Nulling interferometry operating in the mid-infrared (IR) range is one of the techniques being considered for the search of Earth-like exoplanets that may show biological activity. At these wavelengths many life-marker gases present absorption bands, and at the same time, the overwhelming flux of the parent star can be minimized. However, single-mode waveguides need to be used in this approach since they allow high angular resolution while reducing the star flux by ~50 dB. This may allow high spatial resolution measurements of the weak signals from biological activity to be observed against the high background of the light from the star. With this objective in mind, C. Vigreux, E. Barthélémy, and A. Pradel from the Université Montpellier II, in collaboration with L. Bastard and J.-E. Broquin from Minatec-INPG (Grenoble), M. Barillot and S. Ménard from Thales Alenia Space (Cannes), and G. Parent from Nancy-Université, France, fabricated single-mode waveguides operating in the 10–20 μm range based on Te-Ge-Ga glasses.
As reported in the August 1 issue of Optics Letters (DOI: 10.1364/OL.36.002922; p. 2922), the researchers selected this family of glasses because they are transparent between 6 μm and 20 μm wavelengths, films of these materials can be produced by thermal evaporation, their refractive index can be tuned according to their chemical composition, they present good stability, and they do not involve highly toxic or very volatile elements. In particular, the researchers chose Te75Ge15Ga10 as the substrate due to its thermal stability and its relatively low refractive index (3.3960 ± 0.0015 at 10.6 μm), and Te82Ge18 as the waveguide core layer, since it provides a refractive index difference with the substrate of ∆n = 4 × 10-2 and allows design of an efficient aperture for the coupling optics of 1.
With these components, the researchers designed single-mode rib waveguides operating in the 10–20 μm range, that they fabricated by depositing the Te82Ge18 core layer on Te75Ge15Ga10 polished glass disks using thermal co-evaporation. They then used standard UV photolithography to define the rib waveguides, and transferred the pattern to the core layer by reactive ion etching under an atmosphere of a mixture of CHF3, O2, and Ar to achieve a depth of 9 μm. Finally, they polished the input and output facets to obtain the waveguide.
The researchers reported that the overall transmission of these waveguides varies between 15% and 35% for a 1-cm-long device, after correcting from coupling and Fresnel losses, a value they considered very promising to prove the potential of the technology developed. In the future, the research team plans to use these waveguides as a wavefront filter on a nulling interferometer.
