Silver halide single mode fibers for broadband middle infrared stellar interferometry

We report the demonstration of silver halide single mode fibers at lambda=7.5 mu m. The fiber's modal behavior was tested at lambda=10.6 and lambda=7.5 mu m, using near field measurements. Single mode operation was observed for both wavelengths with propagation losses of similar to 16 and similar to 40 dB/m at lambda=10.6 and lambda=7.5 mu m, respectively. A rejection ratio of similar to 30 dB was achieved at lambda=7.5 mu m for a 30 cm long fiber segment. These fibers would be particularly useful for modal filtering in the 6-20 mu m band. Modal filters are essential technology elements in nulling stellar interferometry.