Origin and applications of photosensitivity in germanium-doped silica optical fibers.

Abstract

The photosensitivity of germanium-doped silica glass is studied in optical fiber waveguides excited with visible light. A model is developed to describe the growth dynamics of Bragg gratings photo-induced using the longitudinal writing technique. The model assumes that the change of the dielectric constant is proportional to the square of the local light intensity. Good agreement with the experimental results is obtained by including the thermal effects in the calculations. The photosensitive response is measured as a function of the excitation wavelength. The results establish a clear link between the photosensitivity phenomenon and an ultraviolet absorption band previously attributed to a reduced germanium defect. The intensity of the ultraviolet absorption band is measured by monitoring the associated photoluminescence signal. The relative concentration of defect centers induced by treatment of the optical fibers under reducing atmosphere is deduced from the photoluminescence measurement. The photosensitivity of the optical fibers is qualitatively related to the decrease of the photoluminescence signal observed during the exposure of the fibers to high intensity of visible laser light. The linear and two-photon absorption cross-sections of the defect center are also estimated from the dependence of the luminescence signal on the excitation intensity. The application of photo-induced Bragg gratings to vibration sensing is demonstrated. The amplitude and frequency of the vibrations are detected by monitoring the reflection or the transmission of a probe beam close to the filter resonance. The strain resulting from the presence of vibration modulates the frequency response of the grating. Cross-phase modulation was also used to modulate the transmission of a CW probe beam. The all-optical switching induced by short detuned pump pulses, propagating co-linearly with the probe beam, is investigated numerically and experimentally.