The gamma-ray induced behavior of a saturable absorber in outer space was tested by pump-probe experiments using a femtosecond laser. The test reveals that after gamma-ray exposure of ~120 krad, the modulation depth and saturation fluence of the saturable absorber vary from 6 to 3 % and 40 to 105 μJ/cm 2 , respectively, without no conceivable change in the relaxation time.