The evolution of the index change of type-IIa gratings observed in 28 mol % Ge-SiO2 core fibers with 1.8 mu m core diameter under various strains was measured from the optical spectra, and the induced defects at high and low strains were studied with electron spin resonance. Data will be presented to show that the index modulation (Delta n(mod)) of type-IIa gratings is likely associated with Ge E-' centers. (C) 1999 American Institute of Physics. [S0003-6951(99)04141-8].