Photosensitivity of germanium-doped optical fibers and its enhancement by strain

We review the photosensitivity of germanium-doped optical fibers to ultraviolet light, and especially the role of germanium oxygen deficient centers. We discuss the fabrication of gratings in optical fibers containing such centers.; We show that the growth of red luminescence during ultraviolet light exposure is not correlated to the refractive index change of the glass. We find that the magnitude of the refractive index change depends on the barrier energy for the photochemical transformation of germanium oxygen deficient centers. Therefore, the contribution of each defect to the refractive index change differs according to the defect's barrier height. We propose that the non-radiative energy released into the system determines the magnitude of the index change.; Ultraviolet-induced refractive index changes decay with time. The thermal stability is a function of the barrier energy of the germanium oxygen deficient centers. Defect centers with low barrier energies for grating formation also have low barrier energies for grating decay. Therefore, a uniform UV-light exposure of a fiber grating can functionally remove the contribution of low-barrier-energy sites and make the grating thermally stable. Using this method we fabricate fiber gratings stabilized by ultraviolet light only, eliminating the need for accelerated aging at elevated temperature.; We show for the first time that straining a Ge-doped optical fiber during cw ultraviolet light exposure enhances the fiber's photosensitivity. Strain is therefore an alternative to H₂-loading for enhancement of a fiber's photosensitivity. Refractive index change measurements were made using an in-fiber Mach-Zehnder interferometer. Straining a fiber enhances the photosensitivity of fibers both for Bragg gratings and long period gratings. Strain does not alter the ultraviolet absorption or the rate of photochemical transformation of the glass. Instead, our experiments on the ultraviolet-induced birefringence between the axial and radial directions of the fiber show that the relaxation of the axial stress in strained fibers during exposure accounts for the enhancement of photosensitivity by strain. This implies that optical fibers with enhanced photosensitivity can be fabricated by freezing high axial stress into the fiber core.