NICE

NICE is a laboratory test bench for the LIFE space mission, built at the Exoplanets and Habitability group at ETH Zürich. NICE will demonstrate that a nulling interferometer can achieve the stability and sensitivity required to detect Earth-like exoplanets around sun-like stars.

Previous nulling testbeds, such as the Planet Detection Testbed (PDT) at JPL (Martin 2012) have demonstrated stable nulls over tens of minutes of operation. However, the setup’s throughput was too low to detect planets with realistic flux levels, and never transitioned to cryogenic operation.

To address these two technological gaps, NICE will be the first vryogenically cooled nulling interferometer, operating at 15 K to suppress the thermal background, and also the first nulling testbed to achieve the high throughput required for planet characterisation of ≈ 20 %. We aim for similar stability levels as the PDT of ≈ 0.8 nm RMS in optical path length difference, and ≈ 0.05 % intensity mismatch drift. A more detailed list of requirements can be found in external page Ranganathan2024.

A narrowband 4.6 µm Quantum Cascade Laser and a red alignment laser are the main sources used for the setup. They are overlapped on a dichroic and then enter the star simulator, which splits a single science beam into two beams that are identical in polarisation and chromatic errors. The outputs of the star simulator then represent the entrance apertures of the beam combiner spacecraft of LIFE. A planet simulator will later be installed to simulate an off-axis planet. 

Two pairs of metrology beams from a 1.5 µm laser are also injected in the star simulator. They follow the science beams and monitor any disturbances in the beam paths, such as changes in the optical path lengths, beam positions, and beam angles. These measurements are used to control tip/tilt states and delay lines, to correct for the errors. 

The achromatic phase shifter – shown only schematically in the diagram – is a three-dimensional periscope that inverts the pupil of one beam relative to the other, using only reflections by mirrors. This produces the required relative phase shift of 180 degrees between the beams for nulling.

 The science beams are combined in a modified Mach-Zehnder interferometer and coupled into a spatial filter to greatly reduce wavefront errors. A camera after the spatial filter records the nulled output intensity.

Currently, we achieve instantaneous null depths of ≈ 2e-5, and average nulls of ≈ 5e-5 during a one minute recording. A short section of a null measurement is shown below. Work on meeting our null depth (<1e-5) and stability (<1nm RMS) requirements are ongoing, and we look forward to achieving these in the near future.  

There are several areas that have to be addressed for NICE to achieve the requirements:

• Throughput improvements
• Simultaneous nulling of both polarisation modes
• Broadband nulling
• Stabilising beam positions and angles with closed-loop control to improve intensity stability
• Implementation of a subsystem to mimic a planet signal
• Transition to a cryogenic setup

There are several requirements for NICE to go cryogenic. But one of the most important requirements is repeatability. In cryogenic conditions, the components of NICE are not accessible. So, the optics have to be built with a predetermined alignment procedure, that can produce functional units of NICE in a repeatable manner. An alignment procedure is devised and the GIF below shows the alignment of NICE in steps of functional units.

Once the alignment is completed, active elements viz., the delay line and tip-tilt mirrors (shown with blue and green arrows in the optical setup) is used to maintain the alignment of the setup. A metrology system is used in the setup to measure the drift in the alignment and forms a closed loop feedback with the active elements to correct for the drift.  

For More information on NICE refer to this paper

Jonah Hansen -  
Thomas Birbacher –

The NICE Team

In alphabetical order:

●     Adrian M. Glauser (Head of labs)

●     Eckhart Spalding (Postdoc)

●     Germain Garreau (Postdoc)

●     Jonah T. Hansen (Postdoc)

●     Julio P. Jiménez (Optical engineer)

●     Sascha P. Quanz (PI of LIFE)

●     Thomas Birbacher (PhD student)