Development of low-loss mid-infrared ultrafast laser inscribed waveguides

Abstract. The mid-infrared (mid-IR) is a spectral region (≈2 to 20  μm) that is of key importance in astronomy for applications such as exoplanet imaging and spectroscopic analysis. Long baseline stellar interferometry is the only imaging technique that offers the possibility to achieve milli-arcsecond angular resolution in the mid-IR. At the heart of such an interferometer is the beam combining instrument, which enables coherent beam combination of the signals from each baseline. In comparison to bulk-optic beam combiners, beam combiners that utilize photonic planar light wave circuits for interferometry provide a more scalable and stable platform. The current generation of beam combination circuits are fabricated using conventional fabrication technologies, using silica-based materials, and are thus not suitable for operation in the mid-IR. There is, therefore, a need to explore more unconventional waveguide fabrication technologies, capable of enabling the fabrication of low-loss mid-IR waveguides and photonic beam combining circuits. We report on the development of low-loss single-mode waveguides in a gallium lanthanum sulfide glass using ultrafast laser inscription. The optimum waveguides are found to exhibit a propagation loss of 0.25±0.05  dB cm−1.