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Nulling interferometry is an important technique under development for the DARWIN planet finding mission which enables the detection of the weak infrared emission lines of an orbiting planet. This technique requires the use of single mode optical fibers transmitting light as far as possible in the infrared. Tellurium based glasses transmit infrared light far beyond the second atmospheric window but are usually not stable enough toward crystallisation to be drawn into optical fiber. It was recently shown that the GeTe4 initial glass composition could be stabilized by the introduction of few percents of Se. In this paper, we report the development and the optical characterisation of far-infrared transmitting step index optical. The evolution of the refractive index of the glass versus the ratio Se/Te was measured in order to design a multi mode step index fiber. Its core diameter was 200 μm therefore permitting to easily characterise its optical properties. This multimode fiber transmitted light from 4 to about 15 μ m with losses near 20 dB.m-1. Despite these significant losses, the fiber was efficiently used to detect the broad absorption band of the CO2 located at 15 μm. These results are very promising in the context of the DARWIN mission of the European Space Agency.