Thermal stability of photoinduced gratings and paramagnetic centers in Ge- and Ge/P-doped silica optical fibers

The electron-spin-resonance spectra of paramagnetic defects in Ge/P-doped silica fibers prepared with second-harmonic generation (SHG) are compared with those induced in Ge- and Ge/P-doped silica preforms by 5-eV photons. Among the defects observed, the thermal stability of Ge E′-type centers is similar to that of photoinduced Bragg grating efficiency but not of SHG efficiency. The isochronal anneal curves of the Ge(1) and Ge(2) centers in Ge/P-doped silica correlate well with SHG efficiency, but the agreement is not nearly so good in silica containing only Ge. Only the thermal behavior of photoinduced $\text{Ge}\hspace{0.17em}{E}_{d1}^{\prime }$ centers is similar to that of SHG efficiency in both materials, albeit only in the initial stages of annealing.