r to estimate the radiation sensitivity of a data transmission system based on optical fibers, the radiation- induced loss of other included passive components has to be known, too. We have irradiated (at room temperature) a variety of couplers made by different manufacturers at a $+60$/Co source with a dose rate of 1300 rad(SiO$-2$/)/min up to a total dose of 10$+6$/ rad(SiO$-2$). Most of them were fabricated by the Fused Biconical Taper process of 200/280 $mu@m multimode step index fibers (tested at $lambda $EQ 850 nm), and of 10/125 $mu@m single mode fibers (tested at $lambda $EQ 1300 nm and $lambda $EQ 1550 nm). Couplers with 50/125 $mu@m gradient index fibers (tested at $lambda $EQ 850 nm and $lambda $EQ 1300 nm) were also among them. The measured coupler inherent losses were between about 0.01 dB (fused coupler, high OH MM step index fibers, $lambda $EQ 865 nm) and about 50 dB (coupler made of GRIN lenses, $lambda $EQ 865 nm).
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