FIELD: physics, optics.
SUBSTANCE: invention relates to infrared light guides with a large mode field diameter. The light guide has a core and cladding which consists of rods arranged in a hexagonal order. The 98-112 mcm diameter core is made of silver bromide-based crystals containing a solid solution of monovalent thallium bromide-iodide (TIBr0.46I0.54), with the following ratio of components, wt %: silver bromide - 91.0-61.0; solid solution (TIBr0.46I0.54) - 9.0-39.0. The cladding contains rods with diameter of 42-48 mcm with distance of 70-80 mcm between centres thereof, with the following ratio of components thereof, wt %: silver bromide 92.0-64.5; solid solution (TIBr0.46I0.54) 8.0-35.5.
EFFECT: enabling operation at wavelength of 10,6 mcm, enabling propagation of only one lower-order mode within the fundamental band gap.