm laser to demonstrate stability. Results - We achieve an achromatic null depth of 4.39 · 10⁻⁴ with a stability of σ = 5.02 · 10⁻⁴ over a duration of 60 s without closed loop control, and a stabilised narrow-band null of 2.05 · 10⁻⁴ with (Equation presented) over a duration of 120 s. Conclusions - NICE has both demonstrated achromatic operation and closed loop control to stabilise the null. However, the mean null depth and the null stability achieved do not yet meet the requirements, by a factor of 20 and 10⁴ respectively. This will be improved in future iterations.