High-accuracy measurements and analysis of refractive index change induced by photodarkening and thermal bleaching in ytterbium-doped fibers are presented, based on a modal interference method. Photodarkening-induced refractive index change is positive at the ytterbium lasing wavelengths near 1080 nm, and it approaches a saturated level, which is in the order of 10~(-6) approx 10~(-5) for the tested fiber samples. It is found that the value of this refractive index change is linearly proportional to the photo-darkening-induced excess loss at an arbitrary probe wavelength in the visible band. Thermal bleaching can only partially erase the photodarkening-induced refractive index change, leaving a residual index change of (0.2 approx 0.3) X 10~(-5). The influence of the photodarkening-induced refractive index change on fiber lasers is discussed.
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