Chalcogenide and Silver-Halide Single-Mode Fibers for Modal Filtering within the European DARWIN Mission

The DARWIN mission should allow to study the new extrasolar planets and to discover potential traces of primitive life around 2025. Today employed methods in the search for exoplanets are indirect:

  • the planets are detected by observing and measuring the movements which they impose on their central star;
  • the planets are detected by the effects they have on the brightness of the star.

The DARWIN concept is one of an instrument that is capable of separating the light of a planet and that of its central star in order to allow exactly that spectral analysis. It is one of the few instruments that would be capable to detect molecules in the planetary atmosphere such as carbon dioxide (CO₂), water (H₂O), ozone (O₃), and, therefore, oxygen (O₂). The simultaneous presence of oxygen in large amounts, of water, and of carbon dioxide appears to be a good indicator of photosynthesis. The DARWIN mission could gather in near future the first indications that would allow answering the fundamental question: “Does life exist exclusively on Earth?”.

The concept of the DARWIN mission is based on a phenomenon of nulling interferometry which allows for visual detection and interferometric imaging of terrestrial exoplanets orbiting Sun-like stars. Nulling interferometry is a type of interferometry in which two or more signals are mixed to produce observational regions in which the incoming signals cancel themselves out. Such sets of “blind spots” prevent unwanted signals from those areas from interfering with weaker nearby signals. The DARWIN mission was suggested ESA ( the European Space Agency) for detection Earth-like planets orbiting nearby stars and search for evidence of life on these planets.

The DARWIN itself is an interferometric observatory which operates in the mid-infrared. The bandwidth of the instrument ranges from 6.5 μm to 20 μm as absorption lines of the main biomarkers shall be identified at 7-8 μm (methane), 9.6 μm (ozone), 15 μm and 18 μm (carbon dioxide), and 6-8 respectively 17-20 μm (water). A nulling interferometer provides both high on-axis light suppression and high angular resolution.

The ideal wavefront filter is broadband and would cover the entire DARWIN band. The different wavefront filters are inserted after the interferometer core and will not increase the complexity of the instrument. The fiber can be optimized for each subband and different materials can be implemented, for example, chalcogenide glass for the lower DARWIN band and silver halides for the upper band.

Chalcogenide IR-filters are manufactured by drawing preforms or by crucible drawing. If preforms are required they are fabricated by rod-in-tube techniques or by casting methods where the melted core material is cast into a cladding tube. Longer wavelengths can be handled by using polycrystalline materials like silver halides. Such polycrystalline materials must be extruded to thin fibers under high pressure and elevated temperatures. The preforms are extruded several times until a final extrusion produces the thin single-mode fiber to obtain the thin core diameters. The preforms are fabricated:

  • by the mechanical combining of core rods and cladding tubes;
  • by dropping core material into cladding tubes with the aid of capillary forces;
  • by special preform growth methods combining core and cladding directly from the melt avoiding any contamination within the forming process.

Polycrystalline fibers have been manufactured from all three methods.

Single-mode fibers have been successfully produced from extruded polycrystalline silver halide and from drawn chalcogenide glass. Silver halide fibers can be used within the entire DARWIN band from 6.5 – 20 microns whereas chalcogenide IR-fibers are suited up to about 11 microns.

Core / Clad Chalcogenide Infrared (CIR-) fibers are transparent over a wide spectral range from 1 to 6 μm, therefore covering a spectral gap between silica and polycrystalline fibers. CIR fibers are used in various applications, including spectroscopy and process monitoring tasks, IR radiation delivery to and out closed volumes, thermosensing, laser power flexible delivery systems, IR-imaging, etc.

If you would like to by Optromix Chalcogenide IR-fibers, please contact us at info@optromix.com

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