Silver halide single mode fibers for modal filtering in the middle infrared

Modal filters are necessary to the proposed high-performance mid-infrared nulling interferometers, because they can help achieve deeper interferometric nulls. Silver halide fibers of composition AgCl_xBr_1-x (0<x<l) are leading candidates for these purposes, due to their high transparency in 4-20 μm spectral range. We have fabricated silver halide fibers with small cores and small differences between the refractive indices of the core and the clad. These operated as single mode fibers. An outer absorbing layer was applied to strip off cladding modes and reduce the minimum fiber length needed for modal filtering. Short sections of such fibers exhibited round and symmetrical optical field mode distributions with losses of 10-15dB/m at λ=10.6μm. We have tested the modal filtering properties of such short fiber sections. We found that the presence of unsuppressed cladding modes at the output is the main factor limiting the fiber's performance as a modal filter. This effect can be mitigated by appropriate aperturing of the output. With a properly sized aperture, a 10.5 cm long fiber can suppress the power in nonfundamental modes by a factor larger than 17000. If an aperture is not used, the suppression is reduced by a factor of ∼20.