Long baseline optical interferometry, at facilities such as the CHARA array and the VLTI, can facilitate angular resolutions that significantly exceed those provided by a single telescope. As such, long baseline optical interferometry is a key technique in many areas of astronomy . One key component of an operational interferometer is the beam combiner, which interferometrically combines the light collected by the individual telescopes. Beam combiners are usually constructed using bulk optics, but this becomes increasingly challenging as the number of telescopes in the array increases. One approach to solve this is to develop beam combiners based on integrated optics, where the light is routed and combined using integrated optic (IO) waveguides. Such “astrophotonic” beam combiners have already enabled impressive results, playing a key role in instruments such as GRAVITY . Given that the advantages of IO beam combiners are now acknowledged, there is significant interest in developing IO beam combiners that can provide advanced beam combination capabilities e.g., efficient operation in the mid-IR. The lack of materials suitable for developing IO circuits in mid-IR with the traditional lithographic processes, has driven the exploration of ultrafast laser inscription (ULI) as an IO beam combiner manufacturing process, since it can be used to produce optical waveguides in a wide range of glasses .