Photoinscription of Bragg gratings within preform plates of high NA germanosilicate fibers: Searching for an experimental evidence of type II A photosensitivity in preform plates

Bragg gratings were written in the core of 0.1 mm thick preform plates doped by high concentration of germanium. The thick holograms were written using either pulsed or continuous UV light for long exposure time. Temporal growth in grating efficiency was measured at a probe wavelength of 633 nm in the course of the photoimprinting. The dynamics of grating inscriptions in the preform cores have been compared with those recorded when writing gratings within fibers drawn from these preforms. Experiments have shown that the type II A photosensitivity observed in high NA germanosilicate fibers cannot be found when photoimprinting the gratings within the corresponding preform plates. Implications of this observation are discussed.