Single mode mid-infrared silver halide asymmetric flat waveguide obtained from crystal extrusion

A flat waveguide for the middle infrared was made by co-extrusion of two silver halide crystals of different chemical compositions. The transmission of the waveguide and its modal behavior was studied using a Fourier Transform Spectrometer and a dedicated optical bench. Analyzing this spectrum, we were able to obtain the cut-off wavelength of the waveguide. We observed a single mode behavior for wavelengths longer than 8.83mum, in good agreement with the theoretically expected values. This novel procedure is ideal for tailoring the properties of the waveguide for specific applications, in particular the spectral range where it exhibits a single-mode behavior. It can thus be applied to achieve modal filtering for mid-IR astronomical interferometers (e.g. beam combiners, nullers, etc.).