Two-photon absorption coefficients of optical fibers with 240-nm absorption band

Summary form only given. Photosensitivity in germanosilicate fibers has been known to relate to 240-nm absorption band of Ge-related oxygen-deficiency defects. In the blue-green region, Hill-grating formation and second-harmonic generation are attained by two-photon (multiphoton) absorption. However, the spectral dependence of two-photon absorption (TPA) coefficients has not been made clear. The 240-nm absorption and the photosensitivity have been attributed mainly to Ge-Si (or Ge-Ge) defects. However, the density of Ge/sup 2+/ (=Ge:) defects is two orders of magnitude higher. We have previously shown that Ge/sup 2+/ defects are dominant for UV-induced refractive index changes. Here we measure the TPA spectrum of germanosilicate fiber and discuss the origin of TPA.