2ND-HARMONIC GENERATION IN GE-DOPED SILICA FIBERS - EXPERIMENT AND THEORY

Raman and hyper-Raman studies of pure silica and Ge-doped silica fibre s reveals charge transfer from the Ge centre to silica in the fibres u nder pumping by the second harmonic. Self-organization of these charge -transfer excitons results in a strong phase-matched static electric f ield breaking the inversion symmetry and allowing frequency doubling. Owing to the long-range electron transfer the response of the medium i s very slow and a considerable retardation takes place during the prep aration of the fibre by light. This retardation makes the second-harmo nic amplitude grow at the stage of preparation. The retardation vanish es in the stationary state and the second-harmonic amplitude tends to its initial magnitude in the fully prepared fibre. Despite the fact th at the strong phase-matched electric field is still present, the frequ ency-doubling mechanism is now switched off.