Upon irradiation with laser light of specific wavelengths and intensities optical fibers can undergo a slow (minutes to hours) permanent change, which results in new optical phenomena that were not present before irradiation. These phenomena include photoinduced refractive index gratings and photoinduced second harmonic generation in doped silica fibers. Since these phenomena are the result of a photoinduced periodic modification of the optical properties of the material with a periodicity that is determined by the wavelength(s) of irradiation, they are examples of photoinduced organization in glasses. Although the physics of these phenomena is not understood in detail, there is ample experimental evidence that the presence of optically active centers in the glass is of crucial importance for the observation of these phenomena. The optically active centers are due to defects or dopants in the glass. Here we discuss photoinduced SHG in Ge-doped silica fibers and the use of luminescence spectroscopy to characterize different types of Ge-related defects in the fiber.
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